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Raman, infrared and microwave spectra, conformational stability,R0 structural parameters and vibrational assignments of some organoamines,organophosphines, alcohols and substituted four and five membered rings

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RAMAN, INFRARED AND MICROWAVE SPECTRA, CONFORMATIONAL STABILITY, R0
STRUCTURAL PARAMETERS AND VIBRATIONAL ASSIGNMENTS OF SOME
ORGANOAMINES, ORGANOPHOSPHINES, ALCOHOLS AND SUBSTITUTED
FOUR AND FIVE MEMBERED RINGS
A DISSERTATION IN
Chemistry
and
Geosciences
Presented to the Faculty of the University
of Missouri-Kansas City in partial fulfillment of
the requirements of the degree
DOCTOR OF PHILOSOPHY
by
IKHLAS DAOUD DARKHALIL
BACHELOR IN CHEMISTRY
University of Missouri-Kansas City, 2008
Kansas City, Missouri
2014
UMI Number: 3626544
All rights reserved
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a note will indicate the deletion.
UMI 3626544
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IKHLAS DAOUD DARKHALIL
ALL RIGHTS RESERVED
RAMAN, INFRARED AND MICROWAVE SPECTRA, CONFORMATIONAL STABILITY, R0
STRUCTURAL PARAMETERS AND VIBRATIONAL ASSIGNMENTS OF SOME
ORGANOAMINE, ORGANOPHOSPHINES, ALCOHOLS AND SUBSTITUTED
FOUR AND FIVE MEMBERED RINGS.
Ikhlas Darkhalil, Doctor of Philosophy
University of Missouri-Kansas City, 2014
ABSTRACT
The infrared and Raman spectra of compounds with amino, phosphours, silane and
hydroxyl functional groups, as well as some with cyclic skeletal structures have been recorded of
the gas and in condensed phases. Temperature dependent infrared and Raman spectra in xenon
solutions were also recorded. A complete vibrational assignment, conformational stability
determination and adjusted r0 parameters have been obtained for each of the most stable
conformers and in some cases for the less stable conformers. The vibrational assignments were
supported by normal coordinate calculations with scaled force constant from MP2(full)/6-31G(d)
calcualtions from which the fundamental vibrational frequencies, infrared intensities, Raman
activities, depolarization ratios and infrared band contours were predicted.
For ethylamine the enthalpy difference has been determined to be 62 ± 6 cm-1 (0.746 
0.072 kJ mol-1) with the trans conformer the more stable form. For isorpropylamine, the enthalpy
difference of the sample dissolved in Raman xenon has been determined to be 113  11 cm-1 (1.35
 0.13 kJ mol-1) with the trans conformer the more stable form. For n-propylamine, the five
possible conformers have been identified and their relative stabilities obtained with enthalpy
difference relative to Tt for Tg of 79 ± 9 cm-1 (0.9 ± 0.1 kJ/mol); for Gg of 91 ± 26 cm-1 (1.08 ±
0.3 kJ/mol); for Gg' of 135 ± 21 cm-1 (1.61 ± 0.2 kJ/mol); for Gt of 143 ± 11 cm-1 (1.71 ± 0.1
kJ/mol). For 2-cyanoethylamine, the enthalpy differences between the Gg and Gt conformers was
iii
determined to be 75 cm-1 and for the Gg to Tg form 333 cm-1. For 2,2difluoroethylamine, the
enthalpy differences have been determined among the most stable Tt conformer and the second
stable conformer, Gg, to be 83 ± 8 cm-1 (0.99 ± 0.10 kJ/mol), the third stable conformer, Gt, to be
235 ± 11 cm-1 (2.81 ± 0.13 kJ/mol). For 2,2,2 trifluoroethylamine, the enthalpy difference has been
determined to be 267 ± 27 cm-1 (3.19  0.32 kJ mol-1) with the trans conformer the more stable
form. For 2,2,3,3,3-pentafluoropropylamine, the enthalpy difference has been determined between
the more stable Tt conformer and the less stable Tg form to be 280 ± 14 cm-1 (3.35 ± 0.17 kJ/mol).
In case of cyclopentylamine, the four possible conformers have been identified and their relative
stabilities obtained with enthalpy difference relative to t-Ax of 211 ± 21 cm-1 for t-Eq ≥ 227 ± 22
cm-1 for g-Eq ≥ 255 ± 25 cm-1 for g-Ax. For cyclohexylamine, the four possible conformers have
been identified as t-eq> g-eq>t-ax>g-ax.
Microwave spectra for several of the molecules have been investigaged from 10,000 to
21000 MHz with transtions for the most stable conformer and in some cases for the less stable
conformers. By utilizing the rotational constants reported from microwave studies combined with
the structural parameters predicted from the MP2(full)/6-311+G(d,p) calcualtions, adjusted r0
strutral paramerters have been obtained for the most stable conformer(s) of the different molecules
studied.
iv
APPROVAL PAGE
The faculty listed below, appointed by the Dean of the College of Arts and Sciences have
examined a dissertation titled “Raman, Infrared and Microwave Spectra, Conformational Stability,
R0 Structural Parameters and Vibrational Assignments of Some Organoamines, Organophosphines,
Alcohols And Substituted Four And Five Membered Rings,” presented by Ikhlas Darkhalil,
candidate for the Doctor of Philosophy in Chemistry and Geosiences degree, and certify that in
their opinion it is worthy of acceptance.
Supervisory Committee
Professor James R. Durig, Committee Chair
Department of Chemistry
Professor Keith R. Buszek.
Department of Chemistry
Professor Zhonghua Peng
Department of Chemistry
Professor Jejung Lee
Department of Gesciences
Professor James B. Murowchick
Department of Gesciences
v
CONTENTS
ABSTRACT...................................................................................................................................... iii
LIST OF TABLES ............................................................................................................................ xi
LIST OF ILLUSTRATIONS .......................................................................................................... xix
ACKNOWLEDGEMENTS ........................................................................................................... xxv
Chapter
1. INTRODUCTION ......................................................................................................................... 1
2. EXPERIMENTAL AND THEORETICAL METHODS .............................................................. 7
3. MICROWAVE SPECTRA AND CONFORMATIONAL STUDIES OF ETYHL- AMINE
FROM TEMPERATURE DEPENDENT RAMAN SPECTRA OF XENON SOLUTIONS AND
AB INITIO CALCULATIONS ....................................................................................................... 13
Introduction .................................................................................................................................. 13
Experimental and Theoretical Methods ....................................................................................... 15
Microwave Results....................................................................................................................... 18
Vibrational Assignment ............................................................................................................... 18
Conformational Stability.............................................................................................................. 21
Structural Parameters ................................................................................................................... 23
Discussion .................................................................................................................................... 25
4. CONFORMATIONAL AND STRUCTURAL STUDIES OF ISOPROPYL-AMINE FROM
TEMPERATURE DEPENDENT RAMAN SPECTRA OF XENON SOLUTIONS AND AB
INITIO CALCULATIONS .............................................................................................................. 41
Introduction .................................................................................................................................. 41
Experiment and Theoretical Methods .......................................................................................... 45
Vibrational Assignment ............................................................................................................... 47
Conformational Stability.............................................................................................................. 50
Structural Parameters ................................................................................................................... 52
Discussion .................................................................................................................................... 54
5. CONFORMATIONAL AND STRUCTURAL STUDIES OF N-ROPYLAMINE
PROPYLAMINE FROM TEMPERATURE DEPENDENT RAMAN AND FAR INFRARED
SPECTRA OF XENON SOLUTIONS AND AB INITIO CALCULATIONS .............................. 78
Introduction .................................................................................................................................. 78
Experimental ................................................................................................................................ 80
Ab Initio Calculations .................................................................................................................. 82
Vibrational Assignment ............................................................................................................... 83
Conformational Stability.............................................................................................................. 86
Structural Parameters ................................................................................................................... 88
Discussion .................................................................................................................................... 89
vi
6. INFRARED AND RAMAN SPECTRA, R0 STRUCTURAL PARAMETERS,
CONFORMATIONAL STABILITY, AND VIBRATIONAL ASSIGNMENT .......................... 115
OF 2-CYANOETHYLAMINE ...................................................................................................... 115
Introduction ................................................................................................................................ 115
Experimental and Theoretical Methods ..................................................................................... 117
Vibrational Assignment ............................................................................................................. 120
Conformational Stability............................................................................................................ 121
Structural Parameters ................................................................................................................. 123
Discussion .................................................................................................................................. 124
7. CONFORMATIONAL AND STRUCTURAL STUDIES OF 2,2-DIFLUORO-ETHYLAMINE
FROM TEMPERATURE DEPENDENT INFRARED................................................................. 142
SPECTRA OF XENON SOLUTION AND AB INITIO .............................................................. 142
CALCULATIONS ......................................................................................................................... 142
Introduction ................................................................................................................................ 142
Experimental and Theoretical Methods ..................................................................................... 144
Vibrational Assignment ............................................................................................................. 147
Conformational Stability............................................................................................................ 149
Structural Parameters ................................................................................................................. 151
Discussion .................................................................................................................................. 153
8. CONFORMATIONAL AND STRUCTURAL STUDIES OF 2,2,2 TRIFLUOROETHYLAMINE FROM TEMPERATURE DEPENDENT RAMAN SPECTRA OF XENON
SOLUTIONS AND AB INITIO CALCULATIONS .................................................................... 174
Introduction ................................................................................................................................ 174
Experimental and Theoretical Methods ..................................................................................... 177
Vibrational Assignment ............................................................................................................. 179
Conformational Stability............................................................................................................ 182
Structural Parameters ................................................................................................................. 183
Discussion .................................................................................................................................. 185
9. CONFORMATIONAL, VIBRATIONAL, AND STRUCTURAL STUDIES ......................... 204
OF 2,2,3,3,3-PENTAFLUOROPROPYLAMINE FROM RAMAN AND INFRARED SPECTRA
OF GAS, LIQUID, XENON SOLUTIONS, ................................................................................. 204
AND SOLID SUPPORTED BY AB INITIO CALCULATIONS ................................................ 204
Introduction ................................................................................................................................ 204
Experimental .............................................................................................................................. 206
Ab Initio Calculations ................................................................................................................ 208
Vibrational Assignment ............................................................................................................. 210
vii
Conformational Stability............................................................................................................ 212
Structural Parameters ................................................................................................................. 214
Discussion .................................................................................................................................. 214
10. CONFORMATIONAL STABILITY, R0 STRUCTURAL PARAMETERS, VIBRATIONAL
ASSIGNMENTS AND AB INITIO CALCULATIONS OF ETHYLDICHLOROPHOSPHINE.239
Introduction ................................................................................................................................ 239
Experimental and Theoretical Methods ..................................................................................... 241
Vibrational Assignment ............................................................................................................. 242
Structural Parameters ................................................................................................................. 245
Discussion .................................................................................................................................. 246
11. R0 STRUCTURAL PARAMETERS, CONFORMATIONAL STABILITY, BARRIERS TO
INTERNAL ROTATION, AND VIBRATIONAL ASSIGNMENTS FOR TRANS AND
GAUCHE ETHANOL ................................................................................................................... 260
Introduction ................................................................................................................................ 260
Experimental and Theoretical Methods ..................................................................................... 261
Vibrational Assignment ............................................................................................................. 263
Conformational Stability............................................................................................................ 265
Structural Parameters ................................................................................................................. 267
Discussion .................................................................................................................................. 268
12. CONFORMATIONAL AND STRUCTURAL STUDIES OF ETHYNYL- CYCLOPENTANE
FROM TEMPERATUREDEPENDENT RAMAN SPECTRAOF XENON SOLUTIONS AND
AB INITIO ..................................................................................................................................... 288
CALCULATIONS ......................................................................................................................... 288
Introduction ................................................................................................................................ 288
Experimental and Theoretical Methods ..................................................................................... 290
Vibrational Assignment ............................................................................................................. 292
Conformational Stability............................................................................................................ 294
Structural Parameters ................................................................................................................. 295
Discussion .................................................................................................................................. 297
13. MICROWAVE, STRUCTURAL, CONFORMATIONAL, VIBRATIONAL STUDIES AND
AB INITIO CALCULATIONS OF ISOCYANOCYCLOPENTANE.......................................... 318
Introduction ................................................................................................................................ 318
Experimental Methods ............................................................................................................... 320
Theoretical Methods .................................................................................................................. 322
Microwave Results..................................................................................................................... 324
Vibrational Assignment ............................................................................................................. 325
Conformational Stability............................................................................................................ 328
viii
Structural Parameters ................................................................................................................. 330
Discussion .................................................................................................................................. 332
14. MICROWAVE, INFRARED, AND RAMAN SPECTRA, STRUCTURAL PARAMETERS,
VIBRATIONAL ASSIGNMENTS AND THEORETICAL CALCULATIONS OF 1,3DISILACYCLOPENTANE ........................................................................................................... 358
Introduction ................................................................................................................................ 358
Experimental and Theoretical Methods ..................................................................................... 360
Microwave Results..................................................................................................................... 363
Structural Parameters ................................................................................................................. 364
Vibrational Assignment ............................................................................................................. 365
Discussion .................................................................................................................................. 367
15. MICROWAVE, INFRARED, AND RAMAN SPECTRA, STRUCTURAL PARAMETERS,
VIBRATIONAL ASSIGNMENTS AND THEORETICAL CALCULATIONS OF 1,1,3,3TETRAFLUORO-1,3- ................................................................................................................... 386
DISILACYCLOPENTANE ........................................................................................................... 386
Introduction ................................................................................................................................ 386
Experimental and Theoretical Methods ..................................................................................... 388
Microwave Results..................................................................................................................... 392
Structural Parameters ................................................................................................................. 393
Vibrational Assignment ............................................................................................................. 394
Discussion .................................................................................................................................. 395
16. RAMAN AND INFRARED, MICROWAVE SPECTRA, ADJUSTED R0 STRUCTURAL
PARAMETERS , CONFORMATIONAL STABILITY, AND VIBRATIONAL
ASSIGNMENTS OF CYCLOPENTYLAMINE ......................................................................... 415
Introduction ................................................................................................................................ 415
Experimental .............................................................................................................................. 418
Ab Initio Calculations ................................................................................................................ 420
Microwave Results..................................................................................................................... 421
Vibrational Assignment ............................................................................................................. 422
Conformational Stability............................................................................................................ 424
Structural Parameters ................................................................................................................. 426
Discussion .................................................................................................................................. 428
17. MICROWAVE AND INFRARED SPECTRA, ADJUSTED R0 STRUCTURAL
PARAMETERS, CONFORMATIONAL STABILITIES, VIBRATIONAL ASSIGNMENTS AND
THEORETICAL CALCULATIONS OF CYCLOBUTYLCARBOXYLIC ACID CHLORIDE 452
Introduction ................................................................................................................................ 452
Experimental and Theoretical Methods ..................................................................................... 454
ix
Microwave Results..................................................................................................................... 457
Vibrational Assignment ............................................................................................................. 458
Conformational Stability............................................................................................................ 461
Structural Parameters ................................................................................................................. 464
Discussion .................................................................................................................................. 466
Conclusions ................................................................................................................................ 472
18. VIBRATIONAL ASSIGNMENTS, THEORETICAL CALCULATIONS, STRUCTURAL
PARAMETERS
AND
CONFORMER
STABILITY
DETERMINATIONS
OF
CYCLOBUTYLDICHLOROSILANE .......................................................................................... 498
Introduction ................................................................................................................................ 498
Experimental and Computational Methods ............................................................................... 501
Results and Discussion .............................................................................................................. 502
REFERENCES .............................................................................................................................. 515
VITA .............................................................................................................................................. 526
x
LIST OF TABLES
Table
Page
1. Observed and calculateda wavenumbers (cm-1) for trans ethylamine. ......................................... 29
2. Observed and calculateda wavenumbers (cm -1) for gauche ethylamine. ..................................... 30
3. Calculated energies (hartree) and energy differences (cm-1) for the two conformers of
ethylamine. ....................................................................................................................................... 31
4. Structural parametersa and dipole moments for the trans and gauche conformers of ethylamine.
.......................................................................................................................................................... 32
5. Symmetry coordinates for ethylamine. ........................................................................................ 33
6. Temperature and intensity ratios of the trans and gauche bands of ethylamine. ......................... 34
7. Comparison of rotational constants (MHz) obtained from modified ab initio MP2(full)/6311+G(d,p) structural parameters and the experimental values from the microwave spectra of trans
and gauche conformers of ethylamine. ............................................................................................ 35
8. Observed and calculateda frequencies (cm-1) for trans. ............................................................... 60
9. Observed and calculateda frequencies (cm-1) for gauche isopropylamine. .................................. 62
10. Calculated energies (hartree) and energy differences (cm-1) for the two conformers and
transition states of isopropylamine .................................................................................................. 64
11. Structural parametersa, rotational constants, and dipole moments for trans and gauche
isopropylamine. ................................................................................................................................ 65
12. Symmetry coordinates for isopropylamine. ................................................................................. 67
13. Temperature and intensity ratios of the trans and gauche bands of isopropylamine. ................ 68
14. Comparison of rotational constants obtained from ab initio MP2(full)/6-311+G(d,p)
predictions, experimental valuesa from microwave spectra, and adjusted r0 structural parameters
for trans isopropylamine. ................................................................................................................. 69
15. Quadratic centrifugal distortion constants (kHz) and quadrupole coupling constants (MHz) for
conformers of isopropylamine. ........................................................................................................ 70
16. Observed and predicted wavenumbers (cm-1) for conformers of n-propylamine. ..................... 93
17. Observed and calculateda wavenumbers (cm-1) for Tt n-propylamine. ..................................... 94
18. Observed and calculateda wavenumbers (cm-1) for Tg n-propylamine. .................................... 96
xi
19. Observed and calculateda wavenumbers (cm-1) for Gg n-propylamine. .................................... 98
20. Observed and calculateda wavenumbers (cm-1) for Gg′ n-propylamine. ................................. 100
21. Observed and calculateda wavenumbers (cm-1) for Gt n-propylamine. ................................... 102
22. Calculated energies in (H) and energy differences (cm-1) for the five conformers of npropylamine. .................................................................................................................................. 104
23. Structural parametersa and rotational constants for n-propylamine from MP2(full)/6311+G(d,p) calculations. ................................................................................................................ 105
24. Symmetry coordinates for n-propylamine. .............................................................................. 107
25. Temperature and intensity ratios of the Tt, Tg, Gg, Gg′, and Gt bands of n-propylamine ...... 108
26. Quadratic centrifugal distortion constants (kHz) and quadrupole coupling constants (MHz) for
conformers of n-propylamine. ....................................................................................................... 109
27. Observed and calculateda frequencies (cm-1) and potential energy distributions (P.E.D.s) for the
Gg conformer of 2-cyanoethylamine. ............................................................................................. 129
28. Observed and calculateda frequencies (cm-1) and potential energy distributions (P.E.D.s) for the
Gt conformer of 2-cyanoethylamine. .............................................................................................. 130
29. Observed and calculateda frequencies (cm-1) and potential energy distributions (P.E.D.s) for the
Tg conformer of 2-cyanoethylamine. ............................................................................................. 131
30. Observed and calculateda frequencies (cm-1) and potential energy distributions (P.E.D.s) for the
Tt (Cs) conformer of 2-cyanoethylamine. ....................................................................................... 132
31. Calculated energies in (H) and energy differences (cm-1) for the five conformers of 2cyanoethylamine. ........................................................................................................................... 133
32. Structural parameters (Å and °) and rotational constants (MHz) for the Gg and Gt conformers
of 2-cyanoethylamine. .................................................................................................................... 134
33. Symmetry coordinates for 2-cyanoethylamine Gg conformer................................................. 135
34. Comparison of experimental rotational constants obtained from microwave spectra and
adjusted r0 structural parameters of 2-cyanoethylamine. ............................................................... 136
35. Quadratic centrifugal distortion constants (kHz) and quadrupole coupling constants (MHz) for
Gg and Gt conformers of 2-cyanoethylamine. ............................................................................... 137
36. Observed and calculateda frequencies for the Trans-trans conformer of 2,2-difluoroethylamine.
........................................................................................................................................................ 158
xii
37. Observed and calculateda frequencies for the Gauche-gauche conformer of 2,2difluoroethylamine. ........................................................................................................................ 159
38. Observed and calculateda frequencies for the Gauche-trans conformer of 2,2difluoroethylamine. ........................................................................................................................ 160
39. Calculated energies in (H) and energy differences (cm-1) for the five conformers of 2,2 ....... 161
40. Structural parameters (Å and degree), rotational constants (MHz), and dipole moment (Debye)
for Trans-trans, Gauche-gauche and Gauche-trans forms of 2,2-difluoroethylamine . ................. 162
41. Symmetry coordinates for 2,2–difluoroethylamine. ................................................................. 163
42. Temperature and intensity ratios of the Trans-trans, Gauche-gauche, and Gauche-trans bands of
2,2-difluoroethylamine................................................................................................................... 164
43. Comparison of rotational constants obtained from ab initio MP2(full)/6-311+G(d,p)
predictions, experimental valuesa from microwave spectra, and adjusted r0 structural parameters
for 2,2-difluoroethylamine. ............................................................................................................ 165
44. Quadratic centrifugal distortion constants (kHz) for conformers of 2,2-difluoroethylamine. . 166
45. Observed and calculateda wavenumbers (cm -1) for trans 2,2,2 trifluoroethylamine. .............. 192
46. Observed and calculateda wavenumbers (cm -1) for gauche 2,2,2 trifluoroethylamine. .......... 193
47. Calculated energies (hartree) and energy differences (cm-1) for the two conformers of 2,2,2
trifluoroethylamine. ....................................................................................................................... 194
48. Structural parametersa, rotational constants, and dipole moments for trans and gauche 2,2,2
trifluoroethylamine. ....................................................................................................................... 195
49. Symmetry coordinates for 2,2,2 trifluoroethylamine. ............................................................... 197
50. Temperature and intensity ratios of the trans and gauche bands of 2,2,2 trifluoroethylamine.
........................................................................................................................................................ 198
51. Comparison of rotational constants obtained from experimental values from microwave spectra
and adjusted r0 structural parameters for trans 2,2,2 trifluoroethylamine...................................... 199
52. Quadratic centrifugal distortion constants (kHz) and quadrupole coupling constants (MHz) for
conformers of 2,2,2 trifluoroethylamine. ....................................................................................... 200
53. Observed and calculateda wavenumbers (cm -1) for trans-trans 2,2,3,3,3pentafluoropropylamine. ................................................................................................................ 218
xiii
54. Observed and calculateda wavenumbers (cm -1) for trans-gauche 2,2,3,3,3pentafluoropropylamine. ................................................................................................................ 220
55. Observed and calculateda wavenumbers (cm -1) for trans-trans 2,2,3,3,3pentafluoropropylamine-ND2. ....................................................................................................... 222
56. Observed and calculateda wavenumbers (cm -1) for trans-gauche 2,2,3,3,3pentafluoropropylamine-ND2. ....................................................................................................... 224
57. Calculated energies in (H) and energy differences (cm-1) for the five conformers of 2,2,3,3,3pentafluoropropylamine. ................................................................................................................ 226
58. Structural parameters (Å and degree), rotational constants (MHz) and dipole moment (debye)
for 2,2,3,3,3-pentafluoropropylamine trans-trans (Cs) and trans-gauche (C1) forms. ................... 227
59. Symmetry coordinates for 2,2,3,3,3-pentafluoropropylamine. ................................................. 229
60. Temperature and intensity ratios of the trans-trans and trans-gauche bands of 2,2,3,3,3pentafluoropropylamine. ................................................................................................................ 230
61. Observed and calculateda wavenumbers (cm -1) for trans ethyldichlorophosphine. ................ 250
62. Observed and calculateda wavenumbers (cm -1) for gauche ethyldichlorophosphine.............. 251
63. Calculated energies (hartree) and energy differences (cm-1) for the two conformers of
ethyldichlorophosphine. ................................................................................................................. 252
64. Structural parametersa, rotational constants, and dipole moments for trans and gauche forms of
ethyldichlorophosphine. ................................................................................................................. 253
65. Symmetry coordinates for ethyldichlorophosphine. ................................................................. 254
66. Temperature and intensity ratios of the gauche and trans bands of ethyldichlorophosphine. . 255
67. Quadratic centrifugal distortion constants (MHz) for trans and gauche forms of
ethyldichlorophosphine. ................................................................................................................. 256
68. Observed and calculated frequencies (cm -1) for trans ethanol. ............................................... 273
69. Observeda and calculated frequencies (cm -1) for gauche ethanol. .......................................... 274
70. Structural parametersa, rotational constants, and dipole moments for trans and gauche ethanol.
........................................................................................................................................................ 275
71. Symmetry coordinates for trans ethanol. ................................................................................. 276
72. Calculated energies (hartree) and energy differences (cm-1) for the two conformers and
transition states of ethanol. ............................................................................................................ 277
xiv
73. Temperature and intensity ratios of the trans and gauche bands of ethanol. ........................... 279
74. Comparison of rotational constants obtained from modified ab initio, MP2/6-311+G(d,p)
structural parameters and those from microwave spectra for ethanol. .......................................... 280
75. Quadratic centrifugal distortion constants (MHz) for trans and gauche-ethanol. .................... 281
76. Calculateda and Observed Frequencies (cm-1) for Ethynylcyclopentane Eq (Cs) Form.......... 303
77. Calculateda and Observed Frequencies (cm-1) for Ethynylcyclopentane Ax (Cs) Form. ........ 305
78. Calculated Electronic Energies (Hartree) for the Eq (Cs) and Energy Differences (cm-1) for Ax
(Cs), Twisted (C1), and Planar (Cs) Forms of Ethynylcyclopentane............................................... 307
79. Structural Parameters (Å and Degree), Rotational Constants (MHz) and Dipole Moment
(Debye) for Ethynylcyclopentane Eq and Ax (Cs) Forms. ............................................................. 308
80. Symmetry Coordinates for Ethynylcyclopentane. .................................................................... 309
81. Temperature and Activity Ratios of the Eq and Ax Bands of Ethynylcyclopentane............... 311
82. Comparison of Rotational Constants (MHz) Obtained from ab Initio MP2(full)/6-311+G(d,p)
Predictions, Experimental Valuesa from Microwave Spectra, and Adjusted r0 Structural Parameters
for Ethynylcyclopentane. ............................................................................................................... 312
83. Rotational transition frequencies (MHz) of the ground vibrational state of the Ax form of
Isocyanocyclopentane .................................................................................................................... 339
84. Experimental rotational and centrifugal distortion constants of the Ax form of
Isocyanocyclopentane .................................................................................................................... 340
85. Observed and calculateda frequencies (cm-1) and potential energy distributions (P.E.D.s) for the
Ax (Cs) conformer of Isocyanocyclopentane. ................................................................................ 341
86. Observed and calculateda frequencies (cm-1) and potential energy distributions (P.E.D.s) for the
Eq (Cs) conformer of Isocyanocyclopentane. ................................................................................ 343
87. Calculated electronic energies (hartree) for the Ax (Cs) and energy differences (cm-1) for Eq
(Cs), Twist (C1) and Planar (Cs) forms of isocyanocyclopentane. ................................................. 345
88. Structural Parameters (Å and Degree), Rotational Constants (MHz) and Dipole Moment
(Debye) for isocyanocyclopentane Ax and Eq (Cs) Forms. ........................................................... 346
89. Symmetry Coordinates for Isocyanocyclopentane. ................................................................... 347
90. Comparison of frequencies (cm-1) of ring fundamentals for the Ax conformer of molecules of
the form c-C5H9-X. ........................................................................................................................ 348
xv
91. Temperature and intensity ratios of the Ax and Eq bands of isocyanocyclopentane. ............. 349
92. Comparison of select structural parameters (Å and Degree) of molecules of the form CN-R.
........................................................................................................................................................ 350
93. Comparison of select structural parameters (Å and Degree) for the Ax conformer of molecules
of the form c-C5H9-XY. ............................................................................................................... 351
94. Rotational transition frequencies (MHz) of the ground vibrational state of c-C3H6Si2H4....... 373
95. Rotational transition frequencies (MHz) of the ground vibrational state of c-C3H6Si2H4....... 374
96. Rotational transition frequencies (MHz) of the ground vibrational state of c-C3H6Si2H4....... 375
97. Experimental rotational and centrifugal distortion constants of the ground vibrational state of cC3H6Si2H4 isotopologues. .............................................................................................................. 376
98. Observed and calculateda frequencies (cm-1) and potential energy distributions (P.E.D.s) for the
twist (C2) conformer of c-C3H6Si2H4. ............................................................................................ 377
99. Calculated energies in Hartrees (H) and energy differences (cm-1) for the three conformers of
c-C3H6Si2H4. .................................................................................................................................. 379
100. Structural parameters (Å and degrees), rotational constants (MHz) and dipole moments
(Debye) for twist conformer c-C3H6Si2H4. .................................................................................... 380
101. Comparison of rotational constants (MHz) obtained from experimental values from
microwave spectra, and from the adjusted r0 structural parameters for c-C3H6Si2H4. .................. 381
102. Symmetry coordinates of c-C3H6Si2H4. ................................................................................. 382
103. Rotational transition frequencies (MHz) of the ground vibrational state of c-C3H6Si2F4. .... 400
104. Rotational transition frequencies (MHz) of the ground vibrational state of c-C3H6Si2F4. .... 402
105. Rotational transition frequencies (MHz) of the ground vibrational state of c-C3H6Si2F4. .... 404
106. Experimental rotational and centrifugal distortion constants of c-C3H6Si2F4 isotopologues 405
107. Calculateda and observed frequencies (cm-1) for the twist form of c-C3H6Si2F4 (C2). .......... 406
108. Calculated energies in Hartrees (H) and energy differences (cm-1) for the two conformers and
transition state of c-C3H6Si2F4. ...................................................................................................... 408
109. Symmetry coordinates of c-C3H6Si2F4................................................................................... 409
xvi
110. Structural parameters (Å and degrees), rotational constants (MHz) and dipole moments
(Debye) for twist conformer c-C3H6Si2F4. ..................................................................................... 410
111. Comparison of rotational constants (MHz) obtained from experimental values from
microwave spectra, and from the adjusted r0 structural parameters for c-C3H6Si2F4. ................... 411
112. Microwave spectrum for the t-Ax form of c-C5H9NH2. Observed wavenumbers of rotational
transitions and deviations of calculated values (MHz) .................................................................. 432
113. Rotational Constants (MHz), Quadratic centrifugal distortion constants (kHz) and quadrupole
coupling constants (MHz) for the 14N isotopomer of the t-Ax conformer of Cyclpentylamine. ... 433
114. Calculateda and Observed Wavenumbers (cm-1) for cyclopentylamine t-Ax Form. ............. 434
115. Calculateda and Observed Wavenumbers (cm-1) for cyclopentylamine t-Eq Form............... 436
116. Calculateda and Observed Wavenumbers (cm-1) for cyclopentylamine g-Eq Form. ............. 438
117. Calculateda and Observed Wavenumbers (cm-1) for cyclopentylamine g-Ax Form. ............ 440
118. Calculated energies in (H) and energy differences (cm-1) for the conformers of
cyclopentylamine. .......................................................................................................................... 442
119. Structural parameters (Å and degrees), rotational constants (MHz) and dipole moments
(Debye) for t-Ax form of Cyclopentylamine. ................................................................................ 443
120. Symmetry Coordinates for cyclopentylamine with respect to the t-Ax. ................................. 444
121. Temperature and intensity ratios of the t-Ax, t-Eq, g-Eq and g-Ax bands of cyclopentylamine.
........................................................................................................................................................ 446
122. Microwave spectrum for the g-Eq form of c-C4H7C(O)35Cl. Observed frequencies of
hyperfine components of rotational transitions (MHz) and deviations of calculated values (kHz).
........................................................................................................................................................ 474
123. Microwave spectrum for the g-Eq form of c-C4H7C(O)37Cl. Observed frequencies of
hyperfine components of rotational transitions (MHz) and deviations of calculated values (kHz).
........................................................................................................................................................ 475
124. Observeda and predicted fundamentalb frequencies for the g-Eq conformer of
cyclobutylcarboxylic acid chloride. ............................................................................................... 476
125. Observeda and predicted fundamentalb frequencies for the g-Ax conformer of
cyclobutylcarboxylic acid chloride. ............................................................................................... 478
126. Observeda and predicted fundamentalb frequencies for the t-Eq conformer of
cyclobutylcarboxylic acid chloride. ............................................................................................... 480
xvii
127. Calculated energiesa in (H) and energy differences (cm-1) for the four possible conformers of
cyclobutylcarboxylic acid chloride ................................................................................................ 482
128. Structural parameters (Å and degrees), rotational constants (MHz) and dipole moments
(Debye) for g-Eq form of cyclobutylcarboxylic acid chloride. ..................................................... 483
129. Symmetry coordinatesa for cyclobutylcarboxylic acid chloride............................................ 485
130. Symmetry coordinatesa for cyclobutylcarboxylic acid chloride............................................ 486
131. Rotational Constants (MHz), Quadratic centrifugal distortion constants (kHz) and quadrupole
coupling constants (MHz) for the 35Cl and 37Cl isotopomers of the g-Eq conformer of
cyclobutylcarboxylic acid chloride. ............................................................................................... 487
132A. Temperature and intensity ratios of the g-Eq and g-Ax bands of cyclobutylcarboxylic acid
chloride. ......................................................................................................................................... 488
133. Comparison of rotational constants (MHz) obtained from modified ab initio MP2(full)/6311+G(d,p) structural parameters and the experimental values from the microwave spectra of g-Eq
conformer of cyclobutylcarboxylic acid chloride. ......................................................................... 491
134A. Structural parametersa of a few acetyl chloride molecules of the form R-C(O)Cl (Å and
degree)............................................................................................................................................ 492
135. Calculated Electronic Energies (Hartree) for the t-Eq and Energy Differences (cm-1) for g-Eq,
t-Ax, and g-Ax Forms of cyclobutyldichlorosilane ....................................................................... 506
136. Observed and calculateda frequencies (cm -1) for Eq-t cyclobutyldichlorosilane. ................. 507
137. Observed and calculateda frequencies (cm -1) for Eq-g cyclobutyldichlorosilane. ................ 508
138. Observed and calculateda frequencies (cm -1) for Ax-t cyclobutyldichlorosilane. ................. 509
139. Observed and calculateda frequencies (cm -1) for Ax-g cyclobutyldichlorosilane. ................ 510
140. Structural parametersa (Å and degrees) and rotational constants (MHz) of
cyclobutyldichlorosilane. ............................................................................................................... 511
xviii
LIST OF ILLUSTRATIONS
Figure
Page
1. Experimental and predicted Raman spectra of ethylamine: (A) Raman spectra of xenon solution
at -100°C; (B) simulated Raman spectrum of trans and gauche conformer with enthalpy difference
of 62 ± 6 cm-1 at -100°C; (C) simulated spectrum of gauche form. ................................................ 36
2. Ethylamine molecule showing atomic numbering. ...................................................................... 37
3. Comparison between the band intensities obtained by microwave spectra for trans and gauche
conformers of ethylamine. ............................................................................................................... 39
4. Raman spectra of ethylamine in xenon solution at different temperatures. ................................. 40
5. Experimental and predicted infrared spectra of isopropylamine: (A) gas; (B) simulated
spectrum of T and G; enthalpy difference of 113 cm-1 at 25°C; (C) simulated spectrum of G
conformer; (D) simulated spectrum of T conformer (D) solid. ....................................................... 71
6. Experimental and predicted Raman spectra of isopropylamine: (A) xenon solution at -100°C;
(B) simulated spectrum of T and G; enthalpy difference of 113 cm-1 at -100°C; (C) simulated
spectrum of G conformer; (D) simulated spectrum of T form......................................................... 72
7. Trans and gauche conformers of isopropylamine showing atomic numbering. .......................... 73
8. Experimental and predicted Raman spectra of gaseous isopropylamine: (A) Experimental; (B)
simulated spectrum of T and G; enthalpy difference of 113 cm-1 at 25°C; (C) simulated spectrum
of G conformer; (D) simulated spectrum of T conformer. .............................................................. 74
9. Raman spectra of the xenon solution (Top) and infrared spectra of the gaseous (Bottom) of
isopropylamine (A) 1510 - 1310 cm-1; (B) 1300 – 1100 cm-1; (C) 900 – 700 cm-1......................... 75
10. Spectra of isopropylamine (A) mid-infrared gas; (B) Raman Xe solution at -100°C. .............. 76
11. Raman spectra of isopropylamine in xenon solution at different temperatures ......................... 77
12. Five conformers of n-propylamine showing atomic numbering.............................................. 110
13. Experimental and predicted infrared spectra of n-propylamine: (A) gas; (B) simulated
spectrum of mixture of Tt, Tg (ΔH = 79 cm-1), Gg (ΔH = 91 cm-1), Gg′ (ΔH = 135 cm-1), and Gt
(ΔH = 143 cm-1) conformers at 25°C.; (C) simulated Tt conformer. ............................................ 111
14. Raman spectra of the xenon solution (Top) and infrared spectra of the gaseous vapor (Bottom)
of n-propylamine (A) 920 - 800 cm-1; (B) 500 – 425 cm-1. ........................................................... 112
15. Experimental and predicted Raman spectra of n-propylamine: (A) xenon solution at -100°C;
(B) simulated spectrum of mixture of Tt, Tg (ΔH = 79 cm-1), Gg (ΔH = 91 cm-1), Gg′ (ΔH = 135 cm1
), and Gt (ΔH = 143 cm-1) conformers at -100°C.; (C) simulated Tt conformer; (D) simulated Tg
xix
conformer; (E) simulated Gg conformer; (F) simulated Gg′ conformer; (G) simulated Gt
conformer. ...................................................................................................................................... 113
16. van’t Hoff plot of ln(Iconf1/Iconf2) as a function of 1/T. ............................................................. 114
17. Five conformers of 2-cyanoethylamine. .................................................................................. 138
18. Experimental and predicted infrared spectra of 2-cyanoethylamine: (A) gas; (B) amorphous
solid; (C) simulated spectrum of mixture of Gg and Gt (ΔH = 75 cm-1) conformers at 25°C; (D)
simulated Gt conformer; (E) simulated Gg conformer. ................................................................. 139
19. Experimental and predicted Raman spectra of 2-cyanoethylamine: (A) liquid; (B) simulated
spectrum of mixture of Gg and Gt (ΔH = 75 cm-1) conformers at 25°C; (C) simulated Gt conformer;
(D) simulated Gg conformer. ......................................................................................................... 140
20. Potential function of rotation around the C-N bond (H6-N1-C2-C3 dihedral angle) for the
gauche C-C-C-N heavy atom structure calculated using MP2(full) method with 6-311+G(2d,2p)
basis set. ......................................................................................................................................... 141
21. Mid-infrared spectra of 2,2-difluoroethylamine (A) gas; (B) Xe solution at -65°C. ............... 167
22. Comparison of experimental and predicted infrared spectra of 2,2-difluoroethylamine: (A)
infrared spectra of the solid; (B) simulated spectrum of Tt conformer. ........................................ 168
23. Comparison of experimental and predicted Raman spectra of 2,2-difluoroethylamine: (A)
observed spectrum of the liquid; (B) simulated spectrum of a mixture of Tt, Gg (ΔH = 83 cm -1), Gt
(ΔH = 235 cm-1) conformers at 25°C; (C) simulated spectrum of Gt conformer; (D) simulated
spectrum of Gg conformer; (E) simulated spectrum of Tt conformer. .......................................... 169
24. Possible stable conformers of 2,2-difluoroethylamine. ........................................................... 170
25. Comparison of experimental and predicted infrared spectra of 2,2-difluoroethylamine: (A)
observed spectrum of Xe solution at -65°C; (B) simulated spectrum of mixture of Tt, Gg (ΔH = 83
cm-1), Gt (ΔH = 235 cm-1) conformers at -65°C; (C) simulated spectrum of Gt conformer; (D)
simulated spectrum of Gg conformer; (E) simulated spectrum of Tt conformer. ......................... 171
26. Temperature (-60 to -100°C) dependent mid-infrared spectrum in the 725-835 cm-1 region of
2,2-difluoroethylamine dissolved in liquid xenon. ........................................................................ 172
27. Raman spectra (650-1700 cm-1) showing the polarized and the depolarized bands for 2,2difluoroethylamine. ........................................................................................................................ 173
28. Experimental and predicted Raman spectra of 2,2,2 trifluoroethylamine: (A) xenon solution at 100°C; (B) simulated spectrum of Trans and gauche; enthalpy difference of 267 cm-1 at -100°C;
(C) simulated spectrum of gauche conformer; (D) simulated spectrum of Trans conformer. ....... 201
29. 2,2,2 trifluoroethylamine molecule showing atomic number .................................................. 202
xx
30. Raman spectra of 2,2,2 trifluoroethylamine in xenon solution at different temperatures. ...... 203
31. Mid-infrared spectra of normal 2,2,3,3,3-pentafluoropropylamine (A) gas; (B) Xe solution at 70°C; (C) simulated spectrum of Tt conformer; (D) infrared spectra of the solid. ....................... 231
32.Comparison of experimental and predicted infrared spectra of 2,2,3,3,3pentafluoropropylamine-ND2: (A) gas; (B) simulated spectrum of Tt and Tg; enthalpy difference
of 280 cm-1 at 25°C; (C) simulated spectrum of Tg conformer; (D) simulated spectrum of Tt
conformer. ...................................................................................................................................... 232
33.Comparison of experimental and predicted Raman spectra of 2,2,3,3,3-pentafluoropropylamine:
(A) gas; (B) simulated spectrum of Tt and Tg; enthalpy difference of 280 cm-1 at 25°C; (C)
simulated spectrum of Tg conformer; (D) simulated spectrum of A′ modes of Tt conformer. ..... 233
34.Comparison of experimental and predicted Raman spectra of 2,2,3,3,3-pentafluoropropylamineND2: (A) gas; (B) simulated spectrum of Tt and Tg; enthalpy difference of 280 cm-1 at 25°C; (C)
simulated spectrum of Tg conformer; (D) simulated spectrum of A′ modes of Tt conformer. ..... 234
35.Comparison of experimental and predicted Raman spectra of 2,2,3,3,3-pentafluoropropylamine:
(A) liquid; (B) simulated spectrum of Tt and Tg; enthalpy difference of 280 cm-1 at 25°C; (C)
simulated spectrum of Tg conformer; (D) simulated spectrum of Tt conformer. ......................... 235
36.Comparison of experimental and predicted Raman spectra of 2,2,3,3,3-pentafluoropropylamineND2: (A) liquid; (B) simulated spectrum of Tt and Tg; enthalpy difference of 280 cm-1 at 25°C;
(C) simulated spectrum of Tg conformer; (D) simulated spectrum of Tt conformer. ................... 236
37. Five conformers of 2,2,3,3,3-pentafluoropropylamine showing atomic numbering. .............. 237
38. Infrared spectra of 2,2,3,3,3-pentafluoropropylamine in xenon solution at different
temperatures. .................................................................................................................................. 238
39. Experimental mid-infrared spectra of ethyldichlorophosphine: observed spectra of xenon
solutions at -70 with fundamentals labeled for both (t = trans and g = gauche) conformers ........ 257
40. Ethyldichlorophosphine molecule showing atomic numbering. .............................................. 258
41. Mid-infrared spectra of ethyldichlorophosphine dissolved in liquefied xenon solution at four
different temperatures with bands used in the enthalpy determination assigned on spectra. ........ 259
42. Ethanol showing atomic numbering. ....................................................................................... 282
43. Mid-infrared spectra of ethanol (A) gas; (B) Xe solution at -60 °C. ....................................... 283
44. Comparison of experimental and predicted infrared spectra of ethanol: (A) Xe solution at -60
ºC; (B) simulated spectrum of mixture of trans and gauche conformers at -60oC with ΔH = 62 cm1
; (C) simulated spectrum of gauche conformer; (D) simulated spectrum of trans conformer. ..... 284
xxi
45. Comparison of experimental and predicted Raman spectra of ethanol: (A) gas [113]; (B)
simulated spectrum of the mixture of trans and gauche conformers at 25ºC with ΔH = 62 cm -1; (C)
simulated spectrum of gauche conformer; (D) simulated spectrum of trans conformer A' block. 285
46. van’t Hoff plot of –ln(I887/I879) as a function of 1/T. ............................................................... 286
47. Predicted (dotted curve, from MP2(full)/6-311+G(2d,2p)) and experimental (solid curve[120])
potential function governing the hydroxyl torsion of ethanol. ...................................................... 287
48. Comparison of experimental and calculated infrared spectra of ethynylcyclopentane: (A)
observed spectrum of gas; (B) simulated spectrum of a mixture of Eq and Ax conformers (ΔH = 94
cm-1) at 25°C; (C) simulated spectrum of Ax conformer; (D) simulated spectrum of Eq conformer;
(E) observed spectrum of solid. ....................................................................................................... 313
49. Comparison of experimental and calculated Raman spectra of ethynylcyclopentane: (A) observed
spectrum of liquid; (B) observed spectrum of Xe solution at -60°C; (C) simulated spectrum of a
mixture of Eq and Ax conformers (ΔH = 94 cm-1) at -60°C; (D) simulated spectrum of Ax
conformer; (E) simulated spectrum of Eq conformer. ..................................................................... 314
50. Conformers of ethynylcyclopentane (A) Eq; (B) Ax. .............................................................. 315
51. Infrared and Raman spectra of ethynylcyclopentane (A) observed mid-infrared spectrum of gas;
(B) Raman spectrum of Xe solution at -60°C. ................................................................................ 316
52. Temperature (-50 to -100°C) dependent Raman spectrum of ethynylcyclopentane dissolved in
liquid xenon. .................................................................................................................................. 317
53. Conformers of isocyanocyclopentane (A) Eq (B) Ax. .............................................................. 352
54. Comparison of experimental and calculated infrared spectra of isocyanocyclopentane: (A)
observed spectrum of gas; (B) observed spectrum of Xe solution at -70°C; (C) observed spectrum of
solid; (D) simulated spectrum of a mixture of Ax and Eq conformers (ΔH = 102 cm-1) at 25°C; (E)
simulated spectrum of Eq conformer; (F) simulated spectrum of Ax conformer. ............................ 353
55. Comparison of experimental and calculated Raman spectra of isocyanocyclopentane: (A)
observed spectrum of liquid; (B) simulated spectrum of a mixture of Ax and Eq conformers (ΔH =
102 cm-1) at 25°C; (C) simulated spectrum of Eq conformer; (D) simulated spectrum of Ax
conformer. ...................................................................................................................................... 354
56. Infrared spectra of isocyanocyclopentane (A) gas; (B) Xe solution at -70°C. ......................... 355
57. Temperature (-70 to -100°C) dependent infrared spectrum of isocyanocyclopentane dissolved
in liquid xenon solution. ................................................................................................................ 356
58. Band contour predictions of Ax and Eq conformer of isocyanocyclopentane. ....................... 357
Figure 59: Comparison of experimental and calculated infrared spectra of c-C3H6Si2H4: (A) observed
spectrum of gas; (B) simulated spectrum of twist conformer........................................................... 383
xxii
60. Comparison of experimental and calculated Raman spectra of c-C3H6Si2H4: (A) observed
spectrum of liquid; (B) simulated spectrum of twist conformer. ...................................................... 384
61. Model of c-C3H6Si2H4 showing atomic numbering.................................................................. 385
62. Comparison of experimental (path length = 9.5 cm & gas pressure = 400 mTorr) and calculated
infrared spectra of c-C3H6Si2F4: (A) observed spectrum of gas; (B) simulated spectrum of twist
conformer. ...................................................................................................................................... 412
63. Comparison of experimental and calculated Raman spectra of c-C3H6Si2F4: (A) observed
spectrum of liquid; (B) simulated spectrum of twist conformer. ...................................................... 413
64. Model of c-C3H6Si2F4 showing atomic numbering. ................................................................. 414
65. Atomic numbering of cyclopentylamine with the t-Ax form shown. ...................................... 447
66. Comparison of experimental and calculated infrared spectra of cyclopentylamine: (A) observed
spectrum of gas; (B) simulated spectrum of a mixture of the four stable conformers of
cyclopentylamine at 25°C; (C) simulated spectrum of g-Ax conformer; (D) simulated spectrum of
g-Eq conformer; (E) simulated spectrum of t-Eq conformer; (F) simulated spectrum of conformer
t-Ax. ............................................................................................................................................... 448
67. Comparison of experimental and calculated Raman spectra of cyclopentylamine: (A) xenon
solution at -100°C; (B) simulated spectrum of a mixture of the four stable conformers of
cyclopentylamine at -100°C; (C) simulated spectrum of g-Ax conformer; (D) simulated spectrum
of g-Eq conformer; (E) simulated spectrum of t-Eq conformer; (F) simulated spectrum of
conformer t-Ax............................................................................................................................... 449
68. Spectra of cyclopentylamine (A) mid-infrared gas; (B) Raman Xe. solution at -100°C. ........ 450
69. Raman spectra of cyclopentylamine in xenon solution at different temperatures. .................. 451
70. Labeled conformers of cyclobutylcarboxylic acid chloride with atomic numbering. ............. 494
71. Mid-infrared spectra of cyclobutylcarboxylic acid chloride (A) spectrum of gas; (B) spectrum
of xenon solution at -80°C with bands used in the enthalpy determination assigned on spectra. . 495
72. Experimental and predicted mid-infrared spectra of cyclobutylcarboxylic acid chloride: (A)
observed spectra of xenon solutions at -80, -90, and -100°C; (B) simulated spectrum of mixture of
g-Eq, g-Ax (ΔH = 91 cm-1), and t-Eq (ΔH = 173 cm-1) conformers at -80°C; (C) simulated g-Eq
conformer; (D) simulated g-Ax conformer; (E) simulated t-Eq conformer. ................................. 496
73. Temperature (-70 to -100°C) dependent mid-infrared spectrum of cyclobutylcarboxylic
chloride dissolved in liquid xenon ................................................................................................. 497
74. Conformers of cyclobutyldichlorosilane ................................................................................... 512
xxiii
75. Comparison of experimental and calculated infrared spectra of cyclobutyldichlorosilane: (A)
observed spectrum of gas; (B) simulated spectrum of a mixture of the four stable conformers of
cyclobutyldichlorosilane at 25°C; (C) simulated spectrum of g-Ax conformer; (D) simulated spectrum
of t-Ax conformer; (E) simulated spectrum of g-Eq conformer; (F) simulated spectrum of conformer
t-Eq ................................................................................................................................................. 513
76. Raman spectra of the Liquid from 1500 – 1100 cm-1 .............................................................. 514
xxiv
ACKNOWLEDGEMENTS
Many people view the pursuit of a Ph.D. degree as both a pleasant and painful experience.
It has been compared to the act of climbing a high peak mountain or hill with each step,
accompanied by frustrations, bitterness, hardships, trust as well as encouragement. It requires the
help and contribution of different individuals. When one is finally at the peak with the beautiful
scenery in sight, they realise that they couldn’t have made it without teamwork. I would therefore
like to offer my sincere gratitude and may thanks to all the individuals who contributed to my
success.
First and foremost, I would like to offer my sincere gratitude to my esteemed supervisor,
Professor James R. Durig for accepting me as his student without any doubts. He has given me
valuable advice together with supervision and guidance that has pointed me towards the right
direction. I have personally learned so much from him. His help and support saw me through my
dissertation successfully for which I am thankful.
Special thanks are in order for Professor Keith R. Buszek, Professor Zhonghua Peng,
Professor Jejung Lee and Professor James B. Murowchick for their continued support and advice,
their commitment and tolerance that got me this far and made me see the weaknesses in my work
and make the necessary improvements to them. My deepest gratitude goes to Professor Mustafa
Badr for his encouraging advice and constant help. I thank you from my heart for all the time and
energy you put in to my success.
My heartfelt thanks also go to Professor Gamil Guirgis. He was of great help especially in
terms of synthesising molecules and offering proper guidance and advice. I would also like to
express my gratitude to Dr. Gounev for his help with the recording of an enormous number of
spectra of the molecules I investigated in my research. My gratitude also goes to Professor Groner
who created better computer programmes that made it easier to conduct the research. My heartfelt
xxv
thanks also go to all the members of staff in the Department of Chemistry as well as the School of
Graduate studies. I would also like to thank all the organizations that offered their generous
support, financial assistance and sponsorship, without which it would not have been possible to
complete this Ph.D. degree with success.
Special thanks go to my parents, family, my in-laws and friends. They have sacrificed so
much in the course of their lives and also offered unconditional love as well as care. They have
been my source of inspiration and motivation and I would not have made it this far without their
support as well as their trust. My brothers and sisters have also been a great help to my success.
They have shown great faith and trust in me that encouraged me to do better.
Lastly, I cannot forget to mention my supportive spouse, Dr. Ahmad Mousa. He has been
my source of motivation and the backbone and source of my happiness. The love and support he
gives to me provided the confidence I needed to get through this Ph.D. degree. I cannot hesitate to
acknowledge the step I made without mentioning him as he made things perceptive on my side.
Understanding the hustle of attaining the Ph.D. degree is a step that requires much support and this
possess the gratitude to my love. Thank you for being part of my life. You are the reason why my
Ph.D. degree was a success. Without forgetting, I would like to thank the Almighty God for
bringing me this far.
It has been one great journey for me with memorable memories that I will cherish for the
rest of my life. I have full confidence that the knowledge as well as training acquired will be
helpful in my career.
xxvi
I dedicate this work to
Mom and Dad,
the love of my life, my husband, Dr. Mousa,
my cherished family and my beloved in laws
for their constant support and unconditional love.
I love you all dearly.
xxvii
CHAPTER 1
INTRODUCTION
Society’s pursuit for advanced knowledge in chemistry can be traced back to ancient
civilizations. Records show that by 1000 BC, the technologies used in different societies formed
the foundation for the current branches of chemistry. The earliest form of chemistry research
practiced was by alchemists who sought to explain the nature of substances and how they
transformed. This protoscience was later disproved after the emergence of chemists that relied on
scientific methods in their studies [1]. Early research in chemistry was to ascertain the nature of
substances used by society; interestingly, the same objective remains in the 21st century. On the
contrary, although early studies in chemistry pertained to simple elements and compound, the
advances achieved in technology allow modern chemists to study complex substances that were
previously considered unfathomable.
Pre-scientific philosophers in Greece are the earliest proposers of the molecular concept
applied in the 21st century in chemistry studies. One philosopher, Leucippus, argued that the
world was made of voids and atoms; another scholar, stated that the universe comprised of basic
elements: earth
, water
, fire
and air
. These elements interacted because of
attraction forces that existed in the universe. Modern molecular conceptualization began in the
19th century because of the experimental studies on pure elements and the ability of atoms from
different elements to combine and form molecules [2].
This study seeks to explain the orientation and relationship of a part of a molecule in
relation to the whole. Modern technology and experimental environments can allow studies on
molecular structure and motion, which is essential in establishing spatial conformers or
orientations that exist in different physical states [3]. These studies may also be used to
1
determine the conformational stability of different parts of a molecular structure. The existence
of two or more forms gives rise to three vital issues in molecular studies: The difference of
energy between the conformers; the energy required for inter-conversion from one
conformational arrangement to another; and the conformational arrangement with the highest
stability [4].
Over the years, studies to determine the conformational stability of various organoamines
and organophosphines have been conducted. The findings indicate that for most molecules, the
conformer with the highest stability is different from the one forecasted using the gauche effect.
According to the gauche effect, maximum interaction of a conformer’s polar bonds or pairs of
non-bonded electrons makes it the one with high stability [5]. On the contrary, in studies on
organophosphines, the conformer possessing the least number of interactions is predicted as the
most stable, with the assumption that a hydrogen atom occupies less space than a pair of nonbonded electrons [6]. The prediction of the gauche effect, however, is effective in determining
the conformer with a high stability in an organophosphate that exists in gaseous form.
A common example used in determining conformational analysis is the compound ethane
(CH3-CH3). The carbon atoms internally rotate around the C-C covalent bond because of the sp3
hybridization. The potential function resulting from this internal rotation is threefold barrier
alternating between the staggered form with high stability and the eclipsed conformation that is
less favored. This concept of internal rotation amidst the threefold barrier is a significant issue in
the field of chemistry [7]. Research conducted on conformers in the early 18th century assumed
the internal rotation was not subject to any restrictions such as the threefold barrier [8]. In the
mid 18th century, it, however, was established that internal rotation in conformers is not free and
is subject to an energy barrier within the substance. After this determination, structural molecular
2
scientists were keen on structures of molecules that allow internal rotation at varying levels of
complexity. Several studies were conducted on this area as researchers sought to establish the
influence of internal rotation and a conformer’s stability.
Phosphorous compounds with trivalent bonds, PH3, belong to the organophosphorous
class; examples of these compounds include triphenylphosphine oxide and the relatively stable
chirals (R) - and (S)-methyl propyl phenyl phosphine [9]. Organoamines are another area of
interest for structural chemists as they try to establish the relative stability and internal movement
within these compounds. Organoamines are formed when an atom of nitrogen bonds with one,
two or three atoms of carbon resulting in the formation of an amino group [7] . Organic amines
can be classified as either primary, tertiary or secondary [10]. Each of these classes has been
subject to vibrational studies as structural chemists try to establish the stability of organoamines
conformers. An example of a compound in this category that has been studied widely is cyclobutyl-amine. The mono-replacement in cyclo-butane with NH2, SH, PH2 or OH produces four
conformers with relatively high stability: axial-gauche, axial-trans, equatorial-gauche and
equatorial-trans. Studies have established that the -gauche category of each conformer has a
lower stability than the -trans one .
The energy function for the ethane molecule is relatively simple, although this is not the
case for complex molecules. This is because of the need to consider other variables in the
conformer that affect its energy minima’s stability. In conformers experiencing internal rotation,
these variables are responsible for the uniqueness of the potential function’s shape. Variables or
forces such as the effects of the double bonds resonance, electron delocalization effects, steric
repulsion and bonding of hydrogen affect the stability of conformers. The appearance of a
function of potential energy is a series of maxima and minima; the minimum is directly related to
3
each conformer’s equilibrium geometry. Internal rotation of asymmetric and symmetric rotors is
shown using a potential function. A –CH3 has a symmetric rotor and hence a 120-degree rotation
causes a unique atom arrangement, with the potential function having three minima possessing
the same energy. A rotor that is asymmetric like the –OCH3 does not have the threefold
symmetry. Energy difference between conformers can be derived from the similar difference of
one minimum to another, ∆E, while the difference between the minima and the adjacent maxima
represents the conformer inter-conversion barrier. A similar potential function may be used to
determine thermodynamic functions in organophosphines and organoamines.
Determining the energy difference in conformers is not easily achievable; thus,
researchers make an assumption on enthalpy difference and energy proximity. This is most
common where the molecule under study is in a gaseous state, which makes its interactions
minimal. This assumption, however, is invalid when dealing with solids and liquids, with the
exception of when some volume is dissolved in extremely inert matrix such as liquefied noble
gas. In such a scenario, the difference in enthalpy between the conformers is determined using
the molecule’s vibrational spectra. Establishing a conformer’s spectral feature requires accurate
assigning of resolving solutions, which reflect parameters in relative intensities. Approaches
used to establish variations in conformers are founded on the interaction between Gibbs
difference in energy for temperature and conformational equilibrium.
ΔG = – RT lnK
K = equilibrium constant
Using ΔH – TΔS instead of ΔG the relationship becomes
ΔS – Entropy change
4
Several procedures are used in determining a molecules conformations and
configurations, and these techniques can be classified into spectroscopic, relaxation and classical
methods. Spectroscopic methods include Electron diffraction, NMR, Raman and infrared; they
are the preferred procedures by chemists when analyzing conformations. Studies and
experiments into spectroscopic techniques have led to formulation of advanced methods such as
Vibrational Circular Dichroism and Raman Optical Activity, which are still under study to
ensure they can successfully isolate and determine a large molecule’s configuration [11]. There
are several spectroscopic methods but the most preferred are Raman and infrared when
conducting conformational analysis [12]. One of the reasons for this preference is because a
conformer’s vibrational spectrum is the Boltzmann mean distributions of its highest stable and
least stable forms. The information derived from the vibrational spectrum corresponds to the
structure and is responsible for a molecule’s unique conformation. A comprehensive
understanding of intermolecular forces can be derived from the attributes of a molecule’s
vibrational frequencies. Another reason for extensive use of Raman is that it was widely studied
in the early 18th century, and so sufficient data is available on conducting analysis using this
technique. Infrared procedures complement Raman and so researchers prefer to use the two
methods during conformational analysis. The availability of chemical instrumentation needed
when using Raman and infrared has also contributed to their widespread use in research on
stability of conformers [13].
The study to be conducted in this dissertation represents a series of theoretical and
experimental research on stability, structural parameters, stability of conformers, internal rotation
barriers and vibrational spectra of organoamines, organophosphines and substituted four and five
member rings. An experimental study on these molecules will provide accurate information on
5
their structure, specter, vibration and conformation for use in subsequent researches.
Organophosphines and organoamines are used extensively as substructures in other molecules
[14], and so a comprehensive understanding of them will foster accurate and flexible use. The
choice of four and five member rings is made because these models provide a simple sample for
studying conformational inter-conversion in complex and large molecules. Forces classified as
steric factors hold these models together, and this causes their conformers to have relatively low
inter-conversion barriers.
6
CHAPTER 2
EXPERIMENTAL AND THEORETICAL METHODS
The mid-infrared spectra of the gas and solid are obtained from 4000 to 220 cm-1 on a
Perkin-Elmer model 2000 Fourier transform spectrometer equipped with a Ge/CsI beamsplitter and
a DTGS detector. Atmospheric water vapor was removed from the spectrometer housing by purging
with dry nitrogen. The gas and solid spectra are obtained with a theoretical resolution of 0.5 cm-1 for
the gas and 2 cm-1 for the solid with 128 interferograms added and truncated. Multiple annealings
are required to obtain satisfactory spectra of the solid.
The mid-infrared spectra (4000 to 400 cm-1) of the sample dissolved in liquefied xenon are
recorded on a Bruker model IFS-66 Fourier transform spectrometer equipped with a globar
source, a Ge/KBr beamsplitter and a DTGS detector. In all cases, 100 interferograms are
collected at 1.0 cm-1 resolution, averaged, and transformed with a boxcar truncation function. For
these studies, a specially designed cryostat cell is used. It consists of a copper cell with a path
length of 4 cm with wedged silicon windows sealed to the cell with indium gaskets. The copper
cell is enclosed in an evacuated chamber fitted with KBr windows. The temperature is
maintained with boiling liquid nitrogen and monitored by two Pt thermo resistors.
The far infrared spectra (600 to 10 cm-1) of the sample dissolved in liquid xenon is
recorded on a Bruker model IFS 66 v/S Fourier transform spectrophotometer equipped with a
Globar source, a 6.0 m Mylar beamsplitter, and a liquid helium cooled Si bolometer. The
sample is contained in a 7 cm cell fitted with Si windows and the sample added as described for
the mid-infrared studies. For all spectra 250 interferograms are collected at 0.5 cm-1 resolution,
averaged, and transformed with a Blackman-Harris three term function.
7
The Raman spectra of the liquid are recorded from 4000 to 40 cm-1 on a Spex model
1403 spectrophotometer equipped with a Spectra-Physics model 2017 argon ion laser operating
on the 514.5 nm line. The laser power used is 1.5 W with a spectral bandpass of 3 cm-1. The
spectra of the liquid are recorded with the sample sealed in a Pyrex glass capillary. The
measurements of the Raman frequencies are expected to be accurate to  2 cm-1.
Alternatively, Raman spectra of the liquid are collected in back-scattering geometry using
the 514.532 nm line of an Argon ion laser as the excitation source, with ~ 22 mW incident on the
diamond cell. A Semrock 514 nm edge filter is used to separate the laser line from the Raman
scattered light. The scattered light is dispersed in a Spectra-pro 500i spectrograph and detected
with a Spec-10 liquid nitrogen cooled CCD detector. Raman spectra are collected using a 2400
g/mm grating with a slit width of 100 μm, which gives spectral resolution of ~0.2 cm-1.
The Raman spectra (4000 to 300 cm-1) of the sample dissolved in liquefied xenon at five
different temperatures (-60 to -100oC) is recorded on a Trivista 557 spectrometer consisting of a
double f = 50 cm monochromator equipped with a 2000 lines mm-1 grating, a f = 70 cm
spectrograph equipped with a 2400 lines mm-1 grating, and a back-illuminated LN2-cooled PI
Acton Spec-10:2 kB/LN 2048 x 512 pixel CCD detector. For all experiments, the 514.5 nm line
of a 2017-Ar S/N 1665 Spectra-Physics argon ion laser is used for Raman excitation, with the
power set to 0.8 Watt. Signals related to the plasma lines are removed by using an interference
filter. The frequencies are calibrated using Neon emission lines, and depending on the setup
used, are expected to be accurate within 0.4 cm-1. The experimental set-up used to investigate the
solutions has been described before [15],[16]. A home-built cell for liquids, equipped with four
quartz windows at right angles, is used to record the spectra.
Microwave spectra are recorded by using a “mini-cavity” Fourier-transform microwave
8
spectrometer [17, 18], at Kent State University. The Fabry-Perot resonant cavity is established
by two 7.5-inch diameter diamond-tip finished aluminum mirrors with a 30.5-cm spherical
radius. The Fabry-Perot cavity resides inside a vacuum chamber formed by a 6-way cross and a
15-inch long, 8-inch diameter extension tube. The two cavity mirrors are nominally separated by
30 cm.
The sample is entrained in a 70:30 Ne-He carrier gas mixture at 2 atm and expanded into
the cavity to attain 4K using a reservoir nozzle made from a modified Series-9 General Valve.
The reservoir nozzle is mounted in a recessed region of the mirror flange, external to the vacuum
chamber, and the expansion passes through a 0.182-inch diameter hole into the resonant cavity.
The center of the expansion is offset from the center of the mirror by 1 inch.
The sample is irradiated by microwave radiation generated by an Agilent Technologies
E8247C PSG CW synthesizer; details of the irradiation and heterodyne detection circuitry can be
found in Ref. [19] The vacuum system can accommodate pulse repetition rates of up to 15 s-1
while maintaining a pressure below 10-4 torr, and the instrument can scan 450 MHz in 6 hours
while averaging 100 shots per scan segment.
On the other hand, the rotational spectra of the sample are studied by using a CP-FTMW
spectrometer developed at the University of Virginia, operating in the 6.5 to 18 GHz range. The
chirped pulse methods used in this study have been described in detail previously [20], so only
the brief details relevant to this experiment are necessary.
The microwave source was a 24 GS/s arbitrary waveform generator, producing a 12-0.5
GHz linear frequency sweep in 1 μs. The pulse was upconverted to 6.5-18 GHz by a 18.95 GHz
phase-locked resonant dielectric oscillator (PDRO), and then amplified by a pulsed 300 W
traveling wave tube amplifier. The amplified pulse is then transmitted through free space
9
between two standard-gain microwave horns, where it interacts with a molecular beam generated
by five pulsed nozzles (General Valve Series 9) operating perpendicular to the propagation
direction of the microwave pulse. On the detection end, the receiver is protected from the high
power pulse by a combination of a PIN diode limiter and single-pole microwave switch. The
resulting molecular free induction decay (FID) was then amplified and digitized directly on a 100
GS/s oscilloscope with 33 GHz of hardware bandwidth, with a 20 μs detection time per FID. Due
to the speed of this excitation and detection process, a sequence of 10 excitation/detection cycles
is possible per gas pulse, and all ten detected FIDs are collected and averaged together before the
next valve injection cycle begins. Phase stability of this experiment over the course of many
valve injection cycles is enabled by locking all the frequency sources and the oscilloscope to a 10
MHz Rb-disciplined quartz oscillator. For this experiment, approximately 78 000 valve injection
cycles of the sample gas were completed at 3.3 Hz to create a time-averaged spectrum of 780
000 molecular FIDs (approximately 6.5 hours of averaging). Additionally, the time domain
resolution afforded by a 20 μs FID generates an average Doppler broadened linewidth of
approximately 130 kHz at FWHM.
The entrained sample for spectral investigation was prepared by balancing the target
molecule sample vapor with approximately 3.4 atm of Ne gas (GTS Welco) for a total sample
concentration of approximately 0.1%. This afforded a frequency-domain dynamic range of
approximately 4000:1 at 780 000 averages.
Ab initio and density functional theory (DFT) calculations are performed with the Gaussian
03 program [21] by using Gaussian-type basis functions. The energy minima with respect to
nuclear coordinates are obtained by the simultaneous relaxation of all geometric parameters by
the gradient method of Pulay [22]. A variety of basis sets, as well as, the corresponding ones
10
with diffuse functions are employed with the Møller-Plesset perturbation method [23] to the
second order MP2 with full electron correlation as well as with density functional theory by the
B3LYP method.
In order to obtain descriptions of the molecular motions involved of the fundamental
modes, a normal coordinate analysis is carried out. The force field in Cartesian coordinates is
obtained with the Gaussian 03 program at the MP2(full) level with the 6-31G(d) basis set. The
internal coordinates used to calculate the G and B matrices are given with the atomic numbering
shown. By using the B matrix [24], the force field in Cartesian coordinates is converted to a
force field in internal coordinates. Subsequently, one or two scaling factors are used to obtain the
fixed scaled force constants and the resulting wavenumbers. A set of symmetry coordinates are
used to determine the corresponding potential energy distributions (P.E.Ds) to described the
fundamental vibrational modes.
The predicted scaled frequencies are used together with a Lorentzian function to obtain
the simulated spectra. Infrared intensities are obtained based on the dipole moment derivatives
with respect to Cartesian coordinates. The derivatives are transformed with respect to normal
coordinates by (u/Qi) =
 ( /X )L , where Q
j
u
j
ij
i
is the ith normal coordinate, Xj is the jth
Cartesian displacement coordinate, and Lij is the transformation matrix between the Cartesian
displacement coordinates and the normal coordinates. The infrared intensities are then calculated
by (N)/(3c2) [(x/Qi)2 + (y/Qi)2 + (z/Qi)2]. A comparison may then be made of the
experimental infrared spectra of the gas and simulated infrared spectra where the predictions of
the isolated molecule are normally close to the values in the gas phase.
Additional support for the vibrational assignments is obtained from the simulated Raman
spectra. The evaluation of Raman activity by using the analytical gradient methods has been
11
developed [25-27] and the activity Sj can be expressed as: Sj = gj(45αj2 + 7βj2), where gj is the
degeneracy of the vibrational mode j, αj is the derivative of the isotropic polarizability, and βj is
the anisotropic polarizability. To obtain the Raman scattering cross sections, the polarizabilities
are incorporated into Sj by multiplying Sj with (1-ρj)/ (1+ρj) where ρj is the depolarization ratio
of the jth normal mode. The Raman scattering cross sections and calculated wavenumbers
obtained from the Gaussian 03 program are used together with a Lorentzian function to obtain
the simulated Raman spectra. The average difference in band center due to going from gas to
liquid is usually less than 3 cm-1, and thus, there is usually little interaction between molecules in
the liquid phase for the majority of the molecules studied. A comparison may therefore be made
of the experimental Raman spectra of the liquid and predicted Raman spectra.
12
CHAPTER 3
MICROWAVE SPECTRA AND CONFORMATIONAL STUDIES OF ETYHLAMINE FROM
TEMPERATURE DEPENDENT RAMAN SPECTRA OF XENON SOLUTIONS AND AB
INITIO CALCULATIONS
Introduction
The determination of the conformational stability of amine molecules have provided
some interesting challenges for scientists over the years. Of particular interest is the ethylamine
molecule, where there have been several studies reported the conformational stabilities. One of
the first studies that was reported of ethylamine [28] indicated the presence of both trans and
gauche conformers, but from this study the conformational stability between the two conformers
was not reported. From another latter study [29] the gauche conformer was reported as slightly
favored by 182 cm-1 (0.52 kcal/mol) than the trans form. Within a few years, the gauche-trans
rotational isomerism of ethylamine in the vapor phase was studied [30] by the analysis of the far
infrared spectra of CH3CH2ND2 and CH3CD2ND2. The gauche form was reported as being more
stable than the trans conformer and a difference of 104 cm-1 of the potential energy minima was
obtained. Another study was reported in 1975 [31] where the predicted energy difference
between the potential energy minima of the gauche form and the trans conformation to be 207
cm-1 (0.592 kcal/mole) with the trans conformer the more stable form. During the same year an
infrared absorption spectra of eight isotopic ethylamine molecules in the vapor phase were
studied [32] and a difference of about 230 cm-1 was obtained between the potential energy
minima of the trans and gauche conformations with the trans form the more stable conformer.
Nevertheless after a decade later, an energy difference of both (gauche-trans) conformers has
been estimated to be 100 ± 10 cm-1 [33] from the intensity changes of the NH2 wagging modes
13
observed on changing the nozzle temperature in the matrix formation. From a microwave
investigation study [34] the enthalpy difference of 100 ± 50 cm-1 was obtained with the trans
form as the more stable conformer. This value is comparable with the energy difference value
obtained from the investigation reported in Ref. [33] but it had a much larger uncertainty value.
Hamada et al. [35] examined the enthalpy difference between the two conformers by an electron
diffraction study where it was reported that the trans form is more stable than the gauche
conformer by 107 cm-1 (306 cal/mol) with a large uncertainty of 200. The abundance of the trans
conformer was reported [35] to be 46(10)%. The most recent study reported on the ethylamine
conformational stability investigation [36] was from a Fourier transform infrared spectra of noble
gas solution where it was reported that the enthalpy difference was 54 ± 4 cm-1 (0.65 ± 0.05
kJ/mol) with the trans conformer the more stable form. From the reported studies up to this time,
there is still a question of which conformer is the more stable form and what the enthalpy
difference is between them. These uncertainties of the more stable form and the value of the
enthalpy difference between them as well as the structural parameters of this important molecule
provide a very clear reason for again investigate this molecule. Therefore, microwave and
conformational stability investigations have been initiated to answer conclusively the questions
of the more stable conformer and the enthalpy difference.
In order to obtain a confident determination of the conformational enthalpy differences, an
investigation of the Raman spectra (4000-50 cm-1) of ethylamine, CH3CH2NH2 dissolved in
liquefied xenon was carried out. Also to aid in identifying vibrations for conformers and to
assign fundamentals for the less stable conformer for determining the enthalpy differences ab
initio calculations have been utilized. The harmonic force fields, infrared intensities, Raman
activities, depolarization ratios, and vibrational frequencies were predicted from MP2(full)/6-
14
31G(d) ab initio calculations with full electron correlation. Additionally MP2(full) ab initio and
Density Functional Theory (DFT) calculations by the B3LYP method have been carried out with
a variety of basis sets up to aug-cc-pVTZ, as well as with diffuse functions to predict energy
difference and the conformational stabilities. The results of these microwave, spectroscopic,
structural, and theoretical studies of ethylamine are reported herein.
Experimental and Theoretical Methods
The sample of ethylamine was purchased from Sigma-Aldrich Chemical Co., with stated
purity of ≥98% and the sample was used without further purifications.
The microwave spectra were recorded by a “mini-cavity” Fourier transform microwave
spectrometer [18], [19] at Kent State University. The Fabry-Perot resonant cavity is established
by two 7.5-inch diameter diamond-tip finished aluminum mirrors with a 30.5-cm spherical
radius. The Fabry-Perot cavity resides inside a vacuum chamber formed by a 6-way cross and a
15-inch long, 8-inch diameter extension tube. The two cavity mirrors are nominally separated by
30 cm.
The sample was entrained in a 70:30 Ne-He carrier gas mixture at 2 atm and expanded
into the cavity to attain 4K by using a reservoir nozzle [18] made from a modified Series-9
General Valve. The reservoir nozzle is mounted in a recessed region of the mirror flange,
external to the vacuum chamber, and the expansion passes through a 0.182-inch diameter hole
into the resonant cavity. The center of the expansion is offset from the center of the mirror by 1
inch.
The sample was irradiated by microwave radiation generated by an Agilent Technologies
E8247C PSG CW synthesizer. The details of the irradiation and heterodyne detection circuitry
can be found in Ref. [19]. The vacuum system can accommodate pulse repetition rates of up to
15
15 s-1 while maintaining a pressure below 10-4 torr, and the instrument can scan 450 MHz in 6
hours while averaging 100 shots per scan segment.
The Raman spectra (4000 to 50 cm-1) of the sample dissolved in liquefied xenon (Fig. 1)
at nine different temperatures (-60 to -100oC) was recorded on a Trivista 557 spectrometer
consisting of a double f = 50 cm monochromator equipped with a 2000 lines mm-1 grating, a f =
70 cm spectrograph equipped with a 2400 lines mm-1 grating, and a back-illuminated LN2cooled PI Acton Spec-10:2 kB/LN 2048 x 512 pixel CCD detector. For all experiments, the
514.5 nm line of a 2017-Ar S/N 1665 Spectra-Physics argon ion laser was used for Raman
excitation, with the power set to 0.8 Watt. Signals related to the plasma lines were removed by
using an interference filter. The frequencies were calibrated by using Neon emission lines, and
depending on the setup used, are expected to be accurate within 0.4 cm-1. The experimental setup used to investigate the solutions has been described before [15],[16]. A home-built cell for
liquids was equipped with four quartz windows at right angles were used to record the spectra.
The infrared and Raman bands chosen as fundamentals along with their proposed assignments
and depolarization values are listed in Tables 1 and 2, respectively.
The LCAO-MO-SCF restricted Hartree-Fock calculations were performed with the
Gaussian-03 program [21] by using Gaussian-type basis functions. The energy minima with
respect to nuclear coordinates were obtained by the simultaneous relaxation of all geometric
parameters by the gradient method of Pulay [22] . Several basis sets as well as the corresponding
ones with diffuse functions were employed with the Møller-Plesset perturbation method [23] to
the second order (MP2(full)), as well as with the density functional theory by the B3LYP
method. The predicted conformational energy differences are listed in Table 3.
16
In order to obtain a complete description of the molecular motions involved in the
fundamental modes of CH3CH2NH2, a normal coordinate analysis has been carried out. The
force field in Cartesian coordinates was obtained with the Gaussian 03 program [21] at the
MP2(full) level with the 6-31G(d) basis set. The internal coordinates used to calculate the G and
B matrices are given in Table 4 with the atomic numbering shown in Fig. 2. By using the B
matrix [24], the force field in Cartesian coordinates was converted to a force field in internal
coordinates. Subsequently, scaling factors of 0.88 for CH stretches and CH2 deformation and
0.90 for all other modes except the heavy atom bends were applied, along with the geometric
average of the scaling factors for the interaction force constants, to obtain the fixed scaled force
field and resultant wavenumbers. A set of symmetry coordinates was used to determine the
corresponding potential energy distributions (P.E.D.s) as can be seen in Table 5. A comparison
between the observed and calculated wavenumbers, along with the calculated infrared intensities,
Raman activities, depolarization ratios and potential energy distributions for the trans and gauche
conformers are listed in Tables 1 and 2, respectively.
Additional predictions for the vibrational assignments were obtained from the simulated
Raman spectra. The evaluation of Raman activity by using the analytical gradient methods has
been developed [15, 25-27], [37] and the activity Sj can be expressed as: Sj = gj(45αj2 + 7βj2),
where gj is the degeneracy of the vibrational mode j, αj is the derivative of the isotropic
polarizability, and βj is the anisotropic polarizability. To obtain the Raman scattering cross
sections, the polarizabilities are incorporated into Sj by multiplying Sj with (1-ρj)/ (1+ρj) where ρj
is the depolarization ratio of the jth normal mode. The Raman scattering cross sections and
calculated wavenumbers obtained from the Gaussian 03 program were used together with a
Lorentzian function to obtain the simulated Raman spectra.
17
Microwave Results
As indicated earlier, there have been many determinations of the enthalpy differences
between the trans and gauche conformers where multiple techniques have been used. Most of
these techniques have not confidently predicted the more stable form and, therefore, we believe
by carrying out a variable temperature microwave experimental study where the temperature will
be lowered the less stable conformer will be diminished greatly and end up leaving the more
stable form to have the major spectral intensity. Unfortunately, the instrument that was available
did not have the capability to regulate the temperatures but with FT- microwave instrument the
spectra are obtained at relative low temperature. However, the instrument at Kent State
University allowed us to dial the ground state transitions of the normal species of each conformer
where we were able to obtain the band intensity difference between the two conformers. In this
technique the bands of the more stable form were more intense and resolved in comparison to the
less stable form where it was easy to distingue between the two conformers and confidently
predict the more stable conformer. For the ethylamine molecule in this study, the bands for trans
conformer were very intense in comparison with the very weak ones for the less stable gauche
form. A comparison between the bands intensities that were predicted by using microwave
instrument in this study are shown in Fig. 3.
Vibrational Assignment
An attempted vibrational assignment from the Raman spectra of gaseous ethylamine was
made [38] where the frequencies were assigned to the fundamentals of CH3CH2NH2 but they did
not distinguish between the trans and gauche conformers. Another latter study was reported
where the infrared spectra of eight isotopomers of ethylamine were examined in the vapor phase
in the 300 -100 cm-1 spectral region [32]. In this study several Q-branch peaks were observed for
18
each isotopic species and assigned to the torsional oscillations of the methyl and amino groups of
the trans and gauche conformers. A decade later, an attempt to complete the vibrational
assignments by the help of the ab initio MO method calculation (4-31G(N*)) of the force
constants followed by a normal coordinate treatment was made [33]. Infrared absorption spectra
of both trans and gauche forms of ethylamine and some of its isotopomers in the gaseous state
and in low temperature matrices of Argon were used in the study. Only nine fundamentals were
assigned as arising from the trans form and seventeen fundamentals were assigned for the gauche
form. Additionally, a theoretical study [36] of the infrared spectra of some isotopomers of
ethylamine was reported. In this study the predicted normal mode frequencies were reported
based on the predicted MP2 level of calculation by using the 6-311G** basis set for trans and
gauche conformers. Seven of the fundamentals of the trans conformer in the finger print region
were not assigned and were listed as “no experimental data available”, which agrees with the
study reported previously by Hamada et al.[33], where a limited number of fundamentals were
assigned. The fact that the gauche conformer is degenerate makes it hard to differentiate between
the bands of the two conformers especially since the fundamentals for both conformers are at
nearly the same frequency.
By the utilization of MP2(full)/6-31G(d) predicted vibrational wavenumbers along with
ab initio predicted intensities, and depolarization ratios along with Raman data from xenon
solutions, it has been possible to confidently assign a significantly large number of the
fundamentals for each of the conformers and to also identify the modes involving mainly the
NH2 group. This is important for obtaining the enthalpy difference since the fundamentals which
will be used for the temperature study must be for a single confidently identified conformer. An
emphasis has been placed on the assignments for the fingerprint region from 1700 to 400 cm-1
19
where the number of overtone and combination bands are usually significantly reduced
compared to those in the higher wavenumber region.
In the current study, the region from 1700 to 400 cm-1 is fairly complex with thirty
predicted fundamentals for both trans and gauche conformers. Most of the assignments of these
bands agree with the previously reported infrared rare gas solution assignments [18] except for
eight fundamentals for the trans form and three fundamentals for the gauche conformer. These
reassignments in this region were based largely on the ab initio predicted band frequencies,
group frequencies, Raman activities of the bands and the previously reported assignments of
similar molecules. The reassignment for the trans conformer will be discussed first. The 18
fundamental previously assigned at 1458 cm-1 is now reassigned to a slightly lower frequency
band at 1453 cm-1 in the Raman spectra of the xenon solutions. Also, the 8 fundamental
previously assigned at 1347 cm-1, is now reassigned to the higher frequency band at 1386 cm-1
again from the spectra of the xenon solutions. Interestingly, this band was predicted at 1385 cm-1.
The 19 CH2 twist fundamental previously assigned at 1374 cm-1 is now reassigned to the band at
1356 cm-1. This band is predicted at 1357 cm-1 with a strong Raman activity of 16.0 Å4/amu
which makes the current assignment more accurate. The 10 fundamental is now reassigned at
1122 cm-1 in the Raman spectra of the xenon solutions which is a slightly higher frequency than
what was previously assigned at 1116 cm-1. The v21 NH2 twist fundamental previously assigned
to the band with frequency of 995 cm-1 which is now reassigned from the Raman spectra to the
band at 983 cm-1. The v12 and v13 fundamentals predicted at 897 and 861 cm-1 and were
previously assigned to the bands at 881 cm-1 and 795 cm-1 are now reassigned to the bands at 891
and 881 cm-1, respectively, again from the Raman data. Finally, the v22 CH2 rock which was not
assigned in the previous study is assigned to the band at 776 cm-1.
20
For the gauche conformer, three bands were reassigned which are 10, 18 and 20 and
these fundamentals were previously assigned to bands at 1467, 995 and 777 cm-1, respectively.
However, in the current study they are now reassigned at 1457, 989 and 881 cm -1, respectively.
The assignments of the remaining bands of the trans conformer agree with the previously
reported assignments within one to three wavenumbers. The other fundamentals for the gauche
form have been assigned in the current study and are largely in agreement with the previous
reported values except for the previous indicated 10, 18 and 20 fundamentals. With these more
accurate assignments of the trans and gauche fundamentals in the region from 1700 to 400 cm-1,
the vibrational assignments for ethylamine molecule are believed to be complete.
Conformational Stability
To determine the enthalpy differences between the two observed conformers of ethylamine
molecule, the sample was dissolved in liquefied xenon and the Raman spectra were recorded as a
function of temperature from -60 to -100°C. Only relatively small interactions are expected to
occur between xenon and the sample though the sample can associate with itself forming a
dimmer, trimer or higher order complexes. Therefore, only small wavenumber shifts are
anticipated for the xenon interactions when passing from the gas phase to the liquid xenon
solutions except for the NH2 modes. A significant advantage of this study is that the conformer
bands are better resolved in comparison with those in the Raman spectrum of the liquid. From ab
initio calculations, the dipole moments of the two conformers are predicted to have similar
values and the molecular sizes of the two conformers are nearly the same, so the ΔH value
obtained from the temperature dependent Raman study is expected to be near to that for the gas
[39-43].
21
Once confident assignments have been made for the fundamentals of both conformers the
task was then to find a pair or pairs of bands from which the enthalpy determination could be
obtained. The bands should be sufficiently resolved so their intensities can be determined. The
fundamentals at 776 and 1051 cm-1 for the trans form and 816 and 1081 cm-1 for the gauche
conformer were initially selected as they are confidently assigned, satisfactory resolved, and a
limited number of overtone and combination bands are possible to contribute to them. The band
at 989 cm-1 with a Raman activity of 1.2 Å4/amu was selected next for the gauche conformer as it
was found that the predicted underlying trans conformer fundamental at 983 cm-1 was not of
sufficient intensity to significantly affect the intensity of the observed band. The band at 1115
cm-1 was selected for the gauche form as it was satisfactory resolved which allowed the
determination of it’s intensity to be measured. Finally, the bands at 891 and 881 cm -1 were used
for the fundamentals of both the trans and gauche conformers. Whether these bands would be
used to determine the enthalpy difference depended on whether the results were consistent with
the values obtained from the initially chosen pairs.
The intensities of the individual bands were measured as a function of temperature (Fig. 4)
and their ratios were determined. By application of the van’t Hoff equation lnK = H/(RT) 
S/R, the enthalpy difference was determined from a plot of lnK versus 1/T, where H/R is the
slope of the line and K is substituted with the appropriate intensity ratios, i.e. Iconf-1 / Iconf-2, etc. It
was assumed that S and α are not functions of temperature in the range studied.
The conformational enthalpy difference (Table 6) was determined to be 75  8 cm-1 for the
1051/1082 band pair from the xenon solution. Also, the enthalpy differences were determined to
be 69  5 cm-1 for the 891/989 pair and 37  5 cm-1 for the 891/881 band pair. The fact that both
891 and 881 cm-1 bands were assigned to both the trans and gauche fundamentals and the
22
881cm-1 fundamental is an NH2 wagging mode explain why the 891/881 band pair gave a slightly
lower enthalpy difference value than what was expected. Moreover, an enthalpy value of 81  9
cm-1 was determined from the band pair 776/1115. An additional enthalpy difference value of 50
 7 cm-1 was also obtained from the 776/816 band pair. An average value was obtained by
utilizing all the data as a single set which gave a ΔH value of 62  4 cm-1, with the trans form the
more stable conformer. These error limits were derived from the statistical standard deviation of
two sigma of the measured intensity data where the data from the five band pairs were taken as a
single set. These error limits do not take into account associations with the liquid xenon or the
interference of overtones and combination bands in near coincidence with the measured
fundamentals. The variations in the individual values are undoubtedly due to these types of
interferences, but by taking several pairs, the effect of such interferences should be greatly
reduced. However, this statistical uncertainty is probably better than what can be expected from
this technique and ,therefore, an uncertainty of about 10% in the enthalpy difference is probably
more realistic i.e. 62  6 cm-1. From the enthalpy difference the abundance of the gauche
conformer present at ambient temperature is estimated to be 60%.
Structural Parameters
Initial structural parameters for the ethylamine were proposed from an electron
diffraction study [35] where partial structural parameters of trans and gauche conformers of
ethylamine (Table 4) were determined by a joint analysis of the electron diffraction intensity
measured in the study and the rotational constants previously reported in the literature. In this
study [35], the C-C and C-N bond lengths were determined for each conformer and the C-H
distances were averaged for both conformers. Only the CCN bond angle was determined for both
the trans and gauche conformers.
23
We [44] have shown that ab initio MP2(full)/6-311+G(d,p) calculations predict the carbonhydrogen r0 structural parameters for more than fifty carbon-hydrogen distances to at least 0.002
Å compared to the experimentally determined [45] values from isolated CH stretching
frequencies which agree to previously determined values from earlier microwave studies.
Therefore, all of the carbon-hydrogen parameters can be taken from the MP2(full)/6-311+G(d,p)
predicted values for the trans and gauche conformers of ethylamine.
We have also found that good structural parameters for hydrocarbons and many
substituted ones can be determined by adjusting the structural parameters obtained from the ab
initio MP2(full)/6-311+G(d,p) calculations to fit the rotational constants obtained from
microwave experimental data by using a computer program “A&M” (Ab initio and Microwave)
developed [45] in our laboratory. In order to reduce the number of independent variables, the
structural parameters are separated into sets according to their types where bond distances in the
same set keep their relative ratio, and bond angles and torsional angles in the same set keep their
difference in degrees. This assumption is based on the fact that errors from ab initio calculations
are systematic. Therefore, it should be possible to obtain “adjusted r0” structural parameters for
the five parameters with three heavy atoms and two NH2 parameters by utilizing the previously
reported rotational constants from the earlier microwave study where twelve rotational constants
were reported for each of the trans [46],[47] and gauche [34], [47] conformers. Therefore, we
have obtained complete structural parameters for both ethylamine conformers.
The resulting adjusted r0 parameters are listed in Table 4, where it is believed that the NC, C-C, and N-H distances should be accurate to ± 0.003 Å, the C-H distances are accurate to ±
0.002 Å, and the angles should be within ± 0.5. The fit of the twenty four determined rotational
constants by the structural parameters for ethylamine conformers are good with the differences
24
being less than 1 MHz as can be seen in Table 7. Therefore, it is believed that the suggested
uncertainties are realistic values and the determined structural parameters are probably as
accurate as can be obtained for the molecule in the gas phase by either electron diffraction or
microwave substituted methods.
Discussion
The vibrational assignments reported herein are based on a significant amount of
information with the Raman spectrum of the xenon solutions and predictions from the scaled ab
initio MP2(full)/6-31G(d) calculations, as well as the vibrational studies reported previously on
ethylamine. The ab initio Raman band activities were also used, but they seemed to be the least
reliable of the data utilized. One of the possible reasons for significant difference in predicted
and observed intensities could be the results of the association of the amine moiety of the
molecule with xenon. The evidence for the van der Waals molecules was the significant
decreases in the NH stretching frequencies from the gas to the solutions i.e. 17 and 19 cm-1 lower
for both the trans and gauche conformers. Finally, it should be noted that there is a very small
amount of the dimer present in the xenon solutions as evidence of the shortage of the band
assignments which are present at approximately ~3225 cm-1 in xenon solutions.
For the trans conformer the average ab initio predicted frequencies for A′ fundamentals
with frequencies of 1700 cm-1 and below, except for the NH2 wagging mode, was 9 cm-1 which
represents 0.8% error. The predicted frequency for the NH2 wag was too high with 0.9 scaling
factor but it can be corrected with a scaling factor of 0.7. However, if this value is used it usually
results in two or three unacceptable P.E.D. values. Since the NH2 wag gives rise to a very intense
infrared band, it is usually better to use 0.9 as the scaling factor so the other fundamentals have
“normal” values. For the trans conformer the average ab initio predicted frequencies for A′′
25
fundamentals was 10 cm-1 which represent 0.8% error. The percent error for the predictions for
the gauche conformer is nearly the same as the values for the trans conformer. Thus, the
relatively small basis set of 6-31G(d) by the MP2(full) calculations with two or three scaling
factors provides excellent predicted frequency values for aiding the vibrational assignments.
For the trans form the mixing is relatively small for the A′ modes except for 10, 12 and 13
which have only 30%S10, 48%S12 and 37%S13, respectively, mixed with two to three other
symmetry coordinates. The A′′ modes 20 (CH3 rock) has a relatively small percentage of 28%S20,
with 26%S19 (CH2 twist), 21%S22 (CH2 rock) and 19%S21 (NH2 twist). Also, 21 (NH2 twist) has a
relatively small percentage of 38%S21, with 30%S20 (CH3 rock), 17%S19 (CH2 twist) and 13%S22
(CH2 rock). Nevertheless, the approximate descriptions for the normal modes provide reasonable
information for the molecular motions of most of the vibrations for the trans conformer. However,
for the gauche conformer there is significantly more mixing with the vibration at 1233 cm-1
assigned as the CH3 rock with only 22%S15 and 20%S18 (NH2 twist) mixed with two other
symmetry coordinates. The 16 fundamental has a significantly small percentage with 16%S16, and
also small values for 26%S21 (CH2 rock) and 15%S15 (CH3 rock). Therefore, several of the
approximate descriptions for the fundamentals of the gauche form are more for bookkeeping,
rather than providing descriptions of the molecular motions.
As can be seen from data in Table 4, the C-C distance is 0.006 Å longer for the trans
conformer compared to the corresponding parameters for the gauche form. This similar difference
was reported from the electron diffraction study [35] but the uncertainty of this parameter from the
electron diffraction results was larger. The other heavy atom distances, as well as the angles
reported in the electron diffraction study are in good agreement with the values reported in this
study.
26
From the conformational study of ethylamine, the enthalpy value obtained from the xenon
solutions must be considered the most confident enthalpy difference determination at this time as
it is doubtful that any other experiments would give an enthalpy value with such a low uncertainty.
The use of enthalpy determinations from five band pairs provides excellent accuracy of the
determination by this technique.
Normally, we would not use the fundamentals associated with the NH2 group for the
conformational stability determinations since they can be significantly affected by molecular
association as the temperature is decreased. However, there did not appear to be a significant
change in the association as the temperature was lowered from -60 to -100°C and thus we used the
NH2 wagging band which gave excellent enthalpy results. Therefore, by using five band pairs for
the determination of the enthalpy difference and the data as a single set, the enthalpy difference
was determined to be 62 ± 6 cm-1.
For comparison purposes with the experimental values, the ab initio energy differences
have been determined by a variety of basis sets as can be seen in Table 3. Both the MP2
calculations up to aug-cc-pVTZ basis set and MP4(SDTQ) calculations up to 6-311G(2df,2pd)
basis set did not succeed in predicting the more stable conformer whereas the basis set with diffuse
functions predicted the gauche conformer to be the more stable form. However, by utilizing the
same basis sets and level of calculations without the diffuse functions the trans form is predicted
to be the more stable form. Nevertheless, the B3LYP calculations are consistent with the values
obtained in this study. This is in contrast with ab initio predictions for aminomethylcyclopropane
[48] where the MP2(full)/6-311+G(2d,2p) calculations gave correctly the gauche form as the more
stable conformer by 66 cm-1 (0.79 kJ mol-1) with an experimental value of 109 ± 11 cm-1 (1.30 ±
0.13 kJ mol-1) whereas the calculations with the same basis set but without diffuse functions
27
predicted the trans form to have the lower energy by 73 cm-1 (0.87 kJ mol-1). This is another
reason why this study was carried out since the ab initio calculations can not be depend on to
predict the correct conformational stability and to identify which conformer is the more stable
form, especially for organoamine molecules. Since these molecules are really important in
biological activities further determinations of the conformational stabilities is desirable for other
substituted amine molecules.
28
Table 1. Observed and calculateda wavenumbers (cm-1) for trans ethylamine.
Vib.
No.
Approximate Descriptions
ab
initio
fixed
scaledb
IR
int.
Raman
act.
IR
Gasc
3344
2967
29
IR rare
gas soln
Gasc
3331
2962
2913
2874
1620
1467
1453
1347
1347
1116
1052
881
795
406
3400
2961
2929
1458
1347
1235
995
Raman
P.E.D.e
Band
Contours
Xenon Liquidc solidc
A*
B*
3505
3325
0.4
82.8
3327
3320
3175 100S1
3
97
A 1 NH2 symmetric stretch
3187
2990
32.8
65.0
2958
2963
2959
99S2
30
70
2 CH3 antisymmetric stretch
3115
2922
32.9
92.2
2910
2914
2916
99S3
- 100
3 CH2 symmetric stretch
3095
2904
16.5 105.6
2878
2868
2870
2867 100S4
71
29
4 CH3 symmetric stretch
1728
1639
25.7
8.9
1626
1620
1611
1628
78S5,21S12
- 100
5 NH2 deformation
1570
1476
2.4
0.3
1465
1467
1468
1465
55S6,39S7
18
82
6 CH2 deformation
CH
antisymmetric
deformation
1554
1459
0.0
30.4
1456
1450
1453
1441
53S
43S
43
57
7
3
7,
6
1468
1385
9.3
3.9
1378
1386
1374
1371
86S8
83
17
8 CH3 symmetric deformation
1426
1352
12.0
1.3
1350
1346
1354
1349
82S9
97
3
9 CH2 wag
1201
1146
13.3
1.9
1117
1122
1124
1148
30S10,28S11,,16S14,13S12
28
72
10 CH3 rock
1116
1060
25.2
5.3
1055
1051
1048
1051
38S11,49S13
100
11 CN stretch
946
897
63.3
8.8
882
891
891
872
48S12,33S10,11S5
100
12 CC stretch
908
861 126.8
4.3
789
881
~802
37S13,33S11,15S12
100
13 NH2 wag
413
406
7.0
1.2
404
405
415
423
81S14
78
22
14 CCN bend
3605
3420
0.0
49.2
3413
3394
3366
3324 100S15
A 15 NH2 antisymmetric stretch
3190
2993
50.0
7.8
2968
2966
2963
2959
81S16,19S17
16 CH3 antisymmetric stretch
3162
2966
6.1 113.3
2921
2925
2929
2926
81S17,19S16
17 CH2 antisymmetric stretch
CH
antisymmetric
deformation
1561
1466
7.5
15.5
1459
1453
1452
1448
91S
18
3
18
1431
1357
0.0
16.0
1350
1356
1354
1349
53S19,38S21
19 CH2 twist
1319
1251
0.0
1.5
1238
1233
1238
1249
28S20,26S19,21S22,19S21
20 CH3 rock
1037
984
1.5
0.6
993
983
1003
38S21,30S20, 17S19, 13S22
21 NH2 twist
801
760
1.0
0.0
776
55S22,33S20
22 CH2 rock
323
307
47.9
2.0
265
266
~274
~270
278
74S23, 23S24
23 NH2 torsion
276
262
13.0
0.6
237
240
383
74S24, 24S23
24 CH3 torsion
a
MP2(full)/6-31G(d) ab initio calculations, scaled wavenumbers, infrared intensities (km/mol), Raman activities (Å 4/amu), depolarization ratios and potential energy
distributions (P.E.D.s).
b
MP2(full)/6-31G(d) fixed scaled wavenumbers with factors of 0.88 for CH stretches and CH 2 deformation, 1.0 for heavy atom bends, and 0.90 for all other modes.
c
Ref [36].
d
Calculated with MP2(full)/6-31G(d) and contributions of less than 10% are omitted.
*
A, B and C values in the last three columns are percentage infrared band contours.
Table 2: Observed and calculateda wavenumbers (cm -1) for gauche ethylamine.
Vib.
No.
30
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
Approximate Descriptions
ab
initio
fixed
scaledb
IR Raman
int.
act.
dp
ratio
IR
Gasc
IR rare
Raman
gas soln
Gasc
Xenon
Liquidc
Band
Contours
P.E.D.e
A*
B*
C*
NH2 antisymmetric stretch
3609
3424 0.0 64.3
0.72
3413
3400
3394
3366 100S1
86
14
NH2 symmetric stretch
3508
3328 0.5 101.2
0.12
3344
3331
3327
3320 100S2
58
38
4
CH3 antisymmetric stretch
3216
3016 21.9 49.5
0.74
2985
2977
2975
56S3,42S4
10
23
67
CH3 antisymmetric stretch
3191
2994 36.4 44.3
0.63
2967
2961
2966
2963
57S4,39S3
16
41
43
CH2 antisymmetric stretch
3145
2951 24.3 86.3
0.56
2921
2929
2925
2926
66S5,30S7
2
24
74
CH3 symmetric stretch
3107
2915 15.9 102.3
0.02
2878
2874
2873
2870
98S6
70
24
6
CH2 symmetric stretch
3037
2849 79.6
99.2
0.26
2842
2839
2842
69S7,31S5
1
63
36
NH2 deformation
1636 33.2
8.6
0.69
1622
1620
1620
1611
78S8,21S20
44
1
55
1724
CH2 deformation
1587
1492 0.5
5.4
0.68
1485
1482
1481
1477
87S9,10S10
65
11
24
CH3 antisymmetric deformation
1564
1469 2.7
25.2
0.75
1465
1467
1457
1468
80S10,11S9
41
57
2
CH3 antisymmetric deformation
1556
1461 5.0
18.5
0.75
1456
1453
1450
1452
89S11
2
5
93
CH2 wag
1487
1407 18.8
6.0
0.64
1397
1395
1395
1392
46S12,30S13,11S18
100
CH3 symmetric deformation
1451
1372 1.7
1.1
0.59
1378
1374
1375
1374
65S13,26S12
9
90
1
CH2 twist
1373
1304 5.5
13.5
0.70
1293
1291
1292
1297
58S14,20S18
84
8
8
CH3 rock
1313
1247 0.6
4.5
0.72
1238
1235
1233
1238
22S15,20S18,19S14,15S21
42
21
37
CH3 rock
1200
1142 19.8
4.9
0.23
1117
1116
1115
1124
16S16,26S21,15S15
3
37
60
CN stretch
1144
1086 7.7
6.8
0.69
1085
1082
1081
1084
59S17,20S19,10S12
83
17
NH2 twist
1042
989 0.8
1.2
0.25
993
995
989
1003
34S18,27S16,17S19
1
82
17
CC stretch
947
898 42.7
5.6
0.48
892
891
891
891
37S19,30S20,11S16,10S17
17
53
30
NH2 wag
916
868 93.4
5.5
0.36
773
777
881
31S20,24S17,15S16,10S8
26
45
29
CH2 rock
833
790 17.0
0.7
0.74
816
817
816
47S21,36S15
11
48
41
CCN bend
426
419 12.0
0.6
0.59
413
413
412
415
80S22
79
21
NH2 torsion
300
285 28.6
1.2
0.73
259
258
~270
55S23, 36S24
47
31
22
CH3 torsion
259
246 25.1
0.9
0.75
218
223
61S24, 38S23
27
48
25
a
MP2(full)/6-31G(d) ab initio calculations, scaled wavenumbers, infrared intensities (km/mol), Raman activities (Å4/amu), depolarization ratios and potential energy
distributions (P.E.D.s).
b
MP2(full)/6-31G(d) fixed scaled wavenumbers with factors of 0.88 for CH stretches and CH2 deformation , 1.0 for heavy atom bends, and 0.90 for all other modes.
c
Ref [36].
d
Calculated with MP2(full)/6-31G(d) and contributions of less than 10% are omitted.
*
A, B and C values in the last three columns are percentage infrared band contour.
Table 3: Calculated energies (hartree) and energy differences (cm-1) for the two conformers of
ethylamine.
Method/Basis Set
MP2(full)/6-31G(d)
MP2(full)/6-31+G(d)
MP2(full)/6-311G(d,p)
MP2(full)/6-311+G(d,p)
MP2(full)/6-311G(2d,2p)
MP2(full)/6-311+G(2d,2p)
MP2(full)/6-311G(2df,2pd)
MP2(full)/6-311+G(2df,2pd)
MP2(full)/cc-pVTZ
MP2(full)/aug-cc-pVTZ
MP4(SDTQ)/6-31G(d)
MP4(SDTQ)/6-31+G(d)
MP4(SDTQ)/6-311G(d,p)
MP4(SDTQ)/6-311+G(d,p)
MP4(SDTQ)/6-311G(2d,2p)
MP4(SDTQ)/6-311+G(2d,2p)
MP4(SDTQ)/6-311G(2df,2pd)
B3LYP/6-31G(d)
B3LYP/6-31+G(d)
B3LYP/6-311G(d,p)
B3LYP/6-311+G(d,p)
B3LYP/6-311G(2d,2p)
B3LYP/6-311+G(2d,2p)
B3LYP/6-311G(2df,2pd)
B3LYP/6-311+G(2df,2pd)
B3LYP/6-311++G(3df,3pd)
B3LYP/aug-cc-pVTZ
a
# basis set
59
71
96
108
132
144
188
200
188
299
59
71
96
108
132
144
188
Energya, E Energy Differences, ∆
trans
gaucheb
0.688245
72
0.698791
-66
0.843036
96
0.848939
-64
0.885439
32
0.890587
-66
0.939665
29
0.944493
-59
0.929315
-23
0.949398
-58
0.737254
99
0.748335
-36
0.901116
128
0.907083
-38
0.943525
62
0.948646
-39
0.999485
61
59
71
96
108
132
144
188
200
243
299
Energy of conformer is given as –(E+134) H
Difference is relative to trans form and given in cm-1
b
31
1.170732
1.179411
1.216916
1.221456
1.223536
1.227837
1.226719
1.230974
1.231814
1.234599
151
25
126
3
104
5
94
3
11
2
Table 4: Structural parametersa and dipole moments for the trans and gauche conformers of
ethylamine.
trans
Electron
diffractionb
Adjusted
r0
MP2(full)/6311+G(d,p)
1.528
1.463
1.095
1.094
1.094
1.094
1.094
1.015
1.015
115.5
111.2
110.6
110.6
109.6
109.6
107.6
107.6
110.2
110.2
108.1
108.1
108.0
106.5
106.6
180.0
-59.81
59.81
121.7
-121.7
58.70
-58.70
1.534
1.466
1.095
1.095
1.095
1.095
1.095
1.015
1.015
116.1
111.2
111.0
111.0
109.6
109.6
107.6
107.6
111.0
111.0
107.8
107.8
107.8
106.1
107.1
180.0
-59.94
59.94
121.9
-121.9
59.57
-59.57
1.531(6)
1.470(10)
1.107(6)
1.107(6)
1.107(6)
1.107(6)
1.107(6)
1.052
1.052
115.0(3)
1.526(3)
1.467(3)
1.099(2)
1.098(2)
1.098(2)
1.101(2)
1.101(2)
1.017(3)
1.017(3)
115.5(5)
111.6(5)
111.1(5)
111.1(5)
111.1(5)
111.1(5)
106.7(5)
106.7(5)
110.8(5)
110.8(5)
107.6(5)
107.6(5)
107.5(5)
105.1(5)
105.1(5)
180.0(5)
-59.81(5)
59.81(5)
121.7(5)
-121.7(5)
58.12(5)
-58.12(5)
1.521
1.465
1.093
1.095
1.092
1.100
1.094
1.015
1.015
109.8
111.0
110.6
110.1
109.2
109.6
113.3
107.6
110.8
109.8
107.9
108.9
108.2
107.1
106.8
177.9
-62.42
57.11
124.9
-118.0
179.6
61.84
1.526
1.469
1.094
1.095
1.092
1.102
1.095
1.015
1.016
110.6
110.9
110.9
110.6
109.2
109.4
113.3
107.5
111.2
110.8
107.6
108.7
107.9
106.7
107.2
178.2
-62.23
57.50
125.3
-118.2
-176.9
63.93
|a|
0.624
0.552
1.057(6)c
1.017
1.064
0.11(1)d
|b|
0.969
0.868
0.764(9)c
0.713
0.444
0.0.65(1)d
|c|
0.980
0.926

0.748
0.710
1.014(15)d
1.513
1.384
1.304(11)c
1.449
a
Bond distances in Å, bond angles in degree and dipole moments in Debye.
b
Ref. [35]
c
Dipole moments determined by microwave study, Ref. [46].
d
Dipole moments determined by microwave study, Ref. [34].
1.354
r(C1C2)
r(C2N3)
r(C1H4)
r(C1H5)
r(C1H6)
r(C2H7)
r(C2H8)
r(N3H9)
r(N3H10)
C1C2N3
H4C1C2
H5C1C2
H6C1C2
H7C2C1
H8C2C1
H7C2C3
H8C2C3
H9N3C2
H10N3C2
H4C1H5
H4C1H6
H5C1H6
H7C2H8
H9N3H10
H4C1C2N3
H5C1C2N3
H6C1C2N3
H7C2C1N3
H8C2C1N3
H9N3C2C1
H10N3C2C1
|tot|
R1
R2
R3
R4
R5
r1
r2
r3
r4
A1
A2
A3
A4
A5
A6
A7
β
1
2
1
2
γ
θ
α1
1
2
3
4
5
6
7
MP2(full)/6311+G(d,p)
gauche
B3LYP/6311+G(d,p)
Parameter
Int
Coor.
32
B3LYP/6311+G(d,p)
Electron
diffractionb
Adjusted
r0
1.524(6)
1.475(10)
1.107(6)
1.107(6)
1.107(6)
1.107(6)
1.107(6)
1.052
1.052
109.7(3)
1.520(3)
1.471(3)
1.098(2)
1.099(2)
1.097(2)
1.108(2)
1.102(2)
1.018(3)
1.019(3)
110.2(5)
111.1(5)
110.7(5)
110.2(5)
110.3(5)
110.7(5)
112.5(5)
106.9(5)
111.2(5)
110.3(5)
107.7(5)
108.8(5)
108.1(5)
106.0(5)
107.0(5)
177.9(5)
-62.42(5)
57.11(5)
124.9(5)
-118.0(5)
-179.9(5)
61.41(5)
1.210(15)d
Table 5: Symmetry coordinates for ethylamine.
Species
A
A
Description
Symmetry Coordinatea
NH2 symmetric stretch
S1 =
r6 + r7
CH3 antisymmetric stretch
S2 =
2r1 r2 r3
CH2 symmetric stretch
S3 =
r4 r5
CH3 symmetric stretch
S4 =
r1 + r2 + r3
NH2 deformation
S5 =

CH2 deformation
S6 =
4
CH3 antisymmetric deformation
S7 =

CH3 symmetric deformation
S8 =
 
CH2 wag
S9 =

CH3 rock
S10 =

CCN antisymmetric stretch
S11 =
R1 R2
CCN symmetric stretch
S12 =
R1 R2
NH2 wag
S13 =

CCN bend
S14 =

NH2 antisymmetric stretch
S15 =
r6 r7
CH3 antisymmetric stretch
S16 =
r2 r3
CH2 antisymmetric stretch
S17 =
r4 r5
CH3 antisymmetric deformation
S18 =

CH2 twist
S19 =

CH3 rock
S20 =

NH2 twist
S21 =

CH2 rock
S22 =

NH2 torsion
S23 =

CH3 torsion
S24 =

aNot normalized.
33
Table 6: Temperature and intensity ratios of the trans and gauche bands of ethylamine.
T(C)
Liquid 60.0
xenon 65.0
70.0
75.0
80.0
85.0
90.0
95.0
100.0

Ha
1/T (10-3 K-1) I1051 / I1082
4.692
4.804
4.923
5.047
5.177
5.315
4.460
5.613
5.775
I891 / I989
I891/ I881b
I776 / I1115
I776 / I816
0.438
0.444
0.442
0.430
0.418
0.417
0.413
0.407
0.392
6.154
6.069
5.978
5.859
5.782
5.704
5.620
5.579
5.553
3.273
3.200
3.163
3.138
3.126
3.105
3.081
3.092
3.058
0.633
0.616
0.601
0.594
0.574
0.567
0.565
0.563
0.556
2.102
2.033
2.033
2.000
1.966
1.967
1.967
1.933
1.923
75  8
69  5
37  5
81  9
50  7
a
Average value H = 62  4 cm-1 ( 0.746  0.048 kJ mol-1) with the trans conformer the more stable form and the
statistical uncertainty (1σ) obtained by utilizing all of the data as a single set.
b
The 881 is NH2 wagging mode.
34
Table 7: Comparison of rotational constants (MHz) obtained from modified ab initio MP2(full)/6311+G(d,p) structural parameters and the experimental values from the microwave spectra of trans
and gauche conformers of ethylamine.
trans
Isotopomers
12
CH3 12CH2 14NH2 a
Rotational
constant
A
B
C
13
31758.33(16)
8749.157(50)
7798.905(50)
Adjusted
r0
31757.9249
8749.7097
7799.2239
0.40
0.55
0.32
A
B
C
31626.213(300)
8512.157(61)
7602.491(96)
31626.8150
8512.1333
7602.3497
0.60
0.02
0.14
12
A
B
C
31152.550(160)
8750.198(36)
7762.243(46)
31151.6910
8749.7086
7762.1261
0.86
0.49
0.12
12
CH3 12CH2 15NH2b
A
B
C
31675.510(160)
8524.285(36)
7614.905(46)
31676.1880
8524.2373
7614.8599
0.68
0.05
0.04
gauche
Isotopomers
Rotational
constant
Experimental
Adjusted
r0
12
A
B
C
32401.068(184)
8941.501(52)
7825.542(48)
32401.5679
8942.0957
7825.8660
0.50
0.59
0.32
13
A
B
C
32270.247(200)
8699.496(41)
7632.252(40)
32270.8480
8699.3907
7632.1178
0.60
0.10
0.13
12
A
B
C
31741.380(196)
8942.217(44)
7786.869(44)
31740.4397
8941.9098
7786.8291
0.94
0.31
0.04
12
A
B
C
32221.620(156)
8729.490(36)
7653.411(36)
32221.4813
8729.3063
7653.2783
0.14
0.18
0.13
CH3 12CH2 14NH2b
CH3 13CH2 14NH2b
CH3 12CH2 14NH2c
CH3 12CH2 14NH2b
CH3 13CH2 14NH2b
CH3 12CH2 15NH2b
a
Experimental
Ref[34]
Ref[47]
c
Ref[46]
b
35
||
||
Figure 1: Experimental and predicted Raman spectra of ethylamine: (A) Raman spectra of xenon
solution at -100°C; (B) simulated Raman spectrum of trans and gauche conformer with enthalpy
difference of 62 ± 6 cm-1 at -100°C; (C) simulated spectrum of gauche form.
36
Figure 2: Ethylamine molecule showing atomic numbering.
37
A.
Trans conformer/3000 shots ( 16547.9 - 16548.9)
Frequencies
B.
Gauche conformer/3000 shots (16983.6 - 16984.6)
Frequencies
38
C.
Gauche conformer/1000 shots ( 16929.4 - 16930.4)
Frequencies
Figure 3: Comparison between the band intensities obtained by microwave spectra for trans and
gauche conformers of ethylamine.
39
Figure 4: Raman spectra of ethylamine in xenon solution at different temperatures.
40
CHAPTER 4
CONFORMATIONAL AND STRUCTURAL STUDIES OF ISOPROPYLAMINE FROM
TEMPERATURE DEPENDENT RAMAN SPECTRA OF XENON SOLUTIONS AND AB
INITIO CALCULATIONS
Introduction
Many
organoamine
molecules
are
extremely
important
biomaterials
including
pharmaceuticals, pesticides, cleaners, food additives, etc. Recently there has also been a great deal
of interest in organoamines in the material sciences and nano-technology fields where
organoamines are used for organic light emitting diodes (OLED) and solar cells, as well as, simple
nano-machine structures and molecular scaffolding for precision nano-engineering.
Many of these molecules have been extensively studied to determine their most stable
conformation and to obtain the energy or enthalpy differences between or among the conformers
present at ambient temperature. Additionally, there has been significant interest in the structural
parameters for many of the small organoamines and for some of them the carbon-nitrogen or
nitrogen-hydrogen bond distances are reported significantly different. Since the organoamines are
such important molecules we embarked again on the infrared, Raman and microwave studies of
several of them which have been supported by ab inito calculations and rare gas solutions to
determine the enthalpy differences between or among the conformers identified. We began this
study [36] with a vibrational spectroscopic investigation of ethylamine in which it was determined
to have the trans conformer more stable by 54 ± 4 cm-1 (0.65  0.05 kJ mol-1) than the gauche
form. This value was in agreement with earlier determined values of 107 ± 70 cm-1 (1.28  0.84 kJ
mol-1) from electron diffraction study [35], 110 ± 50 cm-1 (1.32  0.60 kJ mol-1) from a microwave
investigation [34], and 100 ± 10 cm-1 (1.20  0.12 kJ mol-1) from an infrared study of ethylamine
in an argon matrix [33]. However, several of the earlier studies [29-32] reported the gauche
41
conformer as the more stable form, and it is rather interesting that MP2(full)/6-311+G(2d,2p)
calculations predict [36] the gauche conformer to be the more stable form by 66 cm-1 (0.79 kJ mol1
). However, without the diffuse functions the trans form is predicted to be the more stable
conformer by 32 cm-1 (0.38 kJ mol-1).
These results clearly indicate that the enthalpy difference between the gauche and trans
conformers of the NH2 moiety will need to be determined experimentally rather than by theoretical
predictions of organoprimary amines. Thus, it is of scientific interest to determine the effect of
substituting one or more of the hydrogen atoms on -carbon. One of the first molecules we
investigated was 2-fluoroethylamine [49] where one of the hydrogen atoms on the -carbon was
replaced by a fluorine atom which is expected to have a significant effect on the C-C distance and
it has potential to have five stable conformers, instead of two. With the utilization of variable
temperature infrared spectra of xenon solutions we were able to determine the conformational
stabilities of all five of the conformers of 2-fluoroethylamine [49].
The most stable conformer was the Gg′ form with the Gt conformer the second most stable
form with an enthalpy difference of 62 ± 8 cm-1 (0.74  0.10 kJ mol-1), where the first indicator is
the NCCF dihedral angle (G=gauche or T= trans) and the second one (g=gauche or t= trans) is the
relative position of the lone pair of electrons on the nitrogen atom with respect to the -carbon
atom [Gg′ is noted by the N-H eclipsing the fluoride]. The third, fourth, and fifth conformers in
relative energy are the Tg (262 ± 26 cm-1), Tt (289 ± 45 cm-1), and Gg (520 ± 50 cm-1),
respectively, where it was not possible to determine whether the Tg or Tt was the more stable one.
Thus the fluorine atom appears to have an effect on the conformer stability of the ethylene groups
so we have continued our investigations of molecules with substitutions on the -carbon atom of
ethylamine with the determination of the conformational stabilities and structural parameters of npropylamine which has a substitution of the methyl groups which is replacing one of the hydrogen
42
atoms on the -carbon. However this study is in progress and at the same time we have also
replaced one of the hydrogen atoms on the α-carbon of ethylamine with a methyl group, i.e.
isopropylamine to evaluate the effect of substitution on this carbon atom.
Therefore as a continuation of our study of the organoamines, we have again investigated
vibrational spectra of isopropylamine with particular attention to the assignments for the
fundamentals for both conformers, the enthalpy difference between the conformers and the
structural parameters. In the first relatively complete vibrational investigation, the Raman spectra
of gas, liquid and solid and the infrared spectra of the gas and solid of the normal species as well as
the ND2 isotopologue were investigated from 4000 to 50 cm-1 by Durig et al. [50]. From this study
the enthalpy differences of 156 and 183 cm-1 (1.87 kJ mol-1 and 2.19 kJ mol-1) for the d0 and ND2
species was found from the potential function obtained from the assignment of the far infrared
spectra with the fundamentals of 236 and 221 cm-1 for NH2 torsional modes of the trans and
gauche conformers, respectively. These values were significantly different from the value of 42
cm-1 (0.50 kJ mol-1) reported by Kurgen and Jan [51] from a dilute CCl4 solution with the trans
conformer more stable.
The other studies up to this time of the first extensive spectroscopic investigations were detailed in
this article [50] with energy differences ranging with the gauche conformer more stable to values
near zero between the two forms. However, a more recent study [52] by using high-resolution
infrared spectra of the gas and low temperature Argon matrix investigation resulted in some
significantly different assignments for the fundamentals of both conformers and significantly
larger enthalpy difference than the value from the previously reported [50] potential function. In
addition to our interest in the enthalpy difference and the vibrational assignments we were
interested in the structural parameters since only limited data have been reported. In the initial
microwave study [53] the authors point out that the structure of the isopropyl group is relatively
43
constant in a wide variety of substances, therefore, it was assumed that the structural parameters of
the isopropyl group was nearly the same as those of the groups of seven other molecules so the
CCC was taken as 111.8° and the two C-C distances were equal for the trans conformer at 1.527
A° which reproduced the observed planar moment when the substitution value for the amine H---H
distance. The nine moments of inertia from (CH3)2CHNH2, (CH3)2CHNHD, and (CH3)2CHND2
were fit with ґ(C-N) = 1.49 Å, CCN = 108° and HNC = 109° with estimated uncertainties of ±
0.02 Å and ± 2°. In a later microwave study [54] of (CH3)2CHNH2 and (CH3)2CHND2 by FTmicrowave spectroscopy both centrifugal distortion constants and corrected quadrupole coupling
constants as well as more accurate rotational constants were reported but no information was given
for the structural parameters. However a more recent electron diffraction investigation [55] was
reported where the parameters were further refined by combining the moments of inertia from the
microwave studies. In this study it was assumed that the molecule was 100% trans since the NH 2
change to the gauche form was not expected to affect significantly the heavy atom parameters.
However, we believed that it is possible to obtain the structural parameters for the trans conformer
more accurately than the values previously reported and estimate those for the gauche form which
is in large abundance at ambient temperature. Therefore, the three major goals were to correct
some of the earlier vibrational assignments, obtain a reliable enthalpy difference between the two
conformers from which the concentrations of the two forms could be obtained at ambient
temperature, and determine reliable structural parameters for both conformers.
In order to obtain a more reliable experimental value for the conformational enthalpy difference,
we have investigated the variable temperature (-50 to -100 °C) Raman spectra (3500 to 100 cm-1)
of the sample dissolved in the liquified xenon. We have also reinvestigated the infrared spectra
(4000 to 220 cm-1) of the gas and solid. To support these experimental studies we have also
obtained the harmonic force constants, infrared intensities, Raman activities, depolarization ratios,
44
and vibrational frequencies from MP2(full)/6-31G(d) ab intio calculations. To obtain predictions
on the conformational stabilities we have carried out MP2(full) ab intio and density functional
theory (DFT) calculations by B3LYP method utilizing a variety of basis sets. The r0 structural
parameters have been obtained by combining the MP2(full)/6-311+G(d,p) ab initio predicted
parameters with the previously reported rotational constants obtained from the most recent
microwave study [54]. The results of these spectroscopic, structural, and theoretical studies of
isopropylamine are reported herein.
Experiment and Theoretical Methods
The sample of isopropylamine was purchased from Sigma-Aldrich Chemical Co., with stated
purity of ≥99.5%. The sample was used with no further purifications.
The mid-infrared spectrum of the gas (Fig. 5A) and solid (Fig. 5E) were obtained from 4000
to 220 cm-1 on a Perkin-Elmer model 2000 Fourier transform spectrometer equipped with a Ge/CsI
beamsplitter and a DTGS detector. Atmospheric water vapor was removed from the spectrometer
housing by purging with dry nitrogen. The gas and solid spectra were obtained with a theoretical
resolution of 0.5 cm-1 for the gas and 2 cm-1 for the solid with 128 interferograms added and
truncated. Multiple annealings were required to obtain satisfactory spectra of the solid.
The Raman spectra (3500 to 100 cm-1) of the sample dissolved in liquefied xenon (Fig. 6) at six
different temperatures (-50 to -100oC) were recorded on a Trivista 557 spectrometer consisting of a
double f = 50 cm monochromator equipped with a 2000 lines mm-1 grating, a f = 70 cm
spectrograph equipped with a 2400 lines mm-1 grating, and a back-illuminated LN2-cooled PI
Acton Spec-10:2 kB/LN 2048 x 512 pixel CCD detector. For all experiments, the 514.5 nm line of
a 2017-Ar S/N 1665 Spectra-Physics argon ion laser was used for Raman excitation, with the
power set to 0.8 Watt. Signals related to the plasma lines were removed by using an interference
filter. The frequencies were calibrated using Neon emission lines, and depending on the setup
45
used, are expected to be accurate within 0.4 cm-1. The experimental set-up used to investigate the
solutions has been described before [15], [16]. A home-built liquid cell equipped with four quartz
windows at right angles was used to record the spectra. The infrared and Raman bands assigned as
fundamentals along with their proposed assignments and depolarization values are listed in Tables
8 and 9, respectively.
The ab initio calculations were performed with the Gaussian-03 program [21] using
Gaussian-type basis functions. The energy minima with respect to nuclear coordinates were
obtained by the simultaneous relaxiation of all geometric parameters using the gradient method of
Pulay [22]. A variety of basis sets as well as the corresponding ones with diffuse functions were
employed with the Møller-Plesset perturbation method [23] to the second order MP2 with full
electron correlation as well as with the density functional theory by the B3LYP method. The
predicted conformational energy differences are listed in Table 10.
In order to obtain descriptions of the molecular motions involved in the fundamental modes
of isopropylamine, a normal coordinate analysis was carried out. The force field in Cartesian
coordinates was obtained with the Gaussian 03 program at the MP2(full) level with the 6-31G(d)
basis set. The internal coordinates used to calculate the G and B matrices are given for the trans
and gauche conformers in Table 11 with the atomic numbering shown in Fig. 7. By using the B
matrix [24], the force field in Cartesian coordinates was converted to force constants in internal
coordinates. Subsequently, 0.88 was used as the scaling factor for the CH stretches and
deformations, 0.70 for the NH2 torsion, and 0.90 was used for all other modes excluding the heavy
atom bends to obtain the fixed scaled force constants and resultant wavenumbers. A set of
symmetry coordinates was used (Table 12) to determine the corresponding potential energy
distributions (P.E.D.s). A comparison between the observed and calculated wavenumbers, along
with the calculated infrared intensities, Raman activities, depolarization ratios and potential energy
46
distributions for the trans and gauche conformers of isopropylamine are given in Tables 8 and 9,
respectively.
Vibrational Assignment
The first comprehensive vibrational assignment for isoporpylamine [50] was made by
utilizing the Raman spectra of the gas, liquid, and solid as well as the infrared spectra of the gas
and solid for both the normal species along with the ND2 molecule which made it possible to
identify the modes involving mainly the amines. The vibrational assignments were made mostly
based on existing assignments of corresponding vibrations of similar molecules. In the more recent
report on the vibrational assignments [52] the authors utilized predictions from ab intio
calculations with the 4-31G(N*) basis set and infrared spectral data from an argon matrix along
with higher resolution for the FT-IR spectrum. Although one might expect the improved resolution
would aid in making the vibrational assignment the breath of many of the bands resulted in little
information for making the vibrational assignments but the ab inito predictions provided some
guidance for assigning the fundamentals for the two conformers. Nevertheless there were only six
fundamentals that were assigned as arising from the trans form with a similar number for the
gauche conformer and the rest for both conformers. From the argon matrix, three additional
fundamentals were assigned for each conformer in this region of the spectrum where they could
not be separated from the spectrum of the gas. For the frequencies of the individual conformers
there were only one for each form which differed from the earlier assignments. However, some of
the assignments which were attributed to both conformers differed whether they were originally
assigned as A' or A'' modes while others have different vibrational descriptions or are not assigned.
In fact there were only two that differed frequencies as fundamentals for the assigned bands for
both conformers, one for the fundamentals assigned only to the trans form and two for those
assigned only to the gauche conformer as originally assigned. However, by the utilization of
47
MP2(full)/6-31G(d) predicted vibrational wavenumbers along with ab intio predicted intensities,
and depolarization ratios along with Raman data from xenon solutions it has been possible to
assign a significantly larger number of the fundamentals for each of the conformers. This is
important for obtaining the enthalpy difference since the fundamentals being used for the
temperature study must be for a single confidently identified conformer. Therefore, we have
attempted to assign all of the fundamentals for both conformers except for the carbon-hydrogen
stretching modes. The most emphasis has been placed on the assignments for the fingerprint region
from 1700 to 400 cm-1 where it was expected to find vibrational modes from the (CH3)2CH moiety.
The assignments for the NH2 stretching modes for both conformers were readily made from the
Raman spectra of the xenon solution and from the Raman spectrum of the gas (Fig. 8) and they
agree with the assignments previously reported. The assignments for the carbon-hydrogen
stretches are similar to those previously reported with the frequencies for the fundamentals of the
two conformers nearly the same except for the C-H mode where the ab inito calculations predicts
a difference of 74 cm-1 with the fundamental of the trans form with the lower frequency. There
appears to be a Fermi doublet at 2920 cm-1 and 2868 cm-1 so this lower frequency band is part of
the Fermi doublet and cannot be the lower frequency CH stretching fundamental. Thus the weak
band at 2821 cm-1 is assigned as the CH bend of the trans conformer but it could be the higher
frequency band at 2851 cm-1. However, it is doubtful that the ab initio prediction would be that far
off.
The frequencies for the NH2 scissors and the four CH3 antisymmetric deformations also have
essentially the same frequencies. However, most of the remaining fundamentals in the fingerprint
spectral region have slightly different frequencies for the corresponding modes for the two
conformers. For example, the two CH bends and the symmetric CH3 deformations have slightly
different frequencies for the two conformers. However, these eight fundamentals fall in relatively
48
small frequency range of 1382 to 1339 cm-1 which results in a nondescript infrared band contour
which makes it almost impossible to assign these fundamentals (Fig. 5A) from the infrared
spectrum of the gas. However, the Raman spectrum of the xenon solution shows seven of the eight
bands clearly (Fig. 9A). In the liquid these fundamentals appear as three rather broad lines.
By using the Raman spectrum of the xenon solution the two predicted fundamentals in the 1200
cm-1 region are clearly observed whereas they simply appear as a very weak broad band in the
infrared spectrum of the gas (Fig. 10). There are four fundamentals predicted in the 1100 cm-1
region at 1196, 1185, 1156, and 1146 cm-1 and again these fundamentals are clearly observed in
the Raman spectrum of the xenon solution at 1180, 1173, 1141 and 1128 cm-1.
The two
fundamentals of the trans conformer are clearly observed as Q-branches at 1181 and 1128 cm-1
(60%, 66% C-contour) in the infrared spectrum of the gas (Fig. 9B) (1178 and 1130 cm-1 Raman
spectrum of the gas) but these fundamentals for the gauche conformer have predicted band
contours of 99% B and 82% A type which makes it almost impossible to assign the B-type band
but from the Raman spectrum they are observed at 1173 and 1141 cm-1.
The third and most complex region for making vibrational assignments is from 900 to700 cm-1
(Fig. 9C) where there are four fundamentals. On the high frequency side of the broad band in the
infrared spectrum is a series of Q-branches at 826 cm-1 with two excited state bands falling to
higher frequency and thus must be the NH2 wag for the gauche conformer. The corresponding
fundamental for the trans form is clearly observed at 786 cm-1 with a large number of hot band Qbranches falling to both high and low frequencies. There are also two weak bands at 818 and 810
cm-1 in the infrared spectrum which appear as a relatively strong Raman line at 818 cm-1 (xenon
solution) for the trans conformer with a pronounced Q-branch at 811 cm-1 and a shoulder at the
same frequency in the Raman spectrum. The assignments for the remaining fundamentals for both
conformers are readily made and there is no need to discuss them.
49
These two examples clearly show that the better predictions from the ab initio calculations
compared to the earlier study with predicted intensities and band contours with the Raman spectra
of the xenon solutions it was possible to confidently assign the fundamentals for both conformers
in this spectral region.
Conformational Stability
To determine the enthalpy differences among the two observed conformers of isopropylamine, the
sample was dissolved in liquefied xenon and the Raman spectra were recorded as a function of
temperature from -50 to -100°C. Relatively small interactions are expected to occur between xenon
and the sample though the sample can associate with itself forming a dimmer, trimer or higher
order complex. However, due to the very small concentration of sample self-association is greatly
reduced. Therefore, only small wavenumber shifts are anticipated for the xenon interactions when
passing from the gas phase to the liquefied xenon solutions except for the NH2 modes. A
significant advantage of this study is that the conformer bands are better resolved in comparison
with those in the Raman spectrum of the liquid. From ab initio calculations, the dipole moments of
the two conformers are predicted to have similar values and the molecular sizes of the two
conformers are nearly the same, so the ΔH value obtained from the temperature dependent Raman
study is expected to be near to that for the gas [39-43].
Once confident assignments have been made for the fundamentals of both conformers the task was
then to find a pair or pairs of bands from which the enthalpy determination could be obtained. The
bands should be sufficiently resolved as to determine their intensities. The selection of the bands to
use in the enthalpy determination was complicated due to the presence of high intensity bands for
both conformers and the near overlap of many modes. The fundamentals at 472 and 463 cm-1 were
initially selected for the trans and gauche conformer as they are confidently assigned, satisfactory
resolved, and a limited number of overtone and combination bands are possible. The band at 818
50
cm-1 C-C-C symmetric stretch/CN stretch was then selected for the trans form as it was found that
the intensity was not significantly affected by the underlying NH2 bending mode which is weak in
the Raman effect. The bands at 3390 and 3325 cm-1 were next selected for the trans conformer and
the bands at 3380 and 3314 cm-1 were selected for the gauche form. From the frequency shift of
the NH2 stretching modes it is clear that this group interacts with the xenon atom. However, the
shift for both the trans and gauche conformer bands are similar and, thus, the enthalpy difference is
not expected to be significantly affected. Finally, the bands at 793 and 830 cm-1 (NH2 wag modes)
were also used for the trans and gauche conformers, respectively. Whether these bands would be
used to determine the enthalpy difference depends on whether the results were consistent with the
values obtained from the initially chosen pairs.
The intensities of the individual bands were measured as a function of temperature and their ratios
were determined (Fig. 11). By application of the van’t Hoff equation lnK = H/(RT)  S/R, the
enthalpy difference was determined from a plot of lnK versus 1/T, where H/R is the slope of the
line and K is substituted with the appropriate intensity ratios, i.e. Iconf-1 / Iconf-2, etc. It was assumed
that H, S, and α are not functions of temperature in the range studied.
These nine bands, five trans and four gauche, were utilized for the determination of the enthalpy
difference by combining them to form 20 band pairs. The enthalpy difference was determined from
these 20 band pairs with a value of 113  2 cm-1 (Table 13). This value is in reasonable agreement
with the corresponding predicted values from ab initio calculations (Table 10). This error limit is
derived from the statistical standard deviation of one sigma of the measured intensity data taken as
a single data set, but it does not take into account small associations with the liquid xenon or the
possible presence of overtones and combination bands in near coincidence of the measured
fundamentals. The variations in the individual values are undoubtedly due to these types of
interferences, but by taking several pairs, the effect of such interferences should cancel. However,
51
this statistical uncertainty is probably better than can be expected from this technique and,
therefore, an uncertainty of about 10% in the enthalpy difference is probably more realistic i.e. 113
 11 cm-1. From the enthalpy difference the abundance of the gauche conformer present at ambient
temperature is estimated to be 54 ± 1%.
Structural Parameters
Initial structural parameters for the isopropylamine were proposed from a microwave study
[53] where a partial structure of trans-isopropylamine (Table 11) was determined by assuming the
isopropyl group parameters by reproducing the planar moment for it. This was followed by another
microwave study [54] two decades later where the rotational constants were determined for transisopropylamine as well as 2,2,2-trifluoroethylamine and aminoethanol but no structural parameters
were reported for any of these molecules. From a later electron diffraction study [55] the structural
parameters were determined (Table 11) by fixing the structural parameters for the NH2 moiety to
the corresponding parameters previously reported [56] for the methylamine molecule.
We [44] have shown that ab initio MP2(full)/6-311+G(d,p) calculations predict the carbonhydrogen r0 structural parameters for more than fifty carbon-hydrogen distances to at least 0.002 Å
compared to the experimentally determined [45] values from isolated CH stretching frequencies
which agree to previously determined values from earlier microwave studies. Therefore, all of the
carbon-hydrogen parameters can be taken from the MP2(full)/6-311+G(d,p) predicted values for
the trans conformer of isopropylamine.
We have found that good structural parameters for hydrocarbons and many substituted ones
can be determined by adjusting the structural parameters obtained from the ab initio MP2(full)/6311+G(d,p) calculations to fit the rotational constants obtained from microwave experimental data
by using a computer program “A&M” (Ab initio and Microwave) developed [57] in our
laboratory. In order to reduce the number of independent variables, the structural parameters are
52
separated into sets according to their types where bond distances in the same set keep their relative
ratio, and bond angles and torsional angles in the same set keep their difference in degrees. This
assumption is based on the fact that errors from ab initio calculations are systematic. Therefore, it
should be possible to obtain “adjusted r0” structural parameters for the six parameters with four
heavy atoms and two NH2 parameters by utilizing the previously reported six rotational constants
from the earlier microwave study [54]. Therefore we have obtained the complete structural
parameters for trans isopropylamine.
The resulting adjusted r0 parameters are listed in Table 11, where it is believed that the N-C, C-C,
and N-H distances should be accurate to ± 0.003 Å, the C-H distances are accurate to ± 0.002 Å,
and the angles should be within ± 0.5. The fit of the six determined rotational constants (Table 14)
by the structural parameters for the trans conformer are good with the differences being less than 1
MHz. Therefore, it is believed that the suggested uncertainties are realistic values and the
determined structural parameters are probably as accurate as can be obtained for the molecule in
the gas phase by either electron diffraction or microwave substituted methods.
Additionally, we have also shown that the differences in predicted distances and angles from the
ab initio calculations for different conformers of the same molecule can usually be used as one
parameter with the ab initio predicted differences except for some dihedral angles. Therefore we
have predicted structural parameters for the gauche form of isopropylamine by applying the
corresponding adjustments from the trans conformer to the MP2(full)/6-311+G(d,p) predicted
parameters for the gauche form. These parameters should be close to the actual value except for
the dihedral angles.
53
Discussion
The vibrational assignments reported herein are based on a significant amount of information with
the Raman spectrum of the xenon solution and predictions from the scaled ab inito MP2(full)/631G(d) calculations as well as the predicted infrared bands contours.
The ab intio Raman and infrared band intensities were also used but they seemed to be the
least reliable of the data utilized. One of the possible reasons for significant difference in predicted
and observed intensities could be the results of the association of the amine portion of the molecule
with xenon. The evidence for the van der Waals molecules was the significant decreases in the NH
stretching frequencies from the gas to the solutions i.e. 13 cm-1 lower for the gauche form and 21
and 16 cm-1 for the trans conformer. However, for the other fundamentals the difference between
the frequencies in the gas and xenon solution is one or two wavenumbers except for the CCC
antisymmetric stretch where the difference is five wavenumbers for both conformers. Finally, it
should be noted that there is a very, very small amount of the dimer present in the xenon solution
as evidence of the lack of the bands which are present at approximately 3225 cm-1 in xenon
solutions.
For the trans conformer the ab inito predicted frequencies for A′ fundamentals with
frequencies of 1600 cm-1 and below, except for the NH2 wag was 8 cm-1 which represent 0.6%
error. If all the fundamental frequencies are included the percent error increases to 0.8% since the
CH stretches are poorly predicted. The predicted frequency for the NH2 wag was too high with 0.9
scaling but it can be corrected with a scaling factor of 0.7. However, if this value is used it usually
results in two or three unacceptable P.E.D. values. Since the NH2 wag gives rise to a very intense
infrared band, it is usually better to use 0.9 as the scaling factor so the other fundamentals are
“normal”. The percent error for the A′′ modes for the trans form have similar errors to that of the
A′ modes of the predicted frequencies and the percent error for the predictions for the gauche
54
conformer are nearly the same. Thus the relatively small basis set of 6-31G(d) by the MP2(full)
calculations with two or three scaling factors provides excellent predicted frequencies for aiding
the vibrational assignments.
For the trans form the mixing is relatively small for the A′ modes except for the 12 and 13
which have only 33%S12 and S13, respectively, with 22% and 23% of each others symmetry
coordinate. For the A′′ modes only 25 (CH bend) has a relatively small percentage of 29%S25 with
22%S26 the CH3 symmetric deformation. Therefore, the approximate descriptions for the normal
modes provide reasonable information on the molecular motions of vibrations. However, for the
gauche conformer there is significantly more mixing with the vibration at 1254 cm-1 assigned as the
CH3 rock with only 18%S12 and 30%S27 (NH2 twist) but the band at the 951 cm-1 has 35%S27 so this
lower frequency band is described as the NH2 twist. Therefore several of the approximate
descriptions for the fundamentals of the gauche form are more for bookkeeping than providing
descriptions of the molecular motions.
The adjusted r0 structural parameters have been determined for trans isopropylamine for six
independent parameters (three distances and three angles) but the HNC angle did not change when
the parameters were adjusted to fit the reported rotational constants. In the initial microwave study
[53] only three parameters were determined which was the distance (Å) C-N = 1.49(2) and the
angles (°) NCC = 108(2) and HNC = 109(2). The values obtained in the current study for the
angles NCC and HNC are both within the error limits of the values from the previous study but
again this is somewhat trivial due to the size of these error limits. The distance C-N obtained in the
current study is somewhat lower than that obtained in the microwave study but this is relatively
insignificant due to the large error limit of the previous study. The more recent electron diffraction
work [55] agrees with the current study within the error limits for every parameter except the angle
(°) CCC = 114.4(16) which is 3.4 degrees higher than the value in the current study; however,
55
when the error limits of nearly two degrees is taken into account this difference is also only
minimally significant and so the value from this study is believed to be determined well. Thus, the
structural parameters reported in the current study are believed to be accurate to within the stated
error limits.
It is of interest to compare the structural parameters of isopropylamine with the
corresponding parameters of other amine and isopropyl moieties. The methylamine structure has
been well determined using electron diffraction to have the distances (Å) C-N = 1.471(3), N-H =
1.019(6) and angle (°) HNC = 111.5(7). The N-H distance and HNC angle change relatively little
going from methylamine to isopropylamine; however, the C-N bond distance is slightly shorter in
the isopropylamine though it is within the experimental errors. It is also of interest to compare the
isopropyl group with other isopropyl moieties. The structural parameters of isopropyl chloride
have been determined from a microwave study [58] where the rotational constants of 11
isotopologues were used to determine the r0 structural parameters. The parameters of interest are
the distance (Å) C-C = 1.5202(14) and the angle (°) CCC = 113.31(16). The isopropylamine C-C
distance is longer by 0.010 Å than that for isopropyl chloride though this is lessened somewhat by
the error limits of  0.0044 Å the difference is still significant though not unexpected. The CCC
angle is also significantly smaller for the isopropylamine by 2.31° but this is not unexpected as the
chloride group is a larger substituent. Isopropyl alcohol is also of interest for comparing with
isopropylamine and the structural parameters have been determined by a microwave study [59]
where the parameters for the trans form are the distance (Å) C-C = 1.523(2) and the angle (°)
CCC = 112.3(1). These values have the same trend with the C-C distance longer in the
isopropylamine and the CCC angle shorter, though the differences in this case are less
pronounced. It would be of interest to determine more amine and isopropyl moiety parameters so a
more complete comparison could be made.
56
One of the major goals of this research was the determination of the enthalpy difference
between the two conformers where there had been a large range of the values reported from nearly
zero to a value slightly greater than 400 cm-1 (4.79 kJ mol-1). If only the low frequency fundamentals
at 463, 472, and 818 cm-1 and the four NH2 stretches are used in conjunction with the three low
frequency modes to determine the enthalpy difference a value of 104  4 cm-1 (1.24  0.05 kJ mol-1)
is determined. If the enthalpy difference also includes the four determinations using the NH2
stretches paired with each other the value increases slightly to 108  3 cm-1 (1.29  0.04 kJ mol-1)
and then finally including all twenty band pairs the value is 113  2 cm-1 (1.35  0.03 kJ mol-1)
where the statistical uncertainty is clearly unrealistic. Therefore we have simply indicated a more
reasonable value of  11 cm-1 (0.13 kJ mol-1). This value could be affected by the interaction of the
amine with the xenon solution. However even if the value was increased by 50% it still would not
reach near the most recently reported [52] value of ~400 cm-1. This value was estimated based on
the relative intensities of pairs of bands which were observed in the spectra of the argon matrix.
These investigators utilized the CH3 rock’s relative intensities at 1183 and 1178 cm-1 assuming the 
values would be equal. However, the  value of the gauche conformer is approximately half that of
the trans form which would then indicate the bands should have nearly equal intensities rather than
the 4:1 ratio observed by the authors for these bands in the argon matrix. From the Raman studies
these two bands have nearly equal intensity which does not support the large enthalpy difference of
~400 cm-1. Therefore, the use of the relative intensities in the argon matrix is questionable for
determining the enthalpy difference.
To support the experimental values the ab initio energy differences have been determined by
a variety of basis sets (Table 10). With diffuse functions the values are ~200 cm-1 with the trans
form the more stable conformer, whereas, without diffuse functions the values are slightly less than
100 cm-1 which would be consistent with the values obtained in this study.
57
Also in the earlier study [50] the value of the trans-gauche barrier was determined to be
~1000 cm-1 whereas with the more recent assignments [52] in the far infrared spectrum the splitting
in the third energy level indicates the barrier should be much higher. However, the energy difference
obtained from the various calculations give a value ~950 cm-1 (Table 10) which would be consistent
with the barrier reported earlier in the initial extensive vibrational study.
The only way for a more confident determination of the enthalpy difference would require
the use of a sample in a very low pressure gaseous phase. It is doubtful that this would be an easy
experiment to determine the enthalpy and, therefore, the value obtained from the xenon solution
must be considered the most confident enthalpy difference at this time. The use of enthalpy
determinations from 20 band pairs indicates the accuracy of the determination by this technique.
In the most recent microwave study of isopropylamine the quadrupole coupling constants
were determine from the spectral data as well as quartic centrifugal distortion constants. We have
obtained the quadratic centrifugal distortion constants and the quadrupole coupling constants from
the MP2(full)/6-311+G(d,p) calculations as well as the density functional theory by the B3LYP
method utilizing the same basis set. These data are given in Table 15 and as can be seen from the
values of the quadrupole coupling constants there is reasonable agreement with the predicted values
for both calculations. However, the experiment distortion constants are significantly different than
those predicted by both calculation methods. We have found in the past the ab initio calculations
usually provide fairly good distortion and quadrupole coupling constants.
With the predicted structural parameters for the gauche conformer it is believed that the predicted
rotational constants should be very close to those that would be obtained from a microwave study of
this conformer. Therefore it would be interesting if such a study were carried out since the
information obtained could be very useful in obtaining the gauche barrier using the splittings
observed in the ground state. Nevertheless it is believed that the parameters that have been estimated
58
should have rather small uncertainties with the uncertainties not larger than 0.005 Å for the heavy
atom distances and 1° for the angles.
59
Table 8: Observed and calculateda frequencies (cm-1) for trans.
Vib.
No.
Approximate Descriptions
fixed
scaledb
IR
int.
Raman
act.
IR
Gas
xenon Raman
solution
gasc
60
A 1
2
3
4
5
6
7
NH2 symmetric stretch
CH3 antisymmetric stretch , in-plane, in phase
CH3 antisymmetric stretch , out-of- plane, in phase
CH3 symmetric stretch, in phase
CH stretch
NH2 scissor
CH3 antisymmetric deformation, in-plane, in phase
3318
3009
2990
2910
2823
1633
1478
0.7
40.2
37.6
3.6
72.2
37.8
6.5
106.4
81.7
76.3
183.1
100.6
7.4
1.0
3341
2960
2940
2920
2820
1621
1472
3325
2959
2937
2920
2821
1619
1470
3342
8
9
10
11
12
13
14
15
16
17
18
A 19
20
21
CH3 antisymmetric deformation , out-of- plane, in phase
CH3 symmetric deformation, in phase
CH bend
CH3 rock , out-of- plane, in phase
CH3 rock, in plane, in phase
C-N stretch
NH2 wag
C-C-C symmetric stretch
C-C-N deformation, in-plane
C-C-C symmetric deformation
CH3 torsion, in-plane
NH2 antisymmetric stretch
CH3 antisymmetric stretch, in-plane, in phase
CH3 antisymmetric stretch, out-of- plane, out-of- phase
1465
1391
1350
1196
1146
989
871
810
474
375
270
3414
3005
2988
3.1
17.0
18.0
4.3
21.3
6.5
130.1
1.8
13.0
0.2
0.3
0.0
0.8
17.8
26.1
3.5
8.8
4.2
4.0
7.6
4.2
8.1
0.8
0.1
0.0
65.3
6.1
35.4
1461
1382
1352
1181
1128
978
786
818
472
369c
270
3411c
2960
2930
1458
1382
1354
1180
1128
976
793
818
472
369
272
3390
2959
2932
1460
1388
22
23
24
25
26
27
28
29
CH3 symmetric stretch, out-of- phase
CH3 antisymmetric deformation, in- plane, out-of- phase
CH3 antisymmetric deformation, out-of- phase
CH bend
CH3 symmetric deformation, out-of- phase
NH2 twist
C-C-C antisymmetric stretch
CH3 rock, in plane, out-of- phase
2908
1459
1457
1401
1362
1243
1026
949
31.0 5.3
0.2
1.3
0.3 30.5
17.0 1.2
0.9
2.2
0.0 11.3
0.8
3.9
0.5
2.9
2906
1452
1452
1382
1368
1243
1030
951
2906
1449
1449
1382
1364
1240
1025
948
2932
2918
1469
P.E.D.d
IR Solid
3347
2960
2947
2920
2872
1614
1467
1178
1130
970
792
819
472
368
1459
1385
1350/1353
1180
1132
989
810
814/816
471
370
-
2960
2927
-
2906
1452
1452
1385
1377
1246
1039/1055
948
100S1
58S2, 42S3
57S3,42S2
99S4
99S5
77S6,21S14
82S7,10S12
84S8,10S11
87S9
68S10,10S12,,10S9
49S11,14S17,12S15
33S12,22S13,17S16,15S10
33S13,23S12,14S14,14S11,10S15
58S14,15S6,10S15,10S12
57S15,25S13,10S11
62S16,15S17
65S17,20S16
99S18
100S19
55S20,44S21
55S21,45S20
99S22
40S23,41S24
50S24,43S23
29S25,22S26,15S28,13S27
74S26,16S25
38S27,34S25,10S29
48S28,37S27,10S29
69S29,23S28
Band
Contours
B*
C*
100
20
80
87
13
96
4
3
97
70
30
37
63
2
82
98
40
34
4
34
69
68
53
94
-
98
18
2
60
66
96
66
31
32
47
6
-
-
-
912
0.5
0.3
918
918
907
82S30,11S25
30 CH3 rock, in plane, out-of- phase
413
5.2
0.3
404
405
397
85S31
31 C-C-N deformation, out-of- plane
271
37.3 1.5
270
272
71S32,27S33
32 NH2 torsion
238
71S33,27S32
218
15.6 0.5
236c
33 CH3 torsion, out-of- phase
a
4
MP2(full)/6-31G(d) ab initio calculations, scaled frequencies, infrared intensities (km/mol), Raman activities (Å /amu), depolarization ratios and potential energy
distributions (P.E.D.s)
b
MP2(full)/6-31G(d) fixed scaled frequencies with factors of 0.88 for CH stretches and deformations, 0.70 for the NH2 torsion, 1.0 for heavy atom bends, and 0.90 for
all other modes.
c
Ref [50]
d
Calculated with MP2(full)/6-31G(d) and contributions of less than 10% are omitted.
*
A, B and C values in the last three columns are percentage infrared band contours.
-
61
Table 9: Observed and calculateda frequencies (cm-1) for gauche isopropylamine.
62
Vib.
No.b
Approximate Descriptions
19
1
3
2
20
21
5
4
22
6
7
8
24
23
9
26
10
25
12
11
28
13
27
29
30
15
14
16
31
17
18
NH2 antisymmetric stretch
NH2 symmetric stretch
CH3 antisymmetric stretch , out-of- plane, in phase
CH3 antisymmetric stretch , in-plane, in phase
CH3 antisymmetric stretch, in-plane, in phase
CH3 antisymmetric stretch, out-of- plane, out-of- phase
CH stretch
CH3 symmetric stretch, in phase
CH3 symmetric stretch out-of- phase
NH2 scissor
CH3 antisymmetric deformation, in-plane, in phase
CH3 antisymmetric deformation , out-of- plane, in phase
CH3 antisymmetric deformation, out-of- phase
CH3 antisymmetric deformation, in- plane, out-of- phase
CH3 symmetric deformation, in phase
CH3 symmetric deformation, out-of- phase
CH bend
CH bend
CH3 rock, in plane, in phase
CH3 rock , out-of- plane, in phase
C-C-C antisymmetric stretch
C-N stretch
NH2 twist
CH3 rock, in plane, out-of- phase
CH3 rock, in plane, out-of- phase
NH2 wag
C-C-C symmetric stretch
C-C-N deformation, in-plane
C-C-N deformation, out-of- plane
C-C-C symmetric deformation
CH3 torsion, in-plane
fixed
scaledc
3404
3307
3015
2989
2986
2981
2912
2906
2897
1632
1476
1470
1457
1454
1395
1381
1363
1339
1259
1185
1156
1026
960
939
907
897
802
462
417
376
275
IR
int.
Raman
act.
IR
gas
xenon
solution
Raman
gasd
P.E.D.e
1.6
1.3
17.9
55.1
22.4
13.9
29.0
7.3
17.1
29.1
9.5
5.4
0.7
0.3
11.4
13.9
3.9
10.3
4.3
7.2
26.3
2.8
0.7
14.8
2.9
143.9
7.6
1.2
13.1
0.1
1.0
54.6
85.4
63.3
85.7
42.6
19.1
44.4
160.4
74.5
7.9
0.7
22.6
4.2
28.9
4.7
3.2
6.8
6.0
2.6
3.8
3.3
3.7
6.5
4.6
0.7
1.9
9.5
0.4
1.0
0.2
0.1
3393
3327
2967
2940
2930
2930
2920
2906
2899
1621
1472
1466
1452
1452
1382
1377
1368
1339
1254
1172
1140
1030
951
940
918
826
810
459
406
369 d
270
3380
3314
2968
2937
2932
2932
2920
2906
2899
1619
1470
1463
1449
1449
1382
1375
1364
1338
1256
1173
1141
1025
948
938
918
830
811
463
405
369
272
3393
3328
2968
2945
2932
2932
2918
100S19
100S1
26S3,26S2,24S21,22S20
67S2,18S3,11S21
51S20,39S3
57S21,27S20,13S3
75S5,17S4
44S4,28S22,21S5
63S22,36S4
77S6,21S15
77S7,11S8,10S12
72S8,11S7
75S24,10S23
85S23,10S24
58S9,10S10
38S26,32S9,11S25
32S10,42S26,11S27
54S25,19S10,10S26
18S12,30S27,15S16,10S11
28S11,19S13,11S12,11S10
28S28,19S29,11S11,10S25
30S13,43S12,14S14
35S27,19S11,13S13,12S28
45S29,34S28
79S30,10S25
55S15,17S29,14S6
62S14,24S13,10S11
58S16,18S17
80S31
59S17,24S16
94S18
1469
1449
1449
1370
1338
1259
1178
1033
826
463
406
368
Band
Contours
A*
B*
18
13
14
53
41
41
12
41
35
4
6
35
65
93
2
23
33
2
14
10
78
2
28
39
19
77
83
63
11
69
31
94
99
82
15
18
2
78
44
54
7
91
46
54
26
74
80
17
97
14
56
26
C*
82
73
6
59
46
61
94
945
44
84
90
20
33
4
17
26
6
3
80
22
2
2
3
3
30
74
252
31.4
1.7
221d
223
64S32,19S33
23
20
57
32 NH2 torsion
230
15.2
0.8
236d
238
64S33, 33S32
23
21
56
33 CH3 torsion, out-of- phase
a
MP2(full)/6-31G(d) ab initio calculations, scaled frequencies, infrared intensities (km/mol), Raman activities (Å 4/amu), depolarization ratios and potential
energy distributions (P.E.D.s)
b
In order of decending frequency
c
MP2(full)/6-31G(d) fixed scaled frequencies with factors of 0.88 for CH stretches and deformations, 0.70 for the NH2 torsion,1.0 for heavy atom bends, and 0.90 for all
other modes.
d
Ref [50]
e
Calculated with MP2(full)/6-31G(d) and contributions of less than 10% are omitted.
*
A, B and C values in the last three columns are percentage infrared band contour.
63
Table 10: Calculated energies (hartree) and energy differences (cm-1) for the two conformers
and transition states of isopropylamine
a
Energy , E
Method/Basis Set
# basis set
trans
RHF/6-31G(d)
78
0.285678
MP2(full)/6-31G(d)
78
0.863972
MP2(full)/6-31+G(d)
94
0.877327
MP2(full)/6-31G(d,p)
105
0.941879
MP2(full)/6-31+G(d,p)
121
0.954027
MP2(full)/6-311G(d,p)
126
1.062299
MP2(full)/6-311+G(d,p)
142
1.069475
MP2(full)/6-311G(2d,2p)
173
1.116801
MP2(full)/6-311+G(2d,2p)
189
1.122603
MP2(full)/6-311G(2df,2pd)
246
1.187136
MP2(full)/6-311+G(2df,2pd)
262
1.192526
Average MP2(full)
---------------B3LYP/6-31G(d)
78
1.487268
B3LYP/6-31+G(d)
94
1.497602
B3LYP/6-311G(d,p)
126
1.544412
B3LYP/6-311+G(d,p)
142
1.549361
B3LYP/6-311G(2d,2p)
173
1.552714
B3LYP/6-311+G(2d,2p)
189
1.557380
B3LYP/6-311G(2df,2pd)
246
1.556893
B3LYP/6-311+G(2df,2pd)
262
1.561466
B3LYP/aug-cc-pVTZ
391
1.566119
B3LYP Average
---------------a
Energy of conformer is given as –(E+173) H.
b
Difference is relative to trans form and given in cm-1.
c
Average without diffuse functions / Average with diffuse functions.
d
Difference is relative to trans form and given in cm-1.
e
Difference is relative to guache form and given in cm-1.
64
Energy Differences, ∆
(t g)d
(g t)e
gauche
barrier
barrier
116
910
1026
72
1014
1086
228
807
1035
65
1000
1064
237
774
1011
44
929
973
204
768
972
85
883
968
202
777
979
84
877
962
197
772
969
71 / 201c
834 ± 70
970 ± 6
-4
953
949
117
780
897
15
854
870
125
723
848
16
785
801
134
695
829
26
780
806
138
689
827
140
684
824
85 ± 62
744 ± 64
829 ± 24
b
Table 11: Structural parametersa, rotational constants, and dipole moments for trans and gauche
isopropylamine.
trans
Parameter
r N1-C2
r C4-C2
r C5-C2
r N1-H6
r N1-H7
r C2-H3
r C4-H9
r C4-H11
r C4-H13
r C5-H8
r C5-H10
r C5-H12
 N1C2C4
 N1C2C5
 C4C2C5
 H 6 N 1H 7
 C2N1H6
 C2N1H7
 N1C2H3
 H3C2C4
 H3C2C5
 C2C4H9
 C2C4H11
 C2C4H13
 C2C5H8
 C2C5H10
 C2C5H12
 H9C4H11
H11C4H13
 H9C4H13
 H8C5H10
H10C5H12
 H8C5H12
τH6N1C2H3
τH7N1C2H3
τH8C4C2N1
τH9C5C2N1
A
B
C
|a|
|b|
|c|
|t|
Int.
MP2(full)/
B3LYP/
Coor. 6-311+G(d,p) 6-311+G(d,p)
X1
1.466
1.473
R1
1.522
1.529
R2
1.522
1.529
x1
1.016
1.015
x2
1.016
1.015
r1
1.102
1.103
r2
1.094
1.094
r3
1.095
1.095
r4
1.094
1.093
r5
1.094
1.094
r6
1.095
1.095
r7
1.094
1.093
ε1
108.6
108.9
ε2
108.6
108.9
111.4
η
111.7
106.9
γ
107.2
δ1
110.3
110.9
δ2
110.3
110.9
β1
112.1
111.7
β2
108.1
107.8
β3
108.1
107.8
109.7
110.3
ρ1
111.0
111.1
ρ2
110.8
110.9
ρ3
109.7
110.3
ρ4
111.0
111.1
ρ5
110.8
110.9
ρ6
108.3
108.0
α1
108.0
107.7
α2
109.0
108.7
α3
108.3
108.0
α4
108.0
107.7
α5
109.0
108.7
α6
τ1
60.0
60.0
τ2
-60.0
-60.0
EDb
1.469(13)
1.529(5)
1.529(5)
1.031*
1.031*
1.111*
1.104(15)
1.104(15)
1.104(15)
1.104(15)
1.104(15)
1.104(15)
108.9(9)
108.9(9)
114.4(16)
106.0
111.5*
111.5*
111.8*
MWc
1.49(2)
1.527*
1.527*
108(2)
108(2)
111.8*
109(2)
109(2)
111.1(32)
111.1(32)
111.1(32)
111.1(32)
111.1(32)
111.1(32)
adjusted
r0d
1.465(3)
1.530(3)
1.530(3)
1.019(3)
1.019(3)
1.102(2)
1.094(2)
1.095(2)
1.094(2)
1.094(2)
1.095(2)
1.094(2)
108.9(5)
108.9(5)
111.0(5)
106.9(5)
110.3(5)
110.3(5)
112.1(5)
108.0(5)
108.0(5)
109.7(5)
111.0(5)
110.8(5)
109.7(5)
111.0(5)
110.8(5)
108.3(5)
108.0(5)
109.0(5)
108.3(5)
108.0(5)
109.0(5)
59.0(5)
gauche
MP2(full)/
B3LYP/
Predicted
6-311+G(d,p) 6-311+G(d,p)
r0
1.466
1.472
1.465
1.522
1.530
1.530
1.530
1.537
1.538
1.017
1.017
1.020
1.016
1.015
1.019
1.097
1.097
1.097
1.096
1.096
1.096
1.092
1.092
1.092
1.093
1.093
1.093
1.096
1.096
1.096
1.095
1.094
1.095
1.095
1.095
1.095
108.4
108.8
108.7
113.9
114.0
114.2
111.0
111.5
110.6
106.5
107.0
106.5
109.5
110.5
109.5
110.3
110.9
110.3
106.5
106.1
106.5
108.4
108.0
108.3
108.4
108.1
108.3
110.1
110.6
110.0
110.5
110.6
110.5
111.0
111.2
111.0
110.1
110.6
110.1
111.0
111.0
111.0
111.3
111.5
111.3
108.3
108.0
108.3
109.0
108.7
109.0
107.9
107.6
107.9
108.1
107.9
108.1
108.1
107.8
108.1
108.2
107.9
108.2
-177.0
-178.8
-177.0
-59.0(5)
66.1
62.7
-59.4
-59.9
-59.8(5)
-62.3
-63.0
59.4
59.9
59.8(5)
64.2
63.9
8391.30
8308.59
8331.92(2) 8332.77
8222.93
8135.66
8016.30
7934.61
7977.32(2) 7977.08
7984.19
7896.97
4689.37
4629.71
4657.17(8) 4656.62
4689.08
4625.39
0.00
0.00
0.00
1.28
1.18
0.07
0.07
0.10(4)
0.38
0.36
1.42
1.34
1.19(3)
0.48
0.40
1.42
1.34
1.19(3)
1.41
1.30
a
Bond distances in Å, bond angles in degrees, rotational constants in MHz, and dipole moments in Debye.
b
Ref [55]; asterisk indicates fixed values.
τ3
τ4
65
66.1
-62.3
64.2
8187.89
7924.13
4657.81
c
Structural parameters and dipole moments from ref [53], with rotational constants from ref [54]; asterisk
indicates assumed values.
d
Adjusted structural parameters with experimental rotational constants taken from ref [54].
66
Table 12: Symmetry coordinates for isopropylamine.
Symmetry Coordinatea
Description
















NH2 symmetric stretch
CH3 antisymmetric stretch , in-plane, in phase
CH3 antisymmetric stretch , out-of- plane, in phase
CH3 symmetric stretch, in phase
CH stretch
NH2 scissor
CH3 antisymmetric deformation, in-plane, in phase
CH3 antisymmetric deformation , out-of- plane, in
phase
CH3 symmetric deformation, , in phase
CH bend
CH3 rock , out-of- plane, in phase
CH3 rock, in plane, in phase
C-N stretch
NH2 wag
C-C-C symmetric stretch
C-C-N deformation, in-plane
C-C-C symmetric deformation
CH3 torsion, in-plane
 NH2 antisymmetric stretch
CH3 antisymmetric stretch, in-plane, in phase
CH3 antisymmetric stretch, out-of- plane, out-ofphase
CH3 symmetric stretch, out-of- phase
CH3 antisymmetric deformation, in- plane, out-ofphase
CH3 antisymmetric deformation, out-of- phase
CH bend
CH3 symmetric deformation, out-of- phase
NH2 twist
C-C-C antisymmetric stretch
CH3 rock, in plane, out-of- phase
CH3 rock, in plane, out-of- phase
C-C-N deformation, out-of- plane
NH2 torsion
CH3 torsion, out-of- phase
a
Not normalized.
S1
S2
S3
S4
S5
S6
S7
S8
=
=
=
=
=
=
=
=
x1 + x2
2r4 – r2 – r3 + 2r7 – r5 – r6
r 2 – r 3 + r 5- r 6
r2 + r3 + r4 + r5+ r6 + r7
r1
γ
2α1 – α2 – α3 + 2α4 – α5 – α6
α3 – α2 +α5– α6
S9
S10
S11
S12
S13
S14
S15
S16
S17
S18
=
=
=
=
=
=
=
=
=
=
α1 + α2 + α3 + α4 + α5 + α6 – ρ1 – ρ2 – ρ3 – ρ4 – ρ5 – ρ6
2β1 – β2 – β3
ρ2 – ρ1 + ρ5 – ρ4
2ρ3 – ρ2 – ρ1 + 2ρ6 – ρ5 – ρ4
X1
δ1 + δ2
R1 + R2
ε1 + ε 2
η
τ1 – τ2
S19 = x1 – x2
S20 = 2r4 – r2 – r3 – 2r7 + r5 + r6
S21 = r2 – r3 – r5 + r6
S22 = r2 + r3 + r4 – r5 – r6 – r7
S23 = 2α1 – α2 – α3 – 2α4 + α5+ α6
S24
S25
S26
S27
S28
S29
S30
S31
S32
S33
67
=
=
=
=
=
=
=
=
=
=
α3 – α2 – α5+ α6
β2 – β3
α1 + α2 + α3 – α4 – α5 – α6 – ρ1 – ρ2 – ρ3 + ρ4 + ρ5 + ρ6
δ1 – δ2
R1 – R2
2ρ3 – ρ1 – ρ2 – 2ρ6 + ρ4 + ρ5
ρ2 – ρ1+ ρ4 – ρ5
ε1 – ε2
τ1
τ1 + τ2
Table 13: Temperature and intensity ratios of the trans and gauche bands of isopropylamine.
T(C)
Liquid
xenon
50.0
60.0
70.0
80.0
90.0
100.0
4.584
4.692
4.923
5.177
4.460
5.775

Ha
T(C)
Liquid
xenon
50.0
60.0
70.0
80.0
90.0
100.0
I3390 / I3314
I3325 / I3380
I3325 / I3314
I3390 / I830b
I3325 / I830b
I818 / I830b
I472 / I830b
0.956
1.011
1.056
1.111
1.167
1.233
0.804
0.850
0.888
0.935
0.981
1.037
2.111
2.222
2.278
2.367
2.433
2.544
1.776
1.869
1.916
1.991
2.047
2.140
0.729
0.771
0.805
0.847
0.890
0.941
1.610
1.695
1.737
1.805
1.856
1.941
1.703
1.771
1.873
1.941
2.034
2.119
1.271
1.288
1.305
1.373
1.424
1.475
134  5
134  5
95  6
95  6
135  4
95  6
118  6
85  7
1/T (10-3 K-1) I3390 / I3380
1/T (10-3 K-1)
I472 / I463
I818 / I463
I818 / I3380
I818/ I3314
I3390 / I463
I3325/ I463
I472/ I3380
I472/ I3314
4.584
4.692
4.923
5.177
4.460
5.775
0.781
0.792
0.802
0.844
0.875
0.906
1.047
1.089
1.151
1.193
1.250
1.302
2.233
2.322
2.456
2.544
2.667
2.778
1.879
1.953
2.065
2.140
2.243
2.336
0.448
0.474
0.495
0.521
0.547
0.578
0.990
1.042
1.068
1.109
1.141
1.193
1.667
1.689
1.711
1.800
1.867
1.933
1.402
1.421
1.439
1.514
1.570
1.626
85  7
118  6
118  6
118  6
134  5
95  6
84  7
84  7
I793b/ I3314
I793/ I830b
I793b / I463
0.440
0.442
0.444
0.467
0.489
0.511
0.336
0.355
0.374
0.393
0.411
0.430
0.305
0.322
0.339
0.356
0.373
0.390
0.188
0.198
0.208
0.219
0.229
0.240
131  7
131  8
131  7
130  6

Ha
1/T (10-3 K-1) I793b/ I3380
T(C)
Liquid
xenon
50.0
60.0
70.0
80.0
90.0
100.0
4.584
4.692
4.923
5.177
4.460
5.775

Ha
a
-1
-1
Average value H = 113  2 cm (1.35  0.03 kJ mol ) with the trans conformer the more stable form and the
statistical uncertainty (1σ) obtained by utilizing all of the data as a single set.
b
The 830 and 793 are NH2 wag.
68
Table 14: Comparison of rotational constants obtained from ab initio MP2(full)/6-311+G(d,p)
predictions, experimental valuesa from microwave spectra, and adjusted r0 structural parameters
for trans isopropylamine.
Conformer
(CH3)2CHNH2
(CH3)2CHND2
a
Rotational
constants
MP2(full)/
6-311+G(d,p)
Experimentala
Adjusted r0
||
A
8391.30
8331.92(2)
8332.77
0.85
B
8016.30
7977.32(2)
7977.08
0.24
C
4689.37
4657.17(8)
4656.62
0.55
A
7843.64
7806.17(1)
7806.42
0.25
B
7537.32
7490.59(2)
7489.77
0.82
C
4360.70
4331.79(8)
4332.33
0.54
Ref. [54].
69
Table 15: Quadratic centrifugal distortion constants (kHz) and quadrupole coupling constants
(MHz) for conformers of isopropylamine.
Trans
MP2(full)/
6-311+G(d,p)
B3LYP/
6-311+G(d,p)
Gauche
MP2(full)/
6-311+G(d,p)
Exp.a
B3LYP/
6-311+G(d,p)
∆J
3.894
3.937
7.4(22)
3.793
3.874
∆JK
-2.293
-2.198
-12.94(53)
-1.434
-1.563
∆K
5.719
5.704
-9.6(52)
4.673
4.829
δJ
1.451
1.469
0.1443(83)
1.376
1.414
δK
2.146
2.150
-6.43(72)
2.131
2.100
χaa
2.037
2.253
1.789(2)
-3.354
-3.667
χbb
2.664
2.840
2.566(4)
2.276
2.376
χcc
-4.701
-5.093
-4.355(4)
1.078
1.291
a
Ref [54]
70
Figure 5: Experimental and predicted infrared spectra of isopropylamine: (A) gas; (B) simulated
spectrum of T and G; enthalpy difference of 113 cm-1 at 25°C; (C) simulated spectrum of G
conformer; (D) simulated spectrum of T conformer (D) solid.
71
Figure 6: Experimental and predicted Raman spectra of isopropylamine: (A) xenon solution at 100°C; (B) simulated spectrum of T and G; enthalpy difference of 113 cm-1 at -100°C; (C)
simulated spectrum of G conformer; (D) simulated spectrum of T form.
72
Figure 7: Trans and gauche conformers of isopropylamine showing atomic numbering.
73
Figure 8: Experimental and predicted Raman spectra of gaseous isopropylamine: (A)
Experimental; (B) simulated spectrum of T and G; enthalpy difference of 113 cm -1 at 25°C; (C)
simulated spectrum of G conformer; (D) simulated spectrum of T conformer.
74
Figure 9: Raman spectra of the xenon solution (Top) and infrared spectra of the
gaseous (Bottom) of isopropylamine (A) 1510 - 1310 cm-1; (B) 1300 – 1100 cm-1;
(C) 900 – 700 cm-1.
75
Figure 10: Spectra of isopropylamine (A) mid-infrared gas; (B) Raman Xe solution at -100°C.
76
Figure 11: Raman spectra of isopropylamine in xenon solution at different temperatures
77
CHAPTER 5
CONFORMATIONAL AND STRUCTURAL STUDIES OF NROPYLAMINE PROPYLAMINE
FROM TEMPERATURE DEPENDENT RAMAN AND FAR INFRARED SPECTRA OF
XENON SOLUTIONS AND AB INITIO CALCULATIONS
Introduction
Many organoamines and alcohols are very important biochemicals which play important roles in
many areas. Their conformations and structures have been investigated over the years by several
different techniques particularly microwave, infrared, and Raman spectra as well as electron
diffraction data. A significant number of these studies for the enthalpy difference determinations
between conformers was done in solution by infrared and/or Raman spectra. However, both
amines and alcohols readily form dimers or larger associated species. Therefore, it is necessary to
carry out these studies in very dilute solutions or in the gas phase to obtain reproducible results
when varying the temperatures for enthalpy difference determinations, but variable temperature
measurements in the gas phase results in very difficult experiments. However, we have found that
we can overcome many of the difficulties by using rare gas solutions with very long liquid cells,
i.e. 8 cm long, and the infrared technique. We have now extended this technique to the Raman
spectra.
Recently, we have been investigating the conformation stability and determined the
structural parameters of several organoamines [60-63] and alcohols[64-66]. One of the first amines
studies was ethylamine [36] which was determined to have the trans conformer more stable by 54
 4 cm-1 (0.65 ± 0.05 kJ/mol) than the gauche form. This value was in agreement with earlier
determined values of 107  70 cm-1(1.28 ± 0.84 kJ/mol) from electron diffraction study [35], 110 
50 cm-1(1.32 ± 0.60 kJ/mol) from a microwave investigation [34] , and 100  10 cm-1(1.20 ± 0.12
kJ/mol) from an infrared study of ethylamine in an argon matrix [33]. However, several of the
78
earlier studies [29-32] reported the gauche conformer as the more stable form, and it is rather
interesting that MP2(full)/6-311+G(2d,2p) calculations predict [36] the gauche conformer to be the
more stable form by 66 cm-1 (0.79 kJ/mol). However, without the diffuse functions the trans form
is predicted to be the more stable conformer by 32 cm-1(0.38 kJ/mol).
These results clearly indicate that the enthalpy difference between the gauche and trans conformers
of the NH2 moiety will need to be determined experimentally rather than by theoretical predictions
for organoprimary amines. It is of scientific interest to determine the effect of substituting one or
-carbon. One of the earliest molecules of this type studied was npropylamine, CH3CH2CH2NH2 [67] for which the conformational stability was determined. The
conformers are designated according to the relative position of the amine group (T = trans or G =
gauche) to the methyl group (rotation around the C-C bond) and the second one (t = trans, g =
gauche, g′ = gauche′) to the relative position of the NH2 rotor i.e. rotation around the N-C bond.
These combinations result in five possible conformers i.e. the trans-trans (Tt), trans-gauche (Tg),
gauche-gauche (Gg), gauche-gauche′ (Gg′), and gauche-trans (Gt) conformers shown in Fig. 12. The
most stable conformer was determined to be the Tt form but this determination was for the solid.
However, the most stable conformer in the gas or the liquid need not be the same as found in the
solid so it is of interest to obtain the relative stabilities of the conformers of n-propylamine in the
gaseous state.
Prior to our earlier relatively complete vibrational study [67] of n-propylamine two vibrational
studies were reported [68, 69]. In the earliest of these studies [68] three even earlier studies were
cited [70, 71] , [38] but in none of these investigations was there any discussion of rotational
isomers. In this early study [68] the investigators recorded the infrared spectra of CH3CH2CH2NH2
of the gas and a low temperature Ar matrix which was supported by ab initio calculations with
basis set 4-31G(N*). By utilizing the NH2 bending modes in the 700 cm-1 region and the NH2
79
torsion it was concluded that all five conformers were present in the spectrum taken at ambient
temperature. A similar study was reported [69] a year later where Raman spectrum of the liquid
was utilized to identify conformers and ab initio calculations with a similar basis set 3-21G(N*).
These different investigators identified three conformers Tt, Gt, and one of the gauche forms in the
Raman spectrum along with the predicted ab initio relative conformer stabilities with their relative
populations. Therefore, from significantly larger basis sets and higher level calculations it is
desirable for further evaluation of the potential utilization of ab initio calculations for
conformational studies of organoamines.
As a continuation of our conformational studies of organoamines we initiated variable temperature
(-60 to -100 °C) studies of xenon solutions of n-propylamine by recording Raman spectra from
1175 to 625 cm-1 and infrared spectra from 600 to 40 cm-1. To support the spectroscopic studies we
have carried out more extensive ab initio calculations by utilizing a variety of basis sets. To
support the experimental studies we have also obtained the harmonic force constants, infrared
intensities, Raman activities, depolarization ratios, and vibrational wavenumbers from
MP2(full)/6-31G(d) ab intio calculations. To obtain predictions on the conformational stabilities
we have carried out MP2(full) ab intio and density functional theory (DFT) calculations by the
B3LYP method utilizing a variety of basis sets. The results of these spectroscopic, structural, and
theoretical studies of n-propylamine are reported herein.
Experimental
The sample of n-propylamine was purchased from Sigma-Aldrich Chemical Co., with stated
purity of ≥99%. The sample was further purified by low-temperature, low-pressure fractionation
column and the purity of the sample was verified by comparing the infrared spectrum with that
previously reported [67].
80
The mid-infrared spectrum of the gas (Fig. 13A) and solid (Fig. 13H) were obtained from
4000 to 220 cm-1 on a Perkin-Elmer model 2000 Fourier transform spectrometer equipped with a
Ge/CsI beamsplitter and a DTGS detector. Atmospheric water vapor was removed from the
spectrometer housing by purging with dry nitrogen. The spectra of the gas and solid were obtained
with a theoretical resolution of 0.5 cm-1 for the gas and 2 cm-1 for the solid with 128 interferograms
added and truncated. Multiple annealings were required to obtain satisfactory spectra of the crystalline
solid and the solid was cooled to liquid nitrogen temperatures. The observed fundamental bands with
their assignments are listed in Table 16 for the Tt, Tg, Gg, Gg' and Gt conformers.
The far infrared spectra (600 to 10 cm-1) of the sample dissolved in liquid xenon was recorded on a
Bruker model IFS 66 v/S Fourier transform spectrophotometer equipped with a Globar source, a
6.0 m Mylar beamsplitter, and a liquid helium cooled Si bolometer. The sample was contained in
a 7 cm cell fitted with Si windows and the sample added as described for the mid-infrared studies.
For all spectra 250 interferograms were collected at 0.5 cm-1 resolution, averaged and transformed
with a Blackman-Harris three term function. A portion of the spectrum is shown in (Fig. 14B).
The Raman spectra (3500 to 10 cm-1) of the sample dissolved in liquefied xenon (Fig. 15A) at
three different temperatures (-60 to -100oC) were recorded on a Trivista 557 spectrometer
consisting of a double f = 50 cm monochromator equipped with a 2000 lines mm-1 grating, a f = 70
cm spectrograph equipped with a 2400 lines mm-1 grating, and a back-illuminated LN2-cooled PI
Acton Spec-10:2 kB/LN 2048 x 512 pixel CCD detector. For all experiments, the 514.5 nm line of
a 2017-Ar S/N 1665 Spectra-Physics argon ion laser was used for Raman excitation, with the
power set to 0.8 Watt. Signals related to the plasma lines were removed by using an interference
filter. The wavenumbers were calibrated using Neon emission lines, and depending on the setup
used, are expected to be accurate within 0.4 cm-1. The experimental set-up used to investigate the
solutions has been described before [15, 16]. A home-built liquid cell equipped with four quartz
81
windows at right angles was used to record the spectra. A comparison between the observed and
calculated wavenumbers, along with the calculated infrared intensities, Raman activities,
depolarization ratios and potential energy distributions for the Tt, Tg, Gg, Gg′, and Gt conformers of
the CH3CH2CH2NH2 are listed in Tables 17, 18, 19, 20 and 21, respectively.
Ab Initio Calculations
The LCAO-MO-SCF restricted Hartree-Fock calculations were performed with the
Gaussian-03 program [21] with Gaussian-type basis functions. The energy minima with respect to
nuclear coordinates were obtained by the simultaneous relaxation of all geometric parameters
using the gradient method of Pulay [22]. Several basis sets as well as the corresponding ones with
diffuse functions were employed with the Møller-Plesset perturbation method [23] to second order
(MP2(full)) as well as with the density functional theory by the B3LYP method. The predicted
conformational energy differences are listed in Table 22.
In order to obtain a complete description of the molecular motions involved in the fundamental
modes of n-propylamine, a normal coordinate analysis has been carried out. The force field in
Cartesian coordinates was obtained with the Gaussian 03 program [21] at the MP2(full) level with
the 6-31G(d) basis set. The internal coordinates used to calculate the G and B matrices are given in
Table 23 with the atomic numbering shown in Fig. 12. By using the B matrix [24], the force field
in Cartesian coordinates was converted to a force field in internal coordinates. Subsequently,
scaling factor of 0.88 for CH stretches, 0.90 for all other modes excluding the heavy atom bends
were applied, along with the geometric average of the scaling factors for the interaction force
constants, to obtain the fixed scaled force field and resultant wavenumbers. A set of symmetry
coordinates was used (Table 24) to determine the corresponding potential energy distributions
(P.E.D.s).
82
The vibrational spectra were predicted from the MP2(full)/6-31G(d) calculations. The predicted
scaled wavenumbers were used together with a Lorentzian function to obtain the simulated spectra.
Infrared intensities were obtained based on the dipole moment derivatives with respect to Cartesian
coordinates. The derivatives were transformed with respect to normal coordinates by (u/Qi) =

j
(u/Xj)Lij, where Qi is the ith normal coordinate, Xj is the jth Cartesian displacement
coordinate, and Lij is the transformation matrix between the Cartesian displacement coordinates
and the normal coordinates. The infrared intensities were then calculated by (N)/(3c2) [(x/Qi)2
+ (y/Qi)2 + (z/Qi)2].
Additional information on the vibrational assignments was obtained from the simulated Raman
spectra. The evaluation of Raman activity by using the analytical gradient methods has been
developed [25-27, 37] and the activity Sj can be expressed as: Sj = gj(45αj2 + 7βj2), where gj is the
degeneracy of the vibrational mode j, αj is the derivative of the isotropic polarizability, and βj is the
anisotropic polarizability. To obtain the Raman scattering cross sections, the polarizabilities are
incorporated into Sj by multiplying Sj with (1-ρj)/ (1+ρj) where ρj is the depolarization ratio of the
jth normal mode. The Raman scattering cross sections and calculated wavenumbers obtained from
the Gaussian 03 program were used together with a Lorentzian function to obtain the simulated
Raman spectra.
Vibrational Assignment
In order to determine the enthalpy difference between the five stable conformers it was
essential to have a confident assignment for all of the fundamentals of the most stable conformer
and identify vibrations which cannot be assigned to this form. These additional modes must be
overtones, combination bands, or fundamentals of the other conformers. In the initial complete
vibrational assignment [67] of n-propylamine that was made for the Tt conformer with most of the
83
observed vibrational bands assigned to this conformer as fundamentals except for the C-N stretch
(1097cm-1) as a Gg′ mode and several bands which begin at 842 cm-1 and below. These bands were
assigned for the other conformers.
In this study the complete vibrational assignment for the most stable conformer was made by
utilizing the wavenumbers predicted from the ab initio calculations along with the predicted band
contours, the infrared intensities, Raman activities and the well-known group wavenumbers for the
amine group. First the assignment was made for the most stable form in the infrared spectrum of
the crystalline solid which was annealed several times to verify the observed bands arose from a
single conformer. This conformer was shown to be the Tt form as can be verified from the data in
Fig. 13. It should be noted that the most stable conformer in the fluid state need not be the same as
that in the crystalline solid due to crystal packing factors which can play a major role in
determining the conformer stability in a the solid. This form was later shown to be the most stable
form in the fluid state and the scientific evidence is explained later. The region below 1000 cm-1 is
of particular interest since the spectrum of the vapor is complex but it is this spectral region where
the bands for the conformational determination will be taken. However, this spectral region is
greatly simplified in the spectrum of the solid which allows confident assignments of the Tt
conformer fundamentals.
These assignments should be compared to the previous vibrational assignment [67] where the
wavenumbers were predicted with ab initio calculations by the restricted HF method with a basis
set of 3-21G*. In comparison, for the current study fixed scaled ab initio predicted wavenumbers
were utilized which provided data that allowed for the reassignment of five previously assigned
fundamentals. Firstly the band at 980 and 941 cm-1 in the infrared spectra of the gas and solid,
respectively, was assigned as ν13 in the previous study. However, in the current study this band at
979 cm-1 in the gas is assigned as resulting from two accidently degenerate fundamentals of the Gg
84
and Gt conformers whereas the 941 cm-1 band in the spectra of solid is likely a result of the NH2
wag mode from the dimer or trimer. It is clear from the data in Table 17 that there is no Tt
conformer fundamental predicted in the spectral region from 1000 to 900 cm-1 other than the NH2
wag, which is assigned to the bands 786 (gas) and 967 (solid) cm-1 based on well-known group
wavenumbers and relative infrared intensities (and agrees with the previous assignment). The band
at 1292 (gas) and 1295 (solid) cm-1 in the current study is assigned as the accidentally degenerate
fundamentals ν12 and ν26 due to the ab initio predicted difference in their wavenumbers of 1 cm-1.
This is differs from the previous assignment where the ν12 fundamental is assigned at 1349 cm-1
(gas and solid) and ν26 is assigned at 1301 cm-1 in the gas and shifts to 1294 cm-1 in the spectrum
of the solid. The band at 1217 which is observed in the infrared spectra of the solid in both the
current and previous studies but it was not assigned in the previous study. It is now assigned as ν 27
in the current study based on the predicted scaled wavenumber and the Raman spectra of the xenon
solutions. The assignment of the bands from 900 to 800 cm-1 was of particular interest since it is
where the CCC symmetric stretching modes are expected and they are predicted to have sufficient
separation to be observed in the Raman spectra of the xenon solutions. In the previous study the
band at 876 cm-1 in the infrared spectra of the gas was assigned as the CCC symmetric stretch.
However, in the current study this mode is assigned to the band at 865 cm -1 in the infrared spectra
of the gas which corresponds to the 866 cm-1 band in the Raman spectra of the xenon solutions.
Finally the band at 750 cm-1 in the infrared spectra of the gas previously assigned as ν30 is now
reassigned to the band at 736 cm-1 in the current study based on the ab initio fixed scaled
wavenumbers.
With all of the fundamentals assigned for the most stable Tt conformer in the solid the ab initio
predicted differences were used to assign many of the fundamentals for the other conformers.
These assigned fundamentals (Table 16) were verified by using the spectra of the variable
85
temperature xenon solutions along with the predicted band contours, intensities, and depolarization
values.
Two regions of the spectra are of significant interest due to their complexity and the predicted
spectra which can be observed in Fig. 13 and 15 where the bands for the different conformers are
predicted to be sufficiently separated to determine band intensities for use in obtaining the energy
differences. The bands at 877 and 447 cm-1 in the infrared spectra of the solid which correspond to
the bands at 866 and 448 cm-1 in the Raman spectra of Xenon solutions are assigned as ν15 and ν18
for the Tt conformer, respectively. By utilizing the ab initio predicted wavenumber differences the
bands for the remaining conformers were assigned, and they were verified using their band
contours and relative intensities.
Conformational Stability
To determine the enthalpy differences among the five observed conformers of npropylamine, the sample was dissolved in liquefied xenon and the Raman and far infrared spectra
were recorded as a function of temperature from -60 to -100°C and -70 to -100°C, respectively.
Relatively small interactions are expected to occur between xenon and the sample though the
sample can associate with itself forming a dimmer, trimer or higher order complex. However, due
to the very low concentration of sample self-association is greatly reduced. Therefore, only small
wavenumber shifts are anticipated for the xenon interactions when passing from the gas phase to
the liquefied xenon solutions except for the NH2 modes. A significant advantage of this study is
that the conformer bands are better resolved in comparison with those in the Raman spectrum of
the liquid or far infrared spectra of the gas. From ab initio calculations, the dipole moments of the
five conformers are predicted to have similar values and the molecular sizes of the five conformers
are nearly the same, so the ΔH value obtained from the temperature dependent Raman study is
expected to be near to that for the gas [39-43].
86
Once confident assignments have been made for the fundamentals of the observed conformers the
task was then to find a pair or pairs of bands from which the enthalpy determination could be
obtained. The bands should be sufficiently resolved as to determine their intensities. The selection
of the bands to use in the enthalpy determination was complicated due to the presence of all five
conformers nearly in equal amounts. The fundamentals at 448, 452, 472, 464, and 461 cm-1 were
initially selected for the Tg, Tg, Gg, Gg' and Gt conformers, respectively, as they are confidently
assigned, satisfactory resolved (Fig. 14B), and a limited number of overtone and combination
bands are possible. The fundamentals at 866 cm-1 for the Tt, 876 cm-1 for the Tg, and 835 cm-1 for
the Gt were then selected in the Raman spectra (Fig. 14A) as it was found that the band intensities
were not significantly affected by the underlying NH2 bending modes which are weak in the
Raman effect. The intensities of the individual bands were measured as a function of temperature
and their ratios were determined. By application of the van’t Hoff equation, lnK = H/(RT) 
S/R, the enthalpy differences were determined from a plot of lnK versus 1/T (Fig. 16), where
H/R is the slope of the line and K is substituted with the appropriate intensity ratios, i.e. Iconf-1 /
Iconf-2, etc. It was assumed that H, S, and α are not functions of temperature in the range studied.
These bands, i.e. two for Tt, two for Tg, one for Gg, one for Gg', and two for Gt, were utilized for
the determination of the enthalpy difference by combining them to form the band pairs given in
Table 25. By using the two band pairs for the Tt and Tg conformers the enthalpy difference was
determined to be 79  9 cm-1 (Table 25). The band pair for the Tt and Gg forms was used to obtain
the enthalpy difference of 91  26 cm-1. To determine the enthalpy difference for the Tt and Gg'
conformers the one band pair was used form which a value of 135  21 cm-1 was obtained. Then
the two band pairs for the Tt and Gt forms were used to obtain the enthalpy difference of 143  11
cm-1. This error limit is derived from the statistical standard deviation of one sigma of the
measured intensity data taken as a single data set, but it does not take into account small
87
associations with the liquid xenon or the possible presence of overtones and combination bands in
near coincidence of the measured fundamentals. From these enthalpy differences abundance of the
Tt conformer present at ambient temperature is estimated to be 18 ± 1%, with the remaining values
of 24 ± 1% for the Tg conformer, 23 ± 3% for the Gg conformer, 18 ± 1% for the Gg' conformer
and 18 ± 1% for the Gt form. These values indicate that there are nearly equal amounts of all five
conformers! Though the Tt conformer is clearly the most stable form the other forms are gauche
conformers and each have a degeneracy factor of two, this means that the Tt (18%) conformer is
50% more abundant than a single Tg (12%) and Gg (11%) conformer.
Structural Parameters
It should be possible to estimate r0 structural parameters for the all five conformers of npropylamine by adjusting the MP2(full)/6-311+G(d,p) calculated structural parameters by using
structural parameters from similar molecules as a guideline. The CH bond distances predicted by
MP2(full)/6-311+G(d,p) ab initio calculations can be used without any adjustment as we [44] have
shown that ab initio MP2(full)/6-311+G(d,p) calculations predict the r0 CH distances for more than
fifty substituted hydrocarbons to better than ± 0.002 Å compared to the experimentally determined
values from isolated CH stretching wavenumbers. The heavy atom parameters can be estimated by
using the corresponding parameters in ethylamine [36] and 1-chloropropane [72] to adjust the
corresponding MP2(full)/6-311+G(d,p) calculated parameters. It is believed that the estimated r0
parameters reported herein for n-propylamine have listed uncertainty values that should be within
the experimentally determined parameters. Thus, the listed rotational constants given in Table 23
should make it relatively easy to assign the microwave spectrum from which the experimental
parameters could be obtained.
88
Discussion
The vibrational assignments reported herein are based on a significant amount of
information with the Raman spectra of the xenon solution, infrared spectra of the solid, and
predictions from scaled force constant(s) by ab initio MP2(full)/6-31G(d) calculations as well as
the predicted infrared band contours. The ab initio predicted Raman and infrared band intensities
were also used but they seemed to be the least reliable of the data utilized. One of the possible
reasons for significant difference in predicted and observed intensities could be the results of the
association of the amine portion of the molecule with xenon. The evidence for the van der Waals
molecules was the significant decreases in the NH stretching wavenumbers from the gas to the
solutions i.e. 17 cm-1 lower for the A' mode for the Tt form as well as 22 cm-1 for the A'' mode for
this same conformer. However, for the other fundamentals the difference between the
wavenumbers in the gas and xenon solution averages two wavenumbers except for the CH3 rock
(ν13), NH2 twist (ν28), and NH2 symmetric deformation (ν6) which had difference of 11, 8, and 7
cm-1, respectively. Finally, it should be noted that there is a very, very small amount of the dimer
present in the xenon solution as evidence by the very low intensity of the bands which are present
at approximately 3288 cm-1 in xenon solutions.
For the Tt conformer the fixed scaled ab initio predicted wavenumbers was an average of
16 cm-1 in error for the A′ fundamentals, except for the NH2 wag, which represent 0.95% error. If
all the fundamental wavenumbers are included the percent error increases to 1.36% since the NH2
wag is poorly predicted. The predicted wavenumber for the NH2 wag is too high with 0.9 scaling
but it can be corrected with a scaling factor of 0.7. However, if this value is used it usually results
in two or three unacceptable P.E.D. values. Since the NH2 wag gives rise to a very intense infrared
band with well-known group wavenumbers, it is usually better to use 0.9 as the scaling factor so
the other fundamentals have “normal” P.E.D.s. The predicted errors for the A′′ modes have larger
89
errors compared to the A′ modes with average predicted wavenumbers of 16 cm-1 which represent
1.04% error. Thus the relatively small basis set of 6-31G(d) by the MP2(full) calculations with two
or three scaling factors for the force constants provides good predicted wavenumbers for aiding the
vibrational assignments.
For the Tt form the mixing is relatively small for the A′ modes except for 13 which has only
26%S13 and five other symmetry coordinates with 15%S18 being the largest value of this group. For
the A′′ modes only 27 (CH3 rock) and 28 (NH2 twist) have relatively small percentage values where
27 has 30%S with 28%S30 (CH2 rock) and 10% of four other symmetry coordinates. The NH2 twist
35%S28 with 22%S25 (CH2 twist) and the three other coordinates of 14% and less. Therefore, the
approximate descriptions for the normal modes provide reasonable information on the molecular
motions of the vibrations.
The estimated structural parameters have been obtained (Table 23) in the current study and should
be of considerable value for assigning the microwave spectrum for the conformers of npropylamine. These parameters should be accurate to within  0.005 Å and 1° for the heavy atom
bond distances and angles, respectively. The quadratic centrifugal distortion constants and
quadrupole coupling constants (Table 26) have also been predicted to further aid in a possible study
of the microwave spectrum. Such a study could be useful in determining the structure of npropylamine for comparison to other n-propyl-X molecules and biologically active amines.
To support the experimental values the ab initio energy differences have been determined by a
variety of basis sets (Table 22) which can be compared to the experimentally determined enthalpy
differences listed in Table 25. The order of stability is not well predicted by the ab initio MP2(full)
calculations as the order of stability changes from one basis set to another. The DFT calculations by
the B3LYP method gives somewhat better results by identifying the Tt as being the most stable
90
conformer, but the order of stability changes for the rest of the conformers depending on the size of
the basis set.
In the two previous conformational studies [68, 69] all five conformers were observed in the
experimental spectra and their order of stability was predicted by using ab initio calculations. These
predictions have the Tt conformer as the most stable form and the carbon skeleton Gauche
conformers are predicted as being the predominate fraction at room temperature. However, from
neither study were reliable order of stability predicted for the four conformers. In the later
investigation [67] the spectra of the solid showed the Tt conformer as the most stable form and
reconfirmed that all five conformers are observed in both the Raman and infrared spectra of the
vapor. In the most recent conformational study [73] again the Tt conformer was identified as being
the most stable form and the enthalpy difference was obtained from the variable temperature Raman
spectra of n-propylamine-ND2 in the liquid phase with a value of 142 cm-1 (1.7 kJ/mol) between the
Tt and Gt forms. This value is in excellent agreement with the value obtained in the current study,
which is surprising since the value for enthalpy differences determined from the spectra of liquids is
usually significantly higher.
It would be interesting to obtain the enthalpy differences for 3-aminopropionitrile to determine the
effects of substitution on the 3- position of n-propylamine. It would also be of interest to see how
different 1- position substituents affect the conformational stability of n-propyl-X, i.e. namely if
there are nearly equal amounts of all five conformers for these similar molecules (-PH2, -OH, -SH).
A good example of one of these molecules is n-propylphophine [74] where the Tt form was found to
be most stable. The Gt conformer was determined to be the second most stable form and an enthalpy
difference was obtained from the variable temperature Raman spectra of the gas of 140 ± 5 cm -1
(1.68 ± 0.06 kJ/mol) and 351 ± 20 cm-1 (4.2 ± 0.2 kJ/mol) for the liquid relative to the Tt conformer.
The three remaining conformers were observed but in much lower fractions and their enthalpy
91
differences were not determined. The n-propyl alcohol and thiol have both been studied. The
propanol molecule has been studied [75-77] extensively and the Gt and Gg conformers have both
been identified in the rotational and vibrational spectra but the enthalpy difference has not been
determined in the gas. The propanthiol molecule has been studied [78] and the Tt and Gt conformers
have been identified in the infrared spectra of the gas, liquid, and solid and the Raman spectrum of
the liquid. Again the enthalpy difference has not been determined in the gas and it would be
interesting to obtain the enthalpy difference for comparison with the corresponding propyl -amine
and -phosphine molecules.
92
Table 16: Observed and predicted wavenumbers (cm-1) for conformers of n-propylamine.
Vib.
No.a

































Tt
Tg
IR
Gas
fixed
scaled
Gg
fixed
scaled
IR
Gas
Gt
IR
Gas
NH2 antisymmetric stretch
3422 3417
3424
3417
3425
3417
3432 3417
3433
NH2 symmetric stretch
3326 3345
3329
3345
3328
3345
3339 3345
3333
CH3 antisymmetric stretch
2998 2966
2998
2966
3024
2979
3001 2966
2998
CH3 antisymmetric stretch
2994 2958
2994
2958
2991
2958
2991 2958
2993
CH2 antisymmetric stretch
2963 2937
2967
2937
2949
2913
2966 2937
2962
CH2 antisymmetric stretch
2937 2903
2934
2903
2937
2903
2948 2913
2943
CH3 symmetric stretch
2913 2888
2911
2888
2916
2888
2912 2888
2912
CH2 symmetric stretch
2909 2878
2905
2878
2898
2858
2910 2888
2909
CH2 symmetric stretch
2895 2858
2837
2839
2845
2839
2836 2839
2896
NH2 symmetric deformation
1638 1624
1634
1624
1633
1624
1639 1624
1637
CH3 antisymmetric deformation
1493 1470
1503
1470
1498
1470
1499 1470
1494
CH3 antisymmetric deformation
1486 1463
1489
1463
1494
1470
1492 1470
1487
CH2 deformation
1480 1455
1486
1463
1478
1455
1482 1455
1473
CH2 deformation
1471 1446
1461
1455
1473
1446
1467 1446
1470
CH3 symmetric deformation
1396 1401
1406
1401
1402
1401
1401 1401
1399
CH2 wag
1373 1373
1398
1401
1389
1392
1393 1392
1361
CH2 twist
1355 1357
1329
1308
1352
1350
1348 1350
1356
CH2 wag
1297 1292
1301
1300
1306
1300
1320 1308
1341
CH2 twist
1296 1292
1270
1274
1276
1274
1266 1274
1275
CH3 rock
1224 1209c
1222
1209c
1220
1209c
1200 1209c
1213
CH3 rock
1139 1126
1150
1133
1120
1103
1144 1133
1136
CN stretch
1078 1074
1078
1074
1096
1084
1098 1084
1078
CCC antisymmetric stretch
1026 1022
1034
1022
1020
1029
1030 1022
1020
NH2 twist
1011 1029
1019
1029
977
979
981
988
977
NH2 wag
918
786
910
771
922
786
914
782
910
CCC symmetric stretch
845
865
864
876
866
890
852
854
848
CH2 rock
844
841
845
865
831
854
833
854
810
CH2 rock
726
736
742
749
759
760
754
760
748
CCN bend
448
449
455
452
484
470
475
464
470
NH2 torsion
298
282c
291
282c
327
328c
335
328c
339
CCC bend
270
270c
257
255c
255
255c
286
282c
294
c
c
c
c
CH3 torsion
228
213
229
213
226
213
226
213
235
CCC asymmetric torsion
134
145c
127
136
145c
137
145c
133
a
With respect to Tt conformer.
b
MP2(full)/6-31G(d) fixed scaled wavenumbers with factors of 0.88 for CH stretches, 1.0 for heavy atom bends, and
0.90 for all other modes.
c
Data from the Raman spectra of xenon solutions.
3417
3345
2966
2958
2937
2913
2888
2878
2858
1624
1470
1463
1446
1446
1401
1357
1357
1340
1274
1209c
1126
1074
1029
979
771
865
835
749
464
328c
282c
213c
145c
93
IR
Gas
Gg′
fixed
IR
scaled
Gas
fixed
scaled
Approximate Descriptions
fixed
scaledb
Table 17: Observed and calculateda wavenumbers (cm-1) for Tt n-propylamine.
Vib.
No.
94
A 1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
A 20
21
22
23
24
25
26
27
28
29
30
Approximate Descriptions
ab
initio
NH2 symmetric stretch
CH3 antisymmetric stretch
CH3 symmetric stretch
CH2 symmetric stretch
CH2 symmetric stretch
NH2 symmetric deformation
CH3 antisymmetric deformation
CH2 deformation
CH2 deformation
CH3 symmetric deformation
CH2 wag
CH2 wag
CH3 rock
CN stretch
CCC antisymmetric stretch
NH2 wag
CCC symmetric stretch
CCN bend
CCC bend
NH2 antisymmetric stretch
CH3 antisymmetric stretch
CH2 antisymmetric stretch
CH2 antisymmetric stretch
CH3 antisymmetric deformation
CH2 twist
CH2 twist
CH3 rock
NH2 twist
CH2 rock
CH2 rock
3506
3196
3105
3101
3086
1726
1573
1560
1550
1472
1447
1366
1187
1137
1081
966
889
459
273
3607
3191
3159
3130
1566
1429
1367
1290
1065
889
766
fixed
scaledb
IR
int.
3326
0.3
2998 26.7
2913 41.7
2909 28.7
2895 13.7
1638 26.5
1493
4.6
1480
1.5
1471
0.1
1397
3.2
1373
5.1
1297
8.0
1139
7.1
1078
9.9
1026
2.8
918 147.3
845 71.9
448
3.1
270
4.4
3422
0.0
2994 41.2
2963 38.9
2937
4.1
1486
7.4
1355
0.5
1296
0.5
1224
0.0
1011
0.0
844
1.8
726
2.4
Raman dp
act.
ratio
81.8
66.1
128.4
6.1
116.3
8.7
5.1
2.4
32.1
3.1
1.8
0.7
2.2
7.1
3.1
2.4
7.7
4.7
0.3
48.7
37.5
7.1
120.0
16.2
12.0
12.1
1.5
0.3
0.1
0.0
0.11
0.70
0.01
0.40
0.10
0.70
0.74
0.73
0.75
0.74
0.74
0.72
0.51
0.70
0.54
0.75
0.17
0.32
0.72
0.75
0.75
0.75
0.75
0.75
0.75
0.75
0.75
0.75
0.75
0.75
IR Raman
Gas Xe soln.
3345
2966
2888
2878
2858
1624
1470
1455
1446
1401
1373
1292
1126
1074
1022
786
865
449
3417
2958
2937
2903
1463
1357
1292
1029
841
736
3328
2964
2874
2864
2855
1617
1475
1455
1446
1373
1297
1115
1072
1020
788
866
448
270
3395
2956
2934
2897
1465
1357
1297
1209
1037
842
-
IR Solid
3248
2957/2954
2869
2863
2854
1616
1472
1457
1441
1420
1372
1295
1126
1081
1022
967
877
447
297
3315
2957/2954
2927
2896
1462
1358
1295
1217
1037
858/854
746
P.E.D.c
100S1
100S2
83S3,16S4
77S4,17S3
93S5
78S6,21S16
44S7,33S9,18S8
45S8,42S7,11S9
57S9,37S8
90S10
55S11,32S12
44S12,40S11,10S13
26S13,15S18,14S19, 14S14, 12S17,10S16
54S14,22S15,10S17,10S12
1963S ,14S ,10S
15
17
14
35S16,27S13,11S17,10S6
29S17,25S16,19S13,10S14
42S18,23S18,22S17
59S16,36S18
100S20
95S21
80S22,17S23
79S23,19S22
92S24
49S25,40S28,10S29
63S26,16S25,10S29
30S27,28S30,10S29,10S28,10S26,10S25
35S28,22S25, 14S27,14S26,10S30
38S29,34S27,12S26,10S28
54S30,32S29,11S27
Band
Contours
A*
B*
6
94
24
76
46
54
100
10
90
3
97
9
91
96
4
71
29
76
24
100
100
50
50
98
2
79
21
91
9
100
87
13
81
19
-
313
297 53.9
2.4
0.75
282
562
92S31
31 NH2 torsion
CH
torsion
240
228
6.1
0.3
0.75
213
83S
,10S
32
3
32
31
141
134
0.3
0.0
0.75
145
93S33
33 CCC asymmetric torsion
a
MP2(full)/6-31G(d) ab initio calculations, scaled wavenumbers, infrared intensities (km/mol), Raman activities (Å 4/amu), depolarization ratios and potential
energy distributions (P.E.D.s)
b
MP2(full)/6-31G(d) fixed scaled wavenumbers with factors of 0.88 for CH stretches, 1.0 for heavy atom bends, and 0.90 for all other modes.
c
Calculated with MP2(full)/6-31G(d) and contributions of less than 10% are omitted.
*
A, B and C values in the last three columns are percentage infrared band contours.
-
95
Table 18: Observed and calculateda wavenumbers (cm-1) for Tg n-propylamine.
96
Vib.
No.
Approximate Descriptions
20
1
2
21
23
22
3
5
4
6
8
7
24
9
11
10
12
25
26
27
13
14
15
28
16
17
29
30
18
19
31
NH2 antisymmetric stretch
NH2 symmetric stretch
CH3 antisymmetric stretch
CH3 antisymmetric stretch
CH2 antisymmetric stretch
CH2 antisymmetric stretch
CH3 symmetric stretch
CH2 symmetric stretch
CH2 symmetric stretch
NH2 symmetric deformation
CH2 deformation
CH3 antisymmetric deformation
CH3 antisymmetric deformation
CH2 deformation
CH2 wag
CH3 symmetric deformation
CH2 wag
CH2 twist
CH2 twist
CH3 rock
CH3 rock
CN stretch
CCC antisymmetric stretch
NH2 twist
NH2 wag
CCC symmetric stretch
CH2 rock
CH2 rock
CCN bend
CCC bend
NH2 torsion
ab
Initio
3609
3509
3196
3191
3162
3133
3103
3097
3024
1723
1584
1569
1566
1557
1482
1473
1399
1371
1338
1286
1206
1134
1089
1073
959
909
891
782
466
300
267
fixed
scaledb
3424
3329
2998
2994
2967
2934
2911
2905
2837
1634
1486
1489
1486
1477
1406
1398
1329
1301
1270
1222
1150
1078
1034
1019
910
864
845
742
455
291
257
IR
int.
0.1
0.5
27.6
47.2
13.9
27.7
24.1
19.4
71.4
33.2
2.2
3.4
7.1
0.8
9.7
2.3
0.3
2.8
2.1
0.3
8.6
5.9
1.0
2.8
79.9
2.9
78.2
3.8
8.6
25.9
19.5
Raman dp
act.
ratio
68.3
107.7
65.5
22.2
65.1
64.8
81.3
104.9
84.5
9.1
9.1
0.3
16.9
31.5
2.3
3.2
1.3
22.4
2.5
2.5
4.8
11.3
0.6
0.7
2.7
7.1
2.8
0.2
2.2
1.0
0.7
0.71
0.12
0.68
0.70
0.59
0.61
0.01
0.15
0.23
0.68
0.74
0.50
0.75
0.75
0.61
0.75
0.72
0.73
0.60
0.72
0.26
0.56
0.37
0.73
0.36
0.34
0.74
0.74
0.24
0.70
0.75
IR
gas
3417
3345
2966
2958
2937
2903
2888
2878
2839
1624
1470
1463
1463
1455
1401
1401
1308
1300
1274
1133
1074
1022
1029
771
876
865
749
452
-
xenon
solution
3395
3328
2964
2956
2934
2897
2874
2864
2834
1617
1475
1465
1465
1455
1297
1297
1271
1209
1121
1072
1020
1037
775
876
865
750
452
282
255
P.E.D.c
100S20
100S1
99S2
87S21,11S23
72S23,12S21,10S5
62S22,29S4
99S3
85S5,12S23
69S4,30S22
78S6,21S16
79S8,13S9
60S7,18S8,17S9
92S24
71S9,23S7
60S11,14S12,11S28
85S10
47S12,22S28
54S25,33S26
33S26,14S11,11S12
23S27,22S30,13S28
12S13,17S29,12S17,10S30
54S14,25S15
16S15,18S13, 17S17,17S28 ,11S14
16S28,38S15,15S14,12S12
29S16,19S27,13S26,10S6,10S29
38S17,29S13
25S29,36S16,14S27
60S30,21S29,16S27
44S18,23S19,19S17
42S19,33S31,19S18
46S31,20S32,17S19,16S18
A*
18
73
22
2
74
5
8
46
67
16
87
95
57
95
98
68
80
5
98
34
62
26
26
39
80
47
20
Band
Contours
B*
70
24
78
42
26
87
58
33
84
13
5
43
8
4
28
2
47
30
37
69
39
24
30
56
C*
12
3
100
100
56
8
34
54
100
5
2
24
16
67
19
8
37
5
22
76
20
23
24
241
229
11.0
0.4 0.75
213
69S32,19S31
26
49
32 CH3 torsion
134
127
3.2
0.2 0.73
93S33
9
66
33 CCC asymmetric torsion
a
MP2(full)/6-31G(d) ab initio calculations, scaled wavenumbers, infrared intensities (km/mol), Raman activities (Å 4/amu), depolarization ratios and potential
energy distributions (P.E.D.s)
b
MP2(full)/6-31G(d) fixed scaled wavenumbers with factors of 0.88 for CH stretches, 1.0 for heavy atom bends, and 0.90 for all other modes.
c
Calculated with MP2(full)/6-31G(d) and contributions of less than 10% are omitted.
*
A, B and C values in the last three columns are percentage infrared band contours.
25
25
97
Table 19: Observed and calculateda wavenumbers (cm-1) for Gg n-propylamine.
98
Vib.
No.
Approximate Descriptions
20
1
21
2
23
22
3
5
4
6
8
7
24
9
10
11
12
26
25
27
13
14
28
15
16
29
17
30
18
19
NH2
NH2 symmetric stretch
CH3 antisymmetric stretch
CH3 antisymmetric stretch
antisymmetric stretch
CH2 antisymmetric stretch
CH2 antisymmetric stretch
CH3 symmetric stretch
CH2 symmetric stretch
CH2 symmetric stretch
NH2 symmetric deformation
CH2 deformation
CH3 antisymmetric deformation
CH3 antisymmetric deformation
CH2 deformation
CH3 symmetric deformation
CH2 wag
CH2 wag
CH2 twist
CH2 twist
CH3 rock
CH3 rock
CN stretch
NH2 twist
CCC antisymmetric stretch
NH2 wag
CH2 rock
CCC symmetric stretch
CH3 rock
CCN bend
CCC bend
ab
initio
3610
3509
3224
3188
3144
3130
3109
3089
3033
1721
1578
1575
1558
1553
1478
1463
1425
1375
1345
1283
1172
1156
1076
1030
971
912
876
799
494
335
fixed
scaledb
IR
int.
Raman dp
act. ratio
IR
gas
3425
3328
3024
2991
2949
2937
2916
2898
2845
1633
1498
1494
1478
1473
1402
1389
1352
1306
1276
1220
1120
1096
1021
977
922
866
831
759
484
327
0.2
0.3
15.5
36.3
44.6
21.7
21.6
26.9
76.6
33.6
0.9
8.8
5.4
1.0
10.9
3.6
3.1
1.2
1.6
0.2
12.3
3.6
0.8
4.7
55.1
38.6
53.3
8.6
8.7
0.7
68.5
102.3
35.9
57.2
42.3
84.1
92.1
106.0
94.6
9.2
8.1
4.3
23.9
21.3
2.4
5.0
1.6
10.3
17.3
1.3
2.2
5.8
1.1
7.6
3.1
6.3
3.8
1.5
0.4
0.2
3417
3345
2979
2958
2913
2903
2888
2858
2839
1624
1470
1470
1455
1446
1401
1392
1350
1300
1274
1103
1084
1029
979
786
890
854
760
470
-
0.70
0.12
0.73
0.71
0.36
0.74
0.04
0.12
0.27
0.67
0.69
0.75
0.75
0.75
0.57
0.70
0.71
0.74
0.64
0.60
0.66
0.59
0.70
0.75
0.21
0.25
0.19
0.56
0.73
0.16
xenon
solution
3395
3328
2970
2956
2908
2897
2874
2855
2834
1617
1475
1475
1455
1446
1350
1297
1271
1209
1107
1095
1037
977
788
890
854
756
472
328
P.E.D.c
100S20
100S1
67S21,29S2
70S2,28S21
67S23,20S22,10S4
51S22,26S23,17S4
96S3
93S5
71S4,28S22
78S6,21S16
87S8
72S7,10S9
79S24,10S8
84S9,11S7
52S10,35S11,10S28
28S11,22S10,13S12,10S15,10S28,10S7
54S12,20S10,12S11,10S25
38S26,27S28,10S29
46S25,28S26
25S27,23S30,14S25,11S28
29S13,19S29,13S19
41S14,33S15,10S11
34S28,13S15,11S26,11S27,10S30
23S15,28S1410S26
22S16,23S13,19S29
14S29,23S17,12S27,11S16,10S13
48S17,25S16,17S29
49S30,25S27
44S18,29S19.10S30
37S19,29S18,10S33
Band
Contours
A*
B*
5
17
45
45
42
19
15
9
56
62
25
58
16
82
70
20
54
8
9
62
25
7
91
7
92
72
25
51
28
61
19
4
92
36
64
81
12
100
91
9
4
96
94
100
9
82
34
55
29
2
C*
95
38
13
81
76
44
13
42
2
30
80
38
29
68
9
1
3
21
20
4
7
6
9
11
69
266
255
46.7
1.4 0.75
255 70S31,17S19
41
6
31 NH2 torsion
237
226
6.6
0.5 0.75
213 57S32,16S31,16S33
5
32 CH3 torsion
0.74
143
136
4.9
0.3 0.74
145 68S33,29S32
2
5
33 CCC asymmetric torsion
a
MP2(full)/6-31G(d) ab initio calculations, scaled wavenumbers, infrared intensities (km/mol), Raman activities (Å 4/amu), depolarization ratios and potential
energy distributions (P.E.D.s)
b
MP2(full)/6-31G(d) fixed scaled wavenumbers with factors of 0.88 for CH stretches, 1.0 for heavy atom bends, and 0.90 for all other modes.
c
Calculated with MP2(full)/6-31G(d) and contributions of less than 10% are omitted.
*
A, B and C values in the last three columns are percentage infrared band contours.
53
95
93
99
Table 20: Observed and calculateda wavenumbers (cm-1) for Gg′ n-propylamine.
100
Vib.
No.
Approximate Descriptions
20
1
21
2
23
22
5
3
4
6
7
8
24
9
11
10
12
25
26
13
27
14
28
15
16
29
17
30
18
19
NH2 antisymmetric stretch
NH2 symmetric stretch
CH3 antisymmetric stretch
CH3 antisymmetric stretch
CH2 antisymmetric stretch
CH2 antisymmetric stretch
CH2 symmetric stretch
CH3 symmetric stretch
CH2 symmetric stretch
NH2 symmetric deformation
CH3 antisymmetric deformation
CH2 deformation
CH3 antisymmetric deformation
CH2 deformation
CH2 wag
CH3 symmetric deformation
CH2 wag
CH2 twist
CH2 twist
CH3 rock
CH2 rock
CN stretch
NH2 twist
CCC antisymmetric stretch
NH2 wag
CH2 rock
CCC symmetric stretch
CH3 rock
CCN bend
CCC bend
ab
initio
3618
3519
3199
3188
3162
3143
3104
3102
3023
1728
1580
1573
1562
1546
1476
1468
1421
1390
1335
1258
1202
1157
1084
1034
963
898
877
794
484
345
fixed
scaledb
3432
3339
3001
2991
2966
2948
2912
2910
2836
1639
1499
1492
1482
1467
1401
1393
1348
1320
1266
1200
1144
1098
1030
981
914
852
833
754
475
335
IR
int.
0.2
0.2
30.4
37.6
12.7
37.2
23.4
18.0
71.6
36.4
7.4
1.1
5.6
2.1
9.9
5.4
1.1
4.0
1.4
0.5
11.3
3.6
1.3
6.3
62.8
80.2
10.2
0.1
6.3
11.6
Raman dp
act.
ratio
58.4
102.6
39.3
42.3
83.0
86.1
68.9
106.7
87.1
8.0
2.6
16.0
20.0
18.8
3.3
3.4
0.9
19.7
5.5
4.6
3.1
6.1
2.6
4.9
1.6
3.6
8.0
0.6
0.5
0.5
0.73
0.13
0.73
0.57
0.73
0.37
0.24
0.02
0.23
0.68
0.74
0.72
0.75
0.75
0.63
0.63
0.75
0.74
0.58
0.69
0.39
0.51
0.47
0.74
0.36
0.65
0.19
0.41
0.74
0.42
IR
gas
3417
3345
2966
2958
2937
2913
2888
2888
2839
1624
1470
1470
1455
1446
1401
1392
1350
1308
1274
1133
1084
1022
988
782
854
854
760
464
-
xenon
solution
3395
3328
2964
2956
2934
2908
2874
2874
2834
1617
1475
1475
1455
1446
1350
1297
1271
1209
1121
1095
1020
977
775
854
854
756
464
328
P.E.D.c
100S20
100S2
100S1
47S3,50S4
47S21,50S2
49S2,41S21,10S23
82S23,10S21
65S22,31S4
92S5
97S3
69S4,31S22
78S6,21S16
24S7, 40S24,30S8
53S8,39S7
53S24,23S7,16S8
92S9
45S11,40S10
40S10,17S11,13S12,10S15
59S12,17S10,10S11
41S25,24S26,21S28
48S26,19S27,11S25
20S13,20S28,16S25,14S29,12S19
16S27,22S30,12S29,10S18,10S15
42S14,25S15,11S11
25S28,25S15,16S13
19S15,30S14,23S13,12S12
20S16,20S27,15S29,11S26
17S29,40S16,14S17,11S13,10S6
58S17,21S29,10S14
52S30,26S27
49S18,25S19,10S30
44S19,15S18,13S33,13S31
Band
Contours
A*
B*
C*
41
56
3
18
67
15
52
37
11
9
2
89
22
2
76
18
70
12
14
78
8
41
39
20
14
86
25
47
28
32
2
66
41
2
57
47
30
23
29
34
37
93
3
4
3
91
6
100
33
67
26
13
61
86
9
5
2
1
977
39
59
2
55
6
39
58
4
38
7
24
69
14
28
58
3
21
76
4
89
7
53
11
36
96
4
297
286 20.1
0.8 0.72
282
39S31,29S32,17S18,11S19
75
23
31 NH2 torsion
238
226 34.8
1.0 0.74
213
49S32,45S31
84
14
32 CH3 torsion
144
137
5.5
0.2 0.75
145
81S33,12S32
57
41
33 CCC asymmetric torsion
a
MP2(full)/6-31G(d) ab initio calculations, scaled wavenumbers, infrared intensities (km/mol), Raman activities (Å 4/amu), depolarization ratios and potential
energy distributions (P.E.D.s)
b
MP2(full)/6-31G(d) fixed scaled wavenumbers with factors of 0.88 for CH stretches, 1.0 for heavy atom bends, and 0.90 for all other modes.
c
Calculated with MP2(full)/6-31G(d) and contributions of less than 10% are omitted.
*
A, B and C values in the last three columns are percentage infrared band contours.
2
2
2
101
Table 21: . Observed and calculateda wavenumbers (cm-1) for Gt n-propylamine.
102
Vib.
No.
Approximate Descriptions
20
1
2
21
22
23
3
4
6
10
7
24
8
9
10
25
11
12
26
27
14
15
28
13
16
29
17
30
18
19
NH2 antisymmetric stretch
NH2 symmetric stretch
CH3 antisymmetric stretch
CH3 antisymmetric stretch
CH2 antisymmetric stretch
CH2 antisymmetric stretch
CH3 symmetric stretch
CH2 symmetric stretch
CH2 symmetric stretch
NH2 symmetric deformation
CH3 antisymmetric deformation
CH3 antisymmetric deformation
CH2 deformation
CH2 deformation
CH3 symmetric deformation
CH2 twist
CH2 wag
CH2 wag
CH2 twist
CH3 rock
CN stretch
CCC antisymmetric stretch
NH2 twist
CH3 rock
NH2 wag
CH2 rock
CCC symmetric stretch
CH2 rock
CCN bend
CCC bend
ab
initio
3619
3513
3196
3190
3157
3137
3104
3101
3088
1726
1574
1567
1553
1549
1475
1435
1429
1414
1343
1275
1190
1135
1074
1030
959
894
854
788
480
352
fixed
scaledb
3433
3333
2998
2993
2962
2943
2912
2909
2896
1637
1494
1487
1473
1470
1399
1361
1356
1341
1275
1213
1136
1078
1020
977
910
848
810
748
470
339
IR
int.
0.1
0.3
26.9
37.1
36.5
26.6
16.0
31.8
23.5
22.5
7.1
7.3
0.4
1.9
6.6
0.4
10.7
0.9
2.0
0.3
9.9
13.4
1.5
0.7
159.9
22.2
38.6
12.8
10.4
18.8
Raman dp
act. ratio
41.7
75.6
57.1
33.9
50.2
97.1
106.8
84.0
85.2
6.9
7.9
8.1
15.6
22.2
3.1
13.2
4.2
2.3
10.9
1.5
1.5
5.7
0.9
6.0
1.3
1.2
9.0
1.0
1.1
1.5
0.74
0.1
0.68
0.74
0.71
0.60
0.01
0.16
0.16
0.71
0.75
0.73
0.75
0.75
0.75
0.75
0.73
0.75
0.75
0.65
0.74
0.60
0.45
0.74
0.66
0.26
0.19
0.54
0.72
0.50
IR
gas
3417
3345
2966
2958
2937
2913
2888
2878
2858
1624
1470
1463
1446
1446
1401
1357
1357
1340
1274
1126
1074
1029
979
771
865
835
749
464
-
xenon
solution
3395
3328
2964
2956
2934
2908
2874
2864
2855
1617
1475
1465
1446
1446
1357
1357
1340
1271
1209
1115
1072
1037
977
775
866
835
750
461
328
P.E.D.c
99S20
99S1
91S2
88S21
94S22
88S23
97S3
94S4
97S6
77S10,21S16
45S7,42S24
41S24,34S7,10S8,10S9
81S8,10S24
83S9,10S7
80S10
49S25,21S28,17S12
74S11,10S26
51S12,16S28,10S10,10S29
61S26,14S27
14S27,19S25,14S30,14S28,12S29,10S13
31S14,13S30,11S18
59S15,20S14
29S28,20S13,20S25
17S13,23S14,15S15,11S26,10S27,10S12
39S16,21S17,12S6
39S29,12S27,12S13,10S28
52S17,15S29,10S16,10S14
48S30,23S27,10S30
44S18,27S19
20S19,33S31,17S18,16S33
Band
Contours
A*
B*
17
20
42
70
27
18
16
26
4
60
22
57
17
58
8
44
10
55
19
11
41
44
89
3
2
15
55
33
8
92
66
34
58
8
85
12
41
42
61
17
85
15
48
37
66
27
88
10
73
25
83
17
82
15
50
50
15
15
C*
83
38
3
82
58
36
21
25
48
35
70
15
8
83
12
34
3
17
22
15
7
2
2
3
70
302
294
28.7
1.4 0.71
282
43S31,35S19,14S18
6
40
31 NH2 torsion
247
235
10.9
0.5 0.75
213
64S32,19S31,11S33
6
13
32 CH3 torsion
CCC
asymmetric
torsion
140
133
2.3
0.3
0.72
145
73S
,22S
11
13
33
33
32
a
4
MP2(full)/6-31G(d) ab initio calculations, scaled wavenumbers, infrared intensities (km/mol), Raman activities (Å /amu), depolarization ratios and potential
energy distributions (P.E.D.s)
b
MP2(full)/6-31G(d) fixed scaled wavenumbers with factors of 0.88 for CH stretches, 1.0 for heavy atom bends, and 0.90 for all other modes.
c
Calculated with MP2(full)/6-31G(d) and contributions of less than 10% are omitted.
*
A, B and C values in the last three columns are percentage infrared band contours.
54
81
76
103
Table 22: Calculated energies in (H) and energy differences (cm-1) for the five conformers of npropylamine.
Energya, E
Method/Basis Set
# basis set
Tt
RHF/6-31G(d)
78
0.282475
MP2(full)/6-31G(d)
78
0.858792
MP2(full)/6-31+G(d)
94
0.870987
MP2(full)/6-31G(d,p)
105
0.936882
MP2(full)/6-31+G(d,p)
121
0.947888
MP2(full)/6-311G(d,p)
126
1.057176
MP2(full)/6-311+G(d,p)
142
1.063612
MP2(full)/6-311G(2d,2p)
173
1.111371
MP2(full)/6-311+G(2d,2p)
189
1.116751
MP2(full)/6-311G(2df,2pd)
246
1.181788
MP2(full)/6-311+G(2df,2pd)
262
1.186816
MP2(full)/aug-cc-pVTZ
391
1.193848
MP2 Average
B3LYP/6-31G(d)
78
1.484593
B3LYP/6-31+G(d)
94
1.494156
B3LYP/6-311G(d,p)
126
1.541266
B3LYP/6-311+G(d,p)
142
1.545868
B3LYP/6-311G(2d,2p)
173
1.549595
B3LYP/6-311+G(2d,2p)
189
1.553939
B3LYP/6-311G(2df,2pd)
246
1.553714
B3LYP/6-311+G(2df,2pd)
262
1.558014
B3LYP/aug-cc-pVTZ
391
1.562699
B3LYP Average
a
Energy of conformer is given as –(E+173) H.
b
Difference is relative to trans-trans form and given in cm-1.
104
Energy Differences, ∆b
Tg
0
115
-13
128
-11
139
3
66
-14
62
-4
-14
34  58
180
57
141
36
118
36
103
35
34
82  55
Gg
Gg′
Gt
55
41
7
43
8
-16
5
-79
14
-66
34
43
-9  48
327
195
120
145
92
168
81
173
187
165  73
198
172
13
179
19
26
93
80
17
88
35
15
51  35
327
201
274
179
247
183
244
191
187
226  51
180
68
101
70
108
26
93
2
82
0
88
93
55  44
116
178
79
153
81
166
78
172
173
133  44
Table 23: Structural parametersa and rotational constants for n-propylamine from MP2(full)/6311+G(d,p) calculations.
Parameter
Int.
Coor.
Tt
Pred.
Tt
Tg
Pred.
Tg
Gg
Pred.
Gg
Gg′
Pred.
Gg′
Gt
Pred.
Gt
r C4-N5
X
1.462
1.465(5)
1.465
1.470(5)
1.466
1.471(5)
1.469
1.474(5)
1.462
1.465(5)
r C3-C4
Y1
1.529
1.535(5)
1.522
1.532(5)
1.525
1.531(5)
1.524
1.530(5)
1.532
1.538(5)
r C2-C3
Y2
1.527
1.533(5)
1.526
1.528(5)
1.526
1.532(5)
1.527
1.533(5)
1.527
1.533(5)
r C2-H1ip
r1
1.093
1.093(2)
1.093
1.093(2)
1.094
1.094(2)
1.093
1.093(2)
1.093
1.093(2)
r C2-H6op
r2
1.094
1.094(2)
1.094
1.094(2)
1.095
1.095(2)
1.096
1.096(2)
1.095
1.095(2)
r C2-H7op
r3
1.094
1.094(2)
1.094
1.094(2)
1.091
1.091(2)
1.093
1.093(2)
1.094
1.094(2)
r C3-H8
r4
1.097
1.097(2)
1.097
1.097(2)
1.096
1.096(2)
1.096
1.096(2)
1.097
1.097(2)
r C3-H9
r5
1.097
1.097(2)
1.094
1.094(2)
1.098
1.098(2)
1.095
1.095(2)
1.097
1.097(2)
r C4-H10
r6
1.096
1.096(2)
1.095
1.095(2)
1.100
1.100(2)
1.094
1.094(2)
1.095
1.095(2)
r C4-H11
r7
1.096
1.096(2)
1.102
1.102(2)
1.096
1.096(2)
1.102
1.102(2)
1.096
1.096(2)
r N5-H12
r8
1.015
1.015(5)
1.015
1.015(5)
1.016
1.016(5)
1.014
1.014(5)
1.015
1.015(5)
r N5-H13
r9
1.015
1.015(5)
1.015
1.015(5)
1.014
1.015(5)
1.014
1.014(5)
1.014
1.014(5)
C3C4N5
χ1
116.1
116.0(5)
110.4
110.6(5)
110.4
110.6(5)
110.8
111.0(5)
116.4
116.3(5)
C2C3C4
χ2
112.4
112.2(5)
112.4
112.9(5)
113.0
113.5(5)
112.6
113.1(5)
113.3
113.1(5)
C3C2H1
111.5
111.5(5)
111.3
111.3(5)
111.2
111.2(5)
111.2
111.2(5)
111.5
111.2(5)
110.8
110.8(5)
111.0
111.0(5)
110.3
110.3(5)
110.7
110.7(5)
110.6
110.6(5)
C3C2H7
α1
α2
α3
110.8
110.8(5)
110.7
110.7(5)
110.7
110.7(5)
111.2
111.2(5)
111.5
111.5(5)
H1C2H6
υ1
107.9
107.9(5)
107.9
107.9(5)
107.9
107.9(5)
107.8
107.8(5)
107.9
107.9(5)
H1C2H7
υ2
107.9
107.9(5)
107.8
107.9(5)
108.7
108.7(5)
107.3
107.3(5)
107.3
107.3(5)
H6C2H7
υ3
107.8
107.8(5)
107.8
107.9(5)
107.8
107.8(5)
108.3
108.4(5)
108.0
108.0(5)
C2C3H8
β1
110.0
110.0(5)
109.7
109.7(5)
110.0
110.0(5)
109.5
109.5(5)
109.4
109.4(5)
C2C3H9
β2
110.0
110.0(5)
110.5
110.5(5)
110.0
110.0(5)
109.8
109.8(5)
109.8
109.8(5)
C4C3H8
ω1
108.8
108.8(5)
108.8
108.5(5)
108.5
108.2(5)
108.4
108.2(5)
108.4
108.6(5)
C4C3H9
ω2
108.8
108.8(5)
108.4
108.1(5)
108.7
108.4(5)
108.6
108.4(5)
108.9
109.0(5)
H8C3H9

106.7
106.7(5)
106.8
106.8(5)
106.4
106.4(5)
107.6
107.6(5)
106.7
106.7(5)
C3C4H10
109.2
109.2(5)
109.1
109.1(5)
109.1
109.1(5)
109.2
109.2(5)
109.2
109.2(5)
109.2
109.2(5)
108.7
108.7(5)
109.0
109.0(5)
108.8
108.8(5)
109.1
109.1(5)
107.8
107.8(5)
107.8
107.8(5)
113.2
113.1(5)
107.5
107.4(5)
107.5
107.5(5)
N5C4H11
1
2
η1
η2
107.8
107.8(5)
113.5
113.4(5)
107.7
107.6(5)
113.2
113.1(5)
107.5
107.6(5)
H10C4H11
σ
106.5
106.5(5)
107.1
107.1(5)
107.2
107.2(5)
107.1
107.1(5)
106.6
106.6(5)
C4N5H12
110.1
110.1(5)
109.8
109.8(5)
109.7
109.7(5)
110.2
110.2(5)
110.3
110.3(5)
C4N5H13
ρ1
ρ2
110.1
110.1(5)
110.7
110.7(5)
110.6
110.6(5)
110.5
110.5(5)
110.9
110.9(5)
H12N5H13
µ
106.7
106.7(5)
106.9
106.9(5)
106.7
106.7(5)
106.5
106.5(5)
107.0
107.0(5)
H1C2C3C4
τ1
180.0
180.0(5)
179.9
179.9(5)
-179.6 -179.6(5)
175.0
175.0(5)
177.3
177.3(5)
C2C3C4N5
τ2
180.0
180.0(5)
178.2
178.2(5)
63.0
63.0(5)
62.2
62.2(5)
62.8
62.8(5)
H12N5C4C3
τ3
58.7
58.7(5)
63.3
63.3(5)
67.2
67.2(5)
170.6
170.6(5)
56.1
56.1(5)
H13N5C4C3
τ4
-58.7
-58.7(5)
-175.4 -175.4(5)
-72.0
-72.0(5)
-62.2
-62.2(5)
C3C2H6
C3C4H11
N5C4H10
-179.0 -179.0(5)
105
A
24831.50
B
3695.21
3677.79
3746.95
3706.63
4964.55
4895.64
5085.57
5024.90
4916.11
4897.23
C
3483.91
3466.18
3504.91
3470.09
4256.01
4203.71
4230.66
4183.95
4173.85
4155.53
24716.36 25250.79 25276.54 13899.17 13898.38 13771.73 13721.15 13753.73 13670.20
|a|
1.091
0.066
0.859
0.13
1.431
|b|
1.139
0.72
1.116
0.122
0.066
|c|
0.000
1.178
0.302
1.345
0.622
1.440
1.357
1.562
|t|
a
1.577
1.382
Bond distances in Å and bond angles in degrees.
106
Table 24: Symmetry coordinates for n-propylamine.
Symmetry Coordinatea
Description
S1
S2
S3
S4
S5
S6
S7
S8
S9
S10
S11
S12
S13
S14
S15
S16
S17
S18
S19
S20
S21
S22
S23
S24
S25
S26
S27
S28
S29
S30
S31
S32
S33
















NH2 symmetric stretch
CH3 antisymmetric stretch
CH3 symmetric stretch
CH2 symmetric stretch
CH2 symmetric stretch
NH2 symmetric deformation
CH3 antisymmetric deformation
CH2 deformation
CH2 deformation
CH3 symmetric deformation
CH2 wag
CH2 wag
CH3 rock
CN stretch
CCC antisymmetric stretch
NH2 wag
CCC symmetric stretch
CCN bend
CCC bend
NH2 antisymmetric stretch

CH3 antisymmetric stretch
CH2 antisymmetric stretch
CH2 antisymmetric stretch
CH3 antisymmetric deformation
CH2 twist
CH2 twist
CH3 rock
NH2 twist
CH2 rock
CH2 rock
NH2 torsion
CH3 torsion
CCC asymmetric torsion
a
Not normalized.
107
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
r8 + r9
2r1 – r2 – r3
r1 + r2 + r3
r4 + r5
r6 + r7
µ
υ1 + υ2

σ
α1 + α2 + α3
β1 + β2 – ω1 – ω2
1 + 2 – η1 – η2
-2α1 + α2 + α3
X
Y1 – Y2
ρ1 + ρ2
Y1 + Y 2
χ1
χ2
r8 – r9
r2 – r3
r4 – r5
r6 – r7
υ1– υ2
β1 – β2 – ω1 + ω2
1 – 2 – η1 + η2
α2 + α3
ρ1 – ρ2
β1 – β2 + ω1 – ω2
1 – 2 + η1 – η2
τ1
τ2
τ3
Table 25: Temperature and intensity ratios of the Tt, Tg, Gg, Gg′, and Gt bands of n-propylamine
Liquid
xenon
T(C)
1/T (10-3 K-1)
60.0
70.0
80.0
90.0
100.0
4.692
4.923
5.177
4.460
5.775
Tt → Tg
I448 / I452
I866 / I876
Tt → Gg
Tt → Gg’
Tt → Gt
I448 / I472
I448 / I464
I448 / I461
0.324
0.350
0.357
0.365
0.598
0.652
0.673
0.712
0.366
0.396
0.425
0.441
2.149
0.206
0.216
0.226
0.228
2.265
2.415
I866 / I835
1.391
1.527
1.721
H (cm-1)
84  19
75  1
91  26
135  21
153  22
137  1
H (cm-1)a
79  9
143  11
a
Average value H and statistical uncertainty (1σ) obtained by utilizing all of the data as a single set.
108
Table 26: Quadratic centrifugal distortion constants (kHz) and quadrupole coupling constants
(MHz) for conformers of n-propylamine.
Tt
MP2(full)/
6-31G(d)
MP2(full)/6311+G(d,p)
0.77210
0.79500
∆JK -2.38103
∆K 45.91186
∆J
Tg
B3LYP/6311+G(d,p)
Gg
MP2(full)/
6-31G(d)
MP2(full)/6311+G(d,p)
B3LYP/6311+G(d,p)
MP2(full)/
6-31G(d)
MP2(full)/6- B3LYP/6311+G(d,p) 311+G(d,p)
0.80690
0.77896
0.80250
0.80947
4.35405
4.55703
4.34938
-2.36020
-2.29287
-2.49195
-2.62308
-2.36527
16.84227
18.01789
18.51171
47.28130
49.87881
47.85183
49.33778
51.67174
49.63972
54.72093
59.60832
δJ
0.06336
0.06554
0.06510
0.07232
0.07627
0.07500
1.24263
1.30438
1.20550
δK
0.00085
-0.34058
-0.31943
1.14272
1.21762
1.07767
9.73883
10.65783
10.85218
χaa
-0.5815
-0.5153
-0.5324
2.5317
2.6671
2.8457
0.2757
0.2757
0.2350
χbb
-1.3395
-1.5437
-1.7482
-0.0168
0.2292
0.0386
-2.0578
-2.1702
-2.4283
χcc
1.9210
2.0590
2.2805
-3.0754
-2.8963
-2.8842
1.7821
1.8945
2.1933
Gg′
Gt
MP2(full)/
6-31G(d)
MP2(full)/6311+G(d,p)
B3LYP/6311+G(d,p)
MP2(full)/
6-31G(d)
MP2(full)/6311+G(d,p)
B3LYP/6311+G(d,p)
∆J
5.07813
4.99367
4.68502
4.18922
4.68724
4.33197
∆JK
22.42421
-21.10396
-21.16968
17.38352
-20.33125
-19.25717
∆K 57.47114
54.86628
61.25341
54.80098
58.76218
60.81913
δJ
1.56539
1.51875
1.39858
1.11657
1.36446
1.22193
δK
9.99816
9.69497
10.32446
10.52940
9.50937
9.79386
χaa
2.1054
2.2072
2.4495
-3.3933
-3.5192
-3.5970
χbb
2.3459
2.4971
2.6781
2.3087
2.4557
2.6025
χcc
-4.4513
-4.7043
-5.1276
1.0846
1.0635
0.9945
109
Figure 12: Five conformers of n-propylamine showing atomic numbering.
110
Figure 13: Experimental and predicted infrared spectra of n-propylamine: (A) gas; (B) simulated
spectrum of mixture of Tt, Tg (ΔH = 79 cm-1), Gg (ΔH = 91 cm-1), Gg′ (ΔH = 135 cm-1), and Gt
(ΔH = 143 cm-1) conformers at 25°C.; (C) simulated Tt conformer.
111
Figure 14: Raman spectra of the xenon solution (Top) and infrared spectra of the gaseous vapor
(Bottom) of n-propylamine (A) 920 - 800 cm-1; (B) 500 – 425 cm-1.
112
Figure 15: Experimental and predicted Raman spectra of n-propylamine: (A) xenon solution at 100°C; (B) simulated spectrum of mixture of Tt, Tg (ΔH = 79 cm-1), Gg (ΔH = 91 cm-1), Gg′ (ΔH =
135 cm-1), and Gt (ΔH = 143 cm-1) conformers at -100°C.; (C) simulated Tt conformer; (D)
simulated Tg conformer; (E) simulated Gg conformer; (F) simulated Gg′ conformer; (G) simulated
Gt conformer.
113
Tt → Tg
-1.46
-1.48
R2 = 0.9099
ln(I448 /I472 )
-1.5
-1.52
-1.54
-1.56
-1.58
-1.6
4.8
5
5.2
5.4
5.6
5.8
6
5.6
5.8
6
1000/T(K)
Tt → Gg
-0.98
-1
R2 = 0.8551
ln(I448 /I472 )
-1.02
-1.04
-1.06
-1.08
-1.1
-1.12
-1.14
4.8
5
5.2
5.4
1000/T(K)
Figure 16: van’t Hoff plot of ln(Iconf1/Iconf2) as a function of 1/T.
114
CHAPTER 6
INFRARED AND RAMAN SPECTRA, R0 STRUCTURAL PARAMETERS,
CONFORMATIONAL STABILITY, AND VIBRATIONAL ASSIGNMENT
OF 2-CYANOETHYLAMINE
Introduction
The conformational stability determination of 1,2-disubstituted ethane molecules have
provided some interesting challenges for structural scientists over the years. The 1,2-dihaloethane
molecules provided good examples where it has been shown by infrared gas phase studies [79]
that 1,2-difluoroethane has the gauche conformer more stable than the trans form by 280  30 cm-1
(3.35  0.36 kJ mol-1) whereas 1,2-dichloroethane [80] has the trans conformer more stable than
the gauche form by 323  7 cm-1 (3.87  0.09 kJ mol-1) from variable temperature infrared studies
of xenon solutions. This value was obtained from an average of four gauche/trans conformer pairs.
Eight years later the enthalpy difference was again determined [43] for 1,2-dichloroethane for both
the gas and two different noble gas solutions from variable temperature infrared studies. For the
liquid xenon solution two conformer pairs which differed from those used in the earlier study [80]
were utilized [43] from which a value of 272  11 cm-1 (3.26  0.13 kJ mol-1) was obtained.
However, from the determination in krypton solution only one conformer pair was used which
differed from the two used from the xenon solution. The value from the krypton solution [43] was
318  25 cm-1 (3.81  0.30 kJ mol-1) which is much lower than the value obtained for the gas [43]
from one conformational pair with the enthalpy difference of 410  13 cm-1 (4.90  0.15 kJ mol-1).
These results clearly show that more than one conformer pair needs to be used for enthalpy
difference determinations but there may be slight differences from the two different rare gases.
Since many organoamines are important biomolecules such as aminoacids, proteins, etc, it
is of scientific importance to determine the conformational stabilities of representative
115
organoamine molecules. There have been recent studies of 3-XCH2CH2NH2 molecules where X=
F and NH2 to identify the two or three conformers which are the most stable forms and determine
their enthalpy differences. As a continuation of these scientific investigations, the molecule where
X is the nitrile species was studied since it has been shown that this triple bonded moiety has
significantly different effects on the conformational stabilities of X-methyl-cyclopropanes
compared to molecules where the X is F or other halogens or hydrocarbons. The H2NCH2CH2CN
molecule has been previously investigated by microwave spectroscopy [81] where two conformers
were identified, the Gg and Gt forms (Fig. 17) where the Capital G is for Gauche for the
orientation around the C-C bond and the lower cases g and t for gauche and trans orientation for
the amine, but their enthalpy difference could not be determined. However in this study an
estimated value of 0  2 kJ/mol (0 ± 167 cm-1) was reported and it also was reported that these two
identified conformers were at least 4 kJ/mol (334 cm-1) more stable than the other three possible
conformers. Additionally, all the structural parameters except for the C-C-NH2 angle and the C-CC-N dihedral angle were estimated where the values for these estimated parameters were taken
from the corresponding parameters of similar molecules. By using the assumed parameters and
fitting the experimental rotational constants by changing the values for the angles determined
parameters were obtained for CCN of 108.0(15)° for conformer I (Gg) and 114.0(15)° for form II
(Gt) and for the heavy atom dihedral angles the values of 63(3)° and 59(3)° for conformers I and
II, respectively, were reported.
To obtain the structural information and conformational stabilities of 2-cyanoethylamine,
H2NCH2CH2CN, we have investigated the infrared spectra of the gas and solid and the Raman
spectrum of the liquid. To support these experimental studies we have also obtained the predicted
harmonic force fields, infrared intensities, Raman activities, depolarization ratios, and vibrational
frequencies from MP2/6-31G(d) ab initio calculations with full electron correlation. To obtain
116
predictions for the conformational stabilities we have carried out MP2(full) ab initio and density
functional theory (DFT) calculations by the B3LYP method by utilizing a variety of basis sets. The
r0 structural parameters have been obtained by combining the MP2(full)/6-311+G(d,p) ab initio
predicted parameters with the previously reported rotational constants obtained from the
microwave study [81]. The results of these spectroscopic, structural and theoretical studies of 2cyanoethylamine are reported herein.
Experimental and Theoretical Methods
The sample of 2-cyanoethylamine was purchased from Alfa Aesar with an estimated purity
of 98%. The sample was used without further purification and its purity was verified from the
infrared spectra of the gas.
The infrared spectrum of the gas (Fig. 18A) and solid (Fig. 18B) were obtained from 4000 to
220 cm-1 on a Perkin-Elmer model 2000 Fourier transform spectrometer equipped with a Ge/CsI
beamsplitter and a DTGS detector. Atmospheric water vapor was removed from the spectrometer
housing by purging with dry nitrogen. The spectra of the gas and solid were obtained with a
theoretical resolution of 0.5 cm-1 for the gas and 2 cm-1 for the solid with 128 interferograms added
and truncated. Multiple annealings were performed but a pure crystal could not be obtained, thereby
the solid spectra is that of the amorphous solid at boiling liquid nitrogen temperatures.
The Raman spectra of the liquid (Fig. 19A) were recorded from 4000 to 40 cm-1 on a Spex
model 1403 spectrophotometer equipped with a Spectra-Physics model 2017 argon ion laser
operating on the 514.5 nm line. The laser power used was 1.5 W with a spectral bandpass of 3 cm 1
. The spectra of the liquid were recorded with the sample sealed in a Pyrex glass capillary. The
measurements of the Raman frequencies are expected to be accurate to  2 cm-1. All of the
observed bands in the Raman spectra of the liquid along with their proposed assignments and
117
depolarization values are listed in Tables 27, 28, 29, and 30 for the Gg, Gt, Tg, and Tt conformers,
respectively.
The LCAO-MO-SCF restricted Hartree-Fock calculations were performed with the
Gaussian-03 program [21] using Gaussian-type basis functions. The energy minima with respect
to nuclear coordinates were obtained by the simultaneous relaxation of all geometric parameters
using the gradient method of Pulay [22]. Several basis sets as well as the corresponding ones with
diffuse functions were employed with the Møller-Plesset perturbation method [23] to the second
order (MP2(full)) as well as with the density functional theory by the B3LYP method. The
predicted conformational energy differences are listed in Table 31.
In order to obtain a complete description of the molecular motions involved in the
fundamental modes of H2NCH2CH2CN, a normal coordinate analysis has been carried out. The
force field in Cartesian coordinates was obtained with the Gaussian 03 program [21] at the
MP2(full) level with the 6-31G(d) basis set. The internal coordinates used to calculate the G and B
matrices are given in Table 32. By using the B matrix [24], the force field in Cartesian coordinates
was converted to a force field in internal coordinates. Subsequently, scaling factors of 0.88 for CH
stretches and CH2 deformation, and 0.90 for all other modes except the heavy atom bends were
applied, along with the geometric average of the scaling factors for the interaction force constants,
to obtain the fixed scaled force field and resultant wavenumbers. A set of symmetry coordinates
was used (Table 33) to determine the corresponding potential energy distributions (P.E.D.s). A
comparison between the observed and calculated wavenumbers, along with the calculated infrared
intensities, Raman activities, depolarization ratios and potential energy distributions for the Gg, Gt,
Tg, and Tt conformers are listed in Tables 27, 28, 29, and 30, respectively.
The vibrational spectra were predicted from the MP2(full)/6-31G(d) calculations. The
predicted scaled frequencies were used together with a Lorentzian function to obtain the simulated
118
spectra. Infrared intensities were obtained based on the dipole moment derivatives with respect to
Cartesian coordinates. The derivatives were transformed with respect to normal coordinates by
(u/Qi) =
 ( /X )L ,
j
u
j
ij
where Qi is the ith normal coordinate, Xj is the jth Cartesian
displacement coordinate, and Lij is the transformation matrix between the Cartesian displacement
coordinates and the normal coordinates.
The infrared intensities were then calculated by
(N)/(3c2) [(x/Qi)2 + (y/Qi)2 + (z/Qi)2]. A comparison of the experimental spectra of the
gas and solid and simulated infrared spectra of the mixture of the two most stable conformers with
relative concentrations calculated for the equilibrium mixture at 25ºC by using the experimentally
determined enthalpy difference, pure Gt form, and pure Gg conformer are shown in Fig. 18 (A-E),
respectively. The spectrum of the mixture should be compared to that of the infrared spectrum of
the vapor at room temperature. The predicted spectrum is mostly in good agreement with the
experimental spectrum which shows the utility of the scaled predicted frequencies and predicted
intensities for supporting the vibrational assignment.
Additional support for the vibrational assignments was obtained from the simulated Raman
spectra. The evaluation of Raman activity by using the analytical gradient methods has been
developed [25-27, 37] and the activity Sj can be expressed as: Sj = gj(45αj2 + 7βj2), where gj is the
degeneracy of the vibrational mode j, αj is the derivative of the isotropic polarizability, and βj is the
anisotropic polarizability. To obtain the Raman scattering cross sections, the polarizabilities are
incorporated into Sj by multiplying Sj with (1-ρj)/ (1+ρj) where ρj is the depolarization ratio of the
jth normal mode. The Raman scattering cross sections and calculated wavenumbers obtained from
the Gaussian 03 program were used together with a Lorentzian function to obtain the simulated
Raman spectra. Comparison of experimental Raman spectra of the liquid and the predicted Raman
spectra of the mixture of the two most stable conformers with relative concentrations calculated for
119
the equilibrium mixture at 25ºC by using the experimentally determined enthalpy difference, pure
Gt form, and pure Gg conformer are shown in Fig. 19(A-D). The spectrum of the mixture should
be compared to that of the Raman spectrum of the liquid at room temperature, though it must be
noted that the predicted spectrum is that of a molecule in isolation therefore frequency and
intensity changes are expected. The predicted spectrum is in reasonable agreement with the
experimental spectrum which shows the utility of the predicted Raman spectra for the supporting
vibrational assignments.
Vibrational Assignment
From the microwave spectral study it was concluded that there are two conformers that are
dominant and if there are other conformers present at ambient temperature their energy differences
are sufficiently large so that large fractions of further conformers than the observed ones were
ruled out [81]. However, from a subsequent theoretical study, it was concluded there were four
conformations which could be identified as present in the gas phase at normal temperature since
they all had almost equal total energies within 2 kJ/mol (167 cm-1). These results were obtained by
using the program TEXAS written by Pulay [82]. The essential feature of this program system is
that energy gradients are calculated analytically, and complete geometry optimizations are carried
out by the force relaxation method [22]. For carbon and nitrogen the investigator [83] used a 4-21
Gaussian basis set of Pople et al. [84]. However these results are considerably different from those
which we obtained from our calculations with much larger basis sets (aug-cc-pVTZ) with similar
energies for the Gg and Gt forms but the third conformer had an energy difference of 277 cm-1
(3.31 kJ/mol) from Table 31.
Therefore we began the vibrational assignment expecting most of the observed bands
would be for these two conformers. Also, since the difference between these two forms was the
120
position of the NH2 group, one figures most of the carbon-hydrogen fundamentals will have
similar frequencies. Thus the spectrum of the gas was expected to be relatively simple.
The assignment of the carbon-hydrogen fundamentals were made based on “group
frequencies”, predicted frequencies from the ab initio calculations as well as the predicted
intensities of the infrared bands and the Raman activities. However the depolarization values were
of some utility for making assignments particularly for distinguishing bands for either the Gg or Gt
conformer. The CH2 modes were first assigned and then the NH2 fundamentals and finally the
heavy atom motions (Tables 27 and 28). It is interesting that many of the fundamentals for the Gg
and Gt forms have nearly the same predicted frequencies for the CH2 modes although some of the
P.E.D.s are significantly different, i.e. 13, 14, 15, and 16.
The fundamentals of most interest are those of the heavy atom modes which should be
most sensitive to the gauche(G)/trans(T) conformational changes i.e. rotation around the C-C bond
which should aid in identifying vibrations arising from the forms with the lower abundance. The
most interesting of these vibrations is the C-C-C bend which is observed at 572 cm-1 for the Gg
and Gt conformers but for the Tg this mode is observed at 526 cm-1 and for the Tt conformer at
517 cm-1. However most of the other heavy atom vibrations have nearly the same frequencies.
Conformational Stability
There are five possible conformers of 2-cyanoethylamine (Fig. 17). The first conformational
study for this molecule was a microwave study [81] where the enthalpy difference between Gg and
Gt was estimated to be 0  167 cm-1 (0  2 kJ/mol) from intensity comparisons of the K-1 a-type Rbranch transitions. This method was used since the dipole moments could not be determined. The
remaining three conformers could not be assigned even though there were 20 or so unassigned
lines so it was concluded that the remaining three conformers must be at least 334 cm-1 (4 kJ/mol)
higher in energy. The next conformational study was a theoretical study [83] where the energy
121
differences for the five possible conformers were predicted from ab initio calculations with the 421 gaussian basis set [84]. This basis set is fairly small by modern standards and there is
significant disagreement with the predictions with the aug-cc-pVTZ basis set.
Due to the large uncertainty of the enthalpy difference for the two most stable conformers
and the indefinite prediction of the remaining conformers’ enthalpy differences the energy
differences have been predicted (Table 31) with larger basis sets. Additionally the potential
function for the rotation around the C-N bond was calculated (Fig. 20) by using the MP2(full)
method with the 6-311+G(2d,2p) basis set, and an experimentally determined enthalpy difference
has been obtained from band intensities of the infrared spectra of the vapor at room temperature.
By utilizing the relative band intensities at room temperature from the infrared spectra of the gas
an experimental enthalpy difference can be determined. The bands at 1044 and 1000 cm-1 have
been confidently assigned for the Gg and Gt conformers, respectively. The relative intensities were
measured for these intense Q-branches and from this band pair the enthalpy difference was
determined to be 75 cm-1 in the vapor phase. This value falls within the range given by the
previous microwave study [81], but no uncertainty can be determined due to only one band pair
being sufficiently resolved. The band at 526 cm-1 can be confidently assigned to the Tg conformer
and its band intensity can be measured. With the band intensity of the 1044 cm -1 fundamental for
the Gg form an enthalpy difference of 333 cm-1 (3.98 kJ/mol) was obtained. This value agrees with
the previous microwave determination as well as the ab initio predictions but its certainty is
questionable as the intensity of extremely weak bands is difficult to measure accurately. The
remaining two conformers have significantly higher energy but only the Tt could be found in the
infrared spectra of the gas as a low intensity Q-branch though the intensity could not be measured
with enough accuracy to determine the enthalpy difference experimentally.
122
Structural Parameters
In the microwave study , [81] all but two of the structural parameters were assumed for the
two identified conformers and these parameters were assumed to be the same for both conformers.
The two parameters that were fitted were the NCC angle and the NCCC dihedral angle. Due to
these assumptions the structural parameters from the microwave study have large uncertainties.
Therefore, we have determined the structural parameters for the two conformers by utilizing the
rotational constants previously reported from the microwave study [81]. Only one isotopic species
was studied for the two conformers and, thus, only three rotational constants are available for each
conformer.
We have found that good structural parameters for hydrocarbons and many substituted ones
can be determined by adjusting the structural parameters obtained from the ab initio MP2(full)/6311+G(d,p) calculations to fit the rotational constants obtained from microwave experimental data
by using a computer program “A&M” (Ab initio and Microwave) developed [57] in our
laboratory. In order to reduce the number of independent variables, the structural parameters are
separated into sets according to their types where bond distances in the same set keep their relative
ratio, and bond angles and torsional angles in the same set keep their difference in degrees. This
assumption is based on the fact that errors from ab initio calculations are systematic. Additionally,
we have also shown that the differences in predicted distances and angles from the ab initio
calculations for different conformers of the same molecule can usually be used as one parameter
with the ab initio predicted differences except for some dihedral angles. Therefore, it should be
possible to obtain “adjusted r0” structural parameters for the two conformers which have the
determined rotational constants of 2-cyanoethylamine by utilizing the previously reported values
from the earlier microwave study [81].
123
We [44] have shown that ab initio MP2(full)/6-311+G(d,p) calculations predict the r0
structural parameters for more than fifty carbon-hydrogen distances to at least 0.002 Å compared
to the experimentally determined values from isolated CH stretching frequencies which were
compared [45] to previously determined values from earlier microwave studies. Therefore, all of
the carbon-hydrogen distances can be taken from the MP2(full)/6-311+G(d,p) predicted values for
Gg and Gt conformers of 2-cyanoethylamine. The resulting adjusted r0 parameters are listed in
Table 32, where it is believed that the N-C, C-C and N-H distances should be accurate to ± 0.003
Å, the C-H distances should be accurate to ± 0.002 Å, and the angles should be within ± 0.5.
The fit of the six determined rotational constants (Table 34) by the structural parameters for
the Gg and Gt conformers is remarkably good with the largest differences being 0.80, 0.60 and the
smaller ones of 0.30 and 0.10 MHz. Therefore, it is believed that the suggested uncertainties are
realistic values and the determined structural parameters are probably as accurate as can be
obtained for the molecule in the gas phase by the microwave substitution method.
Discussion
The mixing was extensive for four modes; 16 (21% C-C(-N) stretch, 29% C-N stretch)
which has 6 contribution with 10% or more and 13 (27% NH2 twist), 18 (35% C-C(N) stretch),
20 (35% β-CH2 wag), and 23 (50% C-C-N bend) which have 5 contributions of 10% or more.
Therefore several of the approximate descriptions for the fundamentals of the Gg form are more
for bookkeeping than providing descriptions of the molecular motions.
The assignment was somewhat complicated by the nearly equal amounts of both conformers
present. Therefore, significant reliance on the predicted wavenumbers was necessary to assign
many of the modes with group frequencies also playing a significant role. Nevertheless the
predicted wavenumbers for the Gg and Gt forms had average errors of 11 and 13 cm -1,
respectively. These errors correspond to percent errors of 0.7 and 0.8%. Where the C≡N stretch,
124
NH2 deformation, and NH2 rock were excluded due to the well-known problems the MP2(full)
method has predicting these modes. A significant amount of the error is due to the predictions for
the C-C-N bending mode and the C-C(≡N) stretching mode but the assignments agree with
previously reported group frequencies for these vibrations.
The predicted infrared intensities (Table 27 and 28) from the MP2 calculations for the C≡N stretch
are poorly predicted as can be observed in Fig. 18 where the infrared spectra of the gas is 30 times
more intense than that predicted. Similarly the Raman activities (Fig. 19) for this mode are almost
three times more intense. These large differences are the result of the well-known problem from
the MP2(full) calculations which results in a predicted structural parameter for the triple bond
being nearly 0.013 Å longer than the value determined for the adjusted r0 value (Table 32).
To support the experimental enthalpy difference of 75 cm-1 for Gg to Gt in the vapor state the ab
initio energy differences have been determined by a variety of basis sets (Table 31) for
comparison. It is of particular interest that the energy difference from the MP2(full) method with
the aug-cc-pVTZ basis set gives a value of 73 cm-1 and the 6-311+G(2d,2p) basis set which was
used to generate the predicted potential function gives a value of 83 cm -1. These values of the
energy differences are in excellent agreement with the enthalpy difference for Gg to Gt. The order
of stability by the ab initio MP2(full) calculation with the aug-cc-pVTZ basis set agrees with that
predicted in the previous microwave study [81] and agrees with the spectral information obtained
in the current study. However, it should be noted that the same method without diffuse functions
gives the Gt conformer as being more stable. This is interesting as the diffuse functions seem to
better account for the intramolecular hydrogen bonding of the amine with the cyanide as they
account for the more distant portions of the molecular orbitals. The DFT calculations by the
B3LYP method give the Gt conformer as being more stable for all basis sets though adding diffuse
functions to the basis set lowers the predicted energy difference by approximately 100 cm-1.
125
This order of stability seems to be largely the result of the amine group hydrogen-bonding to the π
orbitals of the cyanide group. This can be demonstrated with the distances (Table 32) between the
amine hydrogen and the cyanide carbon. The order of stability for the first two conformers closely
mirrors this distance. The third (Tg) and fourth (Tt) conformers then are the heavy atom trans
conformers where intramolecular hydrogen bonding becomes impossible, though this is the heavy
atom configuration that is the most stable in similar amine molecules without the possibility of
intramolecular hydrogen bonding. The final possible conformer is the Gg' form where the NH7--C≡N distance is 3.415 Å from the MP2(full)/6-311+G(d,p) calculation which is almost 1 Å larger
than the same distance in the Gg conformer. This intramolecular hydrogen bonding is a significant
influence on both the molecular conformation and structure of these amine molecules as is
demonstrated in both this study and previous studies [49, 61].
In the current study the quadratic centrifugal distortion constants have been predicted by ab initio
MP2(full) and DFT B3LYP calculations with the 6-311+G(d,p) basis set. These data are given in
Table 35 and the values are all reasonably well predicted by the both the MP2(full) and B3LYP
methods. However, the values predicted by the B3LYP method for ∆K and δK are significantly
different than those determined experimentally from the microwave study [81]. The quadrupole
coupling constants for both the Gg and Gt conformers have been predicted by utilizing the
MP2(full) and B3LYP methods with the 6-311+G(d,p) basis set to aid in any future microwave
study.
The adjusted r0 structural parameters have been determined for both the Gg and Gt conformers of
2-cyanoethylamine (Table 32). In the initial microwave study [81] only two parameters were
determined for each conformer Gg [Gt] which were the angle (°) NCC = 108.0(15) [114.0(15)] and
the dihedral angle (°) NCCC = -63(3) [59(3)]. These values have such large error limits that their
agreement with the values obtained from the current study for the NCC angle and the NCCC
126
dihedral angle has little meaning. These large differences are due to the assumed structural
parameters used in the previous study where for the Gg conformer the heavy atom bond distances
rN1-C2 and rC2-C3 are 0.014 and 0.013 Å too long, respectively. For the Gt conformer the heavy
atom bond distance rN1-C2 is 0.022 Å too long, but the rC2-C3 bond distance that was assumed is
within 0.003 Å of that obtained in the present study.
The structure obtained in the current study should be accurate to within its error limits and is the
most accurate structure that can be determined with the currently available information. This
structure can be compared to other similar molecules such as the 2-fluoroethylamine [49] where
the adjusted r0 structural parameters were determined for the Gg [Gt] conformers with values for
the distances (Å) of 1.466(3) [1.461(3)] for the C-N and 1.509(3) [1.516(3)] for the C-C. These
distances have differences of +0.005 [+0.008] and -0.026 [-0.029] Å compared to the
corresponding C-N and C-C bond distances for 2-cyanoethylamine. The difference in the C-N
distance averages to approximately the sum of the error limits meaning it may be significant but
may also be the errors of the methods. However, the difference in C-C distances is very large
though to be expected for a fluoride substituent. This small difference in the C-N bond would seem
to indicate the amine group is similarly affected by a fluorine in the 2-position as a cyanide in the
2-position though this similarity does not extend to identically affected.
These substituted ethyl amines are of interest with properties that often cannot be accurately
predicted. An example of this is the study of the 2-fluoro substituted ethyl amine [49] the Gg to Gt
energy difference is predicted by MP2(full)/aug-cc-pVQZ calculations to be 105 cm-1 which
matches well with the experimentally determined enthalpy difference of 62  8 cm-1. It would
seem from this result that the energy differences are well predicted by ab initio calculations.
However, the next most stable conformer is predicted to have an energy difference of 464 cm -1 but
compared to the experimentally determined enthalpy difference of 262  26 cm-1 it is very poorly
127
predicted with nearly twice the energy. It would be of interest to obtain the enthalpy difference
utilizing the variable temperature Raman spectra of the xenon solutions which would be much
more accurate than the current value. Also of interest is the ethyl amine moiety with the alcohol
substituent which has ten possible conformers and should exhibit both the behaviors of an amine
and alcohol.
128
Table 27: Observed and calculateda frequencies (cm-1) and potential energy distributions (P.E.D.s) for the Gg conformer of 2cyanoethylamine.
129
Vib.
no.
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
Raman
Raman
Contour*
IR
ab
Fixed
IR
dp
c
P.E.D.
b
act.
Liquid
initio
scaled
int.
ratio
A
B
gas
solid
NH2 antisymmetric stretch
99S
3619
3434
2.8
60.6
0.66
3380
3361
3383
93
7
1
NH2 symmetric stretch
99S2
3519
3338
1.2
91.8
0.12
3317
3315
3324
1
79
97S3
3187
2990
2.5
55.9
0.73
2994
2992
2988
9
17
-CH2 antisymmetric stretch
α-CH2 antisymmetric stretch
72S4,26S6
3159
2963
24.3
92.5
0.39
2967
2964
2960
49
38
β-CH2 symmetric stretch
97S5
3125
2931
2.9
87.4
0.11
2937
2930
2931
10
90
α-CH2 symmetric stretch
74S6,26S4
3056
2867
56.3
95.3
0.21
2840
2846
2837
12
89S7,11S18
2221
2208
0.2
32.9
0.27
2244
2245
2247
14
82
CN stretch
NH2 deformation
77S8,22S19
1728
1639
42.2
7.7
0.75
1603
1607
1599
3
79
α-CH2 deformation
98S9
1575
1481
1.0
12.7
0.72
1474
1472
1472
74
22
β-CH2 deformation
95S10
1522
1431
8.6
11.0
0.71
1423
1420
1426
12
53
α-CH2 rock
72S11,16S13
1465
1391
13.6
3.3
0.22
1386
1387
1389
58
42
β-CH2 rock
60S12,23S15
1395
1324
4.7
9.9
0.63
1334
1336
1336
3
79
NH2 twist
27S13,29S15,12S14, 10S12, 10S11
1381
1312
2.5
13.1
0.75
1313
1313
1315
2
93
β-CH2 twist
71S14,10S21,10S15
1273
1209
4.2
5.6
0.41
1216
1219
1219
3
2
α-CH2 twist
31S15,27S13,13S12
1208
1151
0.5
3.0
0.60
1155
1158
1155
86
C-C(-N) stretch
21S16,29S17,11S11,10S21,10S19,10S13
1154
1096
7.1
5.4
0.72
1085
1090
1092
10
20
C-N stretch
31S17,28S20,24S21
1110
1057
15.9
3.6
0.15
1044
1050
1044
4
32
35S18,19S16,12S13,10S21,10S12
1003
956
0.6
2.5
0.70
973
978
976
65
7
C-C(N) stretch
NH2 rock
44S19,18S16, 11S8,10S21
922
878
92.9
3.3
0.53
856
925
953
3
13
β-CH2 wag
35S20,22S17,18S19,10S16,10S8
891
847
60.5
4.7
0.32
850
850
853
6
23
α-CH2 wag
24S21,40S18,14S16
847
807
2.6
3.6
0.13
801
816
816
29
28
C-C-C bend
35S22,29S26,12S23
578
569
4.1
0.9
0.42
572
576
576
38
22
C-C-N bend
50S23,11S24,11S26,10S27, 10S20
406
401
13.5
1.0
0.69
419
418
423
67
27
66S24,14S23,10S26,10S27
371
370
2.1
2.8
0.75
375
375
381
9
62
C-CN out-of-plane bend
NH2 torsion
86S25
296
296
55.8
3.1
0.75
~430
212
100
32S26,28S22,25S27,10S26
197
196
17.8
1.1
0.71
187
84
3
C-CN in-plane bend
C-C(N) torsion
118
118
4.4
1.4
0.73
118
55S27,17S22,12S26,10S24
7
89
a
MP2(full)/6-31G(d) ab initio calculations, scaled frequencies, infrared intensities (km mol-1), Raman activities (Å4 u-1), depolarization (dp) ratios and potential energy
distributions (P.E.D.s).
b
MP2(full)/6-31G(d) fixed scaled wavenumbers with factors of 0.88 for CH stretches, 1.0 for heavy atom bends, and 0.90 for all other modes.
c
Calculated with MP2(full)/6-31G(d) and contributions of less than 10% are omitted.
*
A, B and C values in the last three columns are percentage infrared band contours.
Approximate description
C
20
74
13
88
4
18
4
35
19
4
95
14
71
64
28
84
70
43
40
6
29
13
4
Table 28: Observed and calculateda frequencies (cm-1) and potential energy distributions (P.E.D.s) for the Gt conformer of 2cyanoethylamine.
130
Vib.
no.
1
2
4
3
6
5
7
8
9
10
15
11
12
14
13
17
16
18
19
20
21
22
24
23
25
26
27
Raman
Raman
Contour*
IR
ab
Fixed
IR
dp
P.E.D.c
b
act.
Liquid
initio
scaled
int.
ratio
A
B
gas
solid
NH2 antisymmetric stretch
3623
3432
6.9
31.3
0.73
3380
3361
3383 100S1
44
22
NH2 symmetric stretch
3520
3337
1.5
83.9
0.05
3317
3315
3324 100S2
98
α-CH2 antisymmetric stretch
90S4, 10S3
3185
2988
17.3
50.9
0.74
2994
2992
2988
29
11
90S3, 10S4
3163
2968
5.0
82.0
0.66
2967
2964
2960
83
-CH2 antisymmetric stretch
α-CH2 symmetric stretch
98S6
3124
2930
27.0
134.0
0.09
2937
2930
2931
24
48
99S5
3110
2918
5.7
129.4
0.07
2878
2880
2885
76
-CH2 symmetric stretch
89S7,11S18
2220
2207
0.2
54.4
0.19
2244
2245
2247
61
39
CN stretch
NH2 deformation
77S8,22S19
1729
1639
27.9
2.9
0.73
1603
1607
1599
18
35
α-CH2 deformation
98S9
1552
1459
2.0
6.6
0.66
1458
1457
1460
16
31
95S
1530
1439
8.9
7.0
0.74
1423
1420
1426
12
46
-CH2 deformation
10
α-CH2 twist
49S
,31S
10S
1433
1359
1.8
8.0
0.73
1361
1371
1378
42
58
15
13,
11
76S11,10S12
1425
1352
12.6
2.1
0.44
1361
1362
1366
74
24
-CH2 rock
59S12,12S13,11S21
1384
1314
4.1
4.4
0.38
1313
1313
1315
10
59
-CH2 rock
52S14,10S12,10S11
1286
1223
6.6
6.7
0.30
1242
1246
1249
73
4
-CH2 twist
NH2 twist
19S13,26S15,16S14,10S12,10S11
1205
1147
1.9
1.3
0.59
1155
1158
1155
19
5
C-N stretch
54S17,12S14,12S20,10S19
1161
1104
5.4
2.8
0.47
1085
1090
1092
68
C-C(-N) stretch
34S16,18S20,11S18,10S23,10S12
1053
1005
35.1
4.7
0.51
1000
1008
999
66
30
25S18,25S21,21S13,10S22
992
947
0.7
1.7
0.36
966
963
966
79
4
C-C(N) stretch
NH2 rock
53S19,15S8,10S18,10S17
912
866
182.9
2.1
0.08
844
912
937
62
36
30S20,34S16,16S17
881
840
0.9
8.6
0.20
850
850
845
57
43
-CH2 wag
31S21,31S18,11S16,10S19,10S20
826
784
18.8
1.3
0.06
760
757
750
44
56
-CH2 wag
C-C-C bend
33S22,23S26,13S23,10S21
581
572
3.2
0.8
0.31
572
576
576
100
43S24,27S23,11S27,10S20
388
384
2.7
1.7
0.46
398
396
389
40
9
C-CN out-of-plane bend
C-C-N bend
28S23,29S26,17S24,13S25
377
375
7.4
2.4
0.59
375
375
381
22
41
NH2 torsion
77S25,15S24
344
343
37.1
0.4
0.57
245
~430
234
33
17
26S
,30S
,25S
10S
192
192
9.3
1.6
0.75
187
1
22
C-CN in-plane bend
26
22
27,
24
C-C(N) torsion
55S
,16S
,15S
118
118
6.0
1.6
0.72
118
37
48
27
22
26
a
MP2(full)/6-31G(d) ab initio calculations, scaled frequencies, infrared intensities (km mol-1), Raman activities (Å4 u-1), depolarization (dp) ratios and potential energy
distributions (P.E.D.s).
b
MP2(full)/6-31G(d) fixed scaled wavenumbers with factors of 0.88 for CH stretches, 1.0 for heavy atom bends, and 0.90 for all other modes.
c
Calculated with MP2(full)/6-31G(d) and contributions of less than 10% are omitted.
*
A, B and C values in the last three columns are percentage infrared band contours.
Approximate description
C
34
2
60
17
28
24
47
53
42
2
31
23
76
32
4
17
2
51
37
50
77
15
Table 29: Observed and calculateda frequencies (cm-1) and potential energy distributions (P.E.D.s) for the Tg conformer of 2cyanoethylamine.
Vib.
no.
Approximate description
ab
initio
Fixed
scaledb
IR
int.
Raman
act.
dp
ratio
IR
gas
Raman
Liquid
P.E.D.c
Contour*
131
A
B
C
NH2 antisymmetric stretch
100S1
3621
3465
7.0
56.5
0.69
11
50
39
1
NH
symmetric
stretch
99S
3521
3370
1.3
127.7
0.08
99
1
2
2
2
81S
,14S
3186
2972
7.3
22.6
0.40
4
96
3
-CH2 antisymmetric stretch
3
4
α-CH
antisymmetric
stretch
62S
,22S
14S
3165
2951
9.6
77.2
0.69
25
75
4
2
4
6,
3
94S5
3119
2911
5.6
111.9
0.07
19
78
3
5
-CH2 symmetric stretch
76S6,23S4
3063
2865
40.6
107.6
0.16
18
59
23
6
-CH2 symmetric stretch
88S7,12S18
2224
2188
0.1
60.1
0.20
100
7
CN stretch
NH2 deformation
74S8,25S19
1727
1579
34.4
3.0
0.60
56
2
42
8
97S9
1577
1442
1.7
6.8
0.64
20
74
6
9
-CH2 deformation
97S10
1535
1401
5.9
8.5
0.75
100
10
-CH2 deformation
72S11,11S13,10S12
1471
1372
15.7
4.4
0.40
99
1
11
-CH2 rock
α-CH2 twist
43S15,25S13,20S12
1388
1294
2.3
6.9
0.72
83
13
4
15
26S14,29S15,28S12
1362
1270
2.1
6.1
0.74
81
19
14
-CH2 twist
28S12,28S14,13S21,14S11
1307
1215
1.5
0.8
0.28
73
1
26
12
-CH2 rock
NH2 twist
22S13,31S14,17S15
1201
1122
1.6
1.9
0.11
17
23
60
13
C-N stretch
52S17,23S16,10S19
1146
1070
16.3
3.8
0.61
1073
87
3
10
17
α-CH2 wag
25S21,30S17,19S20
1101
1035
9.4
5.3
0.20
42
29
29
21
C-C(-N)
stretch
31S
,22S
,15S
1020
957
1.3
3.9
0.39
46
30
24
16
16
18
13
44S
,13S
,11S
963
915
1.4
3.1
0.16
40
46
14
18
C-C(N) stretch
18
13
22
NH
rock
62S
20S
904
820
141.0
3.0
0.14
27
44
29
19
2
19,
8
45S20,34S21
808
757
5.4
0.2
0.73
803
10
49
41
20
-CH2 wag
C-C-C bend
31S22,40S26,13S16,10S18
526
516
0.2
3.0
0.13
526
526
7
93
22
C-C-N bend
64S23,13S26
395
386
10.0
1.2
0.62
70
5
25
23
80S24,10S27
375
366
0.1
2.7
0.75
78
22
24
C-CN out-of-plane bend
NH2 torsion
92S25
263
299
46.7
0.4
0.48
24
39
37
25
39S26,48S22
165
162
4.7
1.9
0.74
41
51
8
26
C-CN in-plane bend
C-C(N) torsion
83S27,10S24
110
105
10.7
0.4
0.75
4
15
81
27
a
MP2(full)/6-31G(d) ab initio calculations, scaled frequencies, infrared intensities (km mol-1), Raman activities (Å4 u-1), depolarization (dp) ratios and potential energy
distributions (P.E.D.s)
b
MP2(full)/6-31G(d) fixed scaled wavenumbers with factors of 0.88 for CH stretches, 1.0 for heavy atom bends, and 0.90 for all other modes.
c
Calculated with MP2(full)/6-31G(d) and contributions of less than 10% are omitted.
*
A, B and C values in the last three columns are percentage infrared band contours.
Table 30: Observed and calculateda frequencies (cm-1) and potential energy distributions (P.E.D.s) for the Tt (Cs) conformer of 2cyanoethylamine.
Vib.
ab
Fixed
IR
Raman
dp
IR
Raman
P.E.D.c
Contour*
Approximate description
b
no.
initio
scaled
int.
act.
ratio
gas
liquid
A
B
NH2 symmetric stretch
100S2
3522
3341
0.4
87.6
0.10
87
13
1
α-CH2 symmetric stretch
96S6
3131
2937
20.7
47.9
0.12
6
94
2
96S
3108
2915
3.5
114.7
0.08
34
66
3
-CH2 symmetric stretch
5
89S
,11S
2221
2207
0.1
43.4
0.29
4
96
4
CN stretch
7
18
NH
deformation
76S
,23S
1726
1637
29.4
8.7
0.66
12
88
5
2
8
19
α-CH2 deformation
94S9
1558
1465
4.2
6.4
0.73
27
73
6
93S10
1532
1441
3.3
16.5
0.74
9
91
7
-CH2 deformation
α-CH2 rock
75S11,20S12
1434
1361
11.5
8.1
0.62
88
12
8
67S12,24S11
1336
1269
1.7
3.2
0.40
99
1
9
-CH2 rock
C-N stretch
74S17,10S19
1153
1099
6.3
3.8
0.50
1084
23
77
10
C-C(-N) stretch
72S16,10S23
1062
1016
50.3
6.9
0.38
1007
88
12
11
69S18,10S7
973
932
5.3
0.7
0.59
94
6
12
C-C(N) stretch
NH2 rock
61S19,18S8,10S17
879
836
217.1
4.7
0.17
89
11
13
C-C-C bend
32S22,39S26,14S16,10S18
517
508
1.3
4.2
0.17
517
100
14
C-C-N bend
68S23,14S26
383
377
7.9
3.1
0.57
91
9
15
42S26,49S22,10S23
161
160
7.1
1.3
0.75
100
16
C-CN in-plane bend
A"
NH2 antisymmetric stretch
100S1
3626
3440
1.3
50.7
0.75
17
α-CH2 antisymmetric stretch
89S4,11S3
3192
2994
17.6
21.3
0.75
18
89S
,11S
3160
2964
0.7
89.3
0.75
19
-CH2 antisymmetric stretch
3
4
α-CH
twist
55S
,37S
,10S
1427
1354
0.3
11.9
0.75
20
2
15
13
21
50S
,17S
,16S
13S
1348
1279
0.4
6.2
0.75
21
-CH2 twist
14
15
21,
13
17S20,43S14,15S13,15S15
1192
1133
0.0
1.1
0.75
22
-CH2 wag
NH2 twist
31S13,32S20,26S21,10S15
1001
950
0.9
0.5
0.75
23
α-CH2 wag
42S21,38S20,10S27
781
744
1.9
0.1
0.75
749
24
83S24,13S27
378
377
0.0
3.1
0.75
25
C-CN out-of-plane bend
NH2 torsion
95S25
325
325
57.6
2.7
0.75
26
C-C(N) torsion
81S27,10S24
112
111
1.7
0.5
0.75
27
a
MP2(full)/6-31G(d) ab initio calculations, scaled frequencies, infrared intensities (km mol-1), Raman activities (Å4 u-1), depolarization (dp) ratios and potential energy
distributions (P.E.D.s)
b
MP2(full)/6-31G(d) fixed scaled wavenumbers with factors of 0.88 for CH stretches, 1.0 for heavy atom bends, and 0.90 for all other modes.
c
Calculated with MP2(full)/6-31G(d) and contributions of less than 10% are omitted.
*
A, B and C values in the last three columns are percentage infrared band contours.
Sym
block
A'
132
Table 31: Calculated energies in (H) and energy differences (cm-1) for the five conformers of 2cyanoethylamine.
Energya, E
Energy Differences, ∆
Method/Basis Set
# basis
set
Ggb
Gtb
Tgb
Ttb
Gg′b
RHF/6-31G(d)
MP2(full)/6-31G(d)
MP2(full)/6-31+G(d)
MP2(full)/6-31G(d,p)
MP2(full)/6-31+G(d,p)
MP2(full)/6-311G(d,p)
MP2(full)/6-311+G(d,p)
MP2(full)/6-311G(2d,2p)
MP2(full)/6-311+G(2d,2p)
MP2(full)/6-311G(2df,2pd)
MP2(full)/6-311+G(2df,2pd)
MP2(full)/aug-cc-pVTZ
87
87
107
105
125
126
146
169
189
234
254
368
-0.982440
-1.707178
-1.724295
-1.759202
-1.775510
-1.912614
-1.922720
-1.974957
-1.983791
-2.057871
-2.065785
-2.072387
61
-10
117
-28
112
-57
75
-29
83
-40
67
73
100
326
358
307
332
292
326
289
286
283
286
277
323
400
524
329
470
301
441
347
408
349
401
420
517
563
624
521
582
518
549
551
582
549
581
611
-145
-24
-131
-27
-132
-20
-130
-24
-20
159
124
119
111
131
120
117
113
120
110
184
100
178
114
173
110
160
163
556
565
512
508
504
517
484
500
503
B3LYP/6-31G(d)
87
-2.409321
B3LYP/6-31+G(d)
107
-2.422646
B3LYP/6-311G(d,p)
126
-2.478557
B3LYP/6-311+G(d,p)
146
-2.485712
B3LYP/6-311G(2d,2p)
169
-2.486684
B3LYP/6-311+G(2d,2p)
189
-2.493742
B3LYP/6-311G(2df,2pd)
234
-2.493366
B3LYP/6-311+G(2df,2pd)
254
-2.500132
B3LYP/aug-cc-pVTZ
368
-2.506844
a
Energy of conformer is given as –(E+225) H.
b
Difference is relative to Gg form and given in cm-1.
133
Table 32: Structural parameters (Å and °) and rotational constants (MHz) for the Gg and Gt
conformers of 2-cyanoethylamine.
Structural
parameters
r (N1-C2)
r (C2-C3)
r (C3-C4)
r (C4≡N5)
r (N1-H6)
r (N1-H7)
r (C2-H8)
r (C2-H9)
r (C3-H10)
r (C3-H11)
r (NH6--rC≡N)
(NH7--C≡N)
 N1C2C3
 C2C3C4
 C3C4N5
 H6N1H7
 H6N1C2
 H7N1C2
 H8C2N1
 H9C2C3
 H10C3C2
 H11C3C4
 H9C2N1
 H8C2C3
 H10C3C4
 H11C3C2
 H8C2H9

H10C3H11
A
N1C2C3C4
B
C
|a|
|b|
|c|
|t|
Gg
MP2(full)/
6-311+G(d,p)
R1
1.459
R2
1.531
R3
1.464
R4
1.174
r1
1.014
r2
1.016
r3
1.093
r4
1.099
r5
1.094
r6
1.093
3.862
2.651
A1
109.5
A2
110.7
A3
177.7
α1
107.1
α2
110.6
α3
110.5
108.4
1
108.7
2
110.7
3
108.6
4
114.3
1
107.9
2
108.6
3
109.4
4
β1
107.7
β2
108.7
-61.5
1
10433.02
3410.89
2789.35
3.669
2.169
0.859
4.348
a
Ref. [81]
Internal
Coord.
B3LYP/
6-311+G(d,p)
1.461
1.540
1.462
1.153
1.014
1.016
1.093
1.099
1.094
1.093
3.948
2.756
110.5
112.3
178.6
107.4
111.2
111.3
108.2
108.8
110.1
108.5
114.3
107.4
108.3
109.4
107.3
108.1
-64.7
10825.10
3263.00
2718.02
Microwave
a
1.475
1.548
1.463
1.157
1.017
1.017
1.091
1.091
1.091
1.091
108.0(15)
110.5
180.0
111.0
111.0
109.5
109.5
109.5
109.5
-63(3)
10526.46(4)
3387.40(1)
2781.57(1)
Adjusted
r0
1.461(3)
1.535(3)
1.466(3)
1.161(3)
1.014(3)
1.016(3)
1.093(2)
1.099(2)
1.094(2)
1.093(2)
3.884(5)
2.675(5)
109.5(5)
111.1(5)
177.4(5)
107.1(5)
110.6(5)
110.5(5)
107.7(5)
108.8(5)
110.7(5)
109.0(5)
114.9(5)
107.9(5)
107.8(5)
109.4(5)
107.7(5)
108.7(5)
-62.2(10)
10527.02
3388.00
2780.99
134
Gt
MP2(full)/
6-311+G(d,p)
1.452
1.543
1.462
1.174
1.014
1.015
1.093
1.094
1.095
1.095
3.307
2.715
115.8
110.9
177.1
107.6
111.2
111.2
108.3
107.8
110.0
109.0
108.6
108.8
108.4
110.6
107.3
107.8
58.2
10220.02
3367.34
2758.50
2.342
2.813
1.131
3.381
B3LYP/
6-311+G(d,p)
1.452
1.554
1.460
1.154
1.014
1.014
1.093
1.093
1.095
1.095
3.411
2.821
116.7
112.3
177.8
108.3
112.4
112.2
108.3
107.3
110.1
108.7
108.6
108.7
108.4
110.1
106.8
107.2
60.2
10516.76
3249.00
2699.03
Microwavea
1.475
1.548
1.463
1.157
1.017
1.017
1.091
1.091
1.091
1.091
114.0(15)
110.5
180.0
111.0
111.0
109.5
109.5
109.5
109.5
59(3)
10281.38(2)
3367.75(4)
2761.08(4)
Adjusted
r0
1.453(3)
1.545(3)
1.463(3)
1.161(3)
1.014(3)
1.015(3)
1.093(2)
1.094(2)
1.095(2)
1.095(2)
3.318(5)
2.727(5)
116.0(5)
111.1(5)
177.0(5)
107.6(5)
111.1(5)
111.2(5)
108.1(5)
107.8(5)
110.0(5)
108.9(5)
108.5(5)
108.8(5)
108.3(5)
110.6(5)
107.3(5)
107.8(5)
58.2(10)
10282.23
3367. 85
2761.42
Table 33: Symmetry coordinates for 2-cyanoethylamine Gg conformer.
Symmetry Coordinatea
Description
NH2 antisymmetric stretch
S1
=
r1 – r2
NH2 symmetric stretch
S2
=
r1 + r2
-CH2 antisymmetric stretch
S3
=
r5 – r6
α-CH2 antisymmetric stretch
S4
=
r3 – r4
β-CH2 symmetric stretch
S5
=
r5 + r6
α-CH2 symmetric stretch
S6
=
r3 + r4
CN stretch
S7
=
R4
NH2 deformation
S8
=
α1
α-CH2 deformation
S9
=
4β1 – 1 – 2 – 1 – 2
β-CH2 deformation
S10
=
4β2 – 3 – 4 – 3 – 4
α-CH2 rock
S11
=
1 – 2 + 1 – 2
β-CH2 rock
S12
=
3 – 4 + 3 – 4
NH2 twist
S13
=
α2 – α3
β-CH2 twist
S14
=
3 +4 – 3 – 4
α-CH2 twist
S15
=
1 + 2 – 1 – 2
C-C(-N) stretch
S16
=
R2
C-N stretch
S17
=
R1
C-C(N) stretch
S18
=
R3
NH2 rock
S19
=
α2+ α3
β-CH2 wag
S20
=
3 – 4 – 3 + 4
α-CH2 wag
S21
=
1 – 2 – 1 + 2
C-C-C bend
S22
=
A2
C-C-N bend
S23
=
A1
C-CN out-of-plane bend
S24
=
3
NH2 torsion
S25
=
2
C-CN in-plane bend
S26
=
A3
C-C(N) torsion
S27
=
1
a
Not normalized.
135
Table 34: Comparison of experimental rotational constants obtained from microwave spectra and
adjusted r0 structural parameters of 2-cyanoethylamine.
Conformers
Gg
H2NCH2CH2CN
Gt
H2NCH2CH2CN
a
Rotational
constants
Experimentala
Adjusted r0
||
A
B
C
10526.455(38)
3387.403(13)
2781.573(13)
10527.022
3388.004
2780.996
0.6
0.6
0.6
A
B
C
10281.375(19)
3367.7496(43)
2761.0828(39)
10282.233
3367.8466
2761.4188
0.8
0.1
0.3
Ref [81].
136
Table 35: Quadratic centrifugal distortion constants (kHz) and quadrupole coupling constants
(MHz) for Gg and Gt conformers of 2-cyanoethylamine.
Gg
MP2(full)/
6-311+G(d,p)
∆J
Gt
B3LYP/
6-311+G(d,p)
MP2(full)/
6-311+G(d,p)
Exp.a
3.797
B3LYP/
6-311+G(d,p)
3.687
Exp.a
3.88
3.79
4.11(30)
4.066(38)
∆JK
-22.33
-23.50
-22.48(58)
-22.03
-23.69
-22.94(40)
∆K
62.6
78.58
52.8(60)
60.75
73.10
66.41(46)
δJ
1.189
1.146
1.313(17)
1.1550
1.1108
1.2335(71)
δK
7.95
9.94
7.20(75)
7.23
8.02
8.33(27)
χaa
1.9783
2.2298
-3.0510
-3.2699
χbb
2.6797
2.7865
2.2215
2.5317
χcc
-4.6580
-5.0162
0.8294
0.7382
χaa
-1.4340
-1.8227
-1.3839
-1.7231
χbb
-0.2256
-0.1853
-0.3033
-0.3195
χcc
1.6596
2.0080
1.6872
2.0425
N1
N5
a
Ref. [81]
137
Figure 17: Five conformers of 2-cyanoethylamine.
138
Figure 18: Experimental and predicted infrared spectra of 2-cyanoethylamine: (A) gas; (B)
amorphous solid; (C) simulated spectrum of mixture of Gg and Gt (ΔH = 75 cm-1) conformers at
25°C; (D) simulated Gt conformer; (E) simulated Gg conformer.
139
Figure 19: Experimental and predicted Raman spectra of 2-cyanoethylamine: (A) liquid; (B)
simulated spectrum of mixture of Gg and Gt (ΔH = 75 cm-1) conformers at 25°C; (C) simulated Gt
conformer; (D) simulated Gg conformer.
140
141
Figure 20: Potential function of rotation around the C-N bond (H6-N1-C2-C3 dihedral angle) for the gauche C-C-C-N heavy atom structure
calculated using MP2(full) method with 6-311+G(2d,2p) basis set.
CHAPTER 7
CONFORMATIONAL AND STRUCTURAL STUDIES OF 2,2-DIFLUOROETHYLAMINE
FROM TEMPERATURE DEPENDENT INFRARED SPECTRA OF XENON SOLUTION
AND AB INITIO CALCULATIONS
Introduction
We recently initiated a study of the conformational stability and determined the structural
parameters of several organoamines. In these studies we have been utilizing variable temperature
infrared spectra of rare gas solution to obtain the enthalpy differences of the conformers. For the
structural values we have obtained adjusted r0 structural values by utilizing ab initio MP2(full)/6311+G(d,p) predicted parameters for the heavy atoms which are then adjusted to agree with the
microwave determined rotational constants. The parameters for the hydrogen atoms are taken from
the ab initio calculations. By this method we [36] obtained the conformational stability of
ethylamine which was determined to have the trans conformer more stable by 54  4 cm-1 (0.65 ±
0.05 kJ/mol) than the gauche form. This value was in agreement with earlier determined values of
107  70 cm-1(1.28 ± 0.84 kJ/mol) from electron diffraction study [35], 110  50 cm-1(1.32 ± 0.60
kJ/mol) from a microwave investigation [34], and 100  10 cm-1(1.20 ± 0.12 kJ/mol) from an
infrared study of ethylamine in an argon matrix [33]. However, several of the earlier studies [2932] reported the gauche conformer as the more stable form, and it is rather interesting that
MP2(full)/6-311+G(2d,2p) calculations predict [36] the gauche conformer to be the more stable
form by 66 cm-1 (0.79 kJ/mol). However, without the diffuse functions the trans form is predicted
to be the more stable conformer by 32 cm-1(0.38 kJ/mol).
These results clearly indicate that the enthalpy difference between the gauche and trans
conformers of the NH2 moiety will need to be determined experimentally rather than theoretical
predictions for organoprimary amines. Nevertheless, it is of scientific interest to determine the
142
effect of substituting one or m
-carbon. One of the earliest
molecules of this type studied was n-propylamine, CH3CH2CH2NH2 [67] for which the
conformational stability was determined. The most stable conformer was determined to be the Tt
form but this determination was for the solid. However the most stable conformer in the gas or the
liquid need not be the same as found in the solid so it would be of interest to obtain the relative
stabilities of the conformers of n-propylamine in the gaseous state. However, we have determined
the conformational stabilities of all five of the conformers of 2-fluoroethylamine [60] where the
fluorine atom might be expected to have a significant effect on the carbon-carbon distance.
For FCH2CH2NH2, the most stable conformer was the Gg′ form with the Gt conformer the
second most stable form with an enthalpy difference of 62  8 cm-1 (0.74  0.10 kJ/mol), where the
first indicator is the NCCF dihedral angle (G = gauche or T = trans) and the second one (g =
gauche or t = trans) is the relative position of the lone pair of electrons on the nitrogen atom with
-carbon atom [Gg′ is noted by the N-H eclipsing the fluoride]. The third, fourth,
and fifth conformers in relative energy are the Tg (262  26 cm-1), Tt (289  45 cm-1), and Gg (520
 50 cm-1), respectively, where it was not possible to determine whether the Tg or Tt were the
more stable one. Thus the fluorine atom appears to have an effect on the conformer stability of the
-carbon atom of
ethylamine with the determination of the conformational stabilities and structural parameters of
2,2-diflouroethylamine. For these conformational and structural studies of 2,2-difluoroethylamine,
F2CHCH2NH2, we have recorded variable temperature mid-infrared spectra of xenon solutions. To
support these experimental studies we have also obtained the harmonic force fields, infrared
intensities, Raman activities, depolarization ratios, and vibrational frequencies from MP2(full)/631G(d) ab initio calculations with full electron correlation. To obtain predictions on the
conformational stabilities we have carried out MP2(full) ab initio and density functional theory
143
(DFT) calculations by the B3LYP method by utilizing a variety of basis sets. The r 0 structural
parameters have been obtained by combining the MP2(full)/6-311+G(d,p) ab initio predicted
parameters with the previously reported rotational constants obtained from the microwave study
[85]. The results of these spectroscopic, structural and theoretical studies of 2,2-difluoroethylamine
are reported herein.
Experimental and Theoretical Methods
The sample of F2CHCH2NH2 was prepared by the reduction of 2,2-difluoroacetamide
(Oakwood Chemical Co.) with lithium aluminum hydride in dry dibutyl ether. The volatile
material was collected in a tube immersed in liquid nitrogen and first purified by trap-to-trap
distillation and then finally by using a low-pressure, low-temperature purification column.
The mid-infrared spectrum of the gas (Fig. 21A) and solid (Fig. 22A) were obtained from
3500 to 300 cm-1 on a Perkin-Elmer model 2000 Fourier transform spectrometer equipped with a
Ge/CsI beamsplitter and a DTGS detector.
Atmospheric water vapor was removed from the
spectrometer housing by purging with dry nitrogen. The theoretical resolution used to obtain the
spectrum of the gas was 0.5 cm-1 and 128 interferograms were added and transformed with a boxcar
truncation function. For the spectrum of the solid theoretical resolution of 2 cm-1 was used with 128
interferograms added and truncated. Multiple annealings were required to obtain satisfactory spectra
of the solid.
The mid-infrared spectra (3500 to 400 cm-1) of the sample dissolved in liquefied xenon
(Fig. 21B) at ten different temperatures (-55C to -100C) were recorded on a Bruker model IFS66 Fourier transform spectrometer equipped with a globar source, a Ge/KBr beamsplitter and a
DTGS detector. In all cases, 100 interferograms were collected at 1.0 cm-1 resolution, averaged
and transformed with a boxcar truncation function. For these studies, a specially designed cryostat
cell was used. It consists of a copper cell with a path length of 4 cm with wedged silicon windows
144
sealed to the cell with indium gaskets. The temperature was maintained with boiling liquid
nitrogen and monitored by two Pt thermoresistors. After cooling to the designated temperature, a
small amount of the sample was condensed into the cell and the system was then pressurized with
the noble gas, which condensed in the cell, allowing the compound to dissolve. A typical spectrum
is shown in Fig. 21B.
The Raman spectra (Fig. 23) were recorded on a Spex model 1403 spectrophotometer
equipped with a Spectra-Physics model 2017 argon ion laser operating on the 514.5 nm line. The
laser power used was 1.5 W with a spectral bandpass of 3 cm-1. The spectrum of the liquid was
recorded with the sample sealed in a Pyrex glass capillary. The measurements of the Raman
frequencies are expected to be accurate to  2 cm-1. All of the observed bands in the Raman spectra
of the liquid along with their proposed assignments and depolarization values are listed in Tables
36, 37 and 38, for the Tt, Gg and Gt conformers, respectively. The first indicator (G = Gauche or T
= Trans) is for the rotation around the C-C bond (dihedral angle NCCH) and the second one (g =
gauche or t = trans) is for the rotation around the N-C bond which indicates the relative position of
the lone pair of electrons on the nitrogen with respec
-carbon atom.
The LCAO-MO-SCF restricted Hartree-Fock calculations were performed with the
Gaussian-03 program [21] using Gaussian-type basis functions. The energy minima with respect
to nuclear coordinates were obtained by the simultaneous relaxation of all geometric parameters
using the gradient method of Pulay [22]. Several basis sets as well as the corresponding ones with
diffuse functions were employed with the Møller-Plesset perturbation method [23] to the second
order (MP2(full)) as well as with the density functional theory by the B3LYP method. The
predicted conformational energy differences are listed in Table 39.
In order to obtain a complete description of the molecular motions involved in the
fundamental modes of F2CHCH2NH2, a normal coordinate analysis has been carried out. The
145
force field in Cartesian coordinates was obtained with the Gaussian 03 program [21] at the
MP2(full) level with the 6-31G(d) basis set. The internal coordinates used to calculate the G and B
matrices are given in Table 40 with the atomic numbering shown in Fig. 24. By using the B matrix
[24], the force field in Cartesian coordinates was converted to a force field in internal coordinates.
Subsequently, scaling factors of 0.88 for CH stretches and CH2 deformation, 0.70 for NH2 bends
(excluding deformation) and 0.90 for all other modes except torsions and heavy atom bends were
applied, along with the geometric average of the scaling factors for the interaction force constants,
to obtain the fixed scaled force field and resultant wavenumbers. A set of symmetry coordinates
was used (Table 41) to determine the corresponding potential energy distributions (P.E.D.s). A
comparison between the observed and calculated wavenumbers, along with the calculated infrared
intensities, Raman activities, depolarization ratios and potential energy distributions for the transtrans, gauche-gauche and gauche-trans conformers are listed in Tables 36, 37, and 38, respectively.
The vibrational spectra were predicted from the MP2(full)/6-31G(d) calculations. The
predicted scaled frequencies were used together with a Lorentzian function to obtain the simulated
spectra. Infrared intensities were obtained based on the dipole moment derivatives with respect to
Cartesian coordinates. The derivatives were transformed with respect to normal coordinates by
(u/Qi) = j (u/Xj)Lij, where Qi is the ith normal coordinate, Xj is the jth Cartesian
displacement coordinate, and Lij is the transformation matrix between the Cartesian displacement
coordinates and the normal coordinates.
The infrared intensities were then calculated by
(N)/(3c2) [(x/Qi)2 + (y/Qi)2 + (z/Qi)2]. A comparison of the experimental spectra of the
solid and simulated infrared spectra of trans-trans F2CHCH2NH2 is shown in Fig. 22. The infrared
spectra of the xenon solution and the predicted infrared spectra for the pure trans-trans, gauchegauche, and gauche-trans conformers, as well as the mixture of the three conformers with relative
146
concentrations calculated for the equilibrium mixture at -65ºC by using the experimentally
determined enthalpy difference are shown in Fig. 25 (A-E), respectively. The predicted spectrum is
in good agreement with the experimental spectrum which shows the utility of the scaled predicted
frequencies and predicted intensities for supporting the vibrational assignment.
Additional support for the vibrational assignments was obtained from the simulated Raman
spectra. The evaluation of Raman activity by using the analytical gradient methods has been
developed [25-27], [37] and the activity Sj can be expressed as: Sj = gj(45αj2 + 7βj2), where gj is the
degeneracy of the vibrational mode j, αj is the derivative of the isotropic polarizability, and βj is the
anisotropic polarizability. To obtain the Raman scattering cross sections, the polarizabilities are
incorporated into Sj by multiplying Sj with (1-ρj)/ (1+ρj) where ρj is the depolarization ratio of the
jth normal mode. The Raman scattering cross sections and calculated wavenumbers obtained from
the Gaussian 03 program were used together with a Lorentzian function to obtain the simulated
Raman spectra. Comparison of experimental Raman spectra of the liquid and the predicted Raman
spectra for the pure trans-trans, gauche-gauche, and gauche-trans conformers as well as the
mixture of the three conformers with relative concentrations calculated for the equilibrium mixture
at 25°C by using the experimentally determined enthalpy difference are shown in Fig. 23(A-E).
The spectrum of the mixture should be compared to that of the Raman spectrum of the liquid at
room temperature. The predicted spectrum is in reasonable agreement with the experimental
spectrum which shows the utility of the predicted Raman spectra for the supporting vibrational
assignments.
Vibrational Assignment
To be able to determine the enthalpy differences between the conformers it is necessary to
assign the spectra for each of the three conformers which were previously identified from the
microwave spectra of 2,2-difluoroethylamine. To begin the assignments of these conformers ab
147
initio predictions from MP2(full)/6-31G(d) calculations were carried out to obtain the force
constants, frequencies, infrared intensities, band contours, Raman activities, and depolarization
values for all three of the previously identified conformers. Most of these predicted quantities are
listed in Tables 36, 37, and 38 for the Tt, Gg, and Gt conformers, respectively, and in addition to
the frequencies, the infrared intensities and band contours were very valuable to make the initial
assignments for the fundamentals for the spectral region from 400 to 1000 cm-1.
The frequencies for the fundamentals of the Gg and Gt are very similar since the difference
is only the position of the NH2 group so they come as pairs. Thus for these two conformers the
pairs for ν21, ν20, and ν19 are observed at 430, 439: 475, 482: 572, 576 cm-1, respectively, in the
xenon solution with the lower frequency for all three cases arising from the Gt form. These three
modes are CF bends which are extensively mixed but best described as the CF2 rock, CF2 twist,
and CF2 deformation for ν21, ν20, and ν19, respectively. It is interesting that there is only one
fundamental predicted in the gas in this region at 500 cm-1 for the Tt conformer and observed at
507 cm-1 as a clear C-type contour with a predicted 97% C-type. For this conformer this CF2 bend
is primarily a CF2 deformation (85% S13). The next higher frequency fundamental for the Tt
conformer is predicted at 711 cm-1 and observed at 718 cm-1 which has nearly 20% contribution
from four symmetry coordinates with the CF2 rock as the largest amount (24% S12). The three
quite strong bands at 743, 772 and 819 cm-1 are undoubtedly the NH2 rocks of the Gt, Gg and Tt
conformers, respectively, which are always observed at a significantly lower frequency than
predicted from MP2(full)/6-31G(d) calculations. In addition to their intensities the observed
infrared band contours are exactly as predicted so the assignments for these three NH2 has been
confidently made. However, the assignments for the CH2 rocks is relatively difficult to make since
their frequencies, intensities, and band contours are significantly dependent on their mixing with
the NH2 rock as well as possibly heavy atom motions. The predicted frequencies for this mode
148
(CH2 rock) for the Tt, Gg, and Gt forms are 831, 840 and 835 cm-1, respectively, and the pair of
bands at 856 and 847 cm-1 are consistent with expected relative intensities and contours for the Gg
and Gt conformers, respectively and are assigned accordingly. The best candidate for the CH 2 rock
for the Tt form is the band at 814 cm-1 or obscured by the NH2 rock at 824 cm-1. The correct
assignment of the fundamentals discussed so far is very important since it is desirable to use lower
frequency fundamentals for the enthalpy determination. Most of the remaining fundamentals in the
“fingerprint” region were assigned based on “group frequencies”, ab initio predicted frequencies,
band contours, intensities, and changes of intensities with the variable temperatures. These
assignments are all listed in Tables 36, 37 and 38 along with those for the CH and NH stretches.
These tables also include the frequencies observed from the Raman spectrum of the liquid which is
expected to have some significant frequency shifts due to dimerization as a result of hydrogen
bonding of the amino group.
Conformational Stability
To
determine
the
enthalpy
differences
among
the
three
conformers
of
2,2-
difluoroethylamine, the sample was dissolved in liquefied xenon and the mid-infrared spectra were
recorded as a function of temperature from -55 to -100°C. Relatively very small interactions are
expected to occur between xenon and the sample though the sample can associate with itself
forming a dimmer, trimer or higher order complex. However, due to the very small concentration
of sample (~10-4 molar) self association is greatly reduced. Therefore, only small frequency shifts
are anticipated for the xenon interactions when passing from the gas phase to the liquefied xenon
solutions. A significant advantage of this study is that the conformer bands are better resolved in
comparison with those in the infrared spectrum of the gas. From ab initio calculations, the dipole
moments of the two conformers are predicted to have similar values and the molecular sizes of the
149
two conformers are nearly the same, so the ΔH value obtained from the temperature dependent FTIR study is expected to be close to that for the gas [39, 40], [41-43].
Once confident assignments have been made for the fundamentals of the three most abundant
conformers the task was then to find a pair or pairs of bands from which the enthalpy
determination could be obtained. To minimize the effect of combination and overtone bands in the
enthalpy determination it is desirable to have the lowest frequency pair(s) that is possible to use for
the determination. The bands should also be sufficiently resolved as to determine their intensities.
The selection of the bands to use in the enthalpy determination was complicated due to the
presence of all three conformers nearly in equal amounts. The fundamentals at 508 and 819 cm -1
were selected for the Tt conformer band as they are free of interfering bands and in the lower
frequency region of the spectrum. For the Gg and Gt conformers the selection of the bands it was
more difficult since there are few bands resolved sufficiently to determine their intensities. The
fundamentals at 439, 772 cm-1 for the Gg form and 430, 743 cm-1 for the Gt conformer are
relatively free from interfering bands which allowed the determination of their band intensities to
be measured.
The intensities of the individual bands were measured as a function of temperature and their
ratios were determined (Fig. 26). By application of the van’t Hoff equation lnK = H/(RT) 
S/R, the enthalpy difference was determined from a plot of lnK versus 1/T, where H/R is the
slope of the line and K is substituted with the appropriate intensity ratios, i.e. Iconf-1 / Iconf-2, etc. It
was assumed that H, S, and α are not functions of temperature in the range studied.
These six bands, two for each conformer, were utilized for the determination of the enthalpy
difference by combining them to form 12 band pairs. By using the four band pairs for the transtrans and gauche-gauche conformers the enthalpy difference was determined with a value of 83  8
cm-1 (Table 42). Then the four band pairs for the Tt and Gt forms were used to obtain the enthalpy
150
difference of 235  11 cm-1. To determine the enthalpy difference for the Gg and Gt conformers
the four corresponding band pairs were used for which a value of 152  13 cm-1 was obtained.
These values are consistent with the corresponding predicted values from ab initio calculations
with large basis sets (Table 39). This error limit is derived from the statistical standard deviation of
one sigma of the measured intensity data taken as a single data set, but it does not take into account
small associations with the liquid xenon or the interference of overtones and combination bands in
near coincidence with the measured fundamentals. From these enthalpy differences abundance of
the Tt conformer present at ambient temperature is estimated to be 34 ± 1%, with the remainder
values of 45 ± 1% for the Gg conformer and 22 ± 1% for the Gt form.
Structural Parameters
In the microwave study [85], all but two of the structural parameters were assumed for the
three identified conformers. The two parameters that were not assumed but fitted instead were the
CCN angle and the FCCN dihedral angle, where the fluorine atom is involved in hydrogen
bonding. Since the assumed structural parameters were common for all three conformers, it is
likely that they are not very reliable from the microwave study. Therefore, we have again
determined the structural parameters for the three rotamers by utilizing the rotational constants
previously reported from the microwave study [85]. Only one isotopic species was studied for the
three conformers and, thus, only three rotational constants are available for each conformer.
We have found that good structural parameters for hydrocarbons and many substituted ones
can be determined by adjusting the structural parameters obtained from the ab initio MP2/6311+G(d,p) calculations to fit the rotational constants obtained from microwave experimental data
by using a computer program “A&M” (Ab initio and Microwave) developed [57] in our
laboratory. In order to reduce the number of independent variables, the structural parameters are
separated into sets according to their types where bond distances in the same set keep their relative
151
ratio, and bond angles and torsional angles in the same set keep their difference in degrees. This
assumption is based on the fact that errors from ab initio calculations are systematic. Additionally,
we have also shown that the differences in predicted distances and angles from the ab initio
calculations for different conformers of the same molecule can usually be used as one parameter
with the ab initio predicted differences except for some dihedral angles. Therefore, it should be
possible to obtain “adjusted r0” structural parameters for all three conformers of 2,2difluoroethylamine by utilizing the previously reported nine rotational constants from the earlier
microwave study [85] since there are only seven heavy atom parameters to be determined.
We [44] have shown that ab initio MP2(full)/6-311+G(d,p) calculations predict the r0
structural parameters for more than fifty carbon-hydrogen distances to at least 0.002 Å compared
to the experimentally determined values from isolated CH stretching frequencies which were
compared [45] to previously determined values from earlier microwave studies. Therefore, all of
the carbon-hydrogen distances can be taken from the MP2(full)/6-311+G(d,p) predicted values for
Tt, Gg and Gt conformers of 2,2-difluoroethylamine. The resulting adjusted r0 parameters are
listed in Table 40, where it is believed that the N-C and C-C distances should be accurate to ±
0.003 Å, the C-H and N-H distances should be accurate to ± 0.002 Å, and the angles should be
within ± 0.5.
The fit of the nine determined rotational constants (Table 43) by the structural parameters
for the Tt, Gg and Gt conformers is remarkably good with the biggest differences being 0.80, 0.69
and the smaller ones of 0.22, 0.20, 0.17 and the remaining less than 0.1 MHz. Therefore, it is
believed that the suggested uncertainties are realistic values and the determined structural
parameters are probably as accurate as can be obtained for the molecule in the gas phase by either
electron diffraction or microwave substituted methods.
152
Discussion
The vibrational assignment in the lower frequency region could be made relatively easy but
the assignments for the bands in the spectral region from 1000 to 1400 cm-1 was much more
difficult. It should be noted that the approximate description for ν13, ν14 and ν16 for the Gg and Gt
conformers differ significantly but ν16 for the Gt form has only 17% S16 but 31% S12. However, the
NH2 twist (23% S12) is better placed at 1197 cm-1 where the largest contribution is from the CH2
rock (32% S17). Therefore, for many cases the approximate descriptions are more for
“bookkeeping” than conveying the major molecular motions. This is particularly needed when
seven of the fundamentals have contributions of 10% or more from four symmetry coordinates and
three that have five symmetry coordinates contributing 10% or more to the P. E. D. Nevertheless,
it should be noted that the MP2(full)6-31G(d) predicted frequencies with three scaling factors
which had average differences of 8.7 cm-1 and 9.9 cm-1 for the modes with A' and A" symmetry for
the Tt form excluding the NH2 rock which represents percentage error of 0.6% for both species.
The predicted Raman spectrum by using the isolated molecule predictions gives a
reasonable simulated spectrum (Fig. 27) for the liquid where there must be a significant self
association of the molecules. We also predicted and measured the depolarization of the three
deformations are depolarized as predicted with similar results for the bands in the 1300 cm-1
region. The band at 1199 cm-1 has a predicted depolarization value of 0.57 which is consistent with
the observed values. Most of the bands below this frequency are strongly polarized but with the
presence of three conformers it appears that the predicted depolarization values are in satisfactory
agreement with the observed values.
To determine the conformational stabilities we initially used the bands at 508 (Tt), 439
(Gg) and 430 (Gt) which are mainly heavy atom motions which should not be significantly
affected by molecular association. As can be seen from the data in Table 42 the enthalpy difference
153
between the Tt and Gg form is 100 ± 13 cm-1 (1.20 ± 0.16 kJ/mol). Normally, we would not use
the fundamentals associated with NH2 group since they can be significantly affected by molecular
association as the temperature is decreased. However, there did not appear to be a significant
decrease in the association as the temperature was lowered from -60 to -100°C so we also then
used the relatively strong band from the NH2 rocks which were well separated and had excellent
signal-to-noise ratios. Therefore, by using four pairs for the determination of this enthalpy
difference and the data as a single set the enthalpy difference was determined to be 83 ± 8 cm -1
(0.99 ± 0.10 kJ/mol). Considering the uncertainties the two values agree but the value obtained
from four pairs is expected to be a better value since the problem of overtone and combination
bands contributing to the intensities of either fundamental should average out. Therefore, the
reported enthalpy differences should be appropriate for comparison to ab initio predicted values or
enthalpy differences obtained for other substituted organoamines. The ab initio predicted energy
difference from the MP2(full)/aug-cc-pVTZ (345 basis sets) calculation gave a value of 72 cm-1
(0.86 kJ/mol) with the Tt form more stable than the Gg form. Similarly the Gt conformer energy
difference between it and the Tt form was predicted to be 209 cm-1 (2.50 kJ/mol) which is in
excellent agreement with the experimental enthalpy difference of 235 ± 11 cm-1 (2.81 ± 0.13).
However, the density functional theory calculations by the B3LYP method gave a similar value of
93 cm-1 (1.11 kJ/mol) for the energy difference between the Tt and Gg forms but the predicted
energy value for the Tt and Gg was a much smaller value of 136 cm-1 (1.63 kJ/mol). Nevertheless,
the predicted values must be considered to be in excellent agreement with the experimental values.
In the earlier microwave study the quadratic centrifugal distortion constants were
determined for all three of the conformers and we have predicted the values from the predicted
force constants from the ab initio MP2(full) calculations with two different basis sets. Similar
calculations have also been carried out by density functional theory calculations by the B3LYP
154
method with the same basis sets. For the Tt conformer the predicted values from the MP2
calculations do not differ significantly between the two basis sets. This is also true of the
predictions from the B3LYP except for the δK constant which is predicted more than one-half the
value with the larger basis set (Table 44). In general, the predicted values are in good agreement
with the experimental determined values except for δK from the B3LYP calculations. However, for
the Gg conformer there is a significantly value for ΔK by both calculations where the value is quite
large compared to the experimentally determined value. A similar problem also is found for the Gt
form where the predicted values with the smaller basis set are more than twice the experimental
values. Even with the large basis set the predicted values are ~ 20% and ~ 30% larger than the
predicted values from the MP2 and B3LYP calculations, respectively. At this time the reason for
these unusually poor values for only the ΔK constant is not understood.
One of the major goals of this research was the determination of the r0 structural parameters
for the three conformers which has been done for the six heavy atom parameters. In the microwave
study [85] only two parameters for each conformer were determined which were the N3C2C1 and
4CCN
and the heavy atom distances were assumed to have the same values for the three
conformers. The values obtained earlier for the N3C2C1 angle with an uncertainty of 1.0 degree are
in excellent agreement with the corresponding parameters obtained in this study. However, the
determined dihedral angle does not agree with the values obtained for this angle in the current
study, where, for the Gg form the value of 60 ± 2° is reported in the microwave work [85],
whereas, the value from this study is 64.5 ± 0.5°. For the Gt conformer the value of 61.2 ± 0.5° is
in agreement with the value given earlier [85] of 60 ± 2° but the value of exactly 60° for this
dihedral angle for the Tt form is not in agreement with the value of 58.6 ± 0.5° from our results.
The assumption that the heavy atom distances are the same [85] is a good one for the Tt and Gt
conformers where the differences are 0.001 Å for C1-F4 and C2-N3, and 0.002 Å for the C1-C2 and
155
C1-F6 from the current study. However, for the Gg conformer the C1-C2 is shorter by 0.005 and
0.007 Å than the Gt and Tt forms, respectively but for the C2-N3 distance it is longer by 0.005 and
0.006 Å for these conformers, respectively. However, the major differences between the values
reported earlier and those obtained herein is with the C-F distances which were estimated to be
shorter by an average distance of 0.017 Å and for the C-N and C-C bonds where the average
distances are too long by 0.015 and 0.019 Å, respectively. There is also a significant difference
between the assumed FCF angle and the value obtained in this study which is nearly 2° smaller.
With these exceptions many of the other estimated parameters are in acceptable agreement with
those obtained in the current study.
It is also interesting to compare the heavy atom parameters of 2-fluoroethylamine [49] and
2,2,2-trifluoroethylamine [49] with the corresponding ones obtained in the current study of 2,2difluoroethylamine. It should be noted that the C-C bond is smaller by an average of 0.003 Å for
-carbon which is within the experimental error of ± 0.003 Å for
the heavy atom bond distances in each r0 determination. When one compares the C-N bond
distance, the change is considerable with the addition of the second fluorine atom the distance
decreases by an average of 0.008 Å and with the addition of a further fluorine atom decreases by
0.005 Å. However, the parameter which shows the greatest change due to fluorine substitution on
-carbon is the C-F bond distance(s) which drastically reduces by ~ 0.030 Å each time a fluorine
atom is added. Thus for 2-fluoroethylamine the average C-F distance is 1.399(3) Å, for 2,2difluoroethylamine 1.368(3) Å and for 2,2,2-trifluoroethylamine 1.342(3) Å. Such large changes
have a significant effect on some of the other parameters.
It is well known that intramolecular hydrogen bonding can be a stabilizing factor for the
orientation of molecular coformers. This is important in the case of the fluoride substituted
ethylamine where the fluorine atom(s) and the amine hydrogens have the possibility to form an
156
intramolecular hydrogen bond. This is observed with 2-fluoroethylamine [49] where the most
-carbon atom
and has the least distance separating the hydrogen and fluorine atoms of any of the possible
conformers. The second stable conformer (Gt) likewise has the hydrogen and fluorine in the
eclipsed positions but has a greater distance between them. The energy distances increase greatly
in the conformers where no hydrogen bonding is possible. In the current study of 2,2difluoroethylamine there are two fluorine atoms so the most stable conformer (Tt) allows two
intramolecular hydrogen bonds. The second most stable conformer (Gg) only allows one hydrogen
bond but has a shorter distance between the hydrogen and fluorine atoms. The third most stable
conformer (Gt) then allows for a hydrogen bond but at an increased distance. In the case of 2,2,2trifluoroethylamine the three fluorines allow for two intramolecular hydrogen bonds like in the
diflouro- species so the most stable conformer is the Trans conformer where two hydrogen bonds
may be formed. Clearly intramolecular hydrogen bonding plays a significant role in the
conformational stabilities of these molecules and it would be of interest to see how other halogen
substituents might affect this stability. Also this is a very small sample so it would be of interest to
study other substituents on the β-carbon to evaluate other factors that affect the conformer stability
of the other organoamines.
157
Table 36: Observed and calculateda frequencies for the Trans-trans conformer of 2,2-difluoroethylamine.
Fundamentals
ab
initio
fixed
scaleb
IR
int.
Raman
act.
dp
ratio
IR
gas
Xe soln solid
c
Raman
liquid
P.E.D.d
Band Contours
A
B
158
3525 3344
1.5
72.8
0.11 3356
3348 3373
3336 100S1
10
  NH2 symmetric stretch
3160 2964
62.4
105.6
0.17 2969
2963 3001
2968 96S2
25

 C-H stretch
CH
symmetric
stretch
3126
2932
10.6
72.8
0.18
2906
2914
2954
2914
96S
13


2
3
1737 1611
36.9
7.2
0.69 1624
1621 1621
1619 84S4,14S11
31

 NH2 deformation
CH
deformation
1538
1446
5.9
9.5
0.75
1447
1443
1440
1448
99S
36


2
5
1454 1384
37.0
5.0
0.75 1375
1371 1377
1371 55S6,25S7,10S10
90

 H-C-C in-plane bend
1437 1365
4.5
2.7
0.67 1361
1360 1347
1348 81S7,12S10
12

 CH2 wag
1207 1143
63.3
6.2
0.47 1155
1149 1144
1148 36S8,25S9,14S10
19

 CF2 symmetric stretch
1161 1101
12.2
1.6
0.74 1095
1095 1095
1100 40S9,20S8,16S12,12S14
5

 C-N stretch
C-C
stretch
908
881
60.7
2.9
0.18
875
873
872
870
38S
,29S
,12S
,10S
100


10
11
8
6
984
857
181.7
6.6
0.16
824
819
834
853 25S11,33S9,12S10,10S6
90

 NH2 rock
749
711
25.2
4.3
0.46
718
716
727
727 24S12,21S11,20S8,16S14
26

 CF2 rock
504
500
8.7
1.7
0.75
507
508
500
511 85S13,10S8
3

 CF2 deformation
266
261
17.2
0.2
0.34
266
269
243 58S14,20S12,14S6
67

 C-C-N bend
NH
antisymmetric
stretch
3625
3439
4.6
42.5
0.75
3415
3415
3389
3389
100S
100
 
2
15
3187 2988
14.0
58.3
0.75 2989 ~2990 3022
3000 100S16
- 100

 CH2 antisymmetric stretch
1466 1387
31.8
0.6
0.75 1380
1380 1385
1384 60S17,21S22,16S18
- 100

 H-C-C out-of-plane bend
1423 1302
0.1
17.5
0.75 1307
1303 1318
1306 56S18,20S19,10S21
- 100

 CH2 twist
1246 1151
57.7
1.3
0.75 1181
1182 1189
1187 23S19,28S21,25S20,17S22
- 100

 NH2 twist
1105 1021
81.5
2.1
0.75 1035
1036 1030
1029 58S20,25S19,18S18
- 100

 CF2 antisymmetric stretch
901
831
13.0
2.2
0.75
814
814
830
825 50S21,31S19,14S20
- 100

 CH2 rock
417
397
8.1
0.8
0.75
398
442
427 39S22,28S17,17S23
- 100

 CF2 twist
NH
torsion
301
263
64.4
1.8
0.75
271
284
255
82S
- 100


2
23
140
140
0.9
0.1
0.75
139 92S24
- 100

 torsion
a
MP2(full)/6-31G(d) ab initio calculations, scaled frequencies, infrared intensities (km/mol), Raman activities (Å 4/amu), depolarization ratios and potential energy
distributions (P.E.D.s)
b
Scaled ab initio calculations with factors of 0.88 for CH stretches and CH 2 deformation, 0.70 for NH2 bends (excluding deformation) and 0.90 for all other modes
except torsions and heavy atom bends
c
Frequencies may be significantly shifted due to hydrogen bonding.
d
Values less than 10% are omitted.
C
90
75
87
69
64
10
88
81
95
10
74
97
33
-
Table 37: Observed and calculateda frequencies for the Gauche-gauche conformer of 2,2-difluoroethylamine.
Fundamental
ab
initio
fixed
scaleb
IR
int.
Raman
act.
dp
ratio
IR
gas
Xe soln
Raman
liquid
P.E.D.c
Band Contours
A
B
C
159
3642
3455
5.2 56.7
0.72 3432
3424
3413
100S1
19
79
2
 NH2 antisymmetric stretch
NH
symmetric
stretch
3541
3359
1.8
87.7
0.12
3360
3354
3346
100S
23
35
42

2
2
3190
2993
47.2 63.2
0.33 2994 ~2990
3000
86S3,11S4
2
98
 C-H stretch
3173
2976
8.4 64.2
0.40 2986
2980
2988
65S4,22S5,13S3
- 100
 CH2 antisymmetric stretch
3068
2878
39.3 99.8
0.18 2881
2879
2872
76S5,24S4
21
12
66
 CH2 symmetric stretch
NH
deformation
1723
1597
42.2
7.5
0.70
1608
1606
1602
83S
,15S
46
5
49

2
6
18
1562
1469
4.4 12.1
0.73 1458
1454
1460
99S7
79
17
4
 CH2 deformation
1514
1429
24.2
3.5
0.37 1424
1422
1431
25S8,37S10,16S13,10S21
60
38
2
 H-C-C in-plane bend
1458
1385
28.8
4.8
0.74 1370
1368
1361
54S9,20S20
4
96
 H-C-C out-of-plane bend
1408
1318
16.4
4.9
0.75 1335
1330
1333
30S10,23S8,11S9,11S20,10S21
99
1
 CH2 wag
CH
twist
1346
1259
13.2
8.1
0.61
1279
1276
1277
64S
,11S
63
2
35

2
11
21
1242
1148
37.4
3.2
0.73 1150
1149
1148
20S12,31S17,16S16,10S20
28
62
10
 NH2 twist
1199
1138
58.6
5.0
0.73 1134
1132
1133
35S13,15S14,14S8
13
78
9
 C-C stretch
1160
1104 113.1
3.7
0.32 1095
1095
1100
40S14,30S16
79
14
7
 CF2 symmetric stretch
1128
1072
25.5
6.3
0.11 1079
1073
1062
71S15
73
26
1
 C-N stretch
CF
antisymmetric
stretch
1099
1005
49.2
4.9
0.69
1005
1001
1006
28S
,38S
,11S
98
2

2
16
12
11
917
840
17.1
3.3
0.75
856
856
860
29S17,21S13,21S12,14S14
6
32
62
 CH2 rock
897
791 179.6
3.0
0.22
772
772
779
75S18,13S6
19
14
67
 NH2 rock
583
565
5.7
2.0
0.35
578
576
576
40S19,18S14,16S21
3
2
95
 CF2 deformation
494
483
6.7
0.8
0.69
485
482
484
20S20,25S22,13S21,12S8
78
17
5
 CF2 twist
CF
rock
445
439
4.9
1.2
0.46
439
439
439
15S
,45S
,13S
67
13
21

2
21
19
13
305
273
36.8
1.2
0.75
280
266
27S22,39S23,16S20,12S9
58
42
 C-C-N bend
246
231
32.6
1.1
0.75
- 56S23,19S22,11S20
34
64
2
 NH2 torsion
150
149
5.7
0.1
0.75
- 93S24
47
45
9
 torsion
a
4
MP2(full)/6-31G(d) ab initio calculations, scaled frequencies, infrared intensities (km/mol), Raman activities (Å /amu), depolarization ratios and potential energy
distributions (P.E.D.s)
b
Scaled ab initio calculations with factors of 0.88 for CH stretches and CH 2 deformation, 0.70 for NH2 bends (excluding deformation) and 0.90 for all other modes except
torsions and heavy atom bends
c
Values less than 10% are omitted.
Table 38: Observed and calculateda frequencies for the Gauche-trans conformer of 2,2-difluoroethylamine.
Fundamental
ab
initio
fixed
scaleb
IR
int.
Raman
act.
dp
ratio
IR
gas
Xe soln.
Raman
liquid
P.E.D.c
Band Contours
A
B
160
3631
3445
3.8
44.6
0.75 3421
3421
3402
100S1
1
70
 NH2 antisymmetric stretch
3528
3346
1.2
77.8
0.12 3358
3350
3354
100S2
- 100
 NH2 symmetric stretch
CH
antisymmetric
stretch
3196
2998
18.5
47.5
0.72
2992
~2990
3000
97S
1
26

2
3
3146
2951
49.1
67.6
0.36 2956
2953
2943
96S4
2
 C-H stretch
3132
2938
12.4 100.9
0.08 2926
2932
2928
98S5
21
71
 CH2 symmetric stretch
1730
1603
33.2
7.2
0.69 1632
1630
1631
83S6,15S18
63
 NH2 deformation
CH
deformation
1540
1447
3.7
10.4
0.75
1447
1443
1448
99S
5
58

2
7
1483
1409
28.2
3.3
0.73 1398
1400
1403
38S8,26S10,14S21,11S16
86
14
 H-C-C in-plane bend
1458
1386
30.1
4.5
0.75 1377
1375
1372
57S9,20S20,10S10
2
98
 H-C-C out-of-plane bend
1378
1310
11.4
11.3
0.75 1317
1309
1315
45S10,14S11,12S20,11S8
99
1
 CH2 wag
1422
1297
3.2
2.3
0.73 1307
1302
1298
52S11,17S12,11S10,10S21
80
4
 CH2 twist
NH
twist
1255
1162
13.4
1.3
0.57
1197
1193
1199
23S
,32S
,17S
,13S
26
6

2
12
17
21
16
1183
1126
67.5
5.7
0.60 1126
1124
1122
36S13,24S16,12S14
11
71
 CF2 symmetric stretch
1165
1102
49.8
2.4
0.48 1095
1095
1100
31S14,30S15,12S23,10S20,10S13
55
43
 CF2 antisymmetric stretch
1152
1095 107.8
3.4
0.57 1087
1087
1090
50S15,32S14,13S13
65
31
 C-N stretch
1053
966
27.4
1.4
0.42
980
978
980
17S16,31S12,15S11,11S13,10S14
26
65
 C-C stretch
CH
rock
864
835
72.6
2.0
0.21
847
845
840
31S
,23S
,17S
79
19

2
17
12
18
930
782 163.1
5.7
0.22
744
743
748
54S18,10S17,10S6,10S16
93
 NH2 rock
573
557
3.4
2.0
0.36
576
572
572
46S19,19S13,12S21
44
2
 CF2 deformation
CF
twist
502
487
11.5
1.9
0.67
475
475
477
19S
,23S
,13S
,12S
,10S
13
73

2
20
23
21
8
17
436
429
12.5
2.1
0.45
431
430
435
17S21,40S19,15S16,10S8
86
13
 CF2 rock
328
282
62.5
2.4
0.75
289
276
80S22,10S23
4
21
 NH2 torsion
246
242
15.6
0.1
0.68
39S23,24S20,21S9,13S22
11
73
 C-C-N bend
143
143
1.2
0.1
0.75
139
94S24
96
4
 torsion
a
MP2(full)/6-31G(d) ab initio calculations, scaled frequencies, infrared intensities (km/mol), Raman activities (Å 4/amu), depolarization ratios and potential
energy distributions (P.E.D.s)
b
Scaled ab initio calculations with factors of 0.88 for CH stretches and CH2 deformation, 0.70 for NH2 bends (excluding deformation) and 0.90 for all other modes
except torsions and heavy atom bends
c
Values less than 10% are omitted.
C
29
73
98
8
37
37
16
68
18
2
4
9
2
7
54
14
1
75
16
-
Table 39: Calculated energies in (H) and energy differences (cm-1) for the five conformers of 2,2
difluoroethylamine.
Energya, E
Energy Differencesb, ∆
Method/Basis Set
TransGaucheGaucheTransGauchetrans
gauche
trans
gauche
gauche′
MP2(full)/6-31G(d)
0.743275
229
347
1074
1203
MP2(full)/6-31+G(d)
0.776801
58
307
942
1251
MP2(full)/6-311G(d,p)
1.037072
175
231
1001
1203
MP2(full)/6-311+G(d,p)
1.055675
96
261
855
1077
MP2(full)/6-311G(2d,2p)
1.130822
131
237
908
1028
MP2(full)/6-311+G(2d,2p)
1.146119
63
213
828
1026
MP2(full)/6-311G(2df,2pd)
1.232509
101
217
862
984
MP2(full)/6-311+G(2df,2pd)
1.247411
72
205
817
868
MP2(full)/6-311++G(2df,2pd)
1.247731
63
202
808
938
MP2(full)/aug-cc-pVTZ
1.266963
72
209
813
988
Average MP2(full)
93  38
222  20
862  65
1014  99
B3LYP/6-31G(d)
1.643218
259
220
1074
1285c
B3LYP/6-31+G(d)
1.671849
77
166
937
1094c
B3LYP/6-311G(d,p)
1.753232
183
173
1048
1209c
B3LYP/6-311+G(d,p)
1.767451
103
178
897
1039c
B3LYP/6-311G(2d,2p)
1.764692
179
175
962
1154c
B3LYP/6-311+G(2d,2p)
1.777312
99
152
860
991c
B3LYP/6-311G(2df,2pd)
1.772253
139
156
916
1092c
B3LYP/6-311+G(2df,2pd)
1.785325
90
142
839
957c
B3LYP/aug-cc-pVTZ
1.797705
90
137
818
927c
B3LYP/aug-cc-pVQZ
1.822513
93
136
821
924c
Average B3LYP
131  38
164  25
917  90
1067  123
a
Energy of Trans-trans conformer is given as –(E+332) H.
b
Differences are relative to Trans-trans form.
c
The dihedral angle τH9N3C2C1 was fixed to 63° in order to calculate B3LYP energies of this conformer.
161
Table 40: Structural parameters (Å and degree), rotational constants (MHz), and dipole moment
(Debye) for Trans-trans, Gauche-gauche and Gauche-trans forms of 2,2-difluoroethylamine .
MP2(full)/6-311+G(d,p)
Trans- Gauche- Gauchetrans
gauche
trans
r C1-C2
R1 1.513
1.506
1.512
r C2-N3
R2 1.451
1.458
1.453
r C1-F4
R3 1.369
1.370
1.369
r C1-F6
R4 1.369
1.365
1.366
r C1-H5
r1 1.092
1.090
1.093
r C2-H7
r2 1.094
1.098
1.093
r C2-H8
r3 1.094
1.092
1.093
r N3-H9
r4 1.014
1.014
1.014
r N3-H10
r5 1.014
1.013
1.014
114.7
θ 115.1
108.6
 N3C2C1
109.6
ρ1 109.9
109.2
 F4C1C2
109.9
ρ2 109.9
109.9
 F6C1C2
107.8
107.4
 F4C1H5
1 107.9
108.8
108.2
 F6C1H5
2 107.9
α
106.8
107.2
107.3
 F4C1F6
μ
113.9
113.5
114.0
 H5C1C2
π1 107.6
107.5
107.2
 H7C2C1
π2 107.6
107.6
108.0
 H8C2C1
108.3
107.7
 H7C2H8
 107.5
ε1 110.4
110.3
110.7
 H9N3C2
ε2 110.4
110.6
111.4
 H10N3C2
γ 107.3
107.5
107.9
 H10N3H9
τ1
58.6
64.0
63.1
 F4CCN
τ2 -58.6
-178.5
-179.4
 F6CCN
τ3 -59.2
-72.1
-63.1
 H9NCC
τ4
59.2
169.0
60.0
 H10NCC
A
7106.51 8985.18
8888.13
B
4201.62 3714.28
3650.35
C
3444.26 2836.48
2808.35
0.060
2.382
1.194
|a|
0.000
1.231
0.315
|b|
1.660
0.357
1.774
|c|
a
Ref. [85]; asterisk indicates assumed values.
Structural
Parameters
Transtrans
1.530*
1.469*
1.350*
1.350*
1.093*
1.093*
1.093*
1.017*
1.017*
114.0(10)
110.0*
110.0*
109.2*
109.20*
108.94*
109.48*
109.48*
109.48*
109.48*
109.48*
109.48*
109.48*
60.0(0)
60.0*
7125.17
4185.89
3441.17
0.090(20)
0.0
1.427(28)
162
MWa
Gauchegauche
1.530*
1.469*
1.350*
1.350*
1.093*
1.093*
1.093*
1.017*
1.017*
109.0(10)
110.0*
110.0*
109.2*
109.20*
108.94*
109.48*
109.48*
109.48*
109.48*
109.48*
109.48*
109.48*
60.0(20)
0.0*
8987.27
3699.88
2828.92
2.042(15)
1.101(29)
~0.0
Gauchetrans
1.530*
1.469*
1.350*
1.350*
1.093*
1.093*
1.093*
1.017*
1.017*
114.0(10)
110.0*
110.0*
109.2*
109.20*
108.94*
109.48*
109.48*
109.48*
109.48*
109.48*
109.48*
109.48*
60.0(20)
60.0*
8900.80
3646.84
2807.57
1.073(14)
0.22(23)
1.483(26)
Adjusted r0 parameters
TransGauche- Gauchetrans
gauche
trans
1.514(3) 1.507(3) 1.512(3)
1.452(3) 1.458(3) 1.453(3)
1.368(3) 1.370(3) 1.369(3)
1.368(3) 1.365(3) 1.366(3)
1.092(2) 1.090(2) 1.093(2)
1.094(2) 1.098(2) 1.093(2)
1.094(2) 1.092(2) 1.093(2)
1.014(2) 1.014(2) 1.014(2)
1.014(2) 1.013(2) 1.014(2)
115.0(5) 108.8(5) 114.8(5)
110.3(5) 109.3(5) 109.7(5)
110.3(5) 110.0(5) 109.9(5)
107.7(5) 108.1(5) 107.4(5)
107.7(5) 108.5(5) 108.2(5)
106.5(5) 107.0(5) 107.1(5)
113.9(5) 113.5(5) 114.0(5)
107.6(5) 107.5(5) 107.2(5)
107.6(5) 107.6(5) 108.0(5)
107.5(5) 108.3(5) 107.7(5)
110.3(5) 110.3(5) 110.7(5)
110.3(5) 110.6(5) 111.4(5)
107.3(5) 107.5(5) 107.9(5)
61.2(5)
58.6(5)
64.5(5)
-58.6(5)
-178.1(5) 178.8(5)
-59.2(5) -72.13(5) -56.6(5)
63.5(5)
59.2(5)
69.0(5)
7125.34
8987.29 8900.72
4186.69
3700.08 3646.80
3440.48
2829.14 2807.65
Table 41: Symmetry coordinates for 2,2–difluoroethylamine.
Symmetry Coordinatea
Description
















NH2 symmetric stretch
C–H stretch
CH2 symmetric stretch
NH2 deformation
CH2 deformation
H–C–C in-plane bend
CH2 wag
CF2 symmetric stretch
C–N stretch
C–C stretch
NH2 rock
CF2 rock
CF2 deformation
C–C–N bend
NH2 antisymmetric stretch
CH2 antisymmetric stretch
H–C–C out-of-plane bend
CH2 twist
NH2 twist
CF2 antisymmetric stretch
CH2 rock
CF2 twist
NH2 torsion
torsion
a
Not normalized.
S1
S2
S3
S4
S5
S6
S7
S8
S9
S10
S11
S12
S13
S14
S15
S16
S17
S18
S19
S20
S21
S22
S23
S24
163
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
r4 + r5
r1
r2 + r3
γ
4 – θ – σ1 – σ2 – π1– π2
4μ – α – ρ1 – ρ2 – δ1 – δ2
σ1 + σ2 – π1– π2
R3 + R4
R2
R1
ε1 + ε 2
ρ1 + ρ3 – δ1 – δ2
4α – μ – ρ1 – ρ2 – δ3 – δ1
4θ –  – σ1 – σ2 – π1– π2
r4 – r5
r2 – r3
ρ1 – ρ2 + δ1 – δ2
σ 1 – σ 2 – π1 + π 2
ε1 – ε2
R3 – R4
σ1 – σ2 + π1– π2
ρ1 – ρ3 – δ1 + δ2
τ3 + τ4
τ1 + τ2
Table 42: Temperature and intensity ratios of the Trans-trans, Gauche-gauche, and Gauche-trans bands of 2,2-difluoroethylamine.
T(C) 1/T (10-3 K-1)
Liquid
xenon
-60
-65
-70
-75
-80
-85
-90
-95
-100
4.692
4.804
4.923
5.047
5.177
5.315
4.460
5.613
5.775
Tt → Gg
Tt → Gt
I508 / I439
I508 / I772
I819 / I439
I819 / I772
I508 / I430
I508 / I743
I819 / I430
6.499
7.029
7.080
7.000
7.238
7.258
7.581
7.596
7.879
1.463
1.477
1.500
1.490
1.462
1.455
1.542
1.565
1.582
6.174
6.596
6.772
6.808
7.143
7.348
7.331
7.440
7.582
1.390
1.386
1.435
1.449
1.442
1.473
1.492
1.532
1.522
6.094
6.496
6.270
6.934
6.508
6.750
7.331
7.834
8.424
2.449
2.600
2.664
2.808
2.912
2.963
3.273
3.464
3.786
5.789
6.096
5.997
6.744
6.422
6.834
7.089
7.672
8.106
164
H (cm-1)
100  13
46  14
120  15
65  6
186  28 264  14 206  20
-1 a
H (cm )
83  8
235  11
a
Average value H and statistical uncertainty (1σ) obtained by utilizing all of the data as a single set.
Gg → Gt
I819 / I743 I439 / I430
2.327
2.440
2.548
2.731
2.874
3.000
3.165
3.393
3.643
0.938
0.924
0.886
0.991
0.899
0.930
0.967
1.031
1.069
284  5
86  30
I439 / I743
I772 / I430
I772 / I743
3.768
3.699
3.763
4.011
4.023
4.083
4.318
4.560
4.805
4.164
4.397
4.180
4.654
4.453
4.641
4.753
5.007
5.324
1.673
1.760
1.776
1.885
1.992
2.037
2.122
2.214
2.393
161  16 141  21
152  13
219  9
Table 43: Comparison of rotational constants obtained from ab initio MP2(full)/6-311+G(d,p)
predictions, experimental valuesa from microwave spectra, and adjusted r0 structural parameters
for 2,2-difluoroethylamine.
Conformer
Trans-trans
Gauche-gauche
Gauche-trans
a
Rotational
constants
MP2(full)/
6-311+G(d,p)
Experimentala
Adjusted r0
||
A
7106.51
7125.17
7125.34
0.17
B
4201.62
4185.89
4186.69
0.80
C
3444.26
3441.17
3440.48
0.69
A
8985.18
8987.27
8987.29
0.02
B
3714.28
3699.88
3700.08
0.20
C
2836.48
2828.92
2829.14
0.22
A
8888.13
8900.80
8900.72
0.09
B
3650.35
3646.84
3646.80
0.04
C
2808.35
2807.57
2807.65
0.08
Ref. [85].
165
Table 44: Quadratic centrifugal distortion constants (kHz) for conformers of 2,2difluoroethylamine.
Trans-trans
MP2(full)/
6-31G(d)
MP2(full)/6311+G(d,p)
∆J
1.9
1.9
∆J
7.03
7.19
K
∆K
-5.94
-5.96
δJ
0.2599
0.2541
δK
1.54
1.40
Gauche-gauche
Exp.
a
1.0(5)
0.799
5
8.34(10)
6.122
6.59(36)
1.562
0.2420(5
8)
1.53(15)
Trans-gauche
MP2(full)/
6-31G(d)
MP2(full)/6311+G(d,p)
∆J
0.7661
0.7993
∆J
5.633
6.175
1.7336
1.0622
K
∆K
δJ
0.17023
0.17916
δK
3.359
3.612
a
MP2(ful
l)/
631G(d)
Exp.a
0.8440(10)
6.576(11)
0.8284(37)
0.19209(68
)
3.979(15)
Ref [85].
166
0.182
8
3.709
MP2(full)/
6311+G(d,p
)
Exp.a
0.8318
0.9455(4
9)
6.623
7.294(15
)
1.016
0.838(14
)
0.1915
4.002
0.2089(1
6)
4.793(61
)
Figure 21: Mid-infrared spectra of 2,2-difluoroethylamine (A) gas; (B) Xe solution at -65°C.
167
Figure 22: Comparison of experimental and predicted infrared spectra of 2,2-difluoroethylamine:
(A) infrared spectra of the solid; (B) simulated spectrum of Tt conformer.
168
Figure 23: Comparison of experimental and predicted Raman spectra of 2,2-difluoroethylamine:
(A) observed spectrum of the liquid; (B) simulated spectrum of a mixture of Tt, Gg (ΔH = 83 cm 1
), Gt (ΔH = 235 cm-1) conformers at 25°C; (C) simulated spectrum of Gt conformer; (D)
simulated spectrum of Gg conformer; (E) simulated spectrum of Tt conformer.
169
Figure 24: Possible stable conformers of 2,2-difluoroethylamine.
170
Figure 25: Comparison of experimental and predicted infrared spectra of 2,2-difluoroethylamine:
(A) observed spectrum of Xe solution at -65°C; (B) simulated spectrum of mixture of Tt, Gg (ΔH
= 83 cm-1), Gt (ΔH = 235 cm-1) conformers at -65°C; (C) simulated spectrum of Gt conformer; (D)
simulated spectrum of Gg conformer; (E) simulated spectrum of Tt conformer.
171
Figure 26: Temperature (-60 to -100°C) dependent mid-infrared spectrum in the 725-835 cm-1
region of 2,2-difluoroethylamine dissolved in liquid xenon.
172
Figure 27: Raman spectra (650-1700 cm-1) showing the polarized and the depolarized bands for 2,2difluoroethylamine.
173
CHAPTER 8
CONFORMATIONAL AND STRUCTURAL STUDIES OF 2,2,2 TRIFLUOROETHYLAMINE
FROM TEMPERATURE DEPENDENT RAMAN SPECTRA OF XENON SOLUTIONS AND
AB INITIO CALCULATIONS
Introduction
There are a very large number of amines which are important biochemical molecules and
therefore have generated a very large amount of scientific interest. Of particular interest has been
the energy or enthalpy difference for molecular amines which have two or more conformers
present at ambient temperature. Many different techniques have been used but vibrational
spectroscopy was initially one of the most popular techniques for several decades but there has
been a significant number of problems. Initially, most of the enthalpy difference determinations
were made by infrared spectra of solutions such as carbon tetrachloride but there were problems
with the amines forming dimers and higher forms of association. For some of the very small
amines there were a few investigations in the gas phase with both infrared and Raman spectral
studies but most of these studies included a single band pair for the enthalpy difference
determination. In general, where there were multiple determinations for the same molecule there
were large variations of the reported values from the different techniques.
In addition to self association there can be significant interaction of the amine with the
solvent. To reduce both types of interaction we initiated studies using xenon or krypton as the
solvents. These are very excellent solvents since very long paths can be used with these liquids so
the solutions can be extremely dilute which greatly reduces the interaction between the molecules.
Other advantages due to the low temperature results in the narrow band contours. There is very
accurate measurement of the temperature and the temperature range is large. Additionally, it is
usually possible to obtain a relatively large number of band pairs.
174
More recently ab initio calculation has been used to predict the enthalpy differences for
several molecules and for some molecules the predicted values give reasonable agreement with the
experimentally determined values. From the conformational stability of ethylamine [36] the trans
form was determined to be more stable by 54 ± 4 cm-1 (0.65 ± 0.05 kJ/mol) than the gauche form.
However, ab initio calculations at the MP2 level with full electron correlation with diffuse
functions predict the gauche conformer as the more stable form by approximately 200 cm-1
whereas, the correct conformer stability is predicted from the same basis set without diffuse
functions.
For aminomethylcyclopropane c–C3H5CH2NH2 [48] the opposite result is obtained where
the basis set with the diffuse functions give the correct conformer stability. Enthalpy
determinations from variable temperature infrared spectra of rare gas solutions give an enthalpy
difference of 109 ± 11 cm-1 (1.30 ± 0.13 kJ/mol) between the most stable gauche–gauche (Gg)
conformer (the first gauche designation, with capital G, for the heavy atom conformation around
the C–C bond, and the second gauche designation, with lower case g, is for the amino torsion,
around the C–N bond) and the second most stable conformation, gauche–trans (Gt). For this
molecule the MP2(full)/6-311+G(d,p) calculations predict Gg form to be more stable by 36 cm-1
whereas the MP2(full)/6-311G(d,p) calculations predict the Gt rotamer to be more stable by 135
cm-1. These results show the limitations in utilizing ab initio calculations to predict the most stable
conformer of these organoamines.
To obtain more enthalpy differences of some significantly different amines we began a
study of 2-fluoroethylamine where the molecule had been investigated by microwave spectra.
From this investigation [86] two of the possible conformers were identified where the Gg' form
was determined to be more stable than the Gt conformer. An enthalpy difference of 35 ± 105 cm-1
175
(0.42 ± 1.25 kJ/mol) was obtained but this very large uncertainty (which is three times the enthalpy
difference) raises the question which form is the more stable conformer.
Since the value obtained from the microwave study provided a questionable stability of
which conformer is more stable we initiated the conformational and structural studies of 2fluorethylamine molecule, where five possible conformations could be present. We recorded
variable temperature mid-infrared spectra of xenon solutions as well as the far infrared spectra of
krypton solutions. From these data, all five possible conformations were identified and their
relative stabilities obtained. The enthalpy differences were determined among the most stable Gg'
conformer and the second stable form, Gt, to be 62 ± 8 cm-1 (0.74 ± 0.10 kJ/mol), the third stable
conformer, Tg, the enthalpy difference of 262 ± 26 cm-1 (3.14 ± 0.3 kJ/mol), the fourth most stable
conformer, Tt, to be 289 ± 45 cm-1 (3.46 ± 0.5 kJ/mol), and the fifth most stable conformer, Gg to
be 520 ± 50 cm-1 (6.24 ± 0.6 kJ/mol) was obtained. These results clearly showed that the variable
temperature studies (-55 to -131 ºC) provided excellent enthalpy differences for this molecule.
In order to improve the determination of the enthalpy difference of the amines we decided
to utilize the Raman effect with xenon or krypton solutions to obtain the enthalpy difference. A
particular interest was the 2,2,2 trifluoroethylamine since there was a large variation in the
predicted energy difference for this molecule from the ab initio calculations which we carried out.
Since there are only two conformers for 2,2,2 trifluoroethylamine it was felt that this molecule
would be a good example as the enthalpy difference could be determined without the complicating
problems associated with multiple conformers. Another factor in choosing this molecule was the
fact that the ab initio calculations indicated that the energy difference would be about ~626 cm -1
(7.49 kJ/mol) which of course should make it really difficult to observe the second conformer.
However, it has been shown that the ab initio predictions do not provide very meaningful energy
difference for these amines. From a previous vibrational assignment [87] there was evidence for
176
the second conformer from the observation of two fundamentals and the large energy difference
value is probably way over estimated.
To support the experimental studies reported herein we have obtained the harmonic force
constants, infrared intensities, Raman activities, depolarization ratios, and vibrational frequencies
from MP2(full)/6-31G(d) ab initio calculations. To obtain predictions on the conformational
stabilities we have carried out MP2(full) ab initio and density functional theory (DFT) calculations
by the B3LYP method by utilizing a variety of basis sets. The r0 structural parameters have been
obtained by combining the MP2(full)/6-311+G(d,p) ab initio predicted parameters with the
previously reported rotational constants obtained from the microwave studies [54, 88]. The results
of these spectroscopic, structural, and theoretical studies of 2,2,2 trifluoroethylamine are reported
herein.
Experimental and Theoretical Methods
The sample of 2,2,2 trifluoroethylamine was purchased from Sigma-Aldrich Chemical Co.,
with stated purity of ≥99.5%. The sample was used with no further purifications.
The Raman spectra (4000 to 300 cm-1) of the sample dissolved in liquefied xenon (Fig. 28)
at five different temperatures (-60 to -100oC) was recorded on a Trivista 557 spectrometer
consisting of a double f = 50 cm monochromator equipped with a 2000 lines mm-1 grating, a f = 70
cm spectrograph equipped with a 2400 lines mm-1 grating, and a back-illuminated LN2-cooled PI
Acton Spec-10:2 kB/LN 2048 x 512 pixel CCD detector. For all experiments, the 514.5 nm line of
a 2017-Ar S/N 1665 Spectra-Physics argon ion laser was used for Raman excitation, with the
power set to 0.8 Watt. Signals related to the plasma lines were removed by using an interference
filter. The frequencies were calibrated using Neon emission lines, and depending on the setup
used, are expected to be accurate within 0.4 cm-1. The experimental set-up used to investigate the
solutions has been described before [15, 16]. A home-built cell for liquids was equipped with four
177
quartz windows at right angles was used to record the spectra. The infrared and Raman bands
chosen as fundamentals along with their proposed assignments and depolarization values are listed
in Tables 45 and 46, respectively.
The LCAO-MO-SCF restricted Hartree-Fock calculations were performed with the
Gaussian-03 program [21] by using Gaussian-type basis functions. The energy minima with
respect to nuclear coordinates were obtained by the simultaneous relaxation of all geometric
parameters by the gradient method of Pulay [22]. Several basis sets as well as the corresponding
ones with diffuse functions were employed with the Møller-Plesset perturbation method [23] to the
second order (MP2(full)) as well as with the DFT by the B3LYP method.
The predicted
conformational energy differences are listed in Table 47.
In order to obtain a complete description of the molecular motions involved in the
fundamental modes of F3CCH2NH2, a normal coordinate analysis has been carried out. The force
field in Cartesian coordinates was obtained with the Gaussian 03 program [21] at the MP2(full)
level with the 6-31G(d) basis set. The internal coordinates used to calculate the G and B matrices
are given in Table 48 with the atomic numbering shown in Fig. 29. By using the B matrix [24], the
force field in Cartesian coordinates was converted to a force field in internal coordinates.
Subsequently, scaling factors of 0.88 for CH stretches and CH2 deformation and 0.90 for all other
modes except heavy atom bends were applied, along with the geometric average of the scaling
factors for the interaction force constants, to obtain the fixed scaled force field and resultant
wavenumbers. A set of symmetry coordinates was used (Table 49) to determine the corresponding
potential energy distributions (P.E.D.s). A comparison between the observed and calculated
wavenumbers, along with the calculated infrared intensities, Raman activities, depolarization ratios
and potential energy distributions for the trans and gauche conformers are listed in Tables 45 and
46, respectively.
178
Additional support for the vibrational assignments was obtained from the simulated Raman
spectra. The evaluation of Raman activity by using the analytical gradient methods has been
developed [25-27, 37] and the activity Sj can be expressed as: Sj = gj(45αj2 + 7βj2), where gj is the
degeneracy of the vibrational mode j, αj is the derivative of the isotropic polarizability, and βj is the
anisotropic polarizability. To obtain the Raman scattering cross sections, the polarizabilities are
incorporated into Sj by multiplying Sj with (1-ρj)/ (1+ρj) where ρj is the depolarization ratio of the
jth normal mode. The Raman scattering cross sections and calculated wavenumbers obtained from
the Gaussian 03 program were used together with a Lorentzian function to obtain the simulated
Raman spectra.
Vibrational Assignment
The first comprehensive vibrational assignment for 2,2,2 trifluoroethylamine from Wolff et
al. [89] was made by utilizing the infrared spectra of the NH2, NHD and ND2 isotopomers for the
gas and crystalline solid. Additionally, the Raman spectra of the NH2 and ND2 isotopomers for the
liquid. This was followed by another vibrational study from Kalasinsky and Anjarla [87] where
both the 2,2,2 trifluoroethylamine and 2,2,2 trifluoroethanol were studied with the infrared and
Raman spectra of the gas. The vibrational assignments were made mostly based on group
frequencies along with existing assignments of corresponding vibrations of similar molecules. In
the first study of the vibrational assignments [89] no bands were assigned for the gauche form but
all of the fundamentals for the trans conformer were assigned. For the later study of the vibrational
assignments [87] again all of the fundamentals were assigned for the trans conformer but only two
fundamentals were assigned for the gauche form. This assignment is reasonable based on the H
value of 1014 cm-1 obtained by these investigators’ study of the enthalpy difference. This result
indicates that the gauche form would be less that 1% abundance with this large enthalpy
difference. However, by the utilization of MP2(full)/6-31G(d) predicted vibrational wavenumbers,
179
ab initio predicted intensities, and depolarization ratios along with Raman data from xenon
solutions it has been possible to assign a significantly larger number of the fundamentals for the
gauche conformer. The relatively large number of bands and their relative intensities observed in
the current study indicate an enthalpy difference much lower than that reported in the earlier study.
Our study indicates that the E value from the ab initio calculations appears to be larger than that
expected based on the abundance of the gauche conformer. A reliable assignment of the
fundamentals for both the trans and gauche conformer is important for obtaining the enthalpy
difference since the bands being used for the temperature study must be for a single confidently
identified conformer. Therefore, we have attempted to assign all of the fundamentals for both
conformers in the region from 1300 to 400 cm-1 where the number of overtone and combination
bands are significantly reduced compared to those in the higher wavenumber region.
In the current study, the fundamentals ν6, ν18 , ν7 and ν19 are assigned to the bands at 1272,
1245, 1162 and 1083 cm-1 ,respectively, in the Raman spectra of the Xenon solutions based on the
ab initio predictions and the Raman activities but in the previous vibrational studies ν 6 was
assigned to the band at 1278 cm-1, ν18 was assigned to the band at 1251 cm-1, ν7 was assigned to the
band at 1173 cm-1, and ν19 was assigned to the band at 1088 cm-1. The differences of 6, 11 and 5
cm-1 probably arise from the fact that these bands are broad in the spectra of the gas which makes
the determinations of the band center difficult to identify rather than from differences due to xenon
solutions. Two of these bands are mixed B-type so their frequencies are frequently higher than the
actual center. It is extremely difficult to correctly assign band center of B-type fundamental
particularly for bands that have excited states on the low frequency side of the fundamental
The next band to be assigned is the fundamental ν8, the CN stretch, where it is assigned at
1122 cm-1. This band is a clearly resolved band in the Raman spectra of the Xenon solutions. In the
first vibrational assignment [89] this fundamental was assigned at 1114 cm-1 but it is clearly an
180
incorrect assignment based on the ab initio prediction as well as the results from previous studies
of similar molecules. However, in the second vibrational assignment [87] it was reassigned to the
band at 1124 cm-1 which agrees with our current assignment.
The next fundamental to be assigned is the NH2 wag (ν9) which is identified in the Raman
spectra of the xenon solutions as the band at 828 cm-1 as the trans conformer. This corresponding
band was previously reported [87] as the band at 828 cm-1 in the infrared spectra of the gas and
822 cm-1 in the Raman spectra of the gas for the gauche conformer. The band at 757 cm-1 is
assigned as a gauche NH2 wagging fundamental. This corresponding band was previously reported
in the first vibrational study [89] at 762 cm-1 from the infrared spectra of the gas for the trans form
as the NH2 wagging fundamental. Also, in the second vibrational assignment study [87], the band
at 761 cm-1 was also assigned to the NH2 wagging mode from the infrared and Raman spectra of
the gas which correspond as well to the band at 757 cm-1 in the Raman spectra of the xenon
solutions assigned to the gauche form.
In the current study, the bands at 536 and 543 cm-1 are assigned as the ν21 and ν12
fundamentals, respectively, supported by their predicted frequencies and the relative intensities. It
is interesting to note that both of the previous vibrational assignment studies [87], [89] had bands
at 543 and 545 cm-1 in the infrared spectra of the gas as the trans CF3 deformations ν21 and ν12,
respectively, which rather surprisingly that both of the studies had the same frequencies for these
assignments.
The assignments of the remaining bands of the trans conformer ν5, ν 10, ν 11, ν 13, ν 17, and
ν22 agree with the assignments previously reported since they are assigned within one or two
wavenumbers. The fundamentals for the gauche form have been assigned in the current study and
were largely not reported in the previous study though several of the bands proposed as
components of trans fundamentals have now been assigned as gauche fundamentals.
181
With these complete and accurate assignments of the trans and gauche fundamentals in the
region from 1300 to 400 cm-1 there are a significant number of bands that are suitable for the
determination of the enthalpy difference between the conformers.
Conformational Stability
To determine the enthalpy differences between the two observed conformers of 2,2,2
trifluoroethylamine, the sample was dissolved in liquid xenon and the Raman spectra were
recorded as a function of temperature from -60 to -100°C. Only relatively small interactions are
expected to occur between xenon and the sample though the sample can associate with itself
forming a dimmer, trimer or higher order complex. Therefore, only small wavenumber shifts are
anticipated for the xenon interactions when passing from the gas phase to the liquid xenon
solutions except for the NH2 modes. A significant advantage of this study is that the conformer
bands are better resolved in comparison with those in the Raman spectrum of the liquid. From ab
initio calculations, the dipole moments of the two conformers are predicted to have similar values
and the molecular sizes of the two conformers are nearly the same, so the ΔH value obtained from
the temperature dependent Raman study is expected to be near to that for the gas [39], [40-43].
Once confident assignments have been made for the fundamentals of both conformers the
task was then to find a pair or pairs of bands from which the enthalpy determination could be
obtained. The bands should be sufficiently resolved so their intensities can be determined. The
fundamentals at 646 cm-1 for the trans and 968 and 635 cm-1 for the gauche conformer were
initially selected as they are confidently assigned, satisfactory resolved, and a limited number of
overtone and combination bands are possible to contribute to them. The band at 1082 cm-1 was
selected next for the trans conformer as it was found that the predicted underlying gauche
fundamental was not of sufficient intensity to significantly affect the intensity of the observed
band. The band at 3367 cm-1 was selected for the gauche form as it was satisfactory resolved
182
which allowed the determination of it’s intensity to be measured. Finally, the bands at 828 and 757
cm-1 (NH2 wagging modes) were also used for the trans and gauche conformers, respectively.
Whether these bands would be used to determine the enthalpy difference depends on whether the
results were consistent with the values obtained from the initially chosen pairs.
The intensities of the individual bands were measured as a function of temperature (Fig. 30)
and their ratios were determined. By application of the van’t Hoff equation lnK = H/(RT) 
S/R, the enthalpy difference was determined from a plot of lnK versus 1/T, where H/R is the
slope of the line and K is substituted with the appropriate intensity ratios, i.e. Iconf-1 / Iconf-2, etc. It
was assumed that S and α are not functions of temperature in the range studied.
These seven bands, three trans and four gauche, were utilized for the determination of the
enthalpy difference by combining them to form 12 band pairs. The enthalpy difference was
determined to have a value of 267  5 cm-1 (Table 50). This error limit is derived from the
statistical standard deviation of one sigma of the measured intensity data taken as a single data set,
but it does not take into account small associations with the liquid xenon or the possible presence
of overtones and combination bands in near coincidence of the measured fundamentals. The
variations in the individual values are undoubtedly due to these types of interferences, but by
taking several pairs, the effect of such interferences should cancel. However, this statistical
uncertainty is probably better than can be expected from this technique and, therefore, an
uncertainty of about 10% in the enthalpy difference is probably more realistic i.e. 267  27 cm-1.
From the enthalpy difference the abundance of the gauche conformer present is estimated to be 35
± 3% at ambient temperature.
Structural Parameters
Initial structural parameters for the 2,2,2 trifluoroethylamine were proposed from a
microwave study [88] where a partial structure of the trans conformer (Table 48) was determined.
183
In this microwave study [88], all but four of the structural parameters were assumed for the trans
conformer. The four parameters that were fitted were the NH bond distance and the angles CCN,
CNH, HNH. This was followed by another microwave study [54] two decades later where the
rotational constants were determined for the trans conformer for 2,2,2 trifluoroethylamine as well
as isopropylamine and aminoethanol but no structural parameters were reported for any of these
molecules. We have again determined the structural parameters by utilizing the rotational constants
previously reported from the microwave studies [54], [88].
We [44] have shown that ab initio MP2(full)/6-311+G(d,p) calculations predict the carbonhydrogen r0 structural parameters for more than fifty carbon-hydrogen distances to at least 0.002 Å
compared to the experimentally determined [45] values from isolated CH stretching frequencies
which agree to previously determined values from earlier microwave studies. Therefore, all of the
carbon-hydrogen parameters can be taken from the MP2(full)/6-311+G(d,p) predicted values for
the trans conformer of 2,2,2 trifluoroethylamine.
We have also found that good structural parameters for hydrocarbons and many substituted
ones can be determined by adjusting the structural parameters obtained from the ab initio
MP2(full)/6-311+G(d,p) calculations to fit the rotational constants obtained from microwave
experimental data by using a computer program “A&M” (Ab initio and Microwave) developed
[45] in our laboratory. In order to reduce the number of independent variables, the structural
parameters are separated into sets according to their types where bond distances in the same set
keep their relative ratio, and bond angles and torsional angles in the same set keep their difference
in degrees. This assumption is based on the fact that errors from ab initio calculations are
systematic. Therefore, it should be possible to obtain “adjusted r0” structural parameters for the six
heavy atoms by utilizing the previously reported six rotational constants from the earlier
184
microwave studies [54], [88]. Therefore, we have obtained the complete heavy atom structural
parameters for the trans form of 2,2,2 trifluoroethylamine.
The resulting adjusted r0 parameters are listed in Table 48, where it is believed that the NC, C-C, and C-F distances should be accurate to ± 0.003 Å, the C-H distances are accurate to ±
0.002 Å, and the angles should be within ± 0.5. The fit of the six determined rotational constants
(Table 51) by the structural parameters for the trans conformer are good with the differences being
less than 0.8 MHz for both B and C rotational constants. The fact that this is a symmetric molecule
where κ value is equal to -0.9814 does not permit an accurate determination of the A rotational
constant. Therefore, it is believed that the suggested uncertainties are realistic values and the
determined structural parameters are probably as accurate as can be obtained for the molecule in
the gas phase by microwave substituted methods.
Additionally, we have also shown that the differences in predicted distances and angles
from the ab initio calculations for different conformers of the same molecule can usually be used
as one parameter with the ab initio predicted differences except for some dihedral angles.
Therefore, we have predicted structural parameters for the gauche form of 2,2,2
trifluoroethylamine by applying the corresponding adjustments from the trans conformer to the
MP2(full)/6-311+G(d,p) predicted parameters for the gauche form. These parameters should be
close to the actual values.
Discussion
The vibrational assignments reported herein are based on a significant amount of
information with the Raman spectrum of the xenon solutions and predictions from the scaled ab
initio MP2(full)/6-31G(d) calculations. The ab initio Raman band intensities were also used but
they seemed to be the least reliable of the data utilized. One of the possible reasons for the
significant difference in predicted and observed intensities could be the results of the association of
185
the amine portion of the molecule with xenon. The evidence for the van der Waals molecules was
the significant decreases in the NH stretching frequencies from the gas to the solutions i.e. 13 cm -1
for the trans conformer and 15 cm-1 for the gauche form. However, for the other fundamentals the
difference between the frequencies in the gas and xenon solutions is less than ten wavenumbers
except for the CF3 antisymmetric stretch where the difference is eleven and CH2 rock where the
difference is thirteen wavenumbers for the trans conformer.
For the trans conformer the average ab initio predicted frequencies for A′ fundamentals
with all the fundamentals, except for the NH2 wag was 11 cm-1 which represent 0.9% error. The
predicted frequency for the NH2 wag was too high with 0.9 scaling but it can be corrected with a
scaling factor of 0.7. However, if this value is used it usually results in two or three unacceptable
P.E.D. values. Since the NH2 wag gives rise to a very intense infrared band and it is readily
assigned, it is usually better to use 0.9 as the scaling factor so the other fundamentals are “normal”.
The percent error for the A′′ modes for the trans form have similar errors to that of the A′ modes of
the predicted frequencies. Thus, the relatively small basis set of 6-31G(d) by the MP2(full)
calculations with two scaling factors provides excellent predicted frequencies for aiding the
vibrational assignments.
For the trans form the mixing is relatively small for the A′ modes except for 6 (CC stretch)
which has only 22%S6 with 22%S5 of the CH2 wag, 11 (CF3 deformation) which has 39%S11 with
15%S14 of the CCN deformation, and 13 (CF3 rock) which has 22%S13 with 34%S12 of the CF3
deformation. For the A′′ modes only 18 (CF3 antisymmetric stretch) of 27%S18 with 23%S20 of the
CH2 rock and 19 (NH2 twist) 23%S19 with 37%S18 of the CF3 antisymmetric stretch have relatively
small percentage. Therefore, the approximate descriptions for the normal modes provide reasonable
information on the molecular motions of vibrations. However, for the gauche conformer there is
significantly more mixing with the vibration at 1062 cm-1 assigned as the NH2 twist with only
186
19%S13 and 42%S12 (CN stretch). Therefore, several of the approximate descriptions for the
fundamentals of the gauche form are more for bookkeeping than providing descriptions of the
molecular motions.
One of the major goals of this research was the determination of the enthalpy difference
between the two conformers where there has been only a single value reported. In the previous
vibrational study [87] an enthalpy difference of 1014 cm-1 was determined from a variable
temperature study for the gas phase. This value is significantly different than the value obtained
from the current study and as can be seen from the data in Table 50 the enthalpy difference
between the trans and gauche form is 267 ± 27 cm-1.
Normally, we would not use the fundamentals associated with the NH2 group since they
can be significantly affected by molecular association as the temperature is decreased. However,
there did not appear to be a significant change in the association as the temperature was lowered
from -60 to -100°C so we used the NH2 wag bands which gave excellent enthalpy results.
Therefore, by using twelve pairs for the determination of this enthalpy difference and the data as a
single set the enthalpy difference was determined to be 267 ± 5 cm-1. This statistical uncertainty
value is clearly smaller than the method is expected to provide.
The enthalpy value obtained from the xenon solutions must be considered the most confident
enthalpy difference at this time as it is doubtful that any other experiments would give an enthalpy
value with such a low uncertainty as the one determined in this study. The use of enthalpy
determinations from 12 band pairs provides excellent accuracy of the determination by this
technique.
In the most recent microwave study of 2,2,2 trifluoroethylamine the quadrupole coupling
constants and the quartic centrifugal distortion constants were determine from the spectral data. We
have predicted the quadratic centrifugal distortion constants and the quadrupole coupling constants
187
from the MP2(full)/6-311+G(d,p) calculations as well as the DFT by the B3LYP method by
utilizing the same basis set. These data are given in Table 52 and as can be seen from the values of
the quadrupole coupling constants there is reasonable agreement with the predicted values from both
techniques as well as the experimental values. However, the experiment centrifugal distortion
constants are very different from those predicted by both calculation methods. We have found in the
past that the ab initio calculations usually provide reasonably good centrifugal distortion constants
and we expected reasonable agreement. Therefore, we believed this disagreement should be
explored.
An inspection of the microwave data from which the experimental data was taken to obtain
the centrifugal distortion constants gives only fourteen J transitions and the values for the K a are
three with 0, nine with 1 and only two with 2. These excludingly small differences in the K a values
are not sufficient to obtain meaningful values for Δk and δk centrifugal distortion constants. The
value of 1.99(66) kHz for ΔJK with the listed uncertainty also indicates the probability although with
the extremely large uncertainty it does agree with the theoretical predicted values. Also it should be
noted that the experimentally reported values of 0.033(39) kHz has an uncertainty larger than the
value and the value of 0.560(45) kHz for ΔJ has a much larger uncertainty than usually obtained for
the distortion constants. However, it is believed that the theoretical predicted centrifugal distortion
constants have a value within 10% of the correct experimental values.
In the previous microwave investigation [88], all but four of the structural parameters were
assumed for the trans conformer. The four parameters that were not assumed but fitted instead were
the NH bond and the CCN, CNH, HNH angles. The four determined values with their uncertainties
(Table 48) are in agreement with the values obtained in this study. The values obtained earlier for
the N3C2C1 angle with an uncertainty of 1.0 degree are in excellent agreement with the
corresponding parameters obtained in this study. However, the major differences between the values
188
reported earlier and those obtained herein is with the C-F distances which were estimated to be
shorter by an average distance of 0.009 Å and for the C-N and C-C bonds where the average
distances are longer by 0.027 and 0.017 Å, respectively. The difference between the assumed FCF
angle and the value obtained in this study is nearly 1° smaller. With these exceptions many of the
other estimated parameters are in acceptable agreement with those obtained in the current study.
The predicted r0 structural parameters for the gauche form have been predicted as shown in
Table 48. It is interesting to note that the change from trans to gauche conformer causes several
large structural changes. The heavy atom parameters with significant differences and their respective
predicted structural changes are the -0.010 Å change in the C1-F6 distance and the -5.1° change in
the CCN angle. These structural parameter differences are interesting in that the reduced CCN angle
would seem to decrease the N-H---F distance yet this distance is not significantly changed in the
overall structures. The difference in the C1-F6 distance is very interesting as this is the distance
corresponding with the fluorine that would overlap the nitrogen lone pairs. The reduction in this
distance would seem then to correspond to the reduction of the steric strain from the nitrogen and
fluorine lone pairs. It would be very interesting to determine the rotational constants for use in
determining the adjusted r0 structure of the gauche form so as to evaluate this comparsion.
It is also interesting to compare the heavy atom parameters of 2-fluoroethylamine [60] and
2,2-difluoroethylamine [61] with the corresponding ones obtained in the current study of 2,2,2
trifluoroethylamine. It should be noted that the C-C bond is smaller by an average of 0.002 Å for
substitution of additional fluorine on β-carbon which is within the experimental error of ± 0.003 Å
for the heavy atom bond distances in each r0 determination. When we compare the C-N bond
distance, the change is considerable with the addition of the second fluorine atom the distance
decreases by an average of 0.009 Å and with the addition of a further fluorine atom decreases by
0.005 Å. However, the parameter which shows the greatest change due to fluorine substitution on
189
β-carbon is the C-F bond distance(s) which reduces by ~0.030 Å each time a fluorine atom is
added. Thus for 2-fluoroethylamine the average C-F distance is 1.398(3) Å, for 2,2difluoroethylamine 1.368(3) Å and for 2,2,2 trifluoroethylamine 1.344(3) Å. Such large changes
have a significant effect on some of the other parameters.
The natural population analysis (npa) calculations have been carried out for 2,2,2
trifluoroethylamine, 2,2-difluoroethylamine, and 2-fluoroethylamine for comparison by utilizing
the MP2(full) method at the 6-311+G(d,p) basis set. The trans [gauche] conformers population
analysis for 2,2,2 trifluoroethylamine for the heavy atoms are; C1 = 1.18 [1.19], C2 = -0.20 [-0.20],
N = -0.84 [-0.84], F4 = -0.40 [-0.41], F5 = -0.41 [-0.41], and F6 = -0.41 [-0.40]. As can be seen
there is no significant change in atomic charges going from the trans to the gauche form. This is
interesting when compared to the predicted structure as discussed above where there are significant
differences in the predicted structure of the gauche conformer this indicates that these changes are
due to steric effects rather than electronic effects. The corresponding parameters from 2,2difluoroethylamine [2-fluoroethylamine] are C1 = 0.71 [0.16], C2 = -0.18 [-0.16], N = -0.84 [0.84], F5 = -0.43 [-0.45], and F6 = -0.43. The differences in the C2, F5, and F6 charges are likely
insignificant and there is no difference at all in the N charges among the three npa. The C 1 charge
changes by 0.5 each time a fluorine atom is added. This change in charge is likely the cause of the
change in the fluorine distances as they must come closer to the carbon atom to allow for bonding
to its shrinking electron density.
From the microwave spectrum [88] only the trans form of 2,2,2,-trifluoroethylamine
molecule has been reported. This conformer is a near prolate top which does not permit an accurate
determination of the A rotational constant. Rotational constants for three isotopomers have been
reported with the normal species, the NHD species and the ND2 species, where the uncertainties on
190
the A rotational constants are listed as 14 MHz and 39 MHz for the normal and deuterated species,
respectively. The fit of the rotational constants are reported in Table 51.
It is well known that intramolecular hydrogen bonding can be a stabilizing factor for the
orientation of molecular conformers. This is important in the case of the fluoride substituted
ethylamine where the fluorine atom(s) and the amine hydrogens have the possibility to form an
intramolecular hydrogen bond. This is observed with 2,2,2 trifluoroethylamine where the three
fluorine atoms allow for two intramolecular hydrogen bonds so the most stable conformer is the
trans conformer where two hydrogen bonds may be formed. Clearly intramolecular hydrogen
bonding plays a significant role in the conformational stabilities of such molecules and it would be
of interest to see how other halogen or other substituents might affect the stability of other
organoamines.
191
Table 45: Observed and calculateda wavenumbers (cm -1) for trans 2,2,2 trifluoroethylamine.
Vib.
Approximate Descriptions
No.
ab
initio
fixed
scaledb
IR
int.
Raman
act.
IR
c
Gas
Raman
d
Gas
Xenon
d
gas
IR
liquid
c
Solid
P.E.D.
c
e
Band
Contours
A*
192
3533
3352
2.5
72.9 3371
3370
3357 3372
3358
3270 100S1
2
A 1 NH2 symmetric stretch
3141
2947 11.6
84.0 2957
2958
2949 2957
2954
2965 100S2
20
2 CH2 symmetric stretch
1734
1645 34.5
6.6 1636
1636
1635 1635
1626
1625
76S3,23S9
3 NH2 scissor
1536
1453
4.7
10.0 1446
1445
1440 1444
1445
1445 100S4
16
4 CH2 deformation
1458
1389 40.9
0.5 1398
1380
1381 1368
1371
1391
60S5,19S6,10S10
96
5 CH2 wag
CC
stretch
1339
1288
137.2
1.1
1277
1278
1272
1271
1272
1277
22S
,22S
,19S
,19S
,10S
25
6
6
5
10
11
7
1241
1190 220.6
2.5 1172
1173
1162 1172
1149
1150
58S7,12S5,10S12
60
7 CF3 antisymmetric stretch
1178
1122 13.7
2.4 1114
1124
1122
1113
1115
74S8,12S9
7
8 CN stretch
951
904 233.6
1.0
762
761
828
761
900
49S9,17S3,15S8
94
9 NH2 wag
831
790 28.8
7.1
838
838
836
836
812
809
53S10,24S6,,10S9
77
10 CF3 symmetric stretch
659
648 14.0
2.8
645
645
646
645
650
655
39S11,15S14,12S7,11S10,10S13
7
11 CF3 deformation
547
539
5.8
1.8
545
545
543
542
543
548
48S12,20S11,13S6,12S7
28
12 CF3 deformation
420
416 11.3
2.1
413
414
414
410
417
422
22S13,34S12,18S14,11S6
92
13 CF3 rock
231
229
7.9
0.1
236
240
239
47S14,50S13
70
14 CCN deformation
3448
6.5
42.9 3438
3438
3425
3414
3385 100S15
A 15 NH2 antisymmetric stretch 3634
3006
7.5
54.2 2984
2983
3077
2979
2987 100S16
16 CH2 antisymmetric stretch 3204
1440
1371 14.8
10.1 1362
1362
1363
1330
1361
53S17,37S19
17 CH2 twist
1320
1266 130.4
0.2 1251
1251
1245
1245
1246
27S18,23S20,21S19
18 CF3 antisymmetric stretch
NH
twist
1141
1084
80.0
2.1
1088
1088
1083
1087
1095
23S
37S
,37S
19
2
19,
18
17
910
868 17.9
1.9
873
873
873
873
936
58S20,19S18,19S19
20 CH2 rock
532
526
0.0
0.6
543
543
536
543
535
72S21,13S18
21 CF3 deformation
CF
rock
396
387
24.3
1.4
373
374
372
383
351
46S
,29S
,16S
22
3
22
23
21
304
293 52.1
1.4
269
269
69S23,29S22
23 NH2 torsion
130
128
0.0
0.1
130
97S24
24 CF3 torsion
a
MP2(full)/6-31G(d) ab initio calculations, scaled wavenumbers, infrared intensities (km/mol), Raman activities (Å4/amu), depolarization ratios and potential energy
distributions (P.E.D.s)
b
MP2(full)/6-31G(d) fixed scaled wavenumbers with factors of 0.88 for CH stretches, 1.0 for heavy atom bends, and 0.90 for all other modes.
c
Ref [89]
d
Ref [87]
e
Calculated with MP2(full)/6-31G(d) and contributions of less than 10% are omitted
*
A, B and C values in the last three columns are percentage infrared band contours.
B*
98
80
100
84
4
75
40
93
6
23
93
72
8
30
-
Table 46: Observed and calculateda wavenumbers (cm -1) for gauche 2,2,2 trifluoroethylamine.
Vib.
Approximate Descriptions
No.
ab
initio
fixed
scaledb
IR
int.
Raman
act.
IR
gasc
Raman
gasc
Xenon
Band
Contours
P.E.D.d
A*
B*
193
3561
6.7 55.8
- 100S1
20
49
1 NH2 antisymmetric stretch 3649
NH
symmetric
stretch
3547
3365
2.6
86.2
3382
3367
100S
57
6
2
2
2
3187
2989
9.0 58.2
2975
75S3,25S4
2
13
3 CH2 antisymmetric stretch
3074
2884
39.5 90.5
2865
75S4,25S3
15
40
4 CH2 symmetric stretch
1725
1637
41.5
7.5
1626
75S5,24S15
70
5 NH2 scissor
1557
1472
6.2 11.2
1460 100S6
78
22
6 CH2 deformation
1513
1440
34.9
1.7
1429
51S7,21S9,11S13
61
35
7 CH2 wag
1396
1331
79.7
8.3
1325
52S8,13S10,10S13
2
2
8 CH2 twist
1342
1288 124.7
1.6
1272
20S9,19S16,18S7,19S17
32
66
9 CC stretch
1320
1266 111.9
0.9
1245
21S10,28S13,19S14
8
33
10 CF3 antisymmetric stretch
1239
1188 211.1
3.3
1162
49S11,10S18
66
23
11 CF3 antisymmetric stretch
1160
1103
64.8
5.1
1111
39S12,21S8,16S10,12S13
25
39
12 CN stretch
1122
1066
32.0
4.7
1062
19S13,42S12,12S7
8
73
13 NH2 twist
1005
958 115.5
6.6
968
41S14,22S10,13S15
2
14
14 CH2 rock
888
845 120.6
4.4
757
40S15,21S14,12S5
7
67
15 NH2 wag
832
791
27.7
3.5
828
822
836
53S16,24S9,13S15
35
18
16 CF3 symmetric stretch
650
638
19.7
0.8
635
38S17,16S22,11S11,10S20,10S16
88
4
17 CF3 deformation
552
545
6.2
1.6
546
46S18,20S17,13S9,12S11
58
37
18 CF3 deformation
536
531
2.1
0.9
526
73S19,13S10
11
11
19 CF3 deformation
427
423
1.0
1.4
432
22S20,35S18,19S22
31
20 CF3 rock
371
368
2.1
0.1
70S21,15S19
95
3
21 CF3 rock
248
243
16.0
1.0
35S22,40S20,20S23
41
27
22 CCN deformation
222
213
40.2
1.4
73S23,12S22
40
34
23 NH2 torsion
131
129
4.2
0.2
97S24
3
63
24 CF3 torsion
a
MP2(full)/6-31G(d) ab initio calculations, scaled wavenumbers, infrared intensities (km/mol), Raman activities (Å 4/amu), depolarization ratios and
potential energy distributions (P.E.D.s.
b
MP2(full)/6-31G(d) fixed scaled wavenumbers with factors of 0.88 for CH stretches, 1.0 for heavy atom bends, and 0.90 for all other modes.
c
Ref [87]
d
Calculated with MP2(full)/6-31G(d) and contributions of less than 10% are omitted.
*
A, B and C values in the last three columns are percentage infrared band contours.
C*
31
37
85
45
30
4
96
2
59
11
36
19
84
26
47
8
5
78
69
2
32
26
34
Table 47: Calculated energies (hartree) and energy differences (cm-1) for the two conformers of
2,2,2 trifluoroethylamine.
a
Energy , E
trans
0.7890437
0.8270485
0.8249888
0.8620344
1.1505005
1.1722942
1.2694876
1.2874767
1.3958022
1.4132769
1.3895263
1.4399636
Method/Basis Set
# basis set
MP2(full)/6-31G(d)
98
MP2(full)/6-31+G(d)
122
MP2(full)/6-31G(d,p)
110
MP2(full)/6-31+G(d,p)
134
MP2(full)/6-311G(d,p)
132
MP2(full)/6-311+G(d,p)
156
MP2(full)/6-311G(2d,2p)
174
MP2(full)/6-311+G(2d,2p)
198
MP2(full)/6-311G(2df,2pd)
236
MP2(full)/6-311+G(2df,2pd)
260
MP2(full)/cc-pVTZ
236
MP2(full)/aug-cc-pVTZ
368
Average MP2(full)
B3LYP/6-31G(d)
98
1.8960965
B3LYP/6-31+G(d)
122
1.9284823
B3LYP/6-31G(d,p)
110
1.9054581
B3LYP/6-31+G(d,p)
134
1.9376615
B3LYP/6-311G(d,p)
132
2.0350608
B3LYP/6-311+G(d,p)
156
2.0506047
B3LYP/6-311G(2d,2p)
174
2.0487408
B3LYP/6-311+G(2d,2p)
198
2.0628198
B3LYP/6-311G(2df,2pd)
236
2.0593206
B3LYP/6-311+G(2df,2pd)
260
2.0741500
B3LYP/aug-cc-pVTZ
368
2.0908799
B3LYP Average
a
Energy of conformer is given as –(E+431) H.
b
Difference is relative to trans form and given in cm-1.
194
Energy Differences, ∆
b
gauche
828
694
843
669
783
651
731
648
694
642
681
626
708 ± 72
887
741
887
741
898
728
805
698
764
680
665
772 ± 85
Table 48: Structural parametersa, rotational constants, and dipole moments for trans and gauche
2,2,2 trifluoroethylamine.
Parameter
r C1-C2
trans
Int. MP2(full)/
B3LYP/
Coor. 6-311+G(d,p) 6-311+G(d,p)
R1
1.513
1.524
1.530*
adjusted
r0
1.513(3)
MWb
gauche
MP2(full)/
B3LYP/
Predicted
6-311+G(d,p) 6-311+G(d,p)
r0
1.509
1.519
1.509
r C2-N3
R2
1.447
1.448
1.474*
1.447(3)
1.452
1.453
1.452
r C1-F4
R3
1.344
1.352
1.335*
1.344(3)
1.347
1.356
1.347
r C1-F5
R4
1.347
1.355
1.335*
1.347(3)
1.348
1.356
1.348
r C1-F6
R5
1.347
1.355
1.335*
1.347(3)
1.337
1.344
1.337
r C2-H7
r1
1.092
1.093
1.093*
1.092(2)
1.092
1.092
1.092
r C2-H8
r2
1.092
1.093
1.093*
1.092(2)
1.098
1.099
1.098
r N3-H9
r3
1.013
1.014
1.01(2)
1.013(3)
1.012
1.011
1.012
r N3-H10
r4
1.013
1.014
1.01(2)
1.013(3)
1.013
1.013
1.013
2.655
2.711
2.65(2)
2.655(5)
2.649
2.708
2.652
rN-H---F
 C1C2N3
A1
115.0
115.6
113.8(10)
115.2(5)
109.9
110.7
110.1
 F4C1C2
A2
111.4
111.4
111.03*
111.4(5)
110.4
110.3
110.4
 F5C1C2
111.6
111.9
111.03*
111.6(5)
110.9
111.3
110.9
 F6C1C2
A3
A4
111.6
111.9
111.03*
111.6(5)
112.9
113.2
112.9
 F4C1F6
A5
107.5
107.3
108.5*
107.6(5)
107.5
107.3
107.6
 F4C1F5
A6
107.5
107.3
108.5*
107.6(5)
107.3
107.0
107.4
 F5C1F6
A7
106.9
106.8
108.5*
106.9(5)
107.5
107.3
107.5
 H7C2H8
β
107.9
107.4
109.5*
107.9(5)
108.2
107.7
108.2
 H7C2C1
1
107.0
106.9
109.5*
107.0(5)
106.8
106.6
106.8
 H8C2C1
2
107.0
106.9
109.5*
107.0(5)
106.5
106.3
106.5
 H7C2N3
1
109.8
109.8
109.7(5)
109.6
109.6
109.5
 H8C2N3
2
109.8
109.8
109.7(5)
115.5
115.5
115.4
H9N3H10
γ
108.0
108.5
110.8(10)
108.0(5)
108.0
108.7
108.0
 H9N3C2
α1
111.0
112.0
110(20)
111.0(5)
110.6
111.5
110.6
H10N3C2
111.0
112.0
110(20)
111.0(5)
111.0
112.0
111.0
τ FCCN
α2
1
180.0
180.0
180.0
176.5
176.4
176.5
τ FCCH
2
-57.7
-57.4
-57.7(5)
-64.7
-64.5
-64.7
τ CCNH
3
60.1
61.1
60.1(5)
-76.5
-77.1
-76.5
τ HCNH
4
-60.7
-59.9
-60.7(5)
166.4
165.6
166.4
A
5249.99
5196.94
5295(16)
5249.74
5282.71
5227.04
5282.84
B
2786.83
2745.49
2785.69(1)
2785.38
2808.87
2766.51
2807.59
C
2766.09
2723.22
2762.55(1)
2763.10
2779.67
2735.01
2777.59
|a|
0.86
1.29
1.13(3)
2.65
2.55
|b|
0.17
0.27
0.28(3)
1.91
1.77
|c|
0.00
0.00
0.00(5)
1.24
1.08
|t|
0.88
1.32
1.16(5)
3.49
3.29
195
a
Bond distances in Å, bond angles in degrees, rotational constants in MHz, and dipole moments in Debye.
Ref [88]; * are the assumed structural parameters.
b
196
Table 49: Symmetry coordinates for 2,2,2 trifluoroethylamine.
Symmetry Coordinatea
Description
S1
S2
S3
S4
S5
S6
S7
S8
S9
S10
S11
S12
S13
S14
S15
S16
S17
S18
S19
S20
S21
S22
S23
S24
′













′′

NH2 symmetric stretch
CH2 symmetric stretch
NH2 scissor
CH2 deformation
CH2 wag
CC stretch
CF3 antisymmetric stretch
CN stretch
NH2 wag
CF3 symmetric stretch
CF3 deformation
CF3 deformation
CF3 rock
CCN deformation
NH2 antisymmetric stretch
CH2 antisymmetric stretch
CH2 twist
CF3 antisymmetric stretch
NH2 twist
CH2 rock

CF3 deformation
CF3 rock
NH2 torsion
CF3 torsion
a
Not normalize
197
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
r3 + r4
r1 + r2
γ
β
1– 1 + 1 – 2
R1
2R3 – R4 – R5
R2
α1+ α2
R3 + R4 + R5
2A7 – A6 – A5
A5 + A 6 + A 7 – A2 – A3 – A4
2A2 – A3 – A4
A1
r3 – r4
r1 – r2
1 – 1 – 1 + 2
R4 – R5
α1 – α2
1+ 1 – 1 – 2
A6 – A5
A4 – A3
3
1
Table 50: Temperature and intensity ratios of the trans and gauche bands of 2,2,2
trifluoroethylamine.
T(C) 1/T (10-3 K-1) I1083 / I3367
60.0
70.0
80.0
90.0
-100.0
Liquid
xenon

Ha
4.692
4.923
5.177
4.460
5.775
I1083 / I968
I1083 / I757
3.500
4.000
4.500
5.000
5.500
1.400
1.600
1.800
2.000
2.200
0.500
0.571
0.643
0.714
0.786
287  21
287  21
288  21
I828 / I3367
I828 / I968
I828 / I757
7.000
7.500
8.000
9.000
10.500
2.800
3.000
3.200
3.600
4.200
1.000
1.071
1.143
1.286
1.500
287  21
257  20
257  20
257  20
I646 / I968
I646 / I757
4.000
4.500
5.000
5.500
1.600
1.800
2.000
2.200
0.571
0.643
0.714
0.786
4.000
4.500
5.000
5.500
6.000
2.400
0.857
6.000
258  18
258  18
258  18
T(C) 1/T (10-3 K-1) I646 / I3367
Liquid 60.0
xenon 70.0
80.0
90.0
100.0

Ha
4.692
4.923
5.177
4.460
5.775
a
I1083 / I635
3.500
4.000
4.500
5.000
5.500
I646 / I635
258  18
Average value H = 267  5 cm-1 (3.20  0.02 kJ mol-1) with the trans conformer the more stable form and the
statistical uncertainty (1σ) obtained by utilizing all of the data as a single set.
198
I828 / I635
7.000
7.500
8.000
9.000
10.500
257  20
Table 51: Comparison of rotational constants obtained from experimental values from microwave
spectra and adjusted r0 structural parameters for trans 2,2,2 trifluoroethylamine.
Conformer
Rotational
constants
CF3CH2NH2a
A
5308(14)
5249
B
2785.722(3)
2785.378
0.34
C
2762.578(3)
2763.103
0.52
A
5216(39)
5147
B
2628.714(2)
2627.977
0.74
C
2617.842(2)
2618.638
0.80
A
5247(22)
5199
B
2706.27(1)
2706.16
0.11
C
2685.00(1)
2685.59
0.59
a
CF3CH2ND2
CF3CH2NHD
b
Experimentala
a
Ref. [54].
b
Ref. [88].
199
Adjusted r0
||
Table 52: Quadratic centrifugal distortion constants (kHz) and quadrupole coupling constants
(MHz) for conformers of 2,2,2 trifluoroethylamine.
Trans
MP2(full)/
6-311+G(d,p)
a
B3LYP/
6-311+G(d,p)
Gauche
Exp.a
MP2(full)/
6-311+G(d,p)
B3LYP/
6-311+G(d,p)
∆J
0.455
0.429
0.560(45)
0.446
0.464
∆JK
2.80
2.66
1.99(66)
2.78
3.16
∆K
-1.9548
-1.8128
2.648(47)
-1.9207
-2.2941
δJ
-0.0004
-0.0004
0.0033(39)
0.0024
0.0044
δK
4
4
1
1
χaa
-1.485
-1.526
-1.375(3)
2.609
2.752
χbb
-0.367
-0.527
-0.261(3)
0.160
-0.055
χcc
1.852
2.053
1.636(3)
-2.770
-2.697
124(36)
Ref [54]
200
Figure 28: Experimental and predicted Raman spectra of 2,2,2 trifluoroethylamine: (A) xenon
solution at -100°C; (B) simulated spectrum of Trans and gauche; enthalpy difference of 267 cm-1
at -100°C; (C) simulated spectrum of gauche conformer; (D) simulated spectrum of Trans
conformer.
201
Figure 29: 2,2,2 trifluoroethylamine molecule showing atomic number
202
Figure 30: Raman spectra of 2,2,2 trifluoroethylamine in xenon solution at different temperatures.
203
CHAPTER 9
CONFORMATIONAL, VIBRATIONAL, AND STRUCTURAL STUDIES OF 2,2,3,3,3-PENTA
FLUOROPROPYLAMINE FROM RAMAN AND INFRARED SPECTRA OF GAS, LIQUID,
XENON SOLUTIONS, AND SOLID SUPPORTED BY AB INITIO CALCULATIONS
Introduction
The conformational stability and the structural parameters of organoamines have been of
scientific interest for several decades. This interest has arisen, in part, because of the importance of
organoamines in biochemistry and many life processes. However, the determination of the enthalpy
differences for many of the molecules studied has varied significantly depending on the techniques
utilized to obtain the experimental value, i.e. infrared, Raman, NMR, and microwave spectroscopy as
well as electron diffraction data. Because many of the enthalpy determinations were made from
variable temperature studies of dilute solutions there have been problems with molecular association
due to hydrogen bonding of the amines. To significantly reduce this problem we have utilized rare gas
solutions where the pathlengths can be four to eight centimeters so the solutions can be very dilute,
i.e. ~10-4 M. Since the association of the amine with the rare gas is relatively small the enthalpy
difference in these solutions is similar to the value for the gas [39-43].
By this technique trans-ethylamine was determined [36] to be more stable by 54  4 cm-1
(0.65 ± 0.05 kJ/mol) than the gauche form. This value was in agreement with earlier electron
diffraction [35] determined value of 107  70 cm-1 (1.28 ± 0.84 kJ/mol), a microwave determined
[34] value 110  50 cm-1 (1.32 ± 0.60 kJ/mol), and IR matrix value [33] of 100  10 cm-1 (1.20 ±
0.12 kJ/mol) from an infrared study of ethylamine in an argon matrix [33]. Earlier studies [31, 32]
reported the gauche conformer more stable. The MP2(full)/6-311+G(2d,2p) calculations predict
[36] the gauche form more stable by 66 cm-1 (0.79 kJ/mol), but without diffuse functions the trans
204
form is 32 cm-1 (0.38 kJ/mol) more stable. The enthalpy difference between the gauche and trans
conformers of the NH2 moiety need to be determined experimentally.
It is of scientific interest to determine the effect of substituting one or more of the hydrogen
atoms on the β-carbon of the ethylene moiety. The first molecule investigated 2-fluoroethylamine
[49], FCH2CH2NH2, had been previously investigated in a microwave study [86] and the heavy
atom skeleton was reported to be in the gauche form for the two conformers identified. The gauche
form of the amine group was more stable by 35  105 cm-1 (0.42  1.26 kJ/mol) than the trans
form. Our vibrational investigation [90] in xenon solutions identified all five conformers where the
experimentally determined order of stability is Gg′ > Gt [62  8 cm-1 (0.74  0.10 kJ/mol)] > Tg
(262  26 cm-1) ≥ Tt (289  45 cm-1) > Gg (520  50 cm-1).
The next molecule investigated in this series of studies was the 2,2-difluoroethylamine [61]
where the trans-trans conformer was found to be the most stable form. The first indicator is the
NCCH dihedral angle and the second one is the relative position of the lone pair of electrons on
nitrogen with respect to the β-carbon. The experimentally determined order of stability for the
three identified conformers is Tt > Gg [83 ± 8 cm-1 (0.99 ± 0.10 kJ/mol)] > Gt [235 ± 11 cm-1 (2.81
± 0.13 kJ/mol)]. In addition to the enthalpy differences, the r0 structural parameters were obtained
for the Gg′ and Gt conformers by utilizing the previously reported rotational constants from the
microwave study [85].
As a continuation of these conformational and structural studies of the substituted
ethylamines we chose the molecule where the H atom on the β-carbon of 2,2-difluoroethylamine
was replaced by the trifluoromethyl moiety, CF3CF2CH2NH2. The vibrational study was carried out
by utilizing the Raman spectra of the gas and liquid as well as the infrared spectra of the gas,
variable temperature xenon solutions, and solid. Additionally to investigate the mixing of the NH2
motions with the other atom motions of 2,2,3,3,3-pentafluoropropyl the Raman and infrared
205
spectra of the corresponding ND2 molecule were also recorded. These studies were for determining
the conformational stabilities, identify the most stable forms, obtain predicted r0 structural
parameters, and provide vibrational assignments for both molecules.
To support the vibrational study, we have carried out ab initio calculations with basis sets
up to aug-cc-pVTZ as well as those with diffuse functions, i.e., 6-311+G(2df,2pd). We found that
there can be significant differences in the predicted stabilities of the ethylamine molecules with the
size of basis set as well as with the inclusion of diffuse functions at the MP2 level. We have also
carried out density functional theory (DFT) calculations by the B3LYP method with the same basis
sets. We have calculated optimized geometries, conformational stabilities, harmonic force fields,
infrared intensities, Raman activities and depolarization ratios. The results of these spectroscopic
and theoretical studies are reported herein.
Experimental
The sample of 2,2,3,3,3-pentafluoropropylamine was prepared by the reaction of aqueous
ammonia
and
1,1,3-trihydrotetrafluoropropyl-p-toluenesulfonate
in
the
presence
of
sodiumethylbenzene sulfonate as surfactant as previously reported [91] . The reaction mixture was
heated for 24 hrs. at 175° C with continuous stirring. The compound was verified by infrared and
nuclear magnetic resonance spectroscopy.
The mid-infrared spectrum of the gas phase of CF3CF2CH2NH2 (Fig. 31A) and
CF3CF2CH2ND2 (Fig. 32A) and the solid (Fig. 31D) phase of CF3CF2CH2NH2 were obtained from
3600 to 400 cm-1 on a Perkin-Elmer model 2000 Fourier transform spectrometer equipped with a
Ge/CsI beamsplitter and a DTGS detector. Atmospheric water vapor was removed from the
spectrometer housing by purging with dry nitrogen. The theoretical resolution used to obtain the
spectrum of the gas was 0.5 cm-1 and 128 interferograms were added and transformed with a boxcar
truncation function. For the spectrum of the solid theoretical resolution of 2 cm-1 was used with 128
206
interferograms added and truncated. Multiple annealings were required to obtain satisfactory spectra
of the solid.
The far infrared spectra (380 to 30 cm-1) of the gas phase of CF3CF2CH2NH2 and
CF3CF2CH2ND2 were obtained at a resolution of 0.10 cm-1 on a Nicolet model 8000 interferometer
equipped with a vacuum bench and a liquid-helium cooled Ge bolometer containing a wedged
sapphire filter and polyethylene windows. A 12.5 μ Mylar beam splitter was employed and the sample
was contained in a 1-m cell. Interferograms for both the reference and sample cells were recorded 250
times, averaged, and transformed using a boxcar apodization.
The mid-infrared spectra (3600 to 400 cm-1) of CF3CF2CH2NH2 dissolved in liquefied
xenon (Fig. 31B) at ten different temperatures (-55 to -100C) were recorded on a Bruker model
IFS-66 Fourier transform spectrometer equipped with a globar source, a Ge/KBr beamsplitter and
a DTGS detector. In all cases, 100 interferograms were collected at 1.0 cm-1 resolution, averaged
and transformed with a boxcar truncation function. For these studies, a specially designed cryostat
cell was used. It consists of a copper cell with a path length of 4 cm with wedged silicon windows
sealed to the cell with indium gaskets. The temperature was maintained with boiling liquid
nitrogen and monitored by two Pt thermoresistors. After cooling to the designated temperature, a
small amount of the sample was condensed into the cell and the system was then pressurized with
the noble gas, which condensed in the cell, allowing the compound to dissolve.
The Raman spectra were recorded on a Cary model 82 spectrophotometer equipped with a
Spectra-Physics model 171 argon ion laser operating on the 5145 Å line. The spectra of the gas,
CF3CF2CH2NH2 (Fig. 33A) and CF3CF2CH2ND2 (Fig. 34A), were recorded by using a standard
Cary multipass accessory, and the laser power at the sample was 1 W. The spectra of the liquid
phase, CF3CF2CH2NH2 (Fig. 35A) and CF3CF2CH2ND2 (Fig. 36A), were recorded from the
sample sealed in a glass capillary. The measurements of the Raman wavenumbers are expected to
207
be accurate to  2 cm-1. All of the observed bands in the Raman spectra of the gas and liquid along
with their proposed assignments and depolarization values are listed in Tables 53 (Tt) and 54 (Tg)
for CF3CF2CH2NH2 and Tables 55 (Tt) and 56 (Tg) for CF3CF2CH2ND2. The first indicator is the
NCCC dihedral angle (G = gauche or T = trans) and the second one (g = gauche or t = trans) is the
relative position of the lone pair of electrons on nitrogen with respect to β-carbon.
Ab Initio Calculations
The LCAO-MO-SCF restricted Hartree-Fock calculations were performed with the
Gaussian-03 program [21] using Gaussian-type basis functions. The energy minima with respect to
nuclear coordinates were obtained by the simultaneous relaxation of all geometric parameters
using the gradient method of Pulay [22]. Several basis sets as well as the corresponding ones with
diffuse functions were employed with the Møller-Plesset perturbation method [23] to the second
order (MP2(full)) as well as with the density functional theory by the B3LYP method. The
predicted conformational energy differences are listed in Table 57.
In order to obtain a complete description of the molecular motions involved in the
fundamental modes of 2,2,3,3,3-pentafluoropropylamine, a normal coordinate analysis has been
carried out. The force field in Cartesian coordinates was obtained with the Gaussian 03 program
[21] at the MP2(full) level with the 6-31G(d) basis set. The internal coordinates used to calculate
the G and B matrices are given in Table 58 with the atomic numbering shown in Fig. 37. By using
the B matrix [24], the force field in Cartesian coordinates was converted to a force field in internal
coordinates. Subsequently, scaling factors of 0.88 for CH stretches and CH2 deformation, 0.70 for
NH2 bends (excluding deformation) and 0.90 for all other modes except torsions and heavy atom
bends were applied, along with the geometric average of the scaling factors for the interaction
force constants, to obtain the fixed scaled force field and resultant wavenumbers. A set of
symmetry coordinates was used (Table 59) to determine the corresponding potential energy
208
distributions (P.E.D.s). A comparison between the observed and calculated wavenumbers, along
with the calculated infrared intensities, Raman activities, depolarization ratios and potential energy
distributions for the trans-trans and trans-gauche conformers of the CF3CF2CH2NH2 and
CF3CF2CH2ND2 molecules are listed in Tables 53, 54, 55, and 56, respectively.
The vibrational spectra were predicted from the MP2(full)/6-31G(d) calculations. The
predicted scaled wavenumbers were used together with a Lorentzian function to obtain the
simulated spectra. Infrared intensities were obtained based on the dipole moment derivatives with
respect to Cartesian coordinates. The derivatives were transformed with respect to normal
coordinates by (u/Qi) = j (u/Xj)Lij, where Qi is the ith normal coordinate, Xj is the jth
Cartesian displacement coordinate, and Lij is the transformation matrix between the Cartesian
displacement coordinates and the normal coordinates. The infrared intensities were then calculated
by (N)/(3c2) [(x/Qi)2 + (y/Qi)2 + (z/Qi)2]. A comparison of the infrared spectra of the
solid phase and the predicted infrared spectra for the pure trans-trans conformer are shown in Fig.
31 and the corresponding figure of the –ND2 molecule with the infrared spectra of the vapor phase
and the predicted infrared spectra for the pure trans-trans and trans-gauche conformers, as well as,
the mixture of the two conformers with relative concentrations calculated for the equilibrium
mixture at 25ºC by using the experimentally determined enthalpy difference are shown in Fig. 32.
The predicted spectrum is in good agreement with the experimental spectrum which shows the
utility of the scaled predicted wavenumbers and predicted intensities for supporting the vibrational
assignment.
Additional support for the vibrational assignments was obtained from the simulated Raman
spectra. The evaluation of Raman activity by using the analytical gradient methods has been
developed [25, 27, 37] and the activity Sj can be expressed as: Sj = gj(45αj2 + 7βj2), where gj is the
209
degeneracy of the vibrational mode j, αj is the derivative of the isotropic polarizability, and βj is the
anisotropic polarizability. To obtain the Raman scattering cross sections, the polarizabilities are
incorporated into Sj by multiplying Sj with (1-ρj)/ (1+ρj) where ρj is the depolarization ratio of the
jth normal mode. The Raman scattering cross sections and calculated wavenumbers obtained from
the Gaussian 03 program were used together with a Lorentzian function to obtain the simulated
Raman spectra. Comparison of experimental Raman spectra of the liquid or gas phases of
CF3CF2CH2NH2 and CF3CF2CH2ND2 with the respective predicted Raman spectra for the pure
trans-trans and trans-gauche conformers as well as the mixture of the two conformers with relative
concentrations calculated for the equilibrium mixture at 25°C by using the experimentally
determined enthalpy difference are shown in Fig. 33, 34, 35, and 32. The spectrum of the mixture
should be compared to that of the Raman spectrum of the liquid and/or vapor at room temperature.
The predicted spectrum is in reasonable agreement with the experimental spectrum which shows
the utility of the predicted Raman spectra for the supporting vibrational assignments.
Vibrational Assignment
There is a very large amount of information available in addition to the predicted
fundamental wavenumbers for both the conformers for the “normal” species and the ND2
isotopomers which includes band contours, Raman activities and infrared intensities and
depolarization values. These data for both isotopomers are listed in Tables 53, 54, 55, and 56 for
the Tt and Tg conformers. Additionally, the Raman spectra of the gas has been obtained which
usually provides mainly the A′ bands for molecules which have a plane of symmetry and the data
are more definitive than the depolarization values. Also, the “group wavenumbers” for the various
parts of the molecules, i.e., CF3, NH2, ND2, CH2, and CF2, are well known so vibrational
assignments for the more stable Tt conformer could be made with significant confidence.
However, the fundamental wavenumbers for the Tg form might be expected to be very similar to
210
those for the Tt conformer since the difference is only the position of the NH2 (ND2) group. Thus
many of the modes could be nearly in pairs except when the P.E.D differ extensively. Since one of
the primary goals of this research is the conformational determination the assignments in the
“finger print” region are the most critical for obtaining this information.
The NH2 and CH2 stretches were readily assigned as were the deformations and the CH2
wag. However beginning in the thirteen hundred region there was extensive mixing and the
description of the molecular motion was sometimes rather arbitrary. It became particularly difficult
to assign the fundamentals in the twelve hundred to one thousand wavenumbers where the bands
are very intense in the infrared spectrum but rather weak in the Raman spectrum with extensive
overlap of the bands for the second conformer. Therefore, we emphasized the spectral region from
400 to 1000 cm-1 for assigning all the modes for both conformers so appropriate conformer pairs
could be identified to obtain the enthalpy difference.
Most of these modes are primarily CF motions with the significantly poorer predicted
wavenumbers than compared to those for CH motions. Most of these bands have contributions
from at least three symmetry coordinates with many of them with contributions (10% or more)
from four coordinates. Although many of the descriptions are more for bookkeeping than
describing the molecular motions the choice of the bands for the fundamentals of the two
conformers were distinguishable for most cases.
The fundamentals below 800 cm-1 are mainly CF bands which were not scaled and they are
predicted quite well. These include the CF3 symmetric deformation (13, 720 cm-1), 2 CF3
antisymmetric deformations (14 and 27, 585 cm-1), CF2 deformation (15, 526 cm-1) and the CF2
twist (29, 359 cm-1) for the Tt conformer.
One might expect similar wavenumbers for these modes for the Tg conformer and for some
modes the wavenumbers are relatively close. For example the CF3 antisymmetric deformations at
211
585 and 536 cm-1 for the Tg form are comparable to the assignment of 585 cm-1 for both of these
modes for the Tt form but the CF2 twist is observed at 241 cm-1 for the Tg conformer rather than
near to this mode at 359 cm-1 for the Tt form. However, the CF2 wag for the Tg conformer is
observed at 345 cm-1 but there are five symmetry coordinates with the largest one 20%
contributing to this vibration. Therefore, there is relatively little correspondence between the
vibrational modes of the carbon-fluorine bending modes for the Tt and Tg conformers as can be
observed from the data in Tables 53, 54, 55, and 56.
Conformational Stability
To determine the enthalpy differences among the two observed conformers of 2,2,3,3,3pentafluoropropylamine, the sample was dissolved in liquefied xenon and the mid-infrared spectra
were recorded as a function of temperature from -55 to -100°C. Relatively small interactions are
expected to occur between xenon and the sample though the sample can associate with itself
forming a dimmer, trimer or higher order complex. However, due to the very small concentration
of sample (~10-4 molar) self association is greatly reduced. Therefore, only small wavenumber
shifts are anticipated for the xenon interactions when passing from the gas phase to the liquefied
xenon solutions. A significant advantage of this study is that the conformer bands are better
resolved in comparison with those in the infrared spectrum of the gas. From ab initio calculations,
the dipole moments of the two conformers are predicted to have similar values and the molecular
sizes of the two conformers are nearly the same, so the ΔH value obtained from the temperature
dependent FT-IR study is expected to be close to that for the gas [39, 40, 42, 43].
Once confident assignments have been made for the fundamentals of both conformers the
task was then to find a pair or pairs of bands from which the enthalpy determination could be
obtained. To minimize the effect of combination and overtone bands in the enthalpy determination
it is desirable to have the lowest wavenumber pair(s) that is possible to use for the determination.
212
The bands should also be sufficiently resolved as to determine their intensities. The selection of the
bands to use in the enthalpy determination was complicated due to the presence of high intensity
bands for both conformers and the near overlap of many modes. The fundamentals at 521 and 867
cm-1 were selected for the Tt conformer band as they are free of interfering bands and in the lower
wavenumber region of the spectrum. The band at 585 cm-1 was also selected for the Tt form as the
predicted interfering bands are estimated to be of very low intensity and the fundamental is in the
lower region of the spectrum. For the Tg conformer the selection of the bands was more difficult
since there are few bands with sufficient separation from the corresponding Tt fundamentals to
determine their intensities. The fundamentals at 462 and 638 cm-1 are relatively free from
interfering bands which allowed the determination of their band intensities to be measured.
The intensities of the individual bands were measured as a function of temperature and their
ratios were determined (Fig. 38). By application of the van’t Hoff equation lnK = H/(RT) 
S/R, the enthalpy difference was determined from a plot of lnK versus 1/T, where H/R is the
slope of the line and K is substituted with the appropriate intensity ratios, i.e. Iconf-1 / Iconf-2, etc. It
was assumed that H, S, and α are not functions of temperature in the range studied.
These five bands, three for Tt and two for Tg, were utilized for the determination of the
enthalpy difference by combining them to form 6 band pairs. The enthalpy difference was
determined from these six band pairs with a value of 280  6 cm-1 (Table 60). This value is in
excellent agreement with the corresponding predicted values from ab initio calculations with large
basis sets (Table 57). This error limit is derived from the statistical standard deviation of one sigma
of the measured intensity data taken as a single data set, but it does not take into account small
associations with the liquid xenon or the interference of overtones and combination bands in near
coincidence with the measured fundamentals. The variations in the individual values are
undoubtedly due to these types of interferences, but by taking several pairs, the effect of such
213
interferences should cancel. However, this statistical uncertainty is probably better than can be
expected from this technique and, therefore, an enthalpy difference of 280  14 cm-1 is probably
more realistic. From the enthalpy difference the abundance of the Tg conformer present at ambient
temperature is estimated to be 34 ± 2%.
Structural Parameters
It should be possible to estimate structural parameters for the Tt and Tg conformers of
2,2,3,3,3-pentafluoropropylamine by adjusting the MP2(full)/6-311+G(d,p) calculated structural
parameters using similar molecules as a guideline. The CH bond distances predicted by
MP2(full)/6-311+G(d,p) ab initio calculations can be used as a good prediction without any
adjustment as we [44] have shown that ab initio MP2(full)/6-311+G(d,p) calculations predict the r0
CH distances for more than fifty substituted hydrocarbons to better than ± 0.002 Å compared to the
experimentally determined values from isolated CH stretching wavenumbers. The heavy atom and
NH2 parameters can be estimated by using the corresponding parameters in 2,2-difluoroethylamine
[61] and 2,2,2-trifluoroethylamine [60] to adjust the corresponding MP2(full)/6-311+G(d,p)
calculated parameters. It is believed that the estimated r0 parameters reported herein for 2,2,3,3,3pentafluoropropylamine have listed uncertainty values that should be within the experimentally
determined parameters. Thus, the listed rotational constants given in Table 58 should make it
relatively easy to assign the microwave spectrum from which the experimental parameters could
be obtained.
Discussion
The assignments of the fundamentals for the Tt conformer could be made rather straight
forwardly since they are in such a large proportion (66 ± 2%). However, the assignments for the
Tg conformer was much more difficult than would be expected since change of the conformers is
due to only the rotation of the NH2 group. Nevertheless, the rotation of the NH2 results in a very
214
significant structural change since there is undoubtedly hydrogen bonding with both hydrogen
atoms on the amine with the two fluorine atoms on the -carbon atom. These result in drastic
changes in the wavenumbers for the modes of the Tg conformer compared to the corresponding
modes of the Tt form. Therefore, significant reliance on the predicted wavenumbers was necessary
to assign many of the modes of the Tg conformer. Nevertheless the predicted wavenumbers for the
Tt form had an average error of 11 cm-1 which is 0.9% for the A′ modes and 11 cm-1 for the A′′
modes, which is 1% error. This value is slightly larger than normally obtained with MP2(full)/631G(d) predictions for hydrocarbons. Similarly, the Tg form had an average error of 10 cm -1 which
is 1.0%. The predictions of the wavenumbers for the two ND2 molecules are 0.8% for Tt and 1.1 %
for Tg. A significant amount of the error is due to the prediction for the NH2 (ND2), CH2, and the
symmetric CF3 stretches along with either the NH2 twist or wag. However, the assignments for the
ND2 isotopomers give reasonable agreement with the Teller-Redlich product rule which supports
the assignments.
Because of the large amount of mixing it was difficult in some cases to give a distinct
description of the vibration. This is particular true for several of the vibrations for the Tg
conformer which has no symmetry elements, i.e. 5, 13, 17 and 18 where this last one had
contributions from 7 symmetry coordinates with 6 of them with 10% and the other one 15%. The
description of these vibrations is more for bookkeeping than to provide descriptions for the major
atom motions. For the Tt form with a plane-of-symmetry there is still extensive mixup particularly
with the CF2 modes as shown by the CF2 symmetry stretch (1270 cm-1) with only 13S7 but 29S8 for
the CF3 antisymmetric stretch which is assigned at 1206 cm-1 in the 44S8. Similar mixup is found
for the CF2 wag (1023 cm-1) with 17S10 and 24S12 which is the CF3 symmetric stretch (781 cm-1)
with 25S12 which is the largest value of the four contributions of 10% or more. These values are
215
representative of the P.E.D.s for many of the CF motions in the A′ block as well as many of the
modes for the Tg conformer. Similar mixing is also found for the N- deuterated molecules.
With the confidence of the assignments of the lower wavenumber modes for the Tt and Tg
conformers and the use of 6 band pairs as well as the statistical uncertainty the value for the
enthalpy difference is believed to be well determined. The uncertainty of the value for each
individual pair ranging from a high of 19 cm-1 to a low of 6 cm-1. This small spread is a strong
indication that there is little interference from overtones or combination bands having a significant
effect on the determined enthalpy difference obtained in this study. Usually we expect an error of
ten percent based on this method for obtaining the enthalpy value but we have reduced the
estimated error to only 5% because of the small variations of the individual statistical uncertainties.
Also the degeneracy of the Tg conformer aided the study since a significant amount of this
conformer was present (34  2%) which made the assignment of the fundamentals for it more
confidently made.
The experimentally determined enthalpy difference between the more stable Tt conformer
and the Tg form with the value of 280  14 cm-1 is much larger than expected for the simple
rotation of the NH2 from the trans to the gauche position where such a change for the CH3CH2NH2
molecule has a value of 54  4 cm-1 [36]. However the ab initio and density functional theory
calculations for CF3CF2CH2NH2 give similar energy differences with the largest basis set or with
the average of all the values from the basis sets utilized. This very large value is undoubtedly due
to the intramolecular hydrogen bonding of the amine hydrogens with the fluorine atoms of the βcarbon atom. This effect is clearly demonstrated by the HCF2CH2NH2 molecule where the most
stable conformer is the Tt form and with the rotation of the NH2 moiety the Tg predicted energy
difference is 813 cm-1 from the MP2(full)/aug-cc-pVTZ calculations. Therefore the value of 280 
216
14 cm-1 for the CF3CF2CH2NH2 molecule is a reasonable enthalpy difference which is undoubtedly
affected by the steric effect of the perfluormethyl group.
The estimated structural parameters obtained in the current study should be of considerable
value for assigning the microwave spectrum for both the Tt and Tg conformers. Such a study could
be very useful to determine the role of hydrogen bonding contributing to the relatively large
enthalpy difference between the Tt and Tg forms. Since there are no naturally occurring isotopes
of fluorine there are a rather small number of molecules containing fluorine atoms where the
structures have been determined from the microwave data.
It is interesting to compare the conformation stability of n-propylamine with 2,2,3,3,3pentafluoropropylamine to evaluate the effect of the fluorine on two of the carbon atoms. In our
earlier study [67] of the normal molecule the Raman spectra were used to show the presence of a
least two conformers in the fluid phases. Also it was concluded that the trans-trans (Tt) form was
the conformer present in the solid state and the trans-gauche (Tg) form was the second most stable
conformer in the fluid phases and it had nearly equal energy to the trans-trans form from ab intio
MP2/6-31G(d) calculations. However from the potential function obtained from the observed
wavenumbers for the asymmetric torsion transitions it was calculated that the enthalpy of 251 cm-1
(3.35 kJ/mol) was the difference between the Tt and Tg forms. This value is not significantly
smaller than the value of 280 ±14 cm-1 for the pentafluoride molecule. It would be of interest to
obtain the enthalpy difference of n-propylamine by a variable temperature study of the Raman
spectra in a rare gas solution. Similarly the determinations of the enthalpy differences of
CH3CF2CH2NH2 and HCF2CH2NH2 would be of interest to assess the effect of the fluorine atoms
on the enthalpy differences of these amines.
217
Table 53: Observed and calculateda wavenumbers (cm -1) for trans-trans 2,2,3,3,3-pentafluoropropylamine.
Vib.
No.
218
A 1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
A 20
21
22
23
24
25
26
27
28
29
30
31
Fundamental
NH2 symmetric stretch
CH2 symmetric stretch
NH2 symmetric deformation
CH2 symmetric deformation
CCC antisymmetric stretch
CH2 wag
CF2 symmetric stretch
CF3 antisymmetric stretch
CN stretch
CF2 wag
NH2 wag
CF3 symmetric stretch
CF3 symmetric deformation
CF3 antisymmetric
deformation
CF2 symmetric deformation
CF3 symmetric rock
CCC symmetric stretch
CCN bend
CCC bend
NH2 antisymmetric stretch
CH2 antisymmetric stretch
CH2 twist
CF3 antisymmetric stretch
NH2 twist
CF2 antisymmetric stretch
CH2 rock
CF3 deformation
CF2 rock
CF2 twist
NH2 torsion
CF3 wag
ab
initio
fixed
scaledb
IR
int.
Raman
act.
IR
Gas
3530
3142
1733
1538
1469
1413
1308
1270
1174
1064
949
796
725
592
516
378
348
284
163
3632
3205
1442
1298
1268
1136
904
593
453
374
301
224
3349
2948
1606
1445
1409
1349
1265
1211
1110
1022
847
750
704
582
512
375
339
282
163
3446
3006
1330
1220
1200
1043
834
584
446
360
265
224
2.7
9.0
36.5
4.5
38.2
28.1
37.2
284.3
18.0
133.2
227.3
59.9
0.3
1.8
9.0
7.1
5.9
10.1
1.0
6.6
4.8
24.8
133.0
173.4
46.6
7.4
0.9
0.4
10.6
55.7
5.3
71.5
71.0
6.7
8.1
1.3
2.7
1.5
0.4
3.8
0.3
0.3
9.4
0.5
2.7
0.3
1.8
2.4
0.0
0.1
41.3
45.6
9.1
0.8
0.9
1.9
1.6
1.4
0.2
1.6
1.5
0.1
3371
2962
1637
1450
1395
1350
1270
1206
1095
1023
865
781
720
585
526
338
280
160
3440
2988
1337
1236
1148
1041
~821
585
464
359
271
226
Raman
gas
3372
2961
1634
1449
1394
1349
1271
1207
1095
1022
863
780
718
587
527
375
340
282
158
-
Raman xenon
liquid solution
3360
2954
1632
1445
1391
1343
1270
1196
1087
1017
868
779
724
588
526
378
344
287
154
3420
2982
1338
1232
1134
1039
812
588
465
360
270
220
3354
2949
1628
1439
1389
1338
1271
1206
1089
1019
867
777
712
585
521
3422
2975
1338
1231
1134
1036
804
585
462
-
IR Solid
P.E.D.c
3374
2960
1637/1634
1447
1393
1341
1278
1203
1090
1022
869
760
720
621/615
527/522
373
338
3437
2985
1341
1231
1145
1040
817
585/590
462
358
-
100S1
100S2
83S3,15S11
98S4
31S5,22S6
60S6,16S17
13S7,29S8,13S19,12S16
44S8,19S7,13S5
63S9,10S18
17S10,24S12,,17S9,14S5,10S13
47S11,11S7,11S9,10S3,
25S12,22S7,17S11,17S17
28S13,25S12,13S10,10S11
25S14,15S8,12S16,10S5
44S15,39S14
22S16,24S15,23S14
38S17,25S13,12S18
31S18,32S16,31S10
57S19,22S16,14S18
100S20
100S21
76S22,12S24
64S23,12S27
22S24,28S26,26S25,12S23
41S25,32S24,23S22
51S26,33S24,13S25
39S27,24S23,15S30,12S28,
35S28,40S27,11S31
55S29,13S30,12S31
82S30,12S29
49S31,26S28,23S29
Band
Contours
A*
B*
26
74
44
56
2
98
18
82
96
4
84
16
33
67
- 100
73
27
100
79
21
100
65
35
50
50
17
83
100
100
95
5
20
80
-
108
108
0.02
0.1
108
108
100S32
32 CCC asymmetric torsion
68
68
0.02
0.0
78
78
100S33
33 CF3 torsion
a
MP2(full)/6-31G(d) ab initio calculations, scaled wavenumbers, infrared intensities (km/mol), Raman activities (Å 4/amu), depolarization ratios and potential energy
distributions (P.E.D.s)
b
MP2(full)/6-31G(d) fixed scaled wavenumbers with factors of 0.88 for CH stretches and deformations, 0.7 for NH bends (excluding deformation), 1.0 for heavy atom
bends, and 0.90 for all other modes.
c
Calculated with MP2(full)/6-31G(d) and contributions of less than 10% are omitted.
*
A, B and C values in the last three columns are percentage infrared band contours.
-
219
Table 54: Observed and calculateda wavenumbers (cm -1) for trans-gauche 2,2,3,3,3-pentafluoropropylamine.
220
Vib.
No.
Fundamental
ab
initio
fixed
scaledb
IR
int.
20
1
21
2
3
4
5
6
22
8
23
7
25
13
9
24
26
11
12
18
17
27
14
28
15
26
10
30
29
31
19
32
NH2 antisymmetric stretch
NH2 symmetric stretch
CH2 antisymmetric stretch
CH2 symmetric stretch
NH2 symmetric deformation
CH2 symmetric deformation
CCC antisymmetric stretch
CH2 wag
CH2 twist
CF3 antisymmetric stretch
CF3 antisymmetric stretch
CF2 symmetric stretch
CF2 antisymmetric stretch
CF3 symmetric deformation
CN stretch
NH2 twist
CH2 rock
NH2 wag
CF3 symmetric stretch
CCN bend
CCC symmetric stretch
CF3 antisymmetric deformation
CF3 antisymmetric deformation
CF2 rock
CF2 deformation
CF3 symmetric rock
CF2 wag
NH2 torsion
CF2 twist
CF3 wag
CCC bend
CCC asymmetric torsion
3651
3548
3198
3133
1737
1537
1483
1436
1411
1309
1271
1264
1236
1200
1172
1083
935
888
775
658
614
593
527
467
393
373
357
309
250
223
202
128
3463
3366
3000
2939
1608
1446
1420
1359
1299
1254
1215
1211
1165
1142
1106
1005
841
805
741
641
600
583
522
462
388
364
342
284
243
222
202
127
8.5
3.7
5.3
10.0
36.6
14.5
17.5
4.8
24.3
42.4
227.9
238.6
110.9
153.1
10.1
47.8
104.0
176.1
35.2
22.2
14.3
0.5
2.3
7.4
7.0
4.8
11.2
28.4
16.5
8.2
4.8
1.1
Raman
act.
37.3
65.6
51.1
86.1
5.6
8.8
3.3
0.9
8.1
2.5
1.1
0.6
2.4
0.6
1.8
0.8
1.4
1.0
7.2
3.7
1.8
1.7
0.8
1.1
1.9
1.3
1.5
0.8
0.8
0.1
0.2
0.2
IR
gas
Raman
gas
Raman
liquid
3440
3382
2983
2954
1637
1450
1411
1365
1298
1243
1215
1206
1182
1148
1115
1005
821
750
761
640
617
585
536
464
360
345
283
241
218
192
114
3441
3383
2984
2953
1634
1449
1409
1366
1294
1245
1215
1207
1184
1146
1115
1004
819
749
760
640
616
587
536
462
386
359
340
282
249
216
193
115
3430
3372
2982
2954
1632
1445
1405
1372
1308
1242
1215
1196
1180
1134
1114
1006
1006 006
812
1006
746
768
640
616
588
540
465
387
360
344
287
247
220
192
117
xenon
solution
3437
3366
2975
2949
1628
1439
1404
1364
1298
1244
1215
1206
1180
1134
1112
1001
815
741
756
638
612
585
537
462
-
P.E.D.c
99S20
99S1
97S21
97S2
82S3,16S11
88S4
21S5,13S22,11S13,11S17,11S4,10S19
70S6,10S5
57S22,16S24
33S8,10S19,10S16,10S6
73S23,13S27
16S7,38S8,12S5,10S17
35S25,10S26,10S24,10S6
11S13,18S25,15S12,12S26,11S24
77S9
29S24,1822,15S7,10S12
31S26,20S24,19S11,10S7
41S11,18S26,10S3
47S12,22S13,13S17
15S18,10S10,10S25,10S8,10S28,10S16,10S14
14S17,22S13,19S15,19S7,10S26,10S10
34S27,21S23,12S31,10S28
52S14,10S27,10S8
15S28,24S27,17S15,10S10,10S17
21S15,17S29,17S27,14S31,10S18
16S26,22S29,18S15,15S14
20S10,17S13,15S5,14S17,13S30
47S30,22S16,12S10
28S29,25S18,24S30,10S16
47S31,36S28,10S29
50S19,16S26,10S18
71S32,23S33
Band
Contours
A* B*
C*
29 45
26
2 22
76
26 29
45
1 62
37
20 14
66
62
6
32
94
6
35
65
92
4
4
38 56
6
7
9
84
17 43
40
53 15
32
10 80
10
39 28
33
54 30 165
64 27
9
79 21
89 10
1
5 67
28
66 28
6
64 32
4
9
1
90
57 19
24
14 86
37 26
37
16 62
22
27 54
19
27 17
56
33
1
66
17 62
21
23 60
17
33
CF3 torsion
47
47
0.3
0.1
48
49
- 76S33,23S32
7 15
MP2(full)/6-31G(d) ab initio calculations, scaled wavenumbers, infrared intensities (km/mol), Raman activities (Å 4/amu), depolarization ratios and potential energy
distributions (P.E.D.s)
b
MP2(full)/6-31G(d) fixed scaled wavenumbers with factors of 0.88 for CH stretches and deformations, 0.7 for NH bends (excluding deformation), 1.0 for heavy atom
bends, and 0.90 for all other modes.
c
Calculated with MP2(full)/6-31G(d) and contributions of less than 10% are omitted.
*
A, B and C values in the last three columns are percentage infrared band contours.
a
78
221
Table 55: Observed and calculateda wavenumbers (cm -1) for trans-trans 2,2,3,3,3-pentafluoropropylamine-ND2.
Vib.
No.
222
A 2
1
4
5
6
7
8
3
10
9
17
12
11
14
15
16
13
18
19
A 21
20
22
23
25
26
24
27
28
29
31
30
32
Fundamental
CH2 symmetric stretch
ND2 symmetric stretch
CH2 symmetric deformation
CCC antisymmetric stretch
CH2 wag
CF2 symmetric stretch
CF3 antisymmetric stretch
ND2 symmetric deformation
CF2 wag
CN stretch
CCC symmetric stretch
CF3 symmetric stretch
ND2 wag
CF3 antisymmetric deformation
CF2 symmetric deformation
CF3 symmetric rock
CF3 symmetric deformation
CCN bend
CCC bend
CH2 antisymmetric stretch
ND2 antisymmetric stretch
CH2 twist
CF3 antisymmetric stretch
CF2 antisymmetric stretch
CH2 rock
ND2 twist
CF3 antisymmetric deformation
CF2 rock
CF2 twist
CF3 wag
ND2 torsion
CCC asymmetric torsion
ab
initio
3143
2553
1537
1472
1421
1308
1284
1262
1123
1052
848
756
685
591
512
373
340
274
159
3205
2677
1384
1268
1236
1057
785
593
449
359
236
220
100
fixed
scaledb
2948
2421
1445
1410
1355
1265
1216
1192
1071
1005
800
723
622
579
506
370
330
271
159
3007
2539
1311
1219
1165
980
686
583
444
354
226
197
100
IR
int.
8.8
3.2
3.6
36.6
36.8
36.7
254.1
44.4
47.6
128.7
67.3
49.9
44.9
4.6
8.8
5.1
6.5
14.8
1.1
4.5
4.8
38.4
187.9
126.0
33.5
0.7
0.7
0.01
2.7
0.01
36.8
0.2
Raman
act.
67.7
38.3
8.5
1.1
2.6
1.5
0.9
1.8
3.4
1.8
2.9
3.9
1.9
3.3
0.3
1.3
2.4
0.1
0.1
43.4
22.5
7.3
0.7
1.8
2.3
0.9
1.3
0.3
0.9
0.01
0.9
0.1
IR
gas
2965
2485
1446
1402
1351
1268
1216
1191
1082
1007
800
736
623
582
518
368
363688
326
273
159
2986
2560
1327
1226
1165
980
683
351
224
192
108
Raman
gas
2965
2483
1449
1404
1354
1265
1216
1191
1080
1008
801
737
624
585
520
370
332
273
155
-
Raman
liquid
2956
2462
1444
1404
1354
1270
1217
1192
1078
1003
804
741
623
587
521
375
337
278
150
2986
2540
1326
1226
1168
980
587
447
359
190
108
P.E.D.c
100S2
100S1
98S4
31S5,25S6,12S13,10S12
54S6,17S17,10S13
13S7,29S8,13S19,12S116
31S8,18S3,12S7
50S3,15S8,11S9
12S10,29S9,13S12
40S9,16S12,10S10,10S5
16S17, ,32S7,17S18,10S11
44S12,32S13
52S11,12S7,10S3
29S14,16S8,11S16,10S5
41S15,37S14
2416,3015,23S14,10S5
25S13,36S17,10S10
36S18,29S16,26S10
56S19,20S16,16S18
100S21
100S20
80S22,11S25
72S23,13S27
36S25,32S26,10S22,10S28
34S26,39S25,17S24
73S24,21S26
39S27,24S23,15S31,11S28
35S28,41S27,12S31
61S29,11S30
47S31,29S29,18S28
86S30,10S28
98S32
Band
Contours
A*
B*
100
36
64
45
55
23
77
14
86
100
65
35
33
67
100
10
90
80
20
100
51
49
93
7
80
20
100
40
60
100
96
4
-
CF3 torsion
68
68
0.01
0.02
69
78
99S33
33
MP2(full)/6-31G(d) ab initio calculations, scaled wavenumbers, infrared intensities (km/mol), Raman activities (Å 4/amu), depolarization ratios and potential energy
distributions (P.E.D.s)
b
MP2(full)/6-31G(d) fixed scaled wavenumbers with factors of 0.88 for CH stretches and deformations, 0.7 for ND bends (excluding deformation), 1.0 for heavy atom
bends, and 0.90 for all other modes.
c
Calculated with MP2(full)/6-31G(d) and contributions of less than 10% are omitted.
*
A, B and C values in the last three columns are percentage infrared band contours.
a
223
Table 56: Observed and calculateda wavenumbers (cm -1) for trans-gauche 2,2,3,3,3-pentafluoropropylamine-ND2.
224
Vib.
No.
Fundamental
21
2
20
1
4
5
6
22
8
23
7
3
25
14
9
26
12
13
24
15
27
11
14
28
29
16
17
18
31
30
19
32
CH2 antisymmetric stretch
CH2 symmetric stretch
ND2 antisymmetric stretch
ND2 symmetric stretch
CH2 symmetric deformation
CCC antisymmetric stretch
CH2 wag
CH2 twist
CF3 antisymmetric stretch
CF3 antisymmetric stretch
CF2 symmetric stretch
ND2 symmetric deformation
CF2 antisymmetric stretch
CF2 wag
CN stretch
CH2 rock
CF3 symmetric stretch
CF3 symmetric deformation
ND2 twist
CF2 deformation
CF3 antisymmetric deformation
ND2 wag
CF3 antisymmetric deformation
CF2 rock
CF2 twist
CF3 symmetric rock
CCC symmetric stretch
CCN bend
CF3 wag
ND2 torsion
CCC bend
CCC asymmetric torsion
ab
initio
3199
3134
2691
2566
1537
1481
1436
1346
1297
1291
1264
1262
1209
1171
1114
1015
809
781
747
627
612
592
521
463
383
364
345
271
231
216
194
118
fixed
scaledb
3000
2940
2552
2434
1446
1420
1366
1279
1242
1219
1211
1197
1157
1116
1048
944
758
702
687
598
588
580
512
457
379
361
336
259
226
206
187
117
IR
int.
5.0
9.7
6.0
4.1
14.6
18.7
1.6
43.3
57.9
257.4
174.8
61.6
146.8
122.8
24.6
28.3
25.7
86.9
18.5
22.2
30.3
16.5
4.3
10.2
8.8
1.8
3.6
7.0
2.1
16.8
14.2
2.0
Raman
act.
IR
gas
48.9
82.6
20.7
35.5
9.0
3.2
0.7
5.5
3.7
1.4
0.9
1.6
2.1
0.4
3.2
1.7
3.4
2.4
2.2
1.6
2.6
3.0
0.6
0.8
1.8
1.2
1.0
0.2
0.1
0.3
0.6
0.2
2986
2959
2575
2494
1446
1412
1367
1284
1238
1225
1210
1198
1155
1110
1063
941
764
736
736
613
582
582
496
459
378
358
330
258
224
205
180
118
Raman
gas
2986
2956
2576
2493
1449
1414
1364
1285
1238
1226
1209
1198
1155
1110
1066
941
766
737
737
614
585
585
496
461
379
356
338
258
221
204
180
118
Raman
liquid
2986
2956
2555
2473
1444
1412
1364
1286
1238
1226
1210
1199
1144
1106
1064
943
767
741
741
615
587
587
521
462
380
359
337
260
200
184
117
P.E.D.c
97S21
97S2
100S20
100S1
88S4
21S5,11S17,11S13,11S22,10S4,10S12
68S6,12S5
53S22,10S5,10S8
33S8,17S22,10S14,10S6
43S23,19S3,10S9
10S7,28S8,17S23,10S5
44S3,12S9,12S23
54S25,10S28
13S14,29S26,20S12,13S13
63S9,16S3,10S11
25S26,21S7,15S24,10S5
42S12,17S17,13S13
12S13,20S24,15S11,12S12
48S24,14S26,10S7
19S15,19S13,17S7,10S26
21S27,21S11,13S23,10S28
14S11,17S14,14S27,10S23,10S8
44S14,11S15,10S11
19S28,29S27,12S15
17S29,26S15,12S31,11S27
15S16,25S29,15S15,14S14
14S17,25S10,18S13,14S5,11S19
20S18,34S16,14S30
48S31,29S29,15S28
24S30,27S18,22S28,10S19
40S19,38S30,15S16
63S32,27S33,10S30
Band
Contours
A* B* C*
23 31 46
- 64 36
25 49 26
2 30 68
61
8 31
91
9
76 14 10
96
4
5 52 43
1
5 94
42 53
5
1 49 50
21 43 36
47 53
68 30
2
24 37 39
59 14 27
90
2
8
87
8
5
98
1
1
49 49
2
43 57
22 28 50
68 16 16
43 57
72
- 28
3 80 17
12 88
3 95
2
58
2 40
3 52 45
20 56 24
33
45
45
0.3
0.1
45
48
72S33,27S32
7 18
CF3 torsion
MP2(full)/6-31G(d) ab initio calculations, scaled wavenumbers, infrared intensities (km/mol), Raman activities (Å 4/amu), depolarization ratios and potential energy
distributions (P.E.D.s)
b
MP2(full)/6-31G(d) fixed scaled wavenumbers with factors of 0.88 for CH stretches and deformations, 0.7 for ND bends (excluding deformation), 1.0 for heavy atom
bends, and 0.90 for all other modes.
c
Calculated with MP2(full)/6-31G(d) and contributions of less than 10% are omitted.
*
A, B and C values in the last three columns are percentage infrared band contours.
a
75
225
Table 57: Calculated energies in (H) and energy differences (cm-1) for the five conformers of
2,2,3,3,3-pentafluoropropylamine.
a
Method/Basis Set
# basis set
Energy , E
transtrans
-0.008200
-0.065322
-0.044232
-0.100233
-0.550388
-0.583713
-0.733475
-0.760487
-0.923402
-0.949598
-0.992809
-1.676532
-1.723617
-1.885836
-1.908429
-1.907176
-1.927345
-1.923258
-1.944548
-1.970696
-
MP2(full)/6-31G(d)
143
MP2(full)/6-31+G(d)
179
MP2(full)/6-31G(d,p)
155
MP2(full)/6-31+G(d,p)
191
MP2(full)/6-311G(d,p)
186
MP2(full)/6-311+G(d,p)
222
MP2(full)/6-311G(2d,2p)
243
MP2(full)/6-311+G(2d,2p)
279
MP2(full)/6-311G(2df,2pd)
326
MP2(full)/6-311+G(2df,2pd)
362
MP2(full)/aug-cc-pVTZ
506
Average MP2(full)
B3LYP/6-31G(d)
143
B3LYP/6-31+G(d)
179
B3LYP/6-311G(d,p)
186
B3LYP/6-311+G(d,p)
222
B3LYP/6-311G(2d,2p)
243
B3LYP/6-311+G(2d,2p)
279
B3LYP/6-311G(2df,2pd)
326
B3LYP/6-311+G(2df,2pd)
362
B3LYP/aug-cc-pVTZ
506
B3LYP Average
a
Energy of conformer is given as –(E+669) H.
b
Difference is relative to trans-trans form and given in cm-1.
226
transb
gauche
183
306
203
309
256
344
235
317
215
315
293
282  48
139
292
158
294
179
301
169
300
313
238  74
Energy Differences, ∆
gauchegaucheb
b
gauche 2
trans
667
931
717
833
712
945
714
799
699
907
753
780
749
835
770
777
713
787
767
765
702
727
736  30
797  58
797
1008
804
895
756
975
805
858
813
913
832
838
769
863
814
815
801
800
799  23
885  70
gaucheb
gauche 1
1046
2220
1994
2099
1563
991
1482
1405
1252
1218
1233
1306  193
1799
2321
1960
1994
1776
1770
1632
1678
1776
1856  210
Table 58: Structural parameters (Å and degree), rotational constants (MHz) and dipole moment
(debye) for 2,2,3,3,3-pentafluoropropylamine trans-trans (Cs) and trans-gauche (C1) forms.
Parameter
Int.
Coor.
r H---F
MP2(full)/
6-311+G(d, p)
Tt
Tg
2.596
2.586
B3LYP/
6-311+G(d, p)
Tt
Tg
2.647
2.650
Tt
2.602(5)
Tg
2.585(5)
Estimated r0
r Cα-N
X1
1.448
1.453
1.448
1.453
1.448(5)
1.453(5)
r Cα-Cβ
Y1
1.519
1.514
1.532
1.526
1.517(5)
1.512(5)
r Cβ-Cγ
Y2
1.533
1.535
1.549
1.552
1.531(5)
1.533(5)
r Cβ-F1
Z1
1.361
1.362
1.368
1.370
1.360(5)
1.363(5)
r Cβ-F2
Z2
1.361
1.351
1.368
1.358
1.360(5)
1.352(5)
r Cγ-F1
Z3
1.330
1.329
1.336
1.335
1.331(5)
1.330(5)
r Cγ-F2
Z4
1.338
1.338
1.345
1.346
1.332(5)
1.332(5)
r Cγ-F3
Z5
1.338
1.339
1.345
1.346
1.332(5)
1.333(5)
r N-H1
b1
1.014
1.012
1.014
1.012
1.014(2)
1.012(2)
r N-H2
b2
1.014
1.013
1.014
1.013
1.014(2)
1.013(2)
r Cα-H1
r1
1.092
1.098
1.092
1.098
1.092(2)
1.098(2)
r Cα-H2
r2
1.092
1.092
1.092
1.092
1.092(2)
1.092(2)
 NCαCβ
θ
114.0
109.0
114.5
109.8
114.0(5)
109.0(5)
 CαCβCγ
χ
114.5
113.6
114.6
113.7
114.5(5)
113.6(5)
 CαCβF1
ρ1
110.2
109.9
110.3
110.0
110.6(5)
109.9(5)
 CαCβF2
ρ2
110.2
111.5
110.3
111.6
110.6(5)
111.5(5)
 CγCβF1
ρ3
107.2
106.9
107.2
106.8
107.6(5)
106.9(5)
 CγCβF2
ρ4
107.2
107.0
107.2
107.0
107.6(5)
107.0(5)
 F1CβF2
ρ5
107.2
106.9
107.7
107.4
106.9(5)
106.6(5)
 CβCγF1
ρ6
111.4
111.1
111.8
111.4
112.1(5)
111.8(5)
 CβCγF2
ρ7
109.9
110.4
110.1
110.6
110.3(5)
110.8(5)
 CβCγF3
ρ8
109.9
109.8
110.1
110.1
110.3(5)
110.2(5)
 F1CγF2
α1
108.6
108.5
108.4
108.3
108.0(5)
108.1(5)
 F1CγF3
α2
108.6
108.7
108.4
108.4
108.0(5)
108.1(5)
 F2CγF3
α3
108.4
108.2
108.0
107.9
108.0(5)
107.8(5)
 CαNH1
β1
110.9
110.3
111.9
111.4
110.9(5)
110.3(5)
 CαNH2
β2
110.9
110.9
111.9
112.0
110.9(5)
110.9(5)
 H1NH2
γ
107.8
108.0
108.4
108.7
107.8(5)
108.0(5)
 NCαH1
δ1
110.9
115.1
109.6
115.1
110.9(5)
115.1(5)
 NCαH2
δ2
110.9
109.3
109.6
109.3
110.9(5)
109.3(5)
 CβCαH1
ε1
107.8
107.5
107.7
107.3
107.8(5)
107.5(5)
 CβCαH2
ε2
107.8
107.3
107.7
107.2
107.8(5)
107.3(5)
 H1CαH2
η
108.1
108.3
107.5
107.8
108.1(5)
108.3(5)
τ NCCC
τ1
180.0
-175.5
180.0
-176.1
180.0(5)
-175.5(5)
τ CCCF1
τ2
180.0
-178.5
180.0
-178.7
180.0(5)
-178.5(5)
227
τ CCNH1
τ3
59.9
-164.6
61.0
-161.2
59.9(5)
-164.6(5)
τ CCNH2
τ4
-59.9
75.8
-61.0
76.7
-59.9(5)
75.8(5)
A
2538.1
2539.8
2510.5
2512.0
2552.2
2551.2
B
1236.8
1244.5
1216.5
1223.7
1237.2
1247.2
C
1199.7
1205.4
1177.8
1182.7
1197.9
1208.3
|a|
1.207
1.990
1.257
1.952
|b|
0.696
3.026
0.612
2.782
|c|
0.00
0.609
0.00
0.555
|t|
1.393
3.673
1.398
3.444
228
Table 59: Symmetry coordinates for 2,2,3,3,3-pentafluoropropylamine.
Symmetry Coordinatea
Description

















NH2 symmetric stretch
CH2 symmetric stretch
NH2 symmetric deformation
CH2 symmetric deformation
CCC antisymmetric stretch
CH2 wag
CF2 symmetric stretch
CF3 antisymmetric stretch
CN stretch
CF2 wag
NH2 wag
CF3 symmetric stretch
CF3 symmetric deformation
CF3 antisymmetric deformation
CF2 symmetric deformation
CF3 symmetric rock
CCC symmetric stretch
CCN bend
CCC bend
NH2 antisymmetric stretch
CH2 antisymmetric stretch
CH2 twist
CF3 antisymmetric stretch
NH2 twist
CF2 antisymmetric stretch
CH2 rock
CF3 deformation
CF2 rock
CF2 twist
NH2 torsion
CF3 wag
CCC asymmetric torsion
CF3 torsion
a
Not normalized.
229
S1
S2
S3
S4
S5
S6
S7
S8
S9
S10
S11
S12
S13
S14
S15
S16
S17
S18
S19
S20
S21
S22
S23
S24
S25
S26
S27
S28
S29
S30
S31
S32
S33
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
b1 + b2
r1 + r2
γ
2η – ε1 + ε2 + δ1 + δ2
Y1 – Y2
ε1 + ε2 – δ1 – δ2
Z1 + Z2
2Z3 – Z4 – 2Z5
X1
ρ1 + ρ2 – ρ3 – ρ4
β1 + β2
Z3 + Z4 + Z5
ρ6 + ρ7 + ρ8
α3
2ρ5 – ρ1 + ρ2 + ρ3 + ρ4
2ρ6 – ρ7 + ρ8
Y1 + Y2
θ
χ
b1 – b2
r1 – r2
ε1 – ε2 – δ1 + δ2
Z4 – Z5
β1 – β2
Z1 – Z2
ε1 – ε2 + δ1 – δ2
α1 – α3
ρ1 – ρ2 + ρ3 – ρ4
ρ1 – ρ2 – ρ3 + ρ4
τ3
ρ7 – ρ8
τ1
τ2
Table 60: Temperature and intensity ratios of the trans-trans and trans-gauche bands of 2,2,3,3,3pentafluoropropylamine.
T(C)
Liquid
xenon
55.0
60.0
65.0
70.0
75.0
80.0
85.0
90.0
95.0
100.0
1/T (10-3 K-1)
I521 / I462
I585 / I462
I867 / I462
I521 / I638
I585/ I638
I867 / I638
4.584
4.692
4.804
4.923
5.047
5.177
5.315
4.460
5.613
5.775
2.547
2.459
2.656
2.758
2.859
3.114
3.138
3.209
3.731
3.795
1.447
1.470
1.560
1.644
1.715
1.882
1.982
1.944
2.347
2.358
2.558
2.492
2.701
2.813
2.971
3.137
3.227
3.262
3.849
3.828
1.946
2.009
2.090
2.175
2.308
2.482
2.536
2.711
2.870
3.088
1.106
1.201
1.227
1.296
1.385
1.500
1.602
1.642
1.806
1.919
1.955
2.035
2.125
2.218
2.398
2.500
2.607
2.756
2.961
3.115

Ha
254  19 301  19 259  19
272  6
318  9
276  6
a
-1
-1
Average value H = 280  6 cm (3.35  0.08 kJ mol ) with the trans-trans conformer the more stable
form and the statistical uncertainty (1σ) obtained by utilizing all of the data as a single set.
230
Figure 31: Mid-infrared spectra of normal 2,2,3,3,3-pentafluoropropylamine (A) gas; (B) Xe
solution at -70°C; (C) simulated spectrum of Tt conformer; (D) infrared spectra of the solid.
231
Figure 32: Comparison of experimental and predicted infrared spectra of 2,2,3,3,3pentafluoropropylamine-ND2: (A) gas; (B) simulated spectrum of Tt and Tg; enthalpy difference
of 280 cm-1 at 25°C; (C) simulated spectrum of Tg conformer; (D) simulated spectrum of Tt
conformer.
232
Figure 33: Comparison of experimental and predicted Raman spectra of 2,2,3,3,3pentafluoropropylamine: (A) gas; (B) simulated spectrum of Tt and Tg; enthalpy difference of 280
cm-1 at 25°C; (C) simulated spectrum of Tg conformer; (D) simulated spectrum of A′ modes of Tt
conformer.
233
Figure 34: Comparison of experimental and predicted Raman spectra of 2,2,3,3,3pentafluoropropylamine-ND2: (A) gas; (B) simulated spectrum of Tt and Tg; enthalpy difference
of 280 cm-1 at 25°C; (C) simulated spectrum of Tg conformer; (D) simulated spectrum of A′
modes of Tt conformer.
234
Figure 35: Comparison of experimental and predicted Raman spectra of 2,2,3,3,3pentafluoropropylamine: (A) liquid; (B) simulated spectrum of Tt and Tg; enthalpy difference of
280 cm-1 at 25°C; (C) simulated spectrum of Tg conformer; (D) simulated spectrum of Tt
conformer.
235
Figure 36: Comparison of experimental and predicted Raman spectra of 2,2,3,3,3pentafluoropropylamine-ND2: (A) liquid; (B) simulated spectrum of Tt and Tg; enthalpy
difference of 280 cm-1 at 25°C; (C) simulated spectrum of Tg conformer; (D) simulated spectrum
of Tt conformer.
236
Figure 37: Five conformers of 2,2,3,3,3-pentafluoropropylamine showing atomic numbering.
237
Figure 38: Infrared spectra of 2,2,3,3,3-pentafluoropropylamine in xenon solution at different
temperatures.
238
CHAPTER 10
CONFORMATIONAL STABILITY, R0 STRUCTURAL PARAMETERS, VIBRATIONAL
ASSIGNMENTS AND AB INITIO CALCULATIONS OF ETHYLDICHLOROPHOSPHINE
Introduction
The determination of the conformational stability of CH3CH2PX2 molecules have provided
some interesting challenges for the structural scientists over the years. One of the first phosphine
molecules studied was ethylphosphine. There have been several determinations of the enthalpy
difference between the trans and the gauche conformers of ethylphosphine, CH3CH2PH2 with
infrared and Raman spectroscopy the major methods utilized. An estimate of the enthalpy
difference of ~200 ± 21 cm-1 (560 ± 60 cal/mol) was obtained [92] from the slope of the least
square line fit to the observed data points in the Raman spectrum of the liquid. About a decade
later, an enthalpy difference of 232 ± 2 cm-1 (663 ± 5 cal/mol) was reported [93] which was
determined from the torsional transitions of the asymmetric rotor from the far infrared spectrum.
Another organophosphorus molecule that was investigated was ethyldiflurorphosphine [94], and
from the temperature study of the Raman spectrum of the gas, an enthalpy difference of 56 ± 22
cm-1 (160 ± 62 cal/mol) was obtained. From this study the trans conformer was determined to be
the more stable conformer. Similar studies [94] of the liquid resulted in an enthalpy determination
of 96 ± 12 cm-1 (275 ± 33 cal/mol), and again with the trans conformer the more stable form. From
a temperature study of the microwave spectrum [95], it was found that the gauche conformer is at
least 80 cm-1 (229 cal/mol) more stable than the trans form. Another molecule that was studied was
ethyldimethylphosphine [96] with a conclusion that the gauche form was more stable than the trans
conformer. A previous NMR investigation of ethyldichlorophosphine [97] by Dutasta and Robert
reported that the trans form is more stable than the gauche conformer. In 1977, the most stable
conformer of ethyldichlorophosphine, CH3CH2PCl2, was reported [98] to be the gauche form, as
239
the more predominant form in the gaseous state with the probable existence of only 10 to 30% of a
second conformer present at ambient temperature. This conclusion was obtained from an electron
diffraction study. A decade later, an extensive vibrational study [99] was reported and the enthalpy
difference was reported to be 190 ± 30 cm-1 from the Raman spectrum of the liquid with the trans
form as the more stable conformer. Also, in this study, it was shown that both the trans and gauche
conformers were observed from the infrared spectra of the solid. The gauche conformer was about
65% abundant in the gas phase at ambient temperature. Additionally from this study, it was shown
that the gauche conformer had the higher fundamental vibrational frequencies for the CH2 rock and
PC stretch but lower frequencies for both the CC stretch and CCP bend compared to the similar
modes for the trans conformer. From these studies, there is a question which conformer is the more
stable form and what is the enthalpy difference. These uncertainties of the more stable form as well
as the enthalpy difference of this important molecule provides a clear reason to again investigate
this molecule.
To obtain the enthalpy difference it is essential to correctly assign the fundamentals for
each of the conformers. In order to obtain a very confident experimental value of the
conformational enthalpy difference, the mid-infrared spectra have again been investigated with the
vibrational spectra from 4000 to 400 cm-1 with the sample dissolved in liquefied xenon. To aid in
identifying the vibrational fundamentals for each conformer, ab initio calculations have been
utilized. From the theoretical calculations the harmonic force fields, infrared intensities, Raman
activities, depolarization ratios, and vibrational frequencies were obtained from MP2(full)/631G(d) ab initio calculations with full electron correlation. Additionally MP2(full) ab initio and
Density Functional Theory (DFT) calculations by the B3LYP method have been carried out with a
variety of basis sets up to aug-cc-pVTZ as well as with diffuse functions to predict energy
240
difference and the conformational stabilities. The results of these spectroscopic, structural, and
theoretical studies of ethyldichlorophosphine are reported herein.
Experimental and Theoretical Methods
The sample of ethyldichlorophosphine was purchased from Organometallics, Inc. The
sample was further purified on low-temperature, low-pressure fractionation column and the purity
of the sample was verified by comparing the infrared spectrum with that previously reported.
The mid-infrared spectra (4000 to 400 cm-1) of the sample dissolved in liquefied xenon
were recorded on a Bruker model IFS-66 Fourier transform spectrometer equipped with a globar
source, a Ge/KBr beamsplitter and a DTGS detector (Fig. 39). In all cases, 100 interferograms
were collected at 1.0 cm-1 resolution, averaged and transformed with a boxcar truncation function.
For these studies, a specially designed cryostat cell was used. It consists of a copper cell with a
path length of 4 cm with wedged silicon windows sealed to the cell with indium gaskets. The
copper cell was enclosed in an evacuated chamber fitted with KBr windows. The temperature was
maintained with boiling liquid nitrogen and monitored by two Pt thermo resistors. The observed
bands in the infrared spectra of the xenon solutions along with their proposed assignments are
listed in Tables 61 and 62 for the trans and gauche conformers, respectively.
The LCAO-MO-SCF restricted Hartree-Fock calculations were performed with the
Gaussian-03 program [21] by using Gaussian-type basis functions. The energy minima with
respect to nuclear coordinates were obtained by the simultaneous relaxation of all geometric
parameters by the gradient method of Pulay [22]. Several basis sets as well as the corresponding
ones with diffuse functions were employed with the Møller-Plesset perturbation method [23] to the
second order (MP2(full)) as well as with the DFT by the B3LYP method.
conformational energy differences are listed in Table 63.
241
The predicted
In order to obtain a complete description of the molecular motions involved in the
fundamental modes of CH3CH2PCl2, a normal coordinate analysis has been carried out. The force
field in Cartesian coordinates was obtained with the Gaussian 03 program [21] at the MP2(full)
level with the 6-31G(d) basis set. The internal coordinates used to calculate the G and B matrices
are given in Table 64 with the atomic numbering shown in Fig. 40. By using the B matrix [24],
the force field in Cartesian coordinates was converted to a force field in internal coordinates.
Subsequently, scaling factors of 0.88 for CH stretches and CH2 deformation, and 0.90 for all other
modes except heavy atom bends were utilized, along with the geometric average of the scaling
factors for the interaction force constants, to obtain the fixed scaled force field and resultant
wavenumbers. A set of symmetry coordinates was used (Table 65) to determine the corresponding
potential energy distributions (P.E.D.s). A comparison between the observed and calculated
wavenumbers, along with the calculated infrared intensities, Raman activities, depolarization ratios
and potential energy distributions for the trans and gauche conformers are listed in Tables 61 and
62, respectively.
Vibrational Assignment
The first comprehensive vibrational assignment for ethyldichlorophosphine [99] was made
by utilizing the Raman spectra of the gas, liquid and solid as well as the infrared spectra of the gas
and solid for the normal species along with the CD3CD2PCl2 molecule which made it possible to
identify the modes involving mainly phosphine. In this study, only eight fundamentals were
assigned for the gauche conformer and most of the trans form fundamentals were assigned as well.
The vibrational assignments were made mostly based on existing assignments of corresponding
vibrations of similar molecules. In a more recent report on the vibrational assignments [100], all of
the fundamentals were assigned for the trans and gauche conformers. Emphasis has been placed on
the assignments for the fingerprint region from 1400 to 400 cm-1 where the number of overtone
242
and combination bands are usually significantly reduced compared to those in the higher
wavenumber region, by utilizing the MP2(full)/6-31G(d) predicted vibrational wavenumbers along
with ab initio predicted intensities and depolarization ratios along with infrared data from xenon
solutions.
In this study, most of the assignments of the fundamental agreed with what was previously
reported except for three fundamentals for the trans form and two for the gauche conformer. These
reassignments were based largely on the ab initio predicted band frequencies. For the trans
conformer, three bands were reassigned: 7, 18 and 9. The 7 CH2 wag band which was previously
assigned the 1920 cm-1 band, which was identified as a fundamental in the shoulder region of the
spectra in the current study, is now reassigned at 1282 cm-1 in the infrared spectra of the xenon
solution. The 18 CH2 twist band was previously assigned at 1240 cm-1, but it is now reassigned to
the band at 1267 cm-1. This fundamental is predicted at 1253 cm-1 with an infrared intensity of 0.2
km/mol which makes the current assignment a more appropriate one. The 9 CH3 rock band is
assigned to a slightly higher in frequency band at 979 cm-1. This band was previously assigned to
the 975 cm-1 band which is not the center of the band in the spectra of xenon solution in the current
study. For the gauche conformer, the 11 CH2 twist band previously assigned at 1226 cm-1 is now
reassigned to 1239 cm-1 in the infrared spectra of the xenon solutions. This band is predicted at
1240 cm-1 with an infrared intensity of 0.3 km/mol where the band is well resolved, unlike the
previous assignment. Also, the 14 band previously assigned at 975 cm-1 is now reassigned at 979
cm-1 in the infrared spectra of the xenon solutions. The rest of the bands for both trans and gauche
forms agree with the assignments previously reported since they are within one or two
wavenumbers for the bands assigned in this study.
243
Conformational Stability
To determine the enthalpy differences between the two observed conformers of
ethyldichlorophosphine, the sample was dissolved in liquefied xenon and the infrared spectra were
recorded as a function of temperature from -60 to -100°C (Fig. 41). Relatively small interactions
are expected to occur between xenon and the sample and, therefore, only small wavenumber shifts
are anticipated for the xenon interactions when passing from the gas phase to the liquefied xenon.
A significant advantage of this study is the conformer bands are better resolved in comparison to
those in the infrared spectrum of the vapor. From ab initio calculations, the dipole moments of the
two conformers are predicted to have similar values. Thus, the ΔH value obtained from the
temperature dependent infrared study is expected to be near that for the gas [39-43].
Once confident assignments were made for the fundamentals of both conformers the task was
then to find pairs of bands from which the enthalpy difference could be obtained. It is desirable to
utilize the lower frequency pair(s) in the spectra to reduce the possible number of combination and
overtone bands compared to those possible in the higher frequency ranges. Also, the bands should
be sufficiently resolved to allow for accurate intensity measurements. The fundamentals at 1040,
726, 632, 503 and 488 cm-1 for the trans conformer and those at 1024, 751, 664, 507 and 484 cm-1
for the gauche form were selected as they are confidently assigned, satisfactorily resolved, and a
limited number of overtone and combination bands are possible which allowed the determination
of the band intensities to be confidently measured.
Four temperatures (-60 to -100°C) were chosen to obtain the spectral data for these pairs
and by application of the van’t Hoff equation, –lnK = H/(RT) – S/R, the enthalpy differences
were determined from a plot of –lnK versus 1/T, where H/R is the slope of the line and K is
substituted with the appropriate intensity ratios, i.e. Iconf-1/Iconf-2. It was assumed that H, S and
244
the ratio of the molar absorption coefficients conf-1/conf-2 are not a function of temperature in the
small temperature range studied.
The conformational enthalpy difference (Table 66) was determined to be 100  6 cm-1 for the
1024/1040 band pair and 90  6 cm-1 for the 1024/632 band pair from the xenon solution.
Additionally, an enthalpy value of 74  5 cm-1 was determined from the band pair 751/726. Also,
the enthalpy differences were determined to be 107  9 cm-1 for the 664/1040 pair and 96  4 cm-1
for the 664/632 band pair. Furthermore, an enthalpy difference value of 94  9 cm-1 was
determined from the band pair 507/632 and 74  5 cm-1 from the band pair 507/503. An additional
enthalpy difference value of 90  6 cm-1 was also obtained from the 484/488 band pair. An average
value was obtained by utilizing all the data as a single set which gave a ΔH value of 88  4 cm-1,
with the trans form the more stable conformer. These error limits were derived from the statistical
standard deviation of two sigma of the measured intensity data where the data from the eight band
pairs were taken as a single set. These error limits do not take into account associations with the
xenon or the interference of overtones and combination bands in near coincidence with the
measured fundamentals. The variations in the individual values are undoubtedly due to these types
of interferences, but by taking several pairs, the effect of such interferences should be greatly
reduced. However, this statistical uncertainty is probably better than what can be expected from
this technique and ,therefore, an uncertainty of about 10% in the enthalpy difference is probably
more realistic i.e. 88  9 cm-1. From the enthalpy difference the abundance of the gauche
conformer present at ambient temperature is estimated to be 57%.
Structural Parameters
It should be possible to estimate structural parameters for the trans and gauche conformers
of ethyldichlorophosphine by adjusting the MP2(full)/6-311+G(d,p) calculated structural
245
parameters by using similar molecules as a guideline. The CH bond distances predicted by
MP2(full)/6-311+G(d,p) ab initio calculations can be used as a good prediction without any
adjustment, as we [44] have shown that ab initio MP2(full)/6-311+G(d,p) calculations predict the
r0 structural parameters for more than fifty carbon hydrogen distances to at least ± 0.002 Å
compared to the experimentally determined values from isolated CH stretching frequencies. The
heavy atom parameters can be estimated by using the corresponding parameters in
methyldichlorophosphine [101] and ethyldifluorophosphine [102] to adjust the corresponding
MP2(full)/6-311+G(d,p) calculated parameters. The methyldichlorophosphine [101] structural
parameters of P–C =1.83 Å, P–Cl = 2.04 Å, CPCl = 94.0º and ClPCl = 100.0º have been used
to predict the corresponding heavy atom parameters of ethyldichlorophosphine. The structural
parameters of the ethyldifluorophosphine molecule, C–C =1.53 Å and CCP = 114.9º, have been
used to predict the rest of the heavy atom parameters of ethyldichlorophosphine. The predicted
structural parameters for ethyldichlorophosphine molecule are listed in Table 64. It is believed that
the estimated r0 parameters reported herein for ethyldichlorophosphine have listed uncertainty
values that should be within the experimentally determined parameters.
Discussion
The vibrational assignments reported herein are based on a significant amount of
information from the infrared spectra (4000 - 400 cm-1) dissolved in liquefied xenon and
predictions from the scaled ab initio MP2(full)/6-31G(d) calculations. The evidence for the van der
Waals molecules was the significant decreases in the PCl2 symmetric stretch frequencies from the
gas to the solutions, i.e. 10 cm-1 for both the trans and gauche conformers. The shift of CH3 rock
frequencies from the gas to the solutions, i.e. 15 cm-1 for the trans and gauche conformers also
indicates the interaction. However, for the other fundamentals, the difference between the
frequencies in the gas and xenon solutions is less than eight wavenumbers.
246
For the trans conformer the average ab initio predicted frequencies for the A′ modes with
all the fundamentals was 6 cm-1 which represents 0.36% error. The percent error for the A′′ modes
for the trans form have similar errors to that of the A′ modes of the predicted frequencies. For the
gauche conformer, the average ab initio predicted frequencies with all the fundamentals were 5 cm1
which represents 0.49% error. Thus, the relatively small basis set of 6-31G(d) by the MP2(full)
calculations with two scaling factors provides excellent predicted frequencies for aiding the
vibrational assignments.
For the trans conformer, the mixing is relatively small except for the 9 (CH3 rock), which
has only 36%S9, with 37%S8 of the CC stretch and 19%S7 of the CH2 wag. For the gauche form, the
mixing is relatively small as well for the modes except for 14 (CH3 rock), which has only 29%S14,
with 44%S12 of the CC stretch and 18%S10 of the CH2 wag. Therefore, for most of the vibrations,
the approximate descriptions provide a reasonable indication of the major vibrational motions
involved since the mixing was not very extensive as most of the vibrational mode has two or three
contributions from other symmetry coordinates.
The vibrational assignment that was carried out in 1988 [99] was obtained by utilizing band
contours, depolarization values, isotopic shift factors and group frequencies. However, it was
complicated by the fact that in the fluid phases a high concentration of the gauche conformer was
observed, which resulted in most lines in the Raman effect being polarized. Also, the PCl 2 modes
have been well characterized for CH3PCl2 [103, 104] including normal coordinate calculations
[101]. Most of the bands that were assigned to both conformers seemed to have ambiguity as several
frequencies were listed for the same fundamental.
The enthalpy value obtained from this study must be considered to be the most confident
value obtained at this time, since it is doubtful that any other experiments would give an enthalpy
value as accurate as the one obtained from the xenon solution study. The use of enthalpy
247
determinations from eight band pairs provides excellent accuracy of the determination by this
technique.
We have predicted the quadratic centrifugal distortion constants from the MP2(full)/6311+G(d,p) calculations as well as DFT by the B3LYP method by utilizing the same basis set.
These data are given in Table 67 and as can be seen there is reasonable agreement with the predicted
values from both techniques. We have found in the past that the ab initio calculations usually
provide reasonably good centrifugal distortion constants and we expected reasonable agreement.
It is of interest to compare the structural parameters of ethyldichlorophosphine obtained in
this study with the corresponding parameters from the electron diffraction study [98]. The electron
diffraction work [98] agrees with the current study within the error limits for every parameter for the
trans conformer except for the CCP = 112.0° and ClPCl = 100.1(5)° values which are 6.8 and
3.2 degrees lower than those reported in the current study, respectively. For the gauche form, the
electron diffraction study agrees with the structural parameters in the current study except for CCP
= 116.5(22)° and ClPC = 102.6(13)° which are 4.5 and 8.7 degrees higher than the predicted
structural parameters in this study, respectively. Also, the ClPCl = 99.1(8)° which is 3.8 degrees
lower than the value in the current study. It is interesting to note that the uncertainties of the electron
diffraction parameters are too large when they are compared to the values obtained in the current
study.
The estimated structural parameters have been obtained (Table 64) in the current study and
should be of considerable value for assigning the microwave spectrum for the conformers of
ethyldichlorophosphine. These parameters should be accurate to within  0.005 Å and 1° for the
heavy atom bond distances and angles, respectively. The quadratic centrifugal distortion constants
(Table 67) have also been predicted to further aid in a possible study of the microwave spectrum.
248
Such a study could be useful in determining the structure of ethyldichlorophosphine for comparison
to other X-ethylphospine molecules.
To predict the experimental values, the ab initio energy differences have been determined by
a variety of basis sets (Table 63) which can be compared to the experimentally determined enthalpy
differences listed in Table 66. The failure of the ab initio calculations to predict the more stable form
of the PCl2 moiety is not a surprise. The PCl2 is problematic for the ab initio calculations even when
high basis set are used as shown in this study. Simple comparison would be good with the npropyldichlorophosphine [105], where the ab calculations predict the Gauche-Trans (GT) to be the
most stable rotamer at three utilized levels of the ab initio calculations. The experimental data
showed that the Trans-Trans (TT) was the most stable form and the GT is the second most stable
conformer.
Since many phosphines are important molecules in biological area, it is very useful to
investigate studies of additional phosphines that have potential for bio-utilization. It would be of
interest to investigate X-ethylphosphine molecules to determine which conformer is the more stable
forms, as well as the enthalpy difference and predict good structural parameters. It also would be of
interest to investigate tert-methylbiphosphine to determine the conformational stabilities, vibrational
assignments and structural parameters of this molecule.
249
Table 61: Observed and calculateda wavenumbers (cm -1) for trans ethyldichlorophosphine.
Band
P.E.D.e
IRc
Ramanc
IR
Contours
Gas
Solid
Gas
liquid
Solid Xenond Xenon
B*
C*
3211
3012
9.9
70.9
0.68 2978 2974 2978
2971 2972
2977
2970
99S1
83 17
A 1 CH3 antisymmetric
CH3 symmetric stretch
3125
2931 14.2
99.3
0.06 2907 2912 2916
2905 2904
2904
2934
99S2
61 39
2 stretch
3095
2904
4.3
89.8
0.12 2898 2895 2893
2886 2894
2887
2881 100S3
1 99
3 CH2 symmetric stretch
1568
1472
6.4
9.4
0.71 1468 1458
- 1457
1460
1460
92S4
89 11
4 CH3 deformation
1508
1426
5.6
15.5
0.74 1401 1398 1400
1396 1397
1398
1395
99S5
33 67
5 CH2 deformation
1473
1389
5.8
3.7
0.75 1386 1376
1380 1370
1377
1377
98S6
- 100
6 CH3 deformation
CH
wag
1353
1284
0.9
0.9
0.63
1285
1272
1275
1290
1282
79S
,
11S
1 99
7
2
7
9
1102
1049 22.3
3.8
0.58 1040 1044 1042
1038 1043
1041
1040
50S8, 38S9
87 13
8 CC stretch
1024
972
1.4
3.9
0.66
976
974 ~980
975
973
975
979
36S9, 37S8, 19S7
81 19
9 CH3 rock
CP
stretch
668
636
26.2
10.5
0.39
632
638
631
631
636
632
632
83S
86 14
10
10
525
503 39.4
15.9
0.03
506
496
508
499
494
503
503
65S11, 10S14
8 92
11 PCl2 symmetric stretch
41S12, 27S11,
414
405 10.6
5.1
0.35
403
402
401
400
399
402
91
9
12 CCP bend
25S
14
54S
,
24S
,
PCl
deformation
219
218
1.5
4.8
0.51
213
220
211
213
217
21
79
13
13
12
2
1414, 38S13,
45S
176
175
2.1
3.4
0.73
156
165
171
- 17S
100 14 PCl2 wag
1215
3229
3029
6.7
20.2
0.75 2986 2982
- 2981
2986
2986 15S
99S
A 15 CH3 antisymmetric
stretch
CH
antisymmetric
3152
2957
0.9
77.1
0.75
2948
2936
2933
2935
2935
2940
99S
16
2
16
stretch
1563
1467 10.7
19.8
0.75 1458 1448
1456 1447
1456
1455
91S17
17 CH3 antisymmetric
CH2 twist
1316
1253
0.2
7.1
0.75 1242 1234
1237 1236
1240
1267
62S18, 28S19
18 deformation
41S19,
1062
1007
3.5
2.4
0.75
994
990
990
988
991
991
19 CH3 rock
37S
,17S
18
20
775
739
0.3
0.75
725
722
727
721
726
726
71S20, 23S19
20 CH2 rock
512
486 23.8
11.6
0.75
498
471
471
488
488
96S21
21 PCl2 antisymmetric
stretch
112.4
54S
,
30S
,
torsion
272
262
0.8
0.75
287
288
286
288
22 CH
22
23
3
0.2
12S
24
190
186
2.0
0.75
184
64S23, 36S22
23 PCl2 twist
0.0
97
94
0.4
0.75
96
113
124
92S24
24 PCl2 torsion
a
0.0
MP2(full)/6-31G(d) ab initio calculations, scaled wavenumbers,
infrared intensities (km/mol), Raman activities (Å 4/amu), depolarization ratios and potential energy
distributions (P.E.D.s).
b
MP2(full)/6-31G(d) fixed scaled wavenumbers with factors of 0.88 for CH stretches and CH 2 deformations, 1.0 for heavy atom bends, and 0.90 for all other modes.
c
Ref [99]
d
Ref [100]
e
Calculated with MP2(full)/6-31G(d) and contributions of less than 10% are omitted.
*
A, B and C values in the last three columns are percentage infrared band contour.
Vib.
No.
Approximate
Descriptions
ab
initio
fixed
scaledb
IR
int.
Raman
act.
dp
ratio
250
Table 62: Observed and calculateda wavenumbers (cm -1) for gauche ethyldichlorophosphine.
Band
Contours
Gas
solid Xenond Xenon
A* B* C*
CH3 antisymmetric stretch
3216
3017
8.5
39.8
0.69
2978
2972
2977
2976 60S1,38S2
- 95
5
CH3 antisymmetric stretch
3202
3004
11.9
65.7
0.63
2970
2962 62S2,34S1
9
3 88
CH2 antisymmetric stretch
3178
2981
0.1
65.9
0.74
2940
2947 91S3
16 45 39
CH3 symmetric stretch
3115
2922
15.5
76.5
0.04
2909
2909 97S4
90
3
7
CH2 symmetric
3111
2919
1.5
97.8
0.10
2881
2905 94S5
44 52
4
stretch
CH
deformation
1569
1473
8.3
13.7
0.75
1468
1458
1457
1460
1460
55S
,37S
10
90
3
6
7
CH3 antisymmetric
1562
1467
9.0
18.4
0.73
1448 1455
1456 1447
1456
1455 54S7,37S6
- 88 12
deformation
CH2 deformation
1513
1429
6.1
13.4
0.75 1458 1398 1400
1396 1397
1395
1398 99S8
29 39 32
CH3 deformation
1475
1392
5.3
3.6
0.72 1401 1376
1380 1370
1380
1379 97S9
79
6 15
1386
CH2 wag
1341
1272
1.7
0.1
0.59
1274
1274 78S10,13S14
- 91
9
CH2 twist
1308
1240
0.3
6.0
0.75
1226
1239 58S11,30S13
26
2 72
CC stretch
1090
1038
8.6
4.1
0.28
1024 1026
1022 1022
1024
1024 44S12,40S14
94
4
2
1026
CH3 rock
1063
1010
7.2
3.5
0.73
990
990
988
991
991 34S13,38S11,19S15
42
2 56
994
CH3 rock
1028
975
0.6
4.2
0.68
974 ~980
975
973
975
979 29S14,44S12,18S10
61
- 39
976
CH2 rock
795
758
16.7
1.5
0.21
746
752
750
752
751
751 61S15,26S13
13 15 72
752
CP stretch
704
670
29.4
9.2
0.33
663
665
663
664
664
664 76S16
11 80
9
666
PCl2 antisymmetric stretch
532
507
101.2
12.5
0.44
496
508
499
494
508
507 53S17,30S18
86
8
6
506
PCl2 symmetric stretch
511
486
67.9
14.9
0.21
471
471
484
484 56S18,36S17
6 81 13
498
CCP bend
334
326
2.8
4.3
0.35
320
320
320
323
- 49S19,21S23,10S17
77 17
6
321
PCl2 wag
302
291
3.9
3.6
0.33
288
287
286
288
- 54S20,23S21
26 36 38
287
CH3 torsion
214
206
1.2
1.6
0.61
- 62S21,15S22,13S20
6
2 92
PCl2 deformation
190
189
1.9
4.6
0.69
197
191
195
- 68S22,19S20
52 48
185
PCl2 twist
161
160
0.4
1.9
0.75
186
165
178
188
- 65S23,28S19
12 19 69
175PCl2 torsion
104
101
0.2
0.3
0.75
83
- 89S24
35
- 65
a
MP2(full)/6-31G(d) ab initio calculations, scaled wavenumbers, infrared intensities (km/mol), Raman activities (Å 4/amu), depolarization ratios and potential energy
distributions (P.E.D.s).
b
MP2(full)/6-31G(d) fixed scaled wavenumbers with factors of 0.88 for CH stretches and CH 2 deformations, 1.0 for heavy atom bends, and 0.90 for all other modes.
c
Ref [99]
d
Ref [100]
e
Calculated with MP2(full)/6-31G(d) and contributions of less than 10% are omitted.
*
A, B and C values in the last three columns are percentage infrared band contours.
Vib.
Approximate Descriptions
No.
251
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
ab
initio
fixed
scaledb
IR
int.
Raman
act.
dp
ratio
IRc
Gas solid
2978 2974
Ramanc
Xenon
2971
IR
P.E.D.e
Table 63: Calculated energies (hartree) and energy differences (cm-1) for the two conformers of
ethyldichlorophosphine.
a
Energy , E
Method/Basis Set
RHF/3-21G
MP2(full)/6-31G(d)
MP2(full)/6-31+G(d)
MP2(full)/6-31G(d,p)
MP2(full)/6-31+G(d,p)
MP2(full)/6-311G(d,p)
MP2(full)/6-311+G(d,p)
MP2(full)/6-311G(2d,2p)
MP2(full)/6-311+G(2d,2p)
MP2(full)/6-311G(2df,2pd)
MP2(full)/6-311+G(2df,2pd)
MP2(full)/cc-pVTZ
MP2(full)/aug-cc-pVTZ
# basis set
67
97
117
112
132
144
164
184
204
244
264
232
357
trans
0.372960
1.038787
1.050993
1.080218
1.091920
1.415022
1.038787
1.050993
1.080218
1.091920
1.415022
1.425856
1.546312
B3LYP/6-31G(d)
97
3.017333
B3LYP/6-31+G(d)
117
3.022900
B3LYP/6-31G(d,p)
112
3.017333
B3LYP/6-31+G(d,p)
132
3.022900
B3LYP/6-311G(d,p)
144
3.024538
B3LYP/6-311+G(d,p)
164
3.029856
B3LYP/6-311G(2d,2p)
184
3.120497
B3LYP/6-311+G(2d,2p)
204
3.125080
B3LYP/6-311G(2df,2pd)
244
3.139833
B3LYP/6-311+G(2df,2pd)
264
3.142782
B3LYP/aug-cc-pVTZ
357
3.150202
a
Energy of conformer is given as –(E+1338) H.
b
Difference is relative to trans form and given in cm-1.
252
Energy Differences,
∆
b
gauche
-200
-95
-132
-73
-104
-141
-95
-132
-73
-104
-141
-113
15
-74
-120
-74
-120
-51
-95
-88
-94
-61
-71
-78
Table 64: Structural parametersa, rotational constants, and dipole moments for trans and gauche
forms of ethyldichlorophosphine.
trans
Parameter
Int.
Coor.
MP2(full)/6311+G(d,p)
B3LYP/6311+G(d,p)
gauche
EDb
Predicted
r0
MP2(full)/6311+G(d,p)
B3LYP/6311+G(d,p)
Predicted
r0
EDb
r P1-C2
R1
1.832
1.856
1.84*
1.840(5)
1.830
1.857
1.84*
1.838(5)
r C2-C3
R2
1.525
1.529
1.525(7)
1.525(5)
1.529
1.533
1.531(9)
1.529(5)
r P1-Cl4
R3
2.074
2.114
2.044(1)
2.042(5)
2.073
2.112
2.044(1)
2.041(5)
r P1-Cl5
R4
2.074
2.114
2.044(1)
2.042(5)
2.075
2.114
2.044(1)
2.043(5)
r C2-H6
r1
1.097
1.097
1.115(7)
1.097(2)
1.093
1.092
1.116(9)
1.093(2)
r C2-H7
r2
1.097
1.097
1.115(7)
1.097(2)
1.096
1.096
1.116(9)
1.096(2)
r C3-H8
r3
1.091
1.091
1.115(7)
1.091(2)
1.094
1.094
1.116(9)
1.094(2)
r C3-H9
r4
1.091
1.091
1.115(7)
1.091(2)
1.092
1.092
1.116(9)
1.092(2)
r C3-H10
r5
1.093
1.093
1.115(7)
1.093(2)
1.092
1.093
1.116(9)
1.092(2)
 C3C2P1
A1
118.8
120.1
112.0*
118.8(5)
112.0
112.3
116.5(22)
112.0(5)
 Cl4P1C2
A2
98.8
99.7
94.8(7)
95.3(5)
97.4
97.9
102.6(13)
93.9(5)
 Cl5P1C2
A3
98.8
99.7
94.8(7)
95.3(5)
97.9
98.7
102.6(13)
93.9(5)
 Cl4P1Cl5
A4
102.1
102.0
103.2(5)
101.9
101.9
β1
104.5
103.6
104.5(5)
109.6
109.5
99.1(8)
-
102.9(5)
 H6C2P1
100.1(5)
-
 H7C2P1
β2
104.5
103.6
-
104.5(5)
105.5
104.7
-
105.5(5)
 H6C2C3
1
110.6
111.0
-
110.6(5)
111.2
111.7
-
111.2(5)
 H7C2C3
2
110.6
111.0
-
110.6(5)
110.6
-
110.6(5)
 H8C3C2
3
111.2
111.3
-
111.2(5)
111.6
110.7
111.9
-
111.6(5)
 H9C3C2
4
111.2
111.3
111.3
111.5
109.7
109.7
109.7(5)
109.9
109.8
-
111.3(5)
5
-
111.2(5)
 H10C3C2
 H6C2H7
1
106.9
106.5
-
106.9(5)
107.8
107.6
-
107.8(5)
 H8C3H9
2
108.3
108.4
-
108.3(5)
108.3
108.2
-
108.3(5)
 H9C3H10
3
108.1
108.0
-
108.1(5)
107.9
107.7
-
107.9(5)
 H8C3H10
4
108.1
108.0
107.7
1
51.9
52.0
51.9(5)
174.8
107.5
176.8
-
107.7(5)
τCl4P1C2C3
-
108.1(5)
τCl5P1C2C3
2
-51.9
-52.0
-
-51.9(5)
71.6
73.4
-
71.6(5)
τH6C2P1C3
3
-123.9
-124.5
-
-123.9(5)
-123.9
-124.7
-
-123.9(5)
τH8C3C2P1
4
-60.4
-60.5
-60.4(5)
55.8
5
180.0
180.0
180.0(5)
175.2
55.5
174.8
-
55.8(5)
τH10C3C2P1
-
175.2(5)
A
2554.03
2441.11
-
2705.75
2901.34
2768.46
-
3029.09
B
2141.16
2076.79
-
2166.31
1917.92
1866.21
-
1958.49
C
1521.81
1453.67
-
1590.16
1287.01
1238.48
-
1344.96
|a|
1.96
1.99
-
2.12
2.13
-
|b|
0.26
0.27
-
1.97
1.92
-
|c|
1.98
1.93
-
0.30
0.18
-
|t|
2.79
2.79
2.91
2.88
Bond distances in Å, bond angles in degrees, rotational constants in MHz, and dipole moments in Debye.
b
Ref[98]. * Assumed structural parameters.
a
253
-
109.6(5)
109.9(5)
174.8(5)
Table 65: Symmetry coordinates for ethyldichlorophosphine.
Symmetry Coordinatea
Description
′













′′

S1
S2
S3
S4
S5
S6
S7
S8
S9
S10
S11
S12
S13
S14
S15
S16
S17
S18
S19
S20
S21
S22
S23
S24
CH3 antisymmetric stretch
CH3 symmetric stretch
CH2 symmetric stretch
CH3 deformation
CH2 deformation
CH3 deformation
CH2 wag
CC stretch
CH3 rock
CP stretch
PCl2 symmetric stretch
CCP bend
PCl2 deformation
PCl2 wag
CH3 antisymmetric stretch
CH2 antisymmetric stretch
CH3 antisymmetric deformation
CH2 twist
CH3 rock
CH2 rock

PCl2 antisymmetric stretch
CH3 torsion
PCl2 twist
PCl2 torsion
a
Not normalized.
254
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
2r5 – r3 – r4
r3 + r4 + r5
r1 + r2
22 – 3 – 4
1
2 + 3 + 4 – π3 – π4 – π5
π3 + π4 – β1 – β2
R2
2π5 – π3 – π4
R1
R3 + R4
A1
A4
A2 + A 3
r3 – r4
r1 – r2
3 – 4
β2 + π1 – β1 – π2
π4 – π3
β1 + π1 – β2 – π2
R3 – R4
4
A2 – A3
2
Table 66: Temperature and intensity ratios of the gauche and trans bands of
ethyldichlorophosphine.
T(C) 1/T (10-3 K-1) I1024 / I1040 I1024 / I632
I664 / I1040 I664 / I632
I507 / I632 I507 / I503 I484 / I488
Liquid
-60
4.692
1.048
1.866
1.590
1.045
1.862
0.020
3.011
1.208
xenon
-70
4.923
1.019
1.801
1.532
1.021
1.805
0.190
2.900
1.210
-90
4.460
0.931
1.701
1.446
0.922
1.685
0.018
2.765
1.141
-100
5.775
0.902
1.613
1.412
0.893
1.598
0.017
2.670
1.099
90  6
75  6
107  9
96  4
94  9
74  5
90  6
H (cm-1)a
a
I751 / I726
100  6
-1
-1
Average valueH = 88  4 cm ( 1.0  0.04 kJ mol ) with the trans conformer the more stable form and the
statistical uncertainty (2σ) obtained by utilizing all of the data as a single set.
255
Table 67: Quadratic centrifugal distortion constants (MHz) for trans and gauche forms of
ethyldichlorophosphine.
MP2(full)/
6-31G(d)
trans
MP2(full)/
6-311+G(d,p)
B3LYP/
6-31G(d)
∆J
0.37414
0.37242
0.39566
0.24004
0.22279
0.22378
0.23539
0.24004
∆JK
1.01124
1.23673
1.30955
0.76660
0.72650
0.66857
0.77308
0.76660
∆K
-0.36003
-0.57674
-0.65155
1.29442
1.30416
1.36545
1.27577
1.29442
δJ
0.07642
0.07150
0.08262
0.06587
0.06041
0.05935
0.06469
0.06587
0.56591
0.61502
0.74086
0.58823
0.55832
0.52698
0.58813
0.58823
δK
a
B3LYP/
MP2(full)/
6-311+G(d,p) 6-31G(d)
gauche
MP2(full)/
B3LYP/
6-311+G(d,p)
6-31G(d)
r
Quadratic centrifugal distortion constants are defined according to Watson’s A-reduction for I representation.
256
B3LYP/
6-311+G(d,p)
Figure 39: Experimental mid-infrared spectra of ethyldichlorophosphine: observed spectra of
xenon solutions at -70 with fundamentals labeled for both (t = trans and g = gauche) conformers
257
Figure 40: Ethyldichlorophosphine molecule showing atomic numbering.
258
Figure 41: Mid-infrared spectra of ethyldichlorophosphine dissolved in liquefied xenon solution at
four different temperatures with bands used in the enthalpy determination assigned on spectra.
259
CHAPTER 11
R0 STRUCTURAL PARAMETERS, CONFORMATIONAL STABILITY, BARRIERS TO
INTERNAL ROTATION, AND VIBRATIONAL ASSIGNMENTS FOR TRANS AND
GAUCHE ETHANOL
Introduction
The conformational stabilities and barriers to internal rotation of alcohols in the gas phase or
in solution have provided significant challenges to structural scientist for years for a variety of
reasons. The O-H barriers for those that have been determined are about one third or less than
those obtained for methyl barriers so there is usually only the fundamental vibration and one
excited state in the potential well which makes it difficult to obtain the barriers to internal rotation
from the torsional frequencies. Also the determination of the conformational stability has been
difficult to determine because of the association due to hydrogen bonding which is significant in
the gas phase where vibrations due to the dimer as well as trimers can be observed.
The determination of the structural parameters for ethanol has not been without some serious
problems. The first structural determination was an electron diffraction study [106] by Rouault and
Gallagher that determined the CC and CO bond distances, the CH bond distances as one set, and
the CCO angle. The first complete structural determination was a microwave study [107] where
Kadzhar et al. used the principal moments of inertia obtained for ethanol and seven isotopomers to
determine the structural parameters. This structural determination suffered from the assumption
that the methyl hydrogens were all equivalent and the difference between the secondary moments
of inertia Pcc-Pcc not being equal to zero. The next complete structural determination was another
microwave study [108] by Culot where the rotational spectra of thirteen isotopomers of ethanol
were used to determine the rs structural parameters. In this study [108] it was concluded that the
structural determination by the substitution method was unfavorable and the structural parameters
260
obtained were not accurate, this conclusion was based on the finding that the effect of zero-point
vibrations is rather large. This effect is even larger when the substitution is done on the in-plane
hydrogen atom of the methyl group and the hydrogen atom of the OH group. Culot was aware that
the zero-point vibrational effects due to the torsional oscillation of the methyl group resulted in
less accurate structural parameters especially those involving the H7 coordinates, Fig (42). This
was followed by two microwave studies [109, 110] by Quade et al. where the rotational constants
for the gauche conformer were determined but no structural parameters were obtained in either
study. There are remarkable discrepancies between previously obtained experimental structural
parameters of ethanol and the theoretical predicted structural parameters for this molecule. There
are also a number of complicating factors which undermine the validity of the structural
parameters determined in the previous studies. Additionally, there are currently no structural
parameters determined for the gauche conformer of ethanol. Therefore, to obtain a more complete
determination of the structural parameters for ethanol we have combined the ab initio MP2(full)/6311+G(d,p) predicted parameters with the eighteen experimentally determined rotational constants
[108, 109] for the two forms to obtain the complete structural parameters for both conformers of
ethanol.
Experimental and Theoretical Methods
The sample of ethanol was purchased from Sigma-Aldrich Chemical Co., with stated purity
of >99%. The sample was further purified by low-temperature, low-pressure fractionation column
and the purity of the sample was verified by comparing the infrared spectrum with that previously
reported [111].
The mid-infrared spectrum of the gas (Fig. 43A) was obtained from 4000 to 300 cm-1 on a
Perkin-Elmer model 2000 Fourier transform spectrometer equipped with a Ge/CsI beamsplitter and
a DTGS detector. Atmospheric water vapor was removed from the spectrometer housing by
261
purging with dry nitrogen. The theoretical resolution used to obtain the spectrum of the gas was
0.5 cm-1. One hundred twenty eight interferograms were added and transformed with a boxcar
truncation function. The frequencies for the predicted and observed fundamentals for the trans and
gauche conformers are listed in Tables 68 and 69, respectively.
The mid-infrared spectra (4000 to 400 cm-1) of the sample dissolved in liquefied xenon (Fig.
43B) were recorded on a Bruker model IFS-66 Fourier transform spectrometer equipped with a
globar source, a Ge/KBr beamsplitter and a DTGS detector. In all cases, 100 interferograms were
collected at 1.0 cm-1 resolution, averaged and transformed with a boxcar truncation function. For
these studies, a specially designed cryostat cell was used. It consists of a copper cell with a path
length of 4 cm with wedged silicon windows sealed to the cell with indium gaskets. The copper
cell was enclosed in an evacuated chamber fitted with KBr windows. The temperature was
maintained with boiling liquid nitrogen and monitored by two Pt thermo resistors.
The ab initio calculations were performed with the Gaussian-03 program [21] using
Gaussian-type basis functions. The energy minima with respect to nuclear coordinates were
obtained by the simultaneous relaxation of all geometric parameters using the gradient method of
Pulay [22]. A variety of basis sets as well as the corresponding ones with diffuse functions were
employed with the Møller-Plesset perturbation method [23] to the second order MP2 with full
electron correlation as well as with the density functional theory by the B3LYP method.
In order to obtain descriptions of the molecular motions involved in the fundamental modes
of ethanol, a normal coordinate analysis was carried out. The force field in Cartesian coordinates
was obtained with the Gaussian 03 program at the MP2(full) level with the 6-31G(d) basis set.
The internal coordinates used to calculate the G and B matrices are given for the trans and gauche
conformers in Table 70 with the atomic numbering shown in Fig. 44. By using the B matrix [24],
the force field in Cartesian coordinates was converted to a force field in internal coordinates.
262
Subsequently, 0.88 was used as the scaling factor for the CH stretches and deformations and 0.90
was used for all other modes excluding the heavy atom bend to obtain the fixed scaled force field
and resultant wavenumbers. A set of symmetry coordinates was used (Table 71) to determine the
corresponding potential energy distributions (P.E.D.s). A comparison between the observed and
calculated wavenumbers, along with the calculated infrared intensities, Raman activities,
depolarization ratios and potential energy distributions for the trans and gauche conformers of
ethanol , are given in Tables 68 and 69, respectively.
Vibrational Assignment
To obtain information on the relative conformational enthalpy change between the two
conformers a complete vibrational assignment is required for both conformers from 700 to 1300
cm-1 which is the usual region where conformer pairs can be identified. The lower spectral region
will have the smallest number of combination and overtone bands possible so it is the best area to
obtain the enthalpy difference. Since there is very little change in the force constants with
conformational interchange of the OH rotor most of the fundamentals for the trans and gauche
forms have nearly the same frequencies. Therefore as much information as can be obtained is
needed to make the assignments of the fundamentals of the two forms.
First the vibrational spectrum were predicted from the MP2(full)/6-31G(d) calculations. The
predicted scaled frequencies were used together with a Lorentzian function to obtain the calculated
infrared spectra.
Infrared intensities determined from MP2(full)/6-31G(d) calculations were
obtained based on the dipole moment derivatives with respect to Cartesian coordinates. The
derivatives were transformed with respect to normal coordinates by
(u/Qi) =

j
(u/Xj)Lij,
where Qi is the ith normal coordinate, Xj is the jth Cartesian displacement coordinate, and Lij is the
transformation matrix between the Cartesian displacement coordinates and the normal coordinates.
263
The infrared intensities were then calculated by [(N)/(3c2)] [(x/Qi)2 + (y/Qi)2 + (z/Qi)2].
In Fig. 44 a comparison of the experimental and simulated infrared spectra of ethanol is shown for
the spectral region from 900 to 1500 cm-1. The predicted spectrum is in relatively good agreement
with the experimental spectrum which shows the utility of the scaled predicted frequencies and
intensities for supporting the vibrational assignment.
To further support the vibrational assignments, we have simulated the Raman spectra from
the ab initio MP2(full)/6-31G(d) results. The evaluation of Raman activity by using the analytical
gradient methods has been developed [25-27, 37]. The activity Sj can be expressed as:
Sj = gj (45  2j + 7  j2 ), where gj is the degeneracy of the vibrational mode j, j is the derivative of
the isotropic polarizability, and j is that of the anisotropic polarizability. To obtain the polarized
Raman scattering cross sections, the polarizabilities are incorporated into Sj by multiplying Sj with
(1j)/(1+j), where j is the depolarization ratio of the jth normal mode. The Raman scattering
cross sections and calculated wavenumbers obtained from the Gaussian 03 program were used
together with a Lorentzian function to obtain the simulated Raman spectra of the gas. The
predicted spectrum of the mixture should be compared to that of the Raman spectrum of the gas
(Fig. 45) and the relative intensities of the bands are of use in the vibrational assignment of the
Raman data, particularly for the Raman spectra of the gas.
The earliest complete vibrational assignment was reported by Perchard and Josien [112] who
prepared eleven isotopologues along with the normal species of ethanol for which the vibrations
for the various segments were clearly identified. However the assignments were made based on the
belief that the gauche conformer was the most stable form and in great abundance so most of the
observed bands were assigned as gauche bands. However two years later Barnes and Hallam [111]
carried out a vibrational study of ethanol by utilizing argon matrix isolation and concluded that the
trans conformer was the more stable form and it was in the ratio of 2 to 1 so most of the observed
264
fundamentals were assigned to the trans conformer. From a later Raman spectral study [113] the
trans conformer was determined to be the more stable form from their relative intensities to the
bands assigned to the gauche form. This conclusion was supported by the potential function
determined in that study which was further supported by an earlier microwave study [114].
Therefore we have re-investigated the vibrational spectra and made the assignments by utilizing ab
initio predicted frequencies, infrared intensities and Raman activities along with infrared band
contours. From the extensive predictions from the ab initio calculations it was possible to
confidently assign practically all of the fundamentals for both conformers of ethanol (Tables 68
and 69).
Conformational Stability
To determine the enthalpy differences between the two conformers of CH3CH2OH, the
sample was dissolved in liquefied xenon and the mid-infrared spectra were recorded as a function
of temperature from -55 to -100°C. Relatively small interactions are expected to occur between
xenon and the sample though the sample can associate with itself forming a dimer or trimer.
Nevertheless, only small frequency shifts are anticipated for the xenon interactions when passing
from the gas phase to the liquefied xenon solutions. A significant advantage of this study is that the
conformer bands are better resolved in comparison with those in the infrared spectrum of the gas.
From ab initio calculations, the dipole moments of the two conformers are predicted to have
similar values and the molecular sizes of the two conformers are nearly the same, so the ΔH value
obtained from the temperature dependent FT-IR study is expected to be close to that for the gas
[39-43].
Once confident assignments have been made for the fundamentals of both conformers the
task was then to find a pair or more of bands for which the enthalpy determination could be
obtained. To minimize the effect of combination and overtone bands in the enthalpy determination
265
it is desirable to have the lowest frequency pair that is possible to use for the determination.
Additionally, since there is clear evidence that the ethanol molecules associate in the gas phase it is
certainly going to associate in the xenon solution, although it may be to a lesser amount.
Nevertheless to ensure the association is minimized it is desirable to have vibrational fundamentals
that are not significantly affected by dimerization or further association. Therefore the vibrations
that have a large amount of methyl group (CH3) vibrational motion were initially assessed. There
is a band at 887 cm-1 for the trans conformer which has a large contribution from the CH3 motion
(41% S12) as well as some CC stretch and CO stretch contributions. Therefore the methyl rock and
CC stretch should be minimally affected by association. A similar band at 879 cm-1 for the gauche
conformer is present and the difference in frequency is sufficient to be able to provide band
contours with which to determine the relative intensities from -60 to -90°C. The antisymmetric
rock which is in the lower frequency region could likewise be a good candidate but unfortunately
the band is so weak that there is difficulty in separating the two vibrations even though they are
predicted to be separated by 18 wavenumbers. An effort was also made to use the 1025 cm-1 band
of the trans conformer but it appeared to be significantly affected by association since the
determined enthalpy difference was relatively large. Therefore only one conformer pair was found
that provided appropriate band intensities that could readily be determined as the temperature was
lowered.
The intensities of the infrared bands were measured as a function of temperature and their
ratios were determined. By application of the van’t Hoff equation lnK = H/(RT)  S/R, the
enthalpy difference was determined from a plot of lnK versus 1/T, where H/R is the slope of the
line and K is substituted with the appropriate intensity ratios, i.e. Itrans / Igauche. It was assumed that
H, S, and α are not a function of temperature in the temperature range studied. This pair of
bands was utilized for the determination of the enthalpy difference, and the resulting value of 62 
266
12 cm-1 (Table 73) and the van’t Hoff plot is shown in Fig. 46. This value is consistent with the
predicted value from ab initio calculations with large basis sets (Table 72). This error limit is
derived from the statistical standard deviation of one sigma of the measured intensity data, but it
does not take into account small associations with the liquid xenon or the interference of overtones
and combination bands in near coincidence with the measured fundamentals. Thus, the abundance
of the trans conformer present at ambient temperature is 40 ± 1%.
Structural Parameters
Initial structural parameters for the ethanol were proposed from an early electron diffraction
study [106] where the structures of ethanol and ethyl mercaptan were determined by the visual
method and the values are listed in Table 70. This study was followed by a large number of
microwave investigations of this molecule but in only seven of these studies were structural
parameters reported and in all of these investigations only the parameters for the trans form were
given [107, 108, 115-119]. In the earliest of these studies [115-117] the structural parameters were
transferred from the corresponding parameters for the haloethanes and methyl alcohol and the
rotational constants resulting from these parameters agreed with the experimentally determined
ones. In the two most extensive investigations [107, 108] following the earlier ones the structural
parameters were determined by the substitution method but ethanol is one of the most unfavorable
cases as can be imagined as previously indicated for obtaining the parameters by Kraitchman’s
method and the values are listed in Table 70. Therefore, we believe the determination of the r0
structural parameters will provide much more reliable ones than the rs parameters previously
reported [107, 108].
We have found that good structural parameters for hydrocarbons and many substituted ones
can be determined by adjusting the structural parameters obtained from the ab initio MP2(full)/6311+G(d,p) calculations to fit the rotational constants obtained from microwave experimental data
267
by using a computer program developed [57] in our laboratory. In order to reduce the number of
independent variables, the structural parameters are separated into sets according to their types
where bond distances in the same set keep their relative ratio, and bond angles and torsional angles
in the same set keep their difference in degrees. This assumption is based on the fact that errors
from ab initio calculations are systematic. Additionally, we have also shown that the differences in
predicted distances and angles from the ab initio calculations for different conformers of the same
molecule can usually be used as one parameter with the ab initio predicted differences except for
some dihedral angles. Therefore, it should be possible to obtain “adjusted r0” structural parameters
for both conformers of ethanol by utilizing the previously reported fifteen rotational constants from
the microwave study [108] of the trans conformer along with the three rotational constants for the
gauche conformer [109].
The carbon-hydrogen distances were taken as a single set and the three heavy atom
parameters were then adjusted to provide the best fit of the 15 rotational constants for the trans
conformer. The fit of the reported experimental rotational constants is shown in Table 74 and the
agreement is considered satisfactory. The fit to four of the A rotational constants for the trans
conformer have larger differences than those for the B and C rotational constants but the A
rotational constants are four times larger than the B and C constants so only the A constants for the
normal species and that for the molecule with the two 13C atoms have differences larger than those
for the other molecules which includes those for the gauche form. However by considering the
gauche conformer as part of the set it is believed that the uncertainties in the determined structural
parameters are the same for the gauche conformer as those for the trans form (Table 70).
Discussion
All of the fundamentals chosen by Perchard and Josien [112] are fundamentals in our study
and the bands which they assigned as combination bands are assigned as fundamentals for the trans
268
conformer. All of the fundamentals selected in the matrix study [111] are also fundamentals in the
current study but it was estimated that the amount of the trans conformer was twice the amount of
the gauche form. However our study shows that even though the
trans is more stable the
degeneracy of the gauche conformer results in it being 60  1 % present at room temperature
which resulted in many of the fundamentals assigned to trans bands when they are in fact gauche
fundamentals (Table 68).
The ab initio predicted frequencies are in good agreement with the observed fundamentals
with those for the trans form being predicted with an average difference of 8 cm-1 which is 0.5%
error for the A′ block and slightly poorer with an average difference of 10 cm-1 (0.6 % error) for
the A′′ block these errors arises mainly from the carbon-hydrogen stretches and the CH2 wag. For
the gauche conformer the average difference is 8 cm-1 which is 0.5 % error. The advantage of the
ab initio MP2(full)/6-31G(d) predictions is that the predicted frequencies are usually higher than
the observed values or very close to the observed values.
A comparison of the predicted infrared spectrum with the experimental infrared spectrum of
the xenon solution Fig. 44 indicates the intensities are very helpful in making the vibrational
assignments. The Raman predicted activities are not as valuable but they help particularly when
obtained from the gas for a molecule with some symmetry. Only the symmetric modes have the Qbranches which are quite sharp from which the band centers are easily determined (Fig. 45).
The predicted potential function from the ab initio MP2(full)/6-311+G(2d,2p) calculation is
shown in Fig. 47 which is compared to the experimentally determined potential function [120] for
the O-H internal rotation. The trans to gauche barrier is predicted to be essentially the same at 398
cm-1 as the experimentally determined value of 402.8  0.1 cm-1. However the gauche to gauche
barrier is predicted to be 420 cm-1 which is slightly higher than the experimental value of 399.1 
0.2 cm-1 but the difference shown in the figure is mainly due to the difference between the
269
predicted energy difference of 73 cm-1 compared to the enthalpy difference of 45 cm-1 reported
from the potential function. This enthalpy difference was fixed in the potential function to agree
with the previously determined value of 41.2 cm-1 from microwave data [109]. In the current study
the experimentally determined enthalpy difference of 62  12 cm-1 was obtained which is in
reasonable agreement with the earlier determined value from the microwave study.
We also predicted the barrier to methyl rotation for both conformers. For the trans conformer
the predicted CH3 barrier from the six largest basis sets is 1228 cm-1 compared to the
experimentally determined value of 1185  16 cm-1 (14.18  0.19 kJ/mol) which was obtained
from the torsional frequency [120]. It should be noted that the predicted value from
MP2(full)/6-311+G(2d,2p) with 135 basis sets gives a value of 1234 cm-1 and the calculation from
MP2(full)/aug-cc-pVQZ with 516 basis sets predicts a value of 1240 cm-1(Table 72)! For the
gauche conformer the CH3 barrier was obtained from the double jumps in the Raman spectrum of
the gas from which a value of 1251  2 cm-1 (14.97  0.02 kJ/mol) was obtained which should be
compared to the predicted value of 1324 cm-1. These predictions for both conformers indicate the
ab initio values are reasonably good in providing values comparable to the experimentally
determined ones and do not vary appreciably with basis set size. However the variation is
significant from density functional theory calculations by the B3LYP method (Table 72).
The determination of the enthalpy was difficult because of the obvious association (probably
a dimer) in the xenon solution as the temperature was lowered even with the four cm cell path and
a concentration of about 10-4 molar. Attempts were made to obtain values from two other pairs but
neither one gave a satisfactory van’t Hoff plot. More reliable values for the enthalpy differences
are obtained when several pairs can be used but using the low frequency pair reduces the possible
effects of combination bands affecting the change in intensity due to the temperature change.
270
To support the enthalpy determination the predicted intensities of the infrared bands were
used along with the measured intensities at one temperature from which the enthalpy can be
determined. The value determined from the 887/879 cm-1 pair was 97 cm-1, for the 1025/1058 cm-1
pair 102 cm-1, and the 1025/1066 cm-1 pair 114 cm-1 from which the average is 104 cm-1 with the
trans conformer the more stable form. These values are slightly higher than the experimentally
determined value but they support the relatively small enthalpy difference between the conformers.
The adjusted-r0 parameters indicate the C1-H7 rs distance is entirely too short after taking into
account that the rs parameters are smaller than the corresponding r0 values. A similar problem
should be noted for the C1C2H5(H6)° which has a reported value larger than the r0 value.
However the most dramatic difference is for the C2C3H4 where the rs value is 105.4(1)° whereas
the r0 value is 107.5(5)°. The r(C1-C2) rs distance is 0.006 Å shorter than the r0 value but taking the
reported uncertainties for each parameter they agree. Nevertheless the rs values are remarkably
good considering how unfavorable the ethanol molecule is for the determination of the rs
parameters. Finally it should be noted that the initially reported rs value [107] reported for the C1C2 distance of 1.524 Å is entirely too long compared to the other determination but most of the
other parameters are in agreement with the r0 values. Although no parameters have been reported
for the gauche conformer it is believed that the values reported in the current study are quite
reliable within the listed uncertainties.
The quadratic centrifugal distortion constants have been predicted from ab initio MP2(full)
calculations with two basis sets and from density functional theory calculations by the B3LYP
method with the same basis sets. Two different sets of experimental values have been reported
[108, 121] with those from the most recent study expected to be the most accurate. The B3LYP
calculations usually give values in better agreement with the experimental values. This is true in
the current case except for the JK constant where the experimental value is negative as predicted
271
from the MP2(full) calculations but positive from the B3LYP calculations. Also it should be noted
the difference between the values reported in the initial study [108] and those determined recently
that have much smaller uncertainties. The values initially reported are relatively large compared to
the more recently reported distortion constants. This result could affect the value of the A
rotational constant where its fit is significantly poorer than the fits for the other two constants for
most of the isotopic species.
272
Table 68 : Observed and calculated frequencies (cm -1) for trans ethanol.
Sym. Vib.
block No.
A
273
A
a
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
Fundamental
OH stretch
CH3 antisymmetric stretch
CH3 symmetric stretch
CH2 symmetric stretch
CH2 symmetric deformation
CH3 antisymmetric
deformation
CH2 wag
CH3 symmetric deformation
COH bend
CO stretch
CC stretch
CH3 rock
CCO bend
CH3 antisymmetric stretch
CH2 antisymmetric stretch
CH3 antisymmetric
deformation
CH
2 twist
CH2 rock
CH3 rock
OH torsion
CH3 torsion
ab initioa
3779
3211
3119
3062
1598
1567
1510
1456
1312
1146
1079
937
427
3222
3104
1550
1337
1223
848
316
265
fixed
scaledb
IR
int.c
3585
3012
2926
2872
1503
1472
1431
1375
1248
1090
1025
888
420
3022
2912
1455
1268
1160
804
300
251
22.0
21.0
11.3
59.7
2.1
3.6
16.8
0.9
83.9
18.1
54.6
11.1
13.1
21.9
62.4
5.1
0.2
4.7
0.01
109.0
33.8
Raman
act.d
96.6
54.1
98.6
96.1
5.5
22.6
6.1
0.8
6.6
6.1
2.9
6.1
0.2
44.1
77.9
16.7
13.7
3.6
0.04
4.0
0.7
IR
gas
3676
2992
2922
2888
1500
1480
1450
1367
1241
1090
1027
892
418
2987
2901
1455
1275
1166
801
202.6e
244.4e
Xenon
solution
Argon
Matrix f
Raman
gas g
3648
2961
2920
2868
1501
1483
1446
1367
1238
1086
1025
887
419
2965
2884
1453
1272
1156
797
(3657.6)
2995.6
2939.6
2900.5
1490.2
1462.8
1415.0
(1371.3)
1240.3
1091.7
1025.0
886.7
416.5
2985.4
2953.5
(1445.1)
(1250.8)
(1083.2)
(812.1)
211
264
3675
2887
1460
1460
1430
1245
1093
1026
883
422
P.E.D.h
100S1
99S2
99S3
100S4
92S5
84S6
54S
7,18S
8,15S
9,10S11
77S
8,19S
7
55S9,16S7,15S12
57S10,20S12
51S11,21S9,15S12
32S12,36S10,29S11
84S13,10S12
99S14
99S15
93S16
75S17,21S19
57S18,25S19,15S17
46S19,38S18
82S20,15S21
82S21,16S20
MP2(full)/6-31G(d) predicted values.
MP2(full)/6-31G(d) fixed scaled frequencies with factors of 1.0 for the heavy atom bend, 0.88 for CH stretches and deformations, and 0.90 for all other modes.
c
Scaled infrared intensities in km/mol from MP2(full)/6-31G(d) .
d
Scaled Raman activities (Å4/u) from MP2(full)/6-31G(d) .
e
Ref [120]
f
Ref [111], Bands marked in parenthesis belong to the gauche conformer.
g
Ref [113]
h
Calculated with MP2(full)/6-31G(d) and contributions of less than 10% are omitted.
i
A, B and C values in the last three columns are percentage infrared band contours.
b
Band
Contoursi
A
B
94
6
23
77
89
11
- 100
77
23
79
21
83
17
80
20
52
48
67
69
70
88-
33
31
30
12
-
Table 69: Observeda and calculated frequencies (cm -1) for gauche ethanol.
Vib.
No.
Fundamental
ab
initiob
fixed
scaledc
IR
int.d
Raman
act.e
IR
gas
274
Xeno
n
soluti
on
3636
~299
4
2975
2962
2930
2892
1492
1464
1460
1388
1373
1339
1248
1116
1055
1055
879
800
419
Raman
gas g
P.E.D.h
Band
Contours i
A
B C
17 43 40
5 13 82
8 45 48
29
2 69
67 22
- 70 11
3 95 30
2
25
9 75
6 85
100
68 32
61 15
80 14 24
6
5 73 22
89
62 11
38
66 33
1
43 43
14
30 58 12
39 32 29
14 69 17
OH stretch
3772
3579
18.5
61.1
3662
3659
100S1
1
CH3 antisymmetric stretch
3214
3015
24.7
42.3
2994
63S2,33S3
2
CH3 antisymmetric stretch
3193
2996
44.5
31.4
2987
2985
55S3,20S4,17S2
3
CH2 antisymmetric stretch
3175
2978
6.4 106.3
2972
52S4,18S2,18S4,12S6
4
CH
symmetric
stretch
3105
2912
12.3
106.3
2936
2939
98S5
5
3
CH2 symmetric stretch
3077
2886
61.6
88.9
2912
2914
75S6,25S4
6
CH2 symmetric deformation
1585
1490
0.3
3.0
1493
1460
84S7,12S8
7
CH3 antisymmetric
1561
1466
2.0
26.3
1465
1460
79S8,15S7
8
deformation
CH
antisymmetric
1556
1461
7.0
15.1
1460
1460
91S9
9
3
deformation
CH3 symmetric deformation
1476
1396
30.8
5.8
1394
1395
58S10,31S11
10
CH2 wag
1450
1373
22.5
3.4
1373
1370
45S11,35S10,13S12
11
COH bend
1417
1345
4.4
9.5
1342
1340
35S12,32S13,16S11
12
CH
twist
1327
1259
10.2
5.2
1249
1245
42S13,26S18,13S12,12S14
13
2
CH2 rock/CH3 rock
1174
1119
6.7
4.3
1117
1117
17S14,28S16,16S15,11S19,11S18
14
CO stretch
1120
1063
59.5
5.9
1066
1055
51S15,44S17
15
CH3 rock/COH bend
1099
1044
57.3
1.4
1058
1055
19S16,32S12,15S14,11S18
16
CC stretch
922
875
61.1
7.8
879
883
36S17,31S15,30S16
17
CH3 rock
829
786
3.1
0.2
803
43S18,43S14
18
CCO bend
430
423
15.6
0.6
420
422
81S19,11S16
19
OH torsion
330
313
128.1
3.3
243.1/195.8f
89S20
20
CH3 torsion
285
270
9.4
0.3
274
91S21
21
a
Ref [6] assigned three gauche conformer bands; 3662.2, 1076.6, and 889.4 cm-1.
b
MP2(full)/6-31G(d) predicted values.
c
MP2(full)/6-31G(d) fixed scaled frequencies with factors of 1.0 for the heavy atom bend, 0.88 for CH stretches and deformations, and 0.90 for all other modes.
d
Scaled infrared intensities in km/mol from MP2(full)/6-31G(d) .
e
Scaled Raman activities (Å4/u) from MP2(full)/6-31G(d) .
f
Ref [120]
g
Ref [113]
h
Calculated with MP2(full)/6-31G(d) and contributions of less than 10% are omitted.
i
A, B and C values in the last three columns are percentage infrared band contours.
Table 70: Structural parametersa, rotational constants, and dipole moments for trans and gauche
ethanol.
Parameter
Int.
Coor.
MP2(full)/
B3LYP/
EDb
66311+G(d,p) 311+G(d,p)
0.960
0.962
trans
MWc
MWd
rs
MWe
rs
adjusted
r0f,g
0.936
0.956
0.9710 (6)
0.962 (3)
gauche
MP2(full)/ B3LYP/ adjusted
66r0f,g
311+G(d,p) 311+G(d,p)
0.961
0.963
0.963 (3)
r(O3-H4)
R1
r(C2-O3)
R2
1.426
1.431
1.40
1.426
1.428
1.431 (3)
1.432 (3)
1.424
1.429
1.430 (3)
r(C1-C2)
R3
1.513
1.517
1.50
1.533
1.524
1.512 (3)
1.519 (3)
1.519
1.524
1.523 (3)
r(C1-H7)ip
r1
1.093
1.093
1.08
1.092
1.093
1.0878 (3)
1.095 (2)
1.094
1.095
1.096 (2)
r(C1-H8)op
r2
1.092
1.093
1.08
1.092
1.093
1.0908 (2)
1.094 (2)
1.095
1.095
1.097 (2)
r(C1-H9)op
r3
1.092
1.093
1.08
1.092
1.093
1.0908 (2)
1.094 (2)
1.092
1.093
1.094 (2)
r(C2-H5)
r4
1.097
1.099
1.08
1.092
1.100
1.0980 (4)
1.099 (2)
1.098
1.099
1.100 (2)
r(C2-H6)
r5
1.097
1.099
1.08
1.092
1.100
1.0980 (4)
1.099 (2)
1.091
1.092
1.093 (2)
C2O3H4
π1
107.7
109.0
106.0
104.8
105.4 (1)
107.5 (5)
107.0
108.6
107.0 (5)
O3C2H5
π2
110.6
110.4
109.8 (2)
110.8 (5)
110.6
110.4
110.4 (5)
O3C2H6
π3
110.6
110.4
109.8 (2)
110.8 (5)
105.3
105.1
105.2 (5)
C1C2O3
α
107.6
107.9
C1C2H5
ρ1
110.1
C1C2H6
ρ2
H5C2H6
107.4
107.6
107.77 (2)
107.6 (5)
112.3
113.0
112.5 (5)
110.2
109.8
109.5
110.7 (2)
109.6 (5)
110.3
110.4
110.2 (5)
110.1
110.2
109.8
109.5
110.7 (2)
109.6 (5)
110.5
110.4
110.5 (5)

108.1
107.8
108.0 (2)
108.5 (5)
107.7
107.3
107.7 (5)
H7C1C2
1
110.4
110.4
109.8
110.7
110.5 (1)
110.0 (5)
110.7
110.6
110.7 (5)
H8C1C2
2
110.2
110.5
109.8
110.7
110.5 (1)
109.7 (5)
110.8
111.2
110.8 (5)
H9C1C2
3
110.2
110.5
109.8
110.7
110.5 (1)
109.7 (5)
110.3
110.7
110.2 (5)
H8C1H9
μ1
108.6
108.3
108.8 (2)
109.0 (5)
108.3
108.2
108.4 (5)
H7C1H8
μ2
108.6
108.3
108.8 (2)
109.0 (5)
107.8
107.6
107.9 (5)
H7C1H9
μ3
108.7
108.5
108.8 (2)
109.2 (5)
108.8
108.5
108.8 (5)
H4O3C2C1
τ1
180.0
180.0
180.0
180.0
180.0
180.0
180.0
56.9
60.8
56.9 (5)
H7C1C2 O3
τ2
180.0
180.0
180.0
180.0
180.0
180.0
180.0
177.4
177.2
177.4 (5)
A
35060
35097
34890.8
34252
34417
34173.8
B
9421
9320
9350.8
9350.8
9350.65 (2)
9352.933
9270
9122
9191.5
C
8189
8113
8135.0
8135.0
8135.22 (2)
8136.169
8161
8063
8100.4
|μa|
0.014
0.043
0.058 (100)
1.597
1.419
|μb|
1.903
1.772
1.473 (100)
0.024
0.076
|μc|
0.000
0.000
0.000
1.338
1.233
|μt|
109.5
34890.8 34891.73 (7) 34898.282
1.474 (100)
1.903
1.773
2.084
Bond distances in Å, bond angles in degrees, rotational constants in MHz, and dipole moments in Debye.
b
Ref[106].
c
Assumed structural parameters, Ref[115]
d
Ref[107]
e
Ref[108]
f
This study.
g
Adjusted structural parameters with experimental rotational constants taken from Ref[109] .
a
275
2.660
Table 71: Symmetry coordinates for trans ethanol.
Species Description
A OH stretch
CH3 antisymmetric stretch
CH3 symmetric stretch
CH2 symmetric stretch
CH2 symmetric deformation
CH3 antisymmetric deformation
CH2 wag
CH3 symmetric deformation
COH bend
CO stretch
CC stretch
CH3 rock
CCO bend
A CH3 antisymmetric stretch
CH2 antisymmetric stretch
CH3 antisymmetric deformation
CH2 twist
CH2 rock
CH3 rock
OH torsion
CH3 torsion
*Not normalized.
S1
S2
S3
S4
S5
S6
S7
S8
S9
S10
S11
S12
S13
S14
S15
S16
S17
S18
S19
S20
S21
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
276
Symmetry Coordinate*
R1
2r1  r2  r3
r1 + r2 + r3
r4 + r5
4  ρ1 ρ2 π2 π3
μ1 μ2 μ3
π2+ π3-ρ1-ρ2
μ1+ μ2 + μ3 1  2 3
π1
R2
R3
21  2 3
α
r2 - r3
r4 - r5
μ2  μ3
π2  π3  ρ1 + ρ2
π2  π2 + ρ1  ρ2
2  3


Table 72: Calculated energies (hartree) and energy differences (cm-1) for the two conformers and
transition states of ethanol.
MP2(full)/6-31G(d)
Basis
functions
57
-0.528955
MP2(full)/6-31+G(d)
69
MP2(full)/6-31G(d,p)
btg
-30
CH3
barrierc
1310
CH3
barrierd
1402
(t g)
barriere
477
(g g)
barrierf
639
-0.542510
66
1309
1422
442
554
75
-0.582081
-29
398
495
MP2(full)/6-31+G(d,p)
87
-0.595132
59
448
620
MP2(full)/6-311G(d,p)
90
-0.693295
-108
1206
1321
467
683
MP2(full)/6-311+G(d,p)
102
-0.702059
-5
1252
1401
373
595
MP2(full)/6-311G(2d,2p)
123
-0.740875
14
1337
1492
395
449
MP2(full)/6-311+G(2d,2p)
135
-0.748375
73
1234
1415
392
422
MP2(full)/6-311G(2df,2pd)
174
-0.798083
26
1184
1274
423
444
MP2(full)/6-311+G(2df,2pd)
186
-0.805164
68
1238
1302
399
415
MP2(full)/cc-pVTZ
174
-0.142838
77
1203
1301
MP2(full)/aug-cc-pVTZ
276
-0.811230
70
1268
1345
MP2(full)/aug-cc-pVQZ
516
-0.892742
82
1240
1309
B3LYP/6-31G(d)
57
-1.033805
-106
1215
1313
418
587
B3LYP/6-31+G(d)
69
-1.045630
26
1106
1206
421
483
B3LYP/6-311G(d,p)
90
-1.088124
-79
1192
1256
398
581
B3LYP/6-311+G(d,p)
102
-1.095063
22
1114
1196
367
484
B3LYP/6-311G(2d,2p)
123
-1.095358
-31
1210
1271
337
438
B3LYP/6-311+G(2d,2p)
135
-1.101692
32
1106
1207
342
406
B3LYP/6-311G(2df,2pd)
174
-1.099179
-19
1081
1131
360
444
B3LYP/6-311+G(2df,2pd)
186
-1.105504
38
1056
1134
350
404
B3LYP/cc-pVTZ
174
-1.105993
2
1083
1137
B3LYP/aug-cc-pVTZ
276
-1.110261
49
1059
1137
B3LYP/aug-cc-pVQZ
516
-1.121830
46
1310
1402
Method / Basis set
transa
a
Energy of conformer is given as –(E+154) H.
Difference is relative to trans form and given in cm-1 with a negative value indicating the gauche conformer is more
stable.
c
Methyl torsional barrier for the trans conformer.
d
Methyl torsional barrier for the gauche conformer.
b
277
e
f
Eclipsed form is a transition state that has the OH bond eclipsing the methylene CH bond.
Cis form is a transition state that is rotated 180 from the trans conformer (OH bond eclipses the CC bond).
278
Table 73: Temperature and intensity ratios of the trans and gauche bands of ethanol.
Liquid xenon
H (cm-1)

T(C)
1/T (10-3 K-1)
I887 / I879
60.0
65.0
70.0
75.0
80.0
85.0
90.0
4.692
4.804
4.923
5.047
5.177
5.315
5.460
0.613
0.630
0.645
0.649
0.654
0.658
0.661
62  12

279
Table 74: Comparison of rotational constants obtained from modified ab initio, MP2/6-311+G(d,p)
structural parameters and those from microwave spectra for ethanol.
Isotopomer
trans
CH3CH2 OHa
13
CH3CH2 OHa
CH3 13CH2 OHa
CH3CH2 18OHa
13
CH3 13CH2 OHa
Rotational
constants
Observed
Calculated
||
A
B
C
A
B
C
A
B
C
A
B
C
A
B
C
34891.7
9350.7
8135.2
34748.9
9087.7
7927.9
34125.1
9351.5
8093.0
34446.1
8953.2
7809.8
33975.0
9088.3
7887.0
34898.2
9352.0
8136.1
34752.4
9087.7
7927.8
34122.2
9351.9
8093.2
34445.6
8952.8
7809.4
33969.1
9087.4
7886.2
6.5
1.3
0.9
3.5
0.0
0.1
2.9
0.4
0.2
0.5
0.4
0.4
5.9
0.9
0.8
A
34173.8
34172.1
1.7
B
9191.5
9191.9
0.4
8100.4
8099.6
0.8
gauche
CH3CH2 OHb
C
Values for the rotational constants taken from Ref[108] .
b
Values for the rotational constants taken from Ref[109] .
a
280
Table 75: Quadratic centrifugal distortion constants (MHz) for trans and gauche-ethanol.
gauche- CH3CH2OHa
trans- CH3CH2OHa
MP2(full)/
6-31G(d)
MP2(full)/
6-311+G(d,p)
B3LYP/
6-31G(d)
B3LYP/
6-311+G(d,p)
Exp.b
MP2(full)/
6-31G(d)
MP2(full)/
6-311+G(d,p)
B3LYP/
6-31G(d)
B3LYP/
6-311+G(d,p)
0.0116130
0.0120487
0.0123137
0.0101104
0.0101674
0.0103277
0.0085374
(8)
0.0113609
-0.04685
-0.04692
0.01723
0.02065
-0.02878 (3)
-0.05510
-0.05756
-0.05844
-0.05570
∆K
0.4392
0.4535
0.2107
0.2145
0.2525 (1)
0.5019
0.5038
0.0531
0.5275
δJ
0.0199325
0.0020267
0.0020146
0.0020494
0.0017377
(5)
0.0022464
0.0023068
0.0023786
0.0023832
δK
-0.06045
-0.08170
0.04460
0.04587
0.00668 (2)
-0.05519
-0.05932
-0.06428
-0.07317
∆J
0.0099631
∆JK
a
r
Quadratic centrifugal distortion constants are defined according to Watson’s A-reduction for I representation.
Values for the rotational constants taken from Ref [121].
b
281
Figure 42: Ethanol showing atomic numbering.
282
Figure 43: Mid-infrared spectra of ethanol (A) gas; (B) Xe solution at -60 °C.
283
Figure 44: Comparison of experimental and predicted infrared spectra of ethanol: (A) Xe solution
at -60 ºC; (B) simulated spectrum of mixture of trans and gauche conformers at -60oC with ΔH
= 62 cm-1; (C) simulated spectrum of gauche conformer; (D) simulated spectrum of trans
conformer.
284
Figure 45: Comparison of experimental and predicted Raman spectra of ethanol: (A) gas [113]; (B)
simulated spectrum of the mixture of trans and gauche conformers at 25ºC with ΔH = 62 cm-1; (C)
simulated spectrum of gauche conformer; (D) simulated spectrum of trans conformer A' block.
285
Figure 46: van’t Hoff plot of –ln(I887/I879) as a function of 1/T.
286
Figure 47: Predicted (dotted curve, from MP2(full)/6-311+G(2d,2p)) and experimental (solid
curve[120]) potential function governing the hydroxyl torsion of ethanol.
287
CHAPTER 12
CONFORMATIONAL AND STRUCTURAL STUDIES OF ETHYNYLCYCLOPENTANE
FROM TEMPERATUREDEPENDENT RAMAN SPECTRAOF XENON SOLUTIONS AND
AB INITIO CALCULATIONS
Introduction
Mono-substituted cyclopentanes have been of interest for several decades since
cyclopentane undergoes a ring vibration designated [122] as pseudorotation since there are two
“out-of-plane” vibrational modes which are usually described qualitatively as ring-puckering and
ring-twisting. After the initial prediction [122] of pseudorotation in saturated five-membered rings,
a study followed [123] wherein the authors proposed that fluorocyclopentane as well as some other
mono-substituted cyclopentanes should have the bent conformation (envelope) as the preferred
form. Relatively complete vibrational [124, 125] studies of the cyclopentyl halides (F, Cl, Br, and
I) were carried out utilizing infrared and Raman spectra and it was concluded that the F, Cl, and Br
compounds all had doublets for the carbon-halogen stretches arising from equatorial and axial
conformers for the envelope form of these substituted cyclopentanes. We recently determined
[126, 127] the enthalpy differences between the two stable forms of the chloride and bromide by
variable temperature studies of the infrared spectra of rare gas solutions with values of 145 ± 15
cm-1 (1.73 ± 0.18 kJ/mol) and 233 ± 23 cm-1 (2.79 ± 0.28 kJ/mol), respectively, with the axial
conformer the more stable form for both molecules.
From the initial study[125, 128] of fluorocyclopentane it was also concluded that both the
axial and equatorial forms were present in the fluid phases. However from a later Raman study
[129] it was concluded there was only a single conformer present in the fluid states and it was the
envelope-equatorial (Eq) conformer. This conclusion was consistent with predictions from
CNDO/2 calculations [130] that only the Eq conformer was a stable form. As a continuation of our
288
studies of the conformational stabilities of mono-substituted cyclopentanes we reinvestigated the
infrared and Raman spectra of fluorocyclopentane [131]. The results were quite interesting in that
there is, in fact, a single stable conformer present in the fluid phases but it is neither the envelopeaxial (Ax) nor envelope-equatorial, but the twisted form. These conclusions were based on infrared
spectra of variable temperature xenon solutions, Raman spectra of the liquid and solid, and
microwave spectrum which were supported by ab initio and density functional theory calculations.
Therefore, as a continuation of these studies we have turned our attention to another monosubstituted cyclopentane, i.e. ethynylcyclopentane, c-C5H9CCH.
There has been a
previous investigation of the conformational stability of
ethynylcyclopentane from a microwave study [132] where the Eq conformer was reported to be
more stable by 94  24 cm-1 (1.12  0.29 kJ/mol) than the Ax form. By transferring the parameters
reported for cyanocyclopentane which were obtained from assumed parameters (four) transferred
from analogous molecules and five parameters from the least squares fit of the two isotopic species
of each conformer. The three parameters (angles) were reported for ethynylcyclopentane by fitting
the experimental moments of inertia. However the heavy atom bond distances for
cyanocyclopentane were reported not to be in error by more than 0.02 Å and the angles no more
than 5°. Therefore it is desirable to obtain a more accurate enthalpy difference between the two
conformers and obtain more accurate structural parameters for ethynylcyclopentane, so as to
compare them to the corresponding cyanocyclopentane parameters. Therefore we have
investigated the vibrational spectrum of ethynylcyclopentane with a study of the infrared spectra of
the gas and solid. Additionally we have investigated the Raman spectra of the liquid as well as
spectra of the variable temperatures of xenon solutions. To support the vibrational study, we have
carried out ab initio calculations with basis sets up to aug-cc-pVTZ as well as those with diffuse
functions, i.e., 6-311+G(2df,2pd). We have also carried out density functional theory (DFT)
289
calculations by the B3LYP method with the same basis sets. We have calculated optimized
geometries, conformational stabilities, harmonic force fields, infrared intensities, Raman activities
and depolarization ratios. The results of these spectroscopic, structural, and theoretical studies are
reported herein.
Experimental and Theoretical Methods
The sample of c-C5H9CCH was purchased from GFS Chemicals, Columbus, OH with a
stated purity of 95%. The sample was further purified by a low-temperature, low-pressure
fractionation column and the purity of the sample was verified from the infrared spectra of the gas
and NMR spectrum of the liquid.
The infrared spectrum of the gas (Fig. 48A) and solid (Fig. 48E) were obtained from 3500 to
220 cm-1 on a Perkin-Elmer model 2000 Fourier transform spectrometer equipped with a Ge/CsI
beamsplitter and a DTGS detector. Atmospheric water vapor was removed from the spectrometer
housing by purging with dry nitrogen. The spectra of the gas and solid were obtained with a
theoretical resolution of 0.5 cm-1 for the gas and 2 cm-1 for the solid with 128 interferograms added
and truncated. Multiple annealings were required to obtain satisfactory spectra of the solid.
The Raman spectra of the liquid (Fig. 49A) obtained from 3400 to 60 cm-1 were recorded
on a Spex model 1403 spectrophotometer equipped with a Spectra-Physics model 2017 argon ion
laser operating on the 514.5 nm line. The laser power used was 1.5 W with a spectral bandpass of
3 cm-1. The spectrum of the liquid was recorded with the sample sealed in a Pyrex glass capillary.
The measurements of the Raman frequencies are expected to be accurate to  2 cm-1. All of the
observed bands in the Raman spectra of the liquid along with their proposed assignments and
depolarization values are listed in Tables 76 and 77, for the Eq and Ax conformers, respectively.
The Raman spectra (3500 to 136 cm-1) of the sample dissolved in liquefied xenon (Fig.
49B) at six different temperatures (-50 to -100C) were recorded on a Trivista 557 spectrometer
290
consisting of a double f = 50 cm monochromator equipped with a 2000 lines mm-1 grating, a f = 70
cm spectrograph equipped with a 2400 lines mm-1 grating, and a back-illuminated LN2-cooled PI
Acton Spec-10:2 kB/LN 2048 x 512 pixel CCD detector. For all experiments, the 514.5 nm line of
a 2017-Ar S/N 1665 Spectra-Physics argon ion laser was used for Raman excitation, with the
power set to 0.8 Watt. Signals related to the plasma lines were removed by using an interference
filter. The frequencies were calibrated using Neon emission lines, and depending on the setup
used, are expected to be accurate within 0.4 cm-1. The experimental set-up used to investigate the
solutions has been described earlier [15, 16]. A home-built liquid cell equipped with four quartz
windows at right angles was used to record the spectra.
The ab initio and density functional theory calculations were performed with the Gaussian03 program [21]. The energy minima with respect to nuclear coordinates were obtained by the
simultaneous relaxation of all geometric parameters using the gradient method of Pulay [22].
Several basis sets as well as the corresponding ones with diffuse functions were employed with the
Møller-Plesset perturbation method [23] to the second order (MP2(full)) as well as with the density
functional theory by the B3LYP method. The predicted conformational energy differences are
listed in Table 78.
The vibrational spectra were predicted from the MP2(full)/6-31G(d) calculations. The
predicted scaled frequencies were used together with a Lorentzian function to obtain the simulated
spectra. Infrared intensities were obtained based on the dipole moment derivatives with respect to
Cartesian coordinates. The derivatives were transformed with respect to normal coordinates by
(u/Qi) = j (u/Xj)Lij, where Qi is the ith normal coordinate, Xj is the jth Cartesian
displacement coordinate, and Lij is the transformation matrix between the Cartesian displacement
coordinates and the normal coordinates. The infrared intensities were then calculated by (N)/(3c2)
291
[(x/Qi)2 + (y/Qi)2 + (z/Qi)2]. The infrared spectra of the vapor, solid, and the predicted
infrared spectra for the pure Eq and Ax conformers, as well as the mixture of the two conformers
with relative concentrations calculated for the equilibrium mixture at 25ºC by using the
experimentally determined enthalpy difference are shown in Fig. 48 (A-E), respectively. The
predicted spectrum is in good agreement with the experimental spectrum which shows the utility
of the scaled predicted frequencies and predicted intensities for supporting the vibrational
assignment.
Additional support for the vibrational assignments was obtained from the simulated Raman
spectra. The evaluation of Raman activity by using the analytical gradient methods has been
developed [25-27, 37] and the activity Sj can be expressed as: Sj = gj(45αj2 + 7βj2), where gj is the
degeneracy of the vibrational mode j, αj is the derivative of the isotropic polarizability, and βj is the
anisotropic polarizability. To obtain the Raman scattering cross sections, the polarizabilities are
incorporated into Sj by multiplying Sj with (1-ρj)/(1+ρj) where ρj is the depolarization ratio of the
jth normal mode. The Raman scattering cross sections and calculated wavenumbers obtained from
the Gaussian 03 program were used together with a Lorentzian function to obtain the simulated
Raman spectra. Comparison of experimental Raman spectra of the liquid, xenon solutions, and the
predicted Raman spectra for the pure Eq and Ax conformers, as well as, the mixture of the two
conformers with relative concentrations calculated for the equilibrium mixture at -60°C by using
the experimentally determined enthalpy difference are shown in Fig. 49(A-E). The spectrum of the
mixture should be compared to that of the Raman spectra of the liquid and xenon solutions.
Vibrational Assignment
To determine the enthalpy differences between the two conformers it is necessary to assign
the spectra for both conformers of ethynylcyclopentane. For use in the vibrational assignment ab
initio predictions from MP2(full)/6-31G(d) calculations were carried out to obtain the force
292
constants, frequencies, infrared intensities, band contours, Raman activities, and depolarization
values for both conformers. These predicted quantities, as well as, the group frequencies were used
to aid in the assignment of the infrared and Raman spectra.
In order to obtain a complete description of the molecular motions involved in the
fundamental modes of c-C5H9CCH, a normal coordinate analysis has been carried out. The force
field in Cartesian coordinates was obtained with the Gaussian 03 program [21] at the MP2(full)
level with the 6-31G(d) basis set. The internal coordinates used to calculate the G and B matrices
are given in Table 79 with the atomic numbering shown in Fig. 50. By using the B matrix [24], the
force field in Cartesian coordinates was converted to a force field in internal coordinates.
Subsequently, scaling factors of 0.88 for CH stretches and CH2 deformations and 0.90 for all other
modes except heavy atom bends were applied, along with the geometric average of the scaling
factors for the interaction force constants, to obtain the fixed scaled force field and resultant
wavenumbers. A set of symmetry coordinates was used (Table 80) to determine the corresponding
potential energy distributions (P.E.D.s). A comparison between the observed and calculated
wavenumbers, along with the calculated infrared intensities, Raman activities, depolarization ratios
and potential energy distributions for the Eq and Ax conformers are listed in Tables 76 and 77,
respectively.
The CC modes are of significant note where the experimental and predicted frequencies
are significantly different and so group frequencies and B3LYP predictions can be used to aid in
the assignment of this fundamental. The approximate descriptions given in most cases are
consistent with most of the “group frequencies” previously given for the similar mono-substituted
cyclopentanes [126, 127]. Fortunately there are nearly equal amounts of both conformers so a
nearly complete assignment could be made for both conformers which gave a large number of
fundamentals which could be chosen for enthalpy determinations. Careful consideration was given
293
to choose bands with reasonable intensities and without interference from bands from the other
conformer.
Conformational Stability
To determine the enthalpy differences among the two observed conformers of
ethynylcyclopentane, the sample was dissolved in liquefied xenon and the Raman spectra were
recorded as a function of temperature from -50 to -100°C. Relatively small interactions are
expected to occur between xenon and the sample though the sample can associate with itself
through van der Waals interactions. However, due to the very small concentration of sample (~10-4
molar) self association is greatly reduced. Therefore, only small frequency shifts are anticipated for
the xenon interactions when passing from the gas phase to the liquefied xenon solutions (Fig. 51).
A significant advantage of this study is that the conformer bands are better resolved in comparison
with those in the infrared spectrum of the gas or the Raman spectra of the liquid. From ab initio
calculations, the dipole moments of the two conformers are predicted to have similar values and
the molecular sizes of the two conformers are nearly the same, so the ΔH value obtained from the
temperature dependent Raman study is expected to be close to that for the gas [39-43].
Once confident assignments have been made for the fundamentals of both conformers the
task was then to find pairs of bands from which the enthalpy determination could be obtained. To
minimize the effect of combination and overtone bands in the enthalpy determination it is desirable
to have the lowest frequency pair(s) that is possible for the determination. The bands should also
be sufficiently resolved so reproducible intensities can be obtained. The fundamentals at 464, 505,
770, 861, and 952 cm-1 were selected for the Eq conformer bands as they are free of interfering
bands and in the lower frequency region of the spectrum. For the Ax form the fundamentals at 423
and 930 cm-1 are relatively free from interfering bands which allowed the determination of their
band intensities to be confidently measured. The band at 887 cm-1 has an underlying Eq
294
fundamental, though due to the high activity of the Ax fundamental and the low activity of the
underlying Eq fundamental the band intensity is not significantly affected by the Eq fundamental.
The intensities of the individual bands were measured as a function of temperature and their
ratios were determined (Fig. 52). By application of the van’t Hoff equation lnK = H/(RT) 
S/R, the enthalpy differences were determined from a plot of lnK versus 1/T, where H/R is the
slope of the line and K is substituted with the appropriate intensity ratios, i.e. Iconf-1 / Iconf-2, etc. It
was assumed that H, S, and α are not functions of temperature in the range studied.
These eight bands, five for Eq and three for Ax, were utilized for the determination of the
enthalpy difference by combining them to form 15 band pairs where the enthalpy differences for
each pair are given in Table 81. These 15 band pairs were combined to form a single data set and
the enthalpy difference was determined with a value of 94  2 cm-1 (Table 81). This error limit is
derived from the statistical standard deviation of one sigma of the measured intensity data taken as
a single data set. This error determination does not take into account small associations with the
liquid xenon or the interference of overtones and combination bands in near coincidence with the
measured fundamental intensities. The variations in the individual values are undoubtedly due to
these types of interferences, but by taking several pairs, the effect of such interferences should
cancel. However, this statistical uncertainty is probably better than can be expected from this
technique and, therefore, an uncertainty of about 10% in the enthalpy difference is probably more
realistic i.e. 94  9 cm-1. From the enthalpy difference the abundance of the Ax conformer present
at ambient temperature is estimated to be 39 ± 2%.
Structural Parameters
In the microwave study [133], all but three of the structural parameters were assumed for
the two identified conformers. The three parameters that were not assumed but were fitted were the
CCC’ angle, the CβCαCβ′Cγ′ and CβCβ’CαCC dihedral angles. Since the assumed structural
295
parameters were common for both conformers, while the assumptions are reasonable they
contribute significantly to the uncertainties of the structural parameters obtained in the initial
microwave study. Therefore, we have determined the structural parameters for both rotamers by
utilizing the rotational constants previously reported from the microwave study [133]. Only one
isotopic species was studied for both conformers and, thus, only three rotational constants are
available for each conformer.
We have found that good structural parameters for hydrocarbons and many substituted ones
can be determined by adjusting the structural parameters obtained from the ab initio MP2(full)/6311+G(d,p) calculations to fit the rotational constants obtained from microwave experimental data
by using a computer program “A&M” (Ab initio and Microwave) developed [57] in our
laboratory. In order to reduce the number of independent variables, the structural parameters are
separated into sets according to their types where bond distances in the same set keep their relative
ratio, and bond angles in the same set keep their difference in degrees. This assumption is based on
the fact that errors from ab initio calculations are systematic. Additionally, we have shown that the
differences in predicted distances and angles from the ab initio calculations for different
conformers of the same molecule can usually be used as one parameter with the ab initio predicted
differences except for some dihedral angles. Also it has been shown the C≡C distance is nearly
constant irrespective of the substitutions on it [134]. Therefore, it should be possible to obtain
“adjusted r0” structural parameters for both conformers of ethynylcyclopentane by utilizing the
previously reported six rotational constants from the earlier microwave study [132] by fixing the
C≡C distance and utilizing the sets as indicated.
We [44] have also shown that ab initio MP2(full)/6-311+G(d,p) calculations predict the r0
structural parameters for more than fifty carbon-hydrogen distances to at least 0.002 Å compared
to the experimentally determined values from isolated CH stretching frequencies which were
296
compared [45] to previously determined values from earlier microwave studies. Therefore, all of
the carbon-hydrogen distances can be taken from the MP2(full)/6-311+G(d,p) predicted values for
Eq and Ax conformers of ethynylcyclopentane. The resulting adjusted r0 parameters are listed in
Table 79, where it is believed that the C-C distances should be accurate to ± 0.003 Å, the C-H
distances should be accurate to ± 0.002 Å, and the angles should be within ± 0.5.
The fit of the six determined rotational constants (Table 82) by the structural parameters for
the Eq and Ax conformers is good with the differences being less than 1 MHz where the largest
difference is 0.88 MHz. Therefore, it is believed that the suggested uncertainties are realistic
values and the determined structural parameters are probably as accurate as can be obtained for the
molecule in the gas phase by either electron diffraction or microwave substituted methods.
Discussion
The predicted infrared intensities from the B3LYP calculations for the C≡C stretch are 18
times greater than those predicted from the MP2 calculations. Similarly the Raman activities for
this mode are almost three times more intense. These large differences are the result of the MP2
predicted distance for the triple bond being nearly 0.016 Å longer than the value predicted from the
B3LYP calculations, which are significantly closer to the experimental r0 value (Table 79). There
are smaller predicted intensity differences for the Ring-CCH and C≡C-H in-plane bending modes
where the infrared intensities are predicted 1/12 as intense for the Ring-CCH bend but nearly twice
the intensity for the C≡C-H bend, respectively, from the MP2 calculations compared to those from
the B3LYP calculations.
The assignment was somewhat complicated by the nearly equal amounts of both
conformers present. Therefore, significant reliance on the predicted wavenumbers was necessary to
assign many of the modes with group frequencies also playing a significant role. Nevertheless the
predicted wavenumbers for the Eq and Ax forms had average errors of 11 cm-1 which is 0.7% for
297
the A′ modes and 9 cm-1 for the A′′ modes which is 0.6% error. The C≡C stretch and C≡C-H
bending modes were not included due to the poor ab initio predictions for these modes. A
significant amount of the error is due to the predictions for the -CH2 rock and the -CH2 wag but
the assignments agree with previously reported group frequencies for these modes.
For the Eq conformer, the mixing of the normal modes is relatively minor for the A′
fundamentals except for the -CH2 twist and a ring deformation (ν15), but for the A′′ fundamentals
there are three modes; -CH2 wag, -CH2 rock, and -CH2 rock which have four or more
symmetry coordinates contributing more than 10%. For the Ax conformer there is significantly
more mixing of the modes. This indicates that many of these fundamentals are more complicated
than the relatively simple descriptions they have been given.
When this study was initiated there was some question whether the Eq or Ax conformer is
the more stable form. From the microwave study [133] the Eq conformer was reported to be more
stable by 94  24 cm-1 but ab initio calculations predict the Ax conformer more stable by
approximately 180 cm-1. Also it should be noted that cyanocyclopentane which is a very similar
molecule to ethynylcyclopentane was reported [135] to have the Ax conformer the more stable
form by 109  37 cm-1. Thus it was necessary to make a very definitive determination of the more
stable conformer.
The process used was first to obtain the conformer which was the stable form in the crystal.
The infrared spectrum of the solid (Fig. 48E) was obtained and the spectra was predicted of
gaseous Eq (Fig. 48D) and Ax (Fig. 48C) ethynylcyclopentane. The spectra were compared and as
can be seen from Fig. 48 the spectrum of the solid corresponds to that of the pure Eq conformer
where the bands at 769, 1297, and 1343 cm-1 provide clear evidence of the stable form.
Next the variable temperature spectrum of the sample in liquefied xenon was obtained and
the spectra were confidently assigned. The determined ΔH values conclusively showed that the Eq
298
conformer was the more stable form. The value obtained for the enthalpy difference indicates that
this is not an effect of one conformer associating with the xenon to form a more energetically
stable complex.
With the confidence of the assignments of the lower wavenumber modes for the Eq and Ax
conformers and the use of 15 band pairs as well as the statistical uncertainty the value for the
enthalpy difference is believed to be well determined. The uncertainty of the value for each
individual pair ranges from a high value of 12 cm-1 to a low of 4 cm-1. This relatively small spread
coupled with the fact that overtone and combination bands are not frequently observed in the
Raman spectra indicates that there is little interference from overtones or combination bands
having a significant effect on the determined enthalpy difference obtained in this study. The
experimentally determined enthalpy difference between the more stable Eq conformer and the Ax
form with the value of 94  9 cm-1 is in excellent agreement with the previously reported value
from the microwave study [132] of 94  24 cm-1.
The only way for a determination of the enthalpy difference that would not be altered by
association would require the use of a sample in a very low pressure gaseous phase. It is doubtful
that this would be an easy experiment to determine the enthalpy and, therefore, the value obtained
from the xenon solution must be considered the best determined enthalpy difference at this time.
The use of enthalpy determinations from 15 band pairs indicates the accuracy of the determination
by this technique.
The reported enthalpy differences should be appropriate for comparison to ab initio
predicted values or enthalpy differences obtained for other substituted cyclopentanes. The ab initio
predicted energy difference from the MP2(full)/aug-cc-pVTZ (552 basis sets) calculation gave the
Ax conformer more stable by 180 cm-1 (2.15 kJ/mol) than the Eq form. While the B3LYP/aug-ccpVTZ (552 basis sets) calculation gave the Eq conformer as more stable by 268 cm-1 (3.20 kJ/mol)
299
than the Ax form. These values are both far off in magnitude and the ab initio value even predicts
the wrong conformer as being the more stable form. It is interesting to note for cyanocyclopentane
[135] the ab initio and DFT calculations predict similar energy differences to those predicted in the
current study.
The adjusted r0 structural parameters have been determined for the Eq and Ax forms of
ethynylcyclopentane. In the initial microwave study [132] only three parameters were determined
for the Eq [Ax] form which were the angle (°) CβCγCγ′ = 106(5) [106(5)], as well as, the dihedral
angles (°) CβCαCβ′Cγ′ = 41(5) [39(5)] and CβCβ’CαCC = 130(5) [123(5)]. Due to the large error
limits the values from the current study agree with those from the previous study.
It is also of interest to compare the structural parameters of ethynylcyclopentane with the
corresponding parameters of other monosubstituted-cyclopentanes. The adjusted r0 structure has
been determined for the Ax [Eq] form of cyanocyclopentane [135] with the distances (Å) rCα-Cβ =
1.542(3) [1.543(3)], rCβ-Cγ = 1.542(3) [1.544(3)], rCγ-Cγ′ = 1.559(5) [1.564(5)] and dihedral angle
(°) CαCβCγCγ′ = 25.8(5) [26.1(5)]. There is no significant difference between the cyano- and
ethynyl-cyclopentane ring parameters as the differences are within the respective uncertainties.
The adjusted r0 structure has also been determined for the Ax [Eq] conformers of
bromocyclopentane [127] with the distances (Å) rCα-Cβ = 1.526(5) [1.525(5)], rCβ-Cγ = 1.543(5)
[1.543(5)], rCγ-Cγ′ = 1.555(5) [1.559(5)] and dihedral angle (°) CβCαCβ′Cγ′ = 40.2(5) [44.0(5)].
The rCα-Cβ distance is significantly shorter and the CβCαCβ′Cγ′ dihedral angle is slightly more than
4° larger for the Eq conformer. The remaining parameters are within the respective uncertainties.
The
ab
initio
and
DFT
calculations
predict
similar
energy
differences
for
cyanocyclopentane as those predicted for ethynylcyclopentane. However, the most recent study of
cyanocyclopentane determined [135] the Ax conformer to be more stable than the Eq form. This is
opposite of the order of stability of ethynylcyclopentane. This is of interest since there should be
300
significant similarities between cyanocyclopentane and ethynylcyclopentane. However, it should
be noted that the conformational determination of cyanocyclopentane was made from a single band
pair and only five temperatures were used. The relation of the ethynyl- substituent to the
conformational stability as compared to the cyano- group is explained by Klaebo et al. in their
publication [136] for the six membered ring where the Eq conformer was determined to be the
most stable form for both ethynylcyclohexane and cyanocyclohexane in the liquid and gaseous
phases however in the crystalline solid cyanocyclohexane is more stable in the Ax form while
ethynylcyclohexane is most stable as the Eq conformer. This difference in conformational stability
is according to Klaebo et al. “correlated with a high electronic density at the carbon bonded to the
ring in ethynylcyclohexane lowered in the CCN due to the electronegative N atom with its lone
pair orbital.” A reinvestigation of the order of stability of cyanocyclopentane by using the variable
temperature Raman spectra of the xenon solution is needed since it is doubtful that these two
molecules will have significantly different conformer stabilities.
The cyanomethylcyclopropane [137] and ethynylmethylcyclopropane [138] molecules have
significant differences of the effect on these molecules conformational differences due to the
attached –CH2CN and –CH2C≡CH moieties. The carbon to the ring distances (Å) in the cyano
molecule are 1.519 (cis) and 1.513 (gauche) and similar bonds of the ethynyl molecule are 1.522
(cis) and 1.516 (gauche) so that these distances are nearly the same. However, the experimental
enthalpy difference for the ethynyl molecule is 147  14 cm-1 with the cis conformer the more
stable form, whereas for the cyano molecule the experimental enthalpy difference is 54  4 cm-1
but the gauche conformer is the more stable form. For the ethynyl molecule ab initio MP2(full)6311+G(2d,2p) calculations also predict the cis form the more stable (ΔE = 174 cm-1) conformer
which is in agreement with the experimental results and similarly for the cyano compound the cis
301
form again is predicted to be the more stable form by 134 cm-1 which does not agree with the
experimental determination of the gauche conformer which is the more stable form.
The
similarities
of
the
ring
parameters
between
cyanocyclopentane
and
ethynylcycplopentane and the necessity for experimental determination of the enthalpy difference
between the Eq and Ax conformers of ethynylcyclopentane indicates the monosubstitutedcyclopentanes with triple bond substituents requires more study. The pseudo triple bond isocyanide
substituent is currently an understudied moiety and may help to expand the understanding of this class
of compounds. Also of interest are five membered rings where the substituent is in the ring i.e.
pyrrolidine where the conformers at ambient temperature are the Eq and twist forms instead of Eq and
Ax. The phosphorous compound (phospholane) is also of interest as it may have similar
conformational stabilities and phosphorous compounds have a great degree of bioactivity. The large
change in conformational stability due to differences in the substituent on (or in) the ring make the
substituted five membered rings or interest.
302
Table 76: Calculateda and Observed Frequencies (cm-1) for Ethynylcyclopentane Eq (Cs) Form.
Vib.
No.
Approx. description
Infrared
ab
Fixed IR Raman dp
IR Raman
B3LYP
initio scaledb int.
act. ratio
int.
act.
gas
303
A' 1
2
C≡C-H stretch
-CH2 antisymmetric stretch
3523
3189
3307 57.1
2991 57.3
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
A" 26
-CH2 antisymmetric stretch
-CH2 symmetric stretch
-CH2 symmetric stretch
-CH stretch
C≡C stretch
-CH2 deformation
-CH2 deformation
-CH bend (in-plane)
-CH2 wag
-CH2 wag
-CH2 twist
-CH2 twist
Ring deformation
C-CC stretch
-CH2 rock
Ring deformation
Ring breathing
-CH2 rock
C≡C-H linear bend (in-plane)
Ring deformation
Ring-CCH bend (in-plane)
Ring puckering
C-C≡C linear bend (in-plane)
-CH2 antisymmetric stretch
3174
3127
3116
3075
2166
1582
1560
1423
1371
1347
1272
1225
1110
1063
1008
929
904
786
560
500
489
285
133
3179
2977
2933
2923
2885
2054
1488
1467
1354
1302
1279
1212
1166
1058
1011
959
893
865
747
537
488
472
284
132
2982
27 -CH2 antisymmetric stretch
28 -CH2 symmetric stretch
29 -CH2 symmetric stretch
3162
3117
3113
2966 10.7
2924 26.8
2920 11.1
46.3
55.7
0.25
0.74
3477
3101
77.7
76.8
1.7 119.5
30.9 179.9
25.9 67.9
9.2 99.6
0.6 97.7
0.5
8.5
6.3
8.6
3.4
7.7
0.5
8.6
2.3
2.1
0.5 10.2
0.7
6.6
0.3
5.5
0.0
3.6
0.6
3.5
3.0
1.6
0.4 17.6
2.7
0.4
32.4
1.4
3.7
2.5
14.2
2.5
0.2
5.0
0.0
4.1
13.7 73.0
0.45
0.04
0.23
0.25
0.29
0.68
0.72
0.48
0.74
0.70
0.73
0.46
0.69
0.28
0.29
0.04
0.09
0.19
0.33
0.25
0.53
0.74
0.73
0.75
3085
3051
3043
2997
2208
1523
1501
1378
1328
1311
1230
1190
1066
1020
975
889
864
761
675
521
489
284
133
3092
7.4
36.9
38.8
8.5
18.1
0.9
6.2
2.8
0.9
1.7
0.4
0.4
0.2
0.0
0.6
3.2
0.6
2.5
51.2
3.5
1.2
2.8
0.1
25.1
0.75
0.75
0.75
3069
3038
3037
22.7
8.2
30.6
10.6
25.3
12.0
27.4
65.0
solid
3329 3276/3274
2968 2968/2962
188.8 2968 2968/2962
268.7 2934
2947
63.7 2934
2947
135.1 2887
2869
286.1 2124
2115
2.4 1474
1472
6.6 1456
1454
8.5 1340
1343
2.9 1300
1303
0.9 1247
1297
5.3 1218
1201
3.5 1162
1167
4.2 1052
1055
6.3
1007
5.9
957
958
3.6
884
886
21.7
858
862
0.4
770
769
8.3
645
678
4.3 505
510
2.9
492
491
4.8
282
292
2.3
77.6 2968 2968/2962
23.5
18.5
31.2
2968 2968/2962
2934
2947
2934
2947
Raman
P.E.D.c
Xe
liquid
soln.
3318 3308 96S1
2965 2965 65S2,34S3
2965
2927
2927
2873
2124
1476
1448
1343
1290
1249
1214
1162
1050
1007
952
887
861
770
505
486
286
2965
2965
2931
2931
2873
2117
1471
1451
1342
1290
1249
1216
1168
1011
952
893
858
768
508
490
288
2965
60S3,32S2
92S4
90S5
97S6
82S7,14S16
72S8,28S9
72S9,29S8
37S10,40S12
58S11,23S13
27S12,29S10,26S14
47S13,19S11,12S18,10S17
39S14,16S12,14S10
20S15,18S18,12S11,12S16
33S16,32S15,14S19
32S17,29S14
47S18,20S15
53S19,12S17,11S22
81S20
55S21,13S25,11S23
54S22,18S16,10S17
28S23,45S21
52S24,43S25
34S25,36S23,30S24
78S26,19S27
2965 2965 78S27,16S26
2927 2931 91S28
2927 2931 90S29
Band
Contour
A B C
96 - 4
1 - 99
94
99
49
9
100
70
22
96
88
67
84
69
98
99
3
98
95
3
11
3
1
3
2
-
- 6
- 1
- 51
- 91
- - 30
- 78
- 4
- 12
- 33
- 16
- 31
- 2
- 1
- 97
- 2
- 5
- 97
- 89
- 97
- 99
- 97
- 98
100 -
- 100 - 100 - 100 -
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
a
-CH2 deformation
-CH2 deformation
-CH2 wag
-CH bend (out-of-plane)
-CH2 wag
-CH2 twist
-CH2 twist
Ring deformation
Ring deformation
-CH2 rock
-CH2 rock
Ring deformation
C≡C-H bend (out-of-plane)
Ring-CCH bend (out-of-plane)
C-C≡C (out-of-plane)
Ring twisting
1557
1546
1379
1370
1328
1290
1235
1136
1006
994
853
635
548
457
145
48
1464 2.5
1454 0.5
1310 0.0
1301 0.0
1260 0.2
1227 0.4
1175 0.2
1089 0.1
962 0.7
950 3.9
815 0.0
620 0.2
546 45.6
455 5.7
145 0.0
48 0.0
4.9
22.4
0.0
9.3
6.7
10.1
1.3
2.9
1.9
0.2
1.1
1.1
0.3
3.1
7.2
0.0
0.75
0.75
0.75
0.75
0.75
0.75
0.75
0.75
0.75
0.75
0.75
0.75
0.75
0.75
0.75
0.75
1499
1489
1337
1321
1294
1257
1199
1078
980
955
822
673
630
509
158
29
4.5
0.1
0.1
0.0
0.0
0.5
0.2
0.2
0.7
4.0
0.0
51.4
0.1
7.9
0.6
0.0
1.7 1449
12.6 1449
0.4
4.7 1300
4.0 1266
4.9 1238
0.3
5.1 1074
0.6
957
0.3
948
0.9
8.9 630
1.0
627
1.9
457
6.1
0.0
-
1447
1303
1265
1259
1180
1073
958
947
658
660
485
-
1448
1448
1290
1266
1238
1173
1076
952
946
803
625
464
161
-
1451
1451
1290
1262
1239
1175
1073
952
952
807
466
170
-
68S30,33S31
68S31,33S30
52S32,17S3410S36
51S33,19S32,12S36
31S34,33S35,17S32,15S33
48S35,32S34
41S36,27S39,11S40
46S37,17S41,16S38
35S38,25S37,16S36
24S39,20S40,16S38,16S33,13S36
44S40,19S39,16S41,11S37
59S41,17S40
71S42,15S44
38S43,35S42,18S44
64S44, 36S43
93S45
304
MP2(full)/6-31G(d) ab initio calculations, scaled frequencies, B3LYP/6-311+G(d,p) calculations, infrared intensities (km/mol), Raman activities (Å 4/u), depolarization
ratios (dp) and potential energy distributions (P.E.D.s).
b
Scaled frequencies with scaling factors of 0.88 for CH stretches and CH2 deformations, 1.0 for heavy atom bends, and 0.90 for all other modes.
c
Symmetry coordinates with P.E.D. contribution less than 10% are omitte
-
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
-
Table 77: Calculateda and Observed Frequencies (cm-1) for Ethynylcyclopentane Ax (Cs) Form.
Vib.
No.
A'
305
A"
1
2
3
4
6
5
7
8
9
10
12
11
13
14
15
16
18
19
22
20
17
21
23
24
25
26
27
28
29
30
31
Approx. description
C≡C-H stretch
-CH2 antisymmetric stretch
-CH2 antisymmetric stretch
-CH2 symmetric stretch
-CH stretch
-CH2 symmetric stretch
C≡C stretch
-CH2 deformation
-CH2 deformation
-CH bend (in-plane)
-CH2 wag
-CH2 wag
-CH2 twist
-CH2 twist
Ring deformation
C-CC stretch
Ring deformation
Ring breathing
Ring deformation
-CH2 rock
-CH2 rock
C≡C-H linear bend (in-plane)
Ring-CCH bend (in-plane)
Ring puckering
C-C≡C linear bend (in-plane)
-CH2 antisymmetric stretch
-CH2 antisymmetric stretch
-CH2 symmetric stretch
-CH2 symmetric stretch
-CH2 deformation
-CH2 deformation
ab
initio
Fixed
scaledb
IR
int.
Raman
act.
3523
3192
3175
3132
3120
3105
2160
1580
1557
1410
1390
1352
1268
1218
1106
1028
985
926
863
832
700
557
422
295
124
3180
3166
3123
3106
1555
1543
3307
2994
2978
2938
2926
2913
2048
1486
1464
1341
1322
1284
1207
1160
1052
978
941
883
834
792
673
527
418
294
123
2983
2970
2930
2914
1463
1452
53.1
55.3
0.9
23.4
48.6
10.8
0.3
1.7
5.6
9.0
0.7
0.6
1.3
0.4
0.0
0.4
1.3
0.7
0.5
2.5
0.9
48.5
0.2
0.8
0.1
11.7
10.3
21.3
14.2
3.3
1.0
46.1
52.9
120.8
184.8
136.6
74.6
73.5
10.1
5.8
6.0
1.2
8.3
16.1
0.6
6.1
2.9
2.9
15.1
2.7
4.1
1.4
0.5
2.5
5.2
4.0
68.5
7.7
27.4
9.3
4.8
21.7
dp
B3LYP
ratio
IR
int.
0.25
0.75
0.54
0.04
0.12
0.50
0.28
0.70
0.74
0.55
0.38
0.59
0.74
0.10
0.71
0.43
0.23
0.07
0.26
0.06
0.06
0.08
0.72
0.66
0.72
0.75
0.75
0.75
0.75
0.75
0.75
71.0
66.9
8.7
30.6
69.9
13.1
13.4
2.0
6.1
10.5
0.7
0.2
0.9
0.3
0.0
0.5
1.5
1.2
1.0
1.7
5.5
45.6
4.1
2.7
0.2
25.0
10.8
29.0
15.5
6.3
1.1
3477
3104
3090
3058
3047
3030
2204
1520
1499
1369
1345
1309
1230
1188
1057
991
943
884
818
804
682
674
430
284
120
3097
3072
3046
3032
1496
1486
Raman Infrared
act.
gas
28.3
69.6
160.2
316.3
158.9
69.8
229.4
3.5
4.2
3.0
5.3
6.0
7.3
0.4
6.0
3.6
5.8
15.4
1.5
8.5
2.0
6.7
3.0
3.2
4.1
61.7
29.2
40.5
14.8
2.2
12.6
3329
2968
2968
2934
2934
2887
2124
1474
1456
1340
1287
1212
1041
974
937
893
803
791
665
645
423
282
2968
2968
2934
2887
1449
1449
Raman
Xe
soln.
3318
2965
2965
2927
2927
2873
2117
1476
1448
1334
1313
1283
1214
1039
976
930
887
803
795
665
423
286
2965
2965
2927
2873
1448
1448
P.E.D.
liquid
3308
2965
2965
2931
2931
2873
2107
1471
1451
1333
1312
1282
1216
1036
978
932
893
807
796
665
432
288
2965
2965
2931
2873
1451
1451
Band
Contour
c
96S1
79S2,20S3
70S3,20S2
95S4
64S6,30S5
59S5,35S6
83S7,13S16
78S8,22S9
78S9,23S8
44S10,26S13
54S12,29S11
29S11,24S14,14S10,10S12
30S13,24S11,12S14
30S14,30S10,10S13
32S15,22S18,13S11
22S16,22S14,15S15,13S13
32S18,27S17
61S19,13S15,12S18
30S
,20S
,13S
2220
1516
51S
,19S
,11S2319,12S17,10S16
12S
,32S20,19S22,15S16,10S18
1721
100S
33S23,23S22,22S25,10S17
55S24,42S25
33S25,36S23,33S24
67S26,28S27
72S27,23S26
99S28
89S29,10S26
77S30,24S31
77S31,24S30
A
B
C
71
82
32
56
56
77
56
10
95
11
21
93
5
79
8
97
48
70
95
8
45
67
59
64
-
100
100
100
100
100
100
29
18
68
44
10
0
44
23
44
90
5
89
69
7
95
21
92
3
52
30
5
92
55
33
41
36
-
32
34
35
39
33
36
37
38
40
41
42
43
44
45
-CH2 wag
-CH2 wag
-CH2 twist
-CH2 rock
-CH bend (out-of-plane)
-CH2 twist
Ring deformation
Ring deformation
-CH2 rock
Ring deformation
C≡C-H bend (out-of-plane)
Ring-CCH bend (out-of-plane)
C-C≡C (out-of-plane)
Ring twisting
a
1392
1343
1325
1269
1214
1122
1058
941
855
656
560
499
177
74
1323
1274
1259
1206
1159
1069
1006
904
817
645
557
495
177
74
0.2
0.0
1.4
0.0
0.3
2.2
0.8
3.0
0.0
4.7
42.9
2.9
0.2
0.0
0.4
0.1
15.1
1.4
6.4
7.5
3.4
0.3
0.9
0.7
0.4
2.8
6.1
0.4
0.75
0.75
0.75
0.75
0.75
0.75
0.75
0.75
0.75
0.75
0.75
0.75
0.75
0.75
1347
1315
1283
1231
1167
1087
1022
899
817
673
648
522
186
53
0.4
0.1
1.6
0.1
0.0
2.3
0.3
3.9
0.1
49.0
2.1
6.2
1.1
0.0
0.3
0.1
9.9
0.2
5.0
3.8
3.6
0.5
0.8
7.8
0.5
1.4
5.7
0.6
1320
1266
1162
1069
895
629
629
492
-
1320
1266
1214
1150
1069
1002
803
625
486
180
-
1318
1262
1216
1156
1069
1008
807
630
490
181
-
48S32,21S34,12S33
51S34,39S32
54S35,24S33,12S36
22S39,31S35,27S36,10S33
18S33,27S36,22S37,11S41
22S
,22S
,17S
3637
4039
36S
,22S
,15S3934,13S37,11S33
71S38,13S33
41S40,18S39,16S41,13S37
52S41,14S43,10S39
86S42
37S43,45S44
54S44,34S43
90S45
-
306
MP2(full)/6-31G(d) ab initio calculations, scaled frequencies, B3LYP/6-311+G(d,p) calculations, infrared intensities (km/mol), Raman activities (Å 4/u),
depolarization ratios (dp) and potential energy distributions (P.E.D.s).
b
Scaled frequencies with scaling factors of 0.88 for CH stretches and CH 2 deformations, 1.0 for heavy atom bends, and 0.90 for all other modes.
c
Symmetry coordinates with P.E.D. contribution less than 10% are omitted.
100
100
100
100
100
100
100
100
100
100
100
100
100
100
-
Table 78: Calculated Electronic Energies (Hartree) for the Eq (Cs) and Energy Differences (cm-1)
for Ax (Cs), Twisted (C1), and Planar (Cs) Forms of Ethynylcyclopentane.
Energy Differenceb
# Basis Set
Equatorial (Cs)a
RHF/6-31G(d)
125
0.835565
MP2(full)/6-31G(d)
125
MP2(full)/6-31+G(d)
Method/Basis Set
a
Twist (C1)
Planar (Cs)
187
403
2174
1.767393
-178
92
2406
153
1.782766
-40
246
2467
MP2(full)/6-311G(d,p)
186
1.849138
-149
49
2345
MP2(full)/6-311+G(d,p)
214
1.863058
-44
125
2333
MP2(full)/6-311G(2d,2p)
251
2.037352
-224
85
2475
MP2(full)/6-311+G(2d,2p)
279
2.043042
-152
95
2426
MP2(full)/6-311G(2df,2pd)
350
2.112982
-185
100
2524
MP2(full)/6-311+G(2df,2pd)
378
2.117227
-169
123
2493
MP2(full)/aug-cc-pVTZ
552
2.220728
-180
B3LYP/6-31G(d)
125
2.695951
248
384
1963
B3LYP/6-31+G(d)
153
2.707166
305
455
1916
B3LYP/6-311G(d,p)
186
2.771491
203
344
1847
B3LYP/6-311+G(d,p)
214
2.773979
253
392
1827
B3LYP/6-311G(2d,2p)
251
2.781992
226
360
1782
B3LYP/6-311+G(2d,2p)
279
2.784391
256
394
1772
B3LYP/6-311G(2df,2pd)
350
2.790001
238
374
1813
B3LYP/6-311+G(2df,2pd)
378
2.792043
263
400
1792
B3LYP/aug-cc-pVTZ
552
2.800208
268
411
1810
Energy of conformer is given as -(E + 270) H.
Energy difference related to the Eq conformer.
b
307
Axial (Cs)
2342
Table 79: Structural Parameters (Å and Degree), Rotational Constants (MHz) and Dipole Moment
(Debye) for Ethynylcyclopentane Eq and Ax (Cs) Forms.
Structural
Parameters
rC≡C
rCα-C≡C
rCα-Cβ,Cβ′
rCβ-Cγ, rCβ′-Cγ′
rCγ-Cγ′
rC≡C-H
rCα-H
rCβ-H1, Cβ′-H1
rCβ-H2, Cβ′-H2
rCγ-H1, Cγ′-H1
rCγ-H2, Cγ′-H2
Cα-C≡C
CβCα-C≡C
CβCαCβ′
CαCβCγ
CβCγCγ′
H-C≡C
HCα-C≡C
HCαCβ
H1CβCα
H1CβCγ
H2CβCα
H2CβCγ
H1CβH2
H1CγCβ
H1CγCγ′
H2CγCβ
H2CγCγ′
H1CγH2
CβCαCβ′Cγ′
CβCγCγ′Cβ′
A(MHz)
B(MHz)
C(MHz)
|a|
|b|
Int.
coor.
R1
R2
R3
R4
R5
R4
r1
r2
r3
r4
r5
1
2
1
2
3

ψ
δ
λ1
λ2
λ3
λ4
λ5
π1
π2
π3
π4
π5
1
2
0.042
|c|
|t|
MP2(full)/
6-311+G(d,p)
Eq
Ax
1.220
1.221
1.457
1.463
1.537
1.540
1.539
1.540
1.554
1.553
1.064
1.065
1.099
1.095
1.096
1.093
1.093
1.096
1.093
1.093
1.094
1.093
179.8
179.6
114.3
111.0
101.8
101.4
103.7
103.8
105.7
105.6
179.6
179.8
108.9
109.2
108.6
112.0
108.2
112.9
110.7
113.2
112.8
108.1
113.4
110.4
107.9
108.3
111.5
110.0
112.2
110.2
110.1
111.4
110.3
112.2
107.1
107.4
42.2
42.6
0.0
0.0
6417.27 4235.96
1759.34 2191.33
1477.47 2067.11
0.984
0.803
0.000
0.000
0.216
B3LYP/
6-311+G(d,p)
Eq
Ax
1.204
1.204
1.457
1.464
1.548
1.550
1.544
1.546
1.559
1.557
1.062
1.062
1.098
1.095
1.094
1.091
1.092
1.095
1.092
1.092
1.093
1.092
179.6
179.9
114.7
112.2
102.4
101.9
104.2
104.8
106.0
106.0
179.9
179.9
108.2
108.2
108.3
111.1
108.6
112.6
110.8
113.2
112.4
108.0
113.3
110.4
107.4
107.7
111.4
110.2
112.2
110.2
110.4
111.3
110.3
112.3
106.7
106.9
39.8
38.9
0.0
0.0
6339.43 4331.55
1755.30 2117.51
1469.16 1963.24
0.990
0.819
0.000
0.000
0.073
Microwavea
Eq
1.209b
1.470
1.541
1.541
1.541
1.055b
1.100
1.100
1.100
1.100
1.100
Adjusted r0c
Ax
1.209b
1.470
1.546
1.546
1.546
1.055b
1.100
1.100
1.100
1.100
1.100
Eq
1.211(3)
1.461(3)
1.542(3)
1.541(3)
1.556(3)
1.064(2)
1.099(2)
1.096(2)
1.093(2)
1.093(2)
1.094(2)
179.4(5)
113
110
113.7(5)
103.0
101.7
102.6(5)
102
103
103.7(5)
106(5)
106(5)
106.0(5)
179.9(5)
113.9
106.5
108.9(5)
108.8(5)
108.2(5)
109.1(5)
112.8(5)
114.9(5)
109.5
109.5
107.9(5)
111.5(5)
112.8(5)
110.1(5)
109.3(5)
109.5
109.5
107.1(5)
41(5)
39(5)
40.8(5)
0.0
0.0
0.0(5)
6349.37(40) 4264.82(43) 6350.25
1765.18(1) 2168.76(1) 1766.02
1480.55(1) 2032.08(1) 1479.74
Ax
1.211(3)
1.467(3)
1.542(3)
1.542(3)
1.555(3)
1.065(2)
1.095(2)
1.093(2)
1.096(2)
1.093(2)
1.093(2)
179.9(5)
111.5(5)
102.1(5)
103.7(5)
105.9(5)
179.6(5)
109.2(5)
111.2(5)
112.9(5)
114.9(5)
108.1(5)
108.6(5)
108.3(5)
110.0(5)
109.5(5)
111.4(5)
112.7(5)
107.4(5)
41.6(5)
0.0(5)
4265.45
2169.63
2032.11
0.274
0.985
0.832
0.993
0.864
Ref [132], values without uncertainties are taken from Ref[139] .
b
Value from Ref[140]
c
Adjusted parameters using the microwave data from Ref [132] for the given ground states.
a
308
Table 80: Symmetry Coordinates for Ethynylcyclopentane.
Symmetry Coordinatea
Description
A'
A"
C≡C-H stretch
-CH2 antisymmetric stretch
-CH2 antisymmetric stretch
-CH2 symmetric stretch
-CH2 symmetric stretch
-CH stretch
C≡C stretch
-CH2 deformation
-CH2 deformation
-CH bend (in-plane)
-CH2 wag
-CH2 wag
-CH2 twist
-CH2 twist
Ring deformation
C-CC stretch
-CH2 rock
Ring deformation
Ring breathing
-CH2 rock
C≡C-H linear bend (in-plane)
Ring deformation
Ring-CCH bend (in-plane)
Ring puckering
C-C≡C linear bend (in-plane)
-CH2 antisymmetric stretch
-CH2 antisymmetric stretch
-CH2 symmetric stretch
-CH2 symmetric stretch
-CH2 deformation
-CH2 deformation
-CH2 wag
-CH bend (out-of-plane)
-CH2 wag
-CH2 twist
-CH2 twist
Ring deformation
Ring deformation
-CH2 rock
-CH2 rock
Ring deformation
S1
S2
S3
S4
S5
S6
S7
S8
S9
S10
S11
S12
S13
S14
S15
S16
S17
S18
S19
S20
S21
S22
S23
S24
S25
S26
S27
S28
S29
S30
S31
S32
S33
S34
S35
S36
S37
S38
S39
S40
S41
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
309
R4
r4 – r5 + r4′ – r5′
r2 – r3 + r2′ – r3′
r4 + r5 + r4′ + r5′
r2 + r3 + r2′ + r3′
r1
R1
π5 + π5′
λ5 + λ5′
δ + δ′ + 2 ψ
π1 + π2 – π3 – π4 + π1′ + π2′ – π3′ – π4′
λ1 + λ2 – λ3 – λ4 + λ1′ + λ2′ – λ3′ – λ4′
λ1 – λ2 – λ3 + λ4 + λ1′ – λ2′ – λ3′ + λ4′
π1 – π2 – π3 + π4 + π1′ – π2′ – π3′ + π4′
R3 + R4 + R3′ + R4′ – 4R5
R2
λ1 – λ2 + λ3 – λ4 + λ1′ – λ2′ + λ3′ – λ4′
R3 – R4 + R3′ – R4′
R3 + R4 + R3′ + R4′ – R5
π1 – π2 + π3 – π4 + π1′ – π2′ + π3′ – π4′
σ
3θ1 – 2θ2 – 2θ3 + θ2′ + θ3′
2 + 2′
τ1 + τ1′
1
r2 – r3 – r2′ + r3′
r4 – r5 – r4′ + r5′
r4 + r5 – r4′ – r5′
r2 + r3 – r2′ – r3′
π5 – π5′
λ5 – λ5′
π1 + π2 – π3 – π4 – π1′ – π2′ + π3′ + π4′
δ – δ′
λ1 + λ2 – λ3 – λ4 – λ1′ – λ2′ + λ3′ + λ4′
π1 – π2 – π3 + π4 – π1′ + π2′ + π3′ – π4′
λ1 – λ2 – λ3 + λ4 – λ1′ + λ2′ + λ3′ – λ4′
R3 – R4 – R3′ + R4′
R3 + R4 – R3′ – R4′
π1 – π2 + π3 – π4 – π1′ + π2′ – π3′ + π4′
λ1 – λ2 + λ3 – λ4 – λ1′ + λ2′ – λ3′ + λ4′
θ2 – θ3 – θ2′ + θ3′
C≡C-H bend (out-of-plane)
Ring-CCH bend (out-of-plane)
C-C≡C (out-of-plane)
Ring twisting
a
Not normalized.
S42
S43
S44
S45
=
=
=
=
310
τ3
2 – 2′
τ4
τ1 – τ1′
Table 81: Temperature and Activity Ratios of the Eq and Ax Bands of Ethynylcyclopentane.
Liquid
xenon
T(C)
1/T (10-3 K-1)
I770 / I930
I861 / I423
I861 / I887
I861 / I930
I952 / I423
I952 / I887
I952 / I930
50.0
4.584
2.000
5.308
0.608
5.308
1.000
0.115
1.000
60.0
70.0
80.0
90.0
100.0
4.692
4.923
5.177
4.460
5.775
2.077
2.154
2.231
2.308
2.385
5.538
5.769
6.000
6.231
6.462
0.632
0.655
0.678
0.698
0.718
5.538
5.769
6.000
6.231
6.462
1.077
1.115
1.154
1.192
1.231
0.123
0.127
0.130
0.134
0.137
1.077
1.115
1.154
1.192
1.231
94  5
105  5
89  6
105  5
104  12
87  12
104  12

H (cm-1)
a
T(C)
Liquid
xenon
50.0
60.0
70.0
80.0
90.0
100.0
1/T (10-3 K-1)
I464 / I423
I464 / I887
I464 / I930
I505 / I423
I505 / I887
I505 / I930
I770 / I423
I770 / I887
4.584
4.692
4.923
5.177
4.460
5.775
0.885
0.923
0.962
1.000
1.038
1.077
0.101
0.105
0.109
0.113
0.116
0.120
0.885
0.923
0.962
1.000
1.038
1.077
2.154
2.231
2.308
2.385
2.462
2.538
0.247
0.254
0.262
0.270
0.276
0.282
2.154
2.231
2.308
2.385
2.462
2.538
2.000
2.077
2.154
2.231
2.308
2.385
0.229
0.237
0.245
0.252
0.259
0.265
94  5
78  6

H (cm-1)
105  5
91  6
105  5
88  4
72  5
88  4
a
Average value H = 94 ± 2 cm-1 (1.12 ± 0.02 kJ/mol) with the Eq conformer the more stable
form and the statistical uncertainty (1σ) obtained by utilizing all of the data as a single set.
a
311
Table 82: Comparison of Rotational Constants (MHz) Obtained from ab Initio MP2(full)/6311+G(d,p) Predictions, Experimental Valuesa from Microwave Spectra, and Adjusted r0
Structural Parameters for Ethynylcyclopentane.
Conformer
Eq-C5H9CCH
Ax-C5H9CCH
a
Rotational
Constants
MP2(full)/
6-311+G(d,p)
A
6417.27
6349.37(40)
6350.25
0.88
B
1759.34
1765.18(1)
1766.02
0.84
C
1477.47
1480.55(1)
1479.74
0.81
Experimentala
Adjusted r0
||
A
4235.96
4264.82(43)
4265.45
0.63
B
2191.33
2168.76(1)
2169.63
0.87
C
2067.11
2032.08(1)
2032.11
0.03
Ref [132].
312
Figure 48: Comparison of experimental and calculated infrared spectra of ethynylcyclopentane: (A)
observed spectrum of gas; (B) simulated spectrum of a mixture of Eq and Ax conformers (ΔH = 94
cm-1) at 25°C; (C) simulated spectrum of Ax conformer; (D) simulated spectrum of Eq conformer;
(E) observed spectrum of solid.
313
Figure 49: Comparison of experimental and calculated Raman spectra of ethynylcyclopentane: (A)
observed spectrum of liquid; (B) observed spectrum of Xe solution at -60°C; (C) simulated spectrum
of a mixture of Eq and Ax conformers (ΔH = 94 cm-1) at -60°C; (D) simulated spectrum of Ax
conformer; (E) simulated spectrum of Eq conformer.
314
Figure 50: Conformers of ethynylcyclopentane (A) Eq; (B) Ax.
315
Figure 51: Infrared and Raman spectra of ethynylcyclopentane (A) observed mid-infrared spectrum
of gas; (B) Raman spectrum of Xe solution at -60°C.
316
Figure 52: Temperature (-50 to -100°C) dependent Raman spectrum of
ethynylcyclopentane dissolved in liquid xenon.
317
CHAPTER 13
MICROWAVE, STRUCTURAL, CONFORMATIONAL, VIBRATIONAL STUDIES AND AB
INITIO CALCULATIONS OF ISOCYANOCYCLOPENTANE
Introduction
Monosubstituted cyclopentanes have been of interest for many decades since the
cyclopentane ring vibration was proposed to have a motion described as pseudorotation [122].
After the initial prediction of the pseudorotational motion in saturated five-membered rings twelve
years later Pitzer and Donath [123] proposed that several mono-substituted cyclopentanes should
have the bent conformation (envelope) as the preferred form. Relatively complete vibrational [124,
125] studies of the cyclopentyl halides (F, Cl and Br) were carried out utilizing infrared and
Raman spectra and it was concluded that the F, Cl, and Br compounds all had doublets for the
carbon-halogen stretches arising from envelope-equatorial (Eq) and envelope-axial (Ax)
conformers for the envelope form of these halo substituted cyclopentanes. We recently determined
[126, 127] the enthalpy differences between the two stable forms of the chloride and bromide by
variable temperature studies of the infrared spectra of rare gas solutions with the axial conformer
the more stable form for both molecules. Also from the initial study [124, 125] of
fluorocyclopentane it was also concluded that both the Ax and Eq forms were present in the fluid
phases but from a later Raman study [129] it was determined there was only a single conformer
present in the fluid states but it’s conformation was not determined. However from CNDO/2
calculations [130] the predicted result were only one conformer and it was the Eq form.
As a continuation of our studies of the conformational stabilities of mono-substituted
cyclopentanes we reinvestigated the infrared and Raman spectra of fluorocyclopentane [131].
Again a single stable conformer was found in the fluid phases but it is neither the Ax nor Eq form,
but the twisted form. Based on these results it was expected that there may be several other
318
monosubstituted cyclopentane molecules where the previously determined conformer stability is in
error. Therefore we initiated some studies of cyclopentane molecules which might be expected to
have more than one conformer present at ambient temperature. Cyanocyclopentane [141] was the
first of these molecules to be investigated and from the variable temperature studies of the infrared
spectra of rare gas solutions two conformers were identified. It was determined that the conformers
were the axial and equatorial forms and the axial conformer was determined to be the more stable
form. A similar variable temperature study was carried out on ethynylcyclopentane [142] molecule
and it was found that the equatorial conformer was the stable form in contrast to the axial
conformer which was the more stable form for cyanocyclopentane. Currently it is not clear what
factors are important to determine the conformer that is the most stable form for these
monosubstituted cyclopentane molecules. Therefore, as a continuation of these studies of the
monosubstituted cyclopentane molecules we have chosen isocyanocyclopentane, c-C5H9NC as the
next molecule to be investigated for the conformational stabilities and structural parameters.
There
has
not
been
a
previous
report
of
the
conformational
stability
of
isocyanocyclopentane and there is, in general, limited conformational and structural data of
organoisocyanides in the vapor state. We believe it is desirable to obtain an accurate enthalpy
difference between the expected two conformers (Fig. 53) and obtain structural parameters, if
possible, as well as to obtain the identity of the most stable conformer. Therefore we have
investigated the vibrational spectrum of isocyanocyclopentane with a study of the infrared spectra
of the gas, xenon solutions and solid. Additionally we have investigated the Raman spectra of the
liquid as well as infrared spectra of the variable temperatures of the xenon solutions. To support
the vibrational study, we have carried out ab initio calculations with basis sets up to aug-cc-pVTZ
as well as those with diffuse functions, i.e., 6-311+G(2df,2pd). We have also carried out density
functional theory (DFT) calculations by the B3LYP method with the same basis sets. We have
319
calculated optimized geometries, conformational stabilities, harmonic force fields, infrared
intensities, Raman activities and depolarization ratios. The results of these spectroscopic,
structural, and theoretical studies are reported herein.
Experimental Methods
The sample of isocyanocyclopentane was purchased from Acros Organics, with stated
purity of ≥96%. The sample was further purified by low-temperature, low-pressure fractionation
column. The purity of the sample was checked and verified by the infrared spectra.
Microwave spectra of isocyanocyclopentane were recorded on a “mini-cavity” Fouriertransform microwave spectrometer [17, 18] at the Kent State University. The Fabry-Perot resonant
cavity is established by two 7.5-inch diameter diamond-tip finished aluminum mirrors with a 30.5cm spherical radius. The Fabry-Perot cavity resides inside a vacuum chamber formed by a 6-way
cross and a 15-inch long, 8-inch diameter extension tube. One of the cavity mirrors is formed on an
8-inch diameter vacuum flange and mounted on the 6-way cross. The second mirror is mounted on
0.75-inch diameter steel rails that pass through ball bearing brackets mounted inside the extension
arm. A motorized micrometer is used to position the movable mirror over a two-inch travel range.
The two cavity mirrors are nominally separated by 30 cm. The vacuum chamber is pumped by a
Varian VHS-6 diffusion pump (2400 L s-1) backed by a two-stage Edwards E2M30 rotary pump.
The isocyanocyclopentane sample was entrained in 70:30 Ne-He carrier gas mixtures at 2
atm and expanded into the cavity with a reservoir nozzle [18] made from a modified Series-9
General Valve. The reservoir nozzle is mounted in a recessed region of the mirror flange which is
external to the vacuum chamber and the expansion passes through a 0.182-inch diameter hole into
the resonant cavity. The center of the expansion is offset from the center of the mirror by 1 inch.
The sample was irradiated by microwave radiation generated by an Agilent Technologies
E8247C PSG CW synthesizer and details of the irradiation and heterodyne detection circuitry can
320
be found in Reference [19]. Labview software controls the timing of the gas and irradiation pulses,
as well as the detection of any free induction decay signal. The software performs signal averaging
and can scan the spectrometer by stepping both the frequency source and the cavity. Microwave
circuit elements allow for a spectral range from 10.5 to 26 GHz. The digital frequency resolution is
2.5 kHz and governed by the sampling rate and by the length of the free induction decay record.
Rotational transitions are split into Doppler doublets by typically 13 kHz centered at the transition
frequency due to the coaxial orientation of the gas expansion to the cavity axis and the FWHM of
each Doppler component. The assigned microwave lines are listed in Table 83 and the rotational
and centrifugal distortion constants are listed in Table 84.
The mid-infrared spectrum of the gas and solid (Fig. 54) were obtained from 4000 to 220 cm-1
on a Perkin-Elmer model 2000 Fourier transform spectrometer equipped with a Ge/CsI beamsplitter
and a DTGS detector. Atmospheric water vapor was removed from the spectrometer housing by
purging with dry nitrogen. The spectra of the gas and solid were obtained with a theoretical resolution
of 0.5 cm-1 for the gas and 2 cm-1 for the solid with 128 interferograms added and truncated. It needs
to be noted that annealing was attempted multiple times with reduction in some band intensities in the
spectra however a polycrystalline solid of single conformer could not be obtained.
The mid-infrared spectra (3600 to 400 cm-1) of the sample dissolved in liquefied xenon
(Fig. 54B) at ten different temperatures (-65 to -100C) were recorded on a Bruker model IFS-66
Fourier transform spectrometer equipped with a globar source, a Ge/KBr beamsplitter and a DTGS
detector. In all cases, 100 interferograms were collected at 1.0 cm-1 resolution, averaged and
transformed with a boxcar truncation function. For these studies, a specially designed cryostat cell
was used. It consists of a copper cell with a path length of 4 cm with wedged silicon windows
sealed to the cell with indium gaskets. The temperature was maintained with boiling liquid
nitrogen and monitored by two Pt thermoresistors. After cooling to the designated temperature, a
321
small amount of the sample was condensed into the cell and the system was then pressurized with
the noble gas, which condensed in the cell, allowing the compound to dissolve.
The Raman spectra (Fig. 55) were recorded on a Spex model 1403 spectrophotometer
equipped with a Spectra-Physics model 2017 argon ion laser operating on the 514.5 nm line. The
laser power used was 1.5 W with a spectral bandpass of 3 cm-1. The spectrum of the liquid was
recorded with the sample sealed in a Pyrex glass capillary. The measurements of the Raman
frequencies are expected to be accurate to  2 cm-1. All of the observed bands in the Raman spectra
of the liquid along with their proposed assignments and depolarization values are listed in Tables
85 and 86.
Theoretical Methods
The ab initio calculations were performed with the Gaussian-03 program [21] using
Gaussian-type basis functions. The energy minima with respect to nuclear coordinates were
obtained by the simultaneous relaxation of all geometric parameters by using the gradient method
of Pulay [22]. A variety of basis sets as well as the corresponding ones with diffuse functions were
employed with the Møller-Plesset perturbation method [23] to the second order MP2 with full
electron correlation as well as with the density functional theory by the B3LYP method. The
predicted conformational energy differences are listed in Table 87.
In order to obtain descriptions of the molecular motions involved in the fundamental
modes of isocyanocyclopentane, a normal coordinate analysis was carried out. The force field in
Cartesian coordinates was obtained with the Gaussian 03 program at the MP2(full) level with the
6-31G(d) basis set. The internal coordinates used to calculate the G and B matrices are given for
the Ax and Eq conformers in Table 88. By using the B matrix [24], the force field in Cartesian
coordinates was converted to force constants in internal coordinates. Subsequently, 0.88 was used
as the scaling factor for the CH stretches and deformations, and 0.90 was used for all other modes
322
excluding the heavy atom bends and –CN stretch to obtain the fixed scaled force constants and
resultant wavenumbers. A set of symmetry coordinates was used (Table 89) to determine the
corresponding potential energy distributions (P.E.D.s). A comparison between the observed and
calculated wavenumbers, along with the calculated infrared intensities, Raman activities,
depolarization ratios and potential energy distributions for the Ax and Eq conformers of
isocyanocyclopentane are given in Tables 85 and 86, respectively.
The vibrational spectra were predicted from the MP2(full)/6-31G(d) calculations. The
predicted scaled frequencies were used together with a Lorentzian function to obtain the simulated
spectra. Infrared intensities were obtained based on the dipole moment derivatives with respect to
Cartesian coordinates. The derivatives were transformed with respect to normal coordinates by
(u/Qi) =
 ( /X )L ,
j
u
j
ij
where Qi is the ith normal coordinate, Xj is the jth Cartesian
displacement coordinate, and Lij is the transformation matrix between the Cartesian displacement
coordinates and the normal coordinates. The infrared intensities were then calculated by
[(N)/(3c2)] [(x/Qi)2 + (y/Qi)2 + (z/Qi)2]. A comparison of experimental and simulated
infrared spectra of c-C5H9NC is shown in Fig. 54. Infrared spectrum of the gas and the predicted
infrared spectra of the pure Ax and Eq conformers, and the mixture of the two conformers with
relative concentrations calculated for the equilibrium mixture at 25ºC by using the experimentally
determined enthalpy difference are shown in Fig. 54 (A-D). The predicted spectra are of isolated
molecule and should be comparable to the spectrum of the vapor phase. The predicted spectrum is
in good agreement with the experimental spectrum which shows the utility of the scaled predicted
frequencies and predicted intensities for supporting the vibrational assignment.
Additional support for the vibrational assignments was obtained from the simulated Raman
spectra. The evaluation of Raman activity by using the analytical gradient methods has been
323
developed [25-27, 37] and the activity Sj can be expressed as: Sj = gj(45αj2 + 7βj2), where gj is the
degeneracy of the vibrational mode j, αj is the derivative of the isotropic polarizability, and βj is the
anisotropic polarizability. To obtain the Raman scattering cross sections, the polarizabilities are
incorporated into Sj by multiplying Sj with (1-ρj)/ (1+ρj) where ρj is the depolarization ratio of the
jth normal mode. The Raman scattering cross sections and calculated wavenumbers obtained from
the Gaussian 03 program were used together with a Lorentzian function to obtain the simulated
Raman spectra of the isolated molecule. The Raman spectra of the liquid and the predicted Raman
spectra for the pure Ax and Eq conformers and the mixture of the two conformers with relative
concentrations are obtained by using the experimentally determined enthalpy difference (102 cm -1)
and is shown in Fig. 55(A-D). The predicted spectra should be comparable to that of the liquid as
the frequency shift due to the intermolecular interactions of the liquid are relatively small with an
average value of 3 cm-1. The spectrum of the mixture should be compared to that of the Raman
spectrum of the liquid at room temperature. The predicted spectrum is in reasonable agreement
with the experimental spectrum which shows the utility of the predicted Raman spectra for the
supporting vibrational assignments.
Microwave Results
The the rotational spectra were fit to obtain the microwave constants. An overview of the
fit rotational parameters for isocyanocyclopentane and the comparison to ab initio results can be
found in Table 84. The spectrum was satisfactorily fit (Table 83) using a standard Watson semirigid rotor Hamiltonian of the A-reduction type in the Ir representation [143], with 3 kHz RMS
error, better than the experimental uncertainty of approximately 25 kHz (line centers determined to
±12.5 kHz). A summary of the fit can be found in Table 84.
The largest value of J is J = 5 and the largest value of Ka is Ka = 2. This leads to a lack of
information with which to determine the centrifugal distortion constants (CDCs) as the magnitude
324
of the effects are dependent on the energy level. To obtain the CDCs the transitions were fit while
keeping the determined rotational constants fixed. The final fit was then obtained with the CDCs
kept constant at the determined values. These CDCs are well fit except for the K constants which
have higher uncertainty as all of the transitions assigned are Ka = 2 or less.
Vibrational Assignment
In order to determine the enthalpy difference between the stable conformers it is essential
to have a confident assignment for all of the fundamentals of the stable forms and identify
vibrations which cannot be assigned to these conformers. The vibrations of the CH2 group are
expected to be very similar to those found in the usual five membered rings with just carbon atoms
so a discussion of their assignments is not necessary. Therefore, the assignments of the nine
fundamentals of the ring will be provided first and this will be followed by the vibration
assignments involving the C-N≡C modes.
The assignments of the ring fundamentals for the mono substituted five membered rings is
greatly simplified by the pattern in the vibrational spectra which can be clearly demonstrated from
the data in Table 90. The first two ring deformations are ~1050 to 1000 cm-1 and are the first ring
deformations in the A′ and A′′ blocks. The next set of two ring deformations are at ~900 cm-1, one
in each the A′ and A′′ blocks. The A′ block ring deformation in this case is relatively variable in its
position changing with substituent and conformation. This is near the ring breathing mode in the
A′ block which always appears at ~880 cm-1 and changes very little with different substituents.
The next ring deformation is in the A′ block and is lower in frequency than the ring breathing and
this ring deformation appears between 850 to 650 cm-1. This is the deformation most affected by
substituent identity and conformer changes. It also has a great deal of mixing with other modes
which change considerably with changes in conformation and substituent and so its motion is often
split between a number of fundamentals in this region. The remaining ring deformation is located
325
lower in frequency between 650-600 cm-1 and is also relatively variable in frequency with
significant changes from conformational or substituent change. The final ring fundamentals are the
ring puckering and ring twisting modes which are assigned near 300 cm-1 and 100 cm-1,
respectively. These modes are well known to change considerably with substituent and
conformational changes. This well understood pattern of the ring fundamentals aids considerably
in their assignments and therefore they are excellent for use in the enthalpy determination from the
vibrational spectra of the variable temperature xenon solutions.
The heavy atom modes of the five membered ring isocyanocyclopentane also gives a total
of nine fundamentals with five of them in the A′ block and the remaining four are in the A′′ block,
for both Ax and Eq conformers. For the cyclopentane ring of the A′ block there are three ring
deformations, a ring breathing mode, and a ring puckering vibration. The deformations for the Ax
form are assigned at 1040, 929 and 839 cm-1 and the ring breathing fundamental has been assigned
at 889 cm-1 with the fifth fundamental for the A′ block which is a ring puckering fundamental has
been assigned at 271cm-1. Three ring deformations and a ring twisting fundamental are possible in
the A′′ block. The three ring deformations were observed at 1003, 897 and 650 cm -1 but the ring
twist fundamental was not observed however it was predicted at 64 cm-1 with IR intensity of 0.6
km/mol.
There are additional two heavy atoms for the isocyanocyclopentane molecule which
generate six fundamentals with four of them in the A′ block and the other two in the A′′ block. The
N≡C stretch for the Ax conformer was observed at 2138 cm-1 in the A′ block. The C-N stretch and
ring-NC bend (in-plane) were assigned at 973 cm-1 and 407 cm-1 respectively. The C-N≡C in-plane
bend was not observed but it was predicted at 122 cm-1 with IR intensity of 3.7 km/mol and Raman
activity of 2.3 Å4/u. In the A′′ block of the Ax conformer, the ring–NC out-of-plane bend has been
326
observed at 476 cm-1. From the Raman spectra of the liquid the C-N≡C out-of-plane bend has been
assigned at 182 cm-1.
Similar to the Ax conformer, Eq conformer also has three ring deformations, a ring
breathing and a ring puckering fundamentals in the A′ block. Ring deformations are assigned at
1044, 896 and 480 cm-1. The ring breathing is observed at 869 cm-1 and band at 271 cm-1 has been
assigned for the ring puckering fundamental. For the Eq conformer, three ring deformations and a
ring twisting mode occur in the A′′ block. These ring deformations are assigned at 1096, 970 and
629 cm-1.
The band at 2138 cm-1 which was assigned for N≡C stretch of the Ax form, was also
assigned for the similar stretch of the Eq conformer. In the A′ block of the Eq form, the ring-NC
in-plane bend was assigned to the band at 474 cm-1 and the ring-NC out-of-plane bend mode of the
A′′ block was observed at 444 cm-1. The C-NC in-plane bend, ring twisting, and the C-N≡C out-ofplane bend of the Eq form were not assigned because these vibrations were not observed in the
spectra.
The mixing of the vibrations is indicated by the potential energy distributions and practically all of
the modes have major contributions from two or more symmetry coordinates and their
approximate descriptions are given in Tables 85 and 86. In general for the Ax conformer (Table
85) the mixing was extensive for the fundamentals starting at 1287 cm-1 and lower frequencies.
Most of the fundamentals have extensive contributions from four or more modes. The Eq
conformer (Table 86) is similar, with extensive mixing starting from the vibration at 1310 cm-1 and
below. However, the Eq form shows a significant reduction in mixing with most of the modes with
contributions of 10% or more from three modes. In the Ax conformer the descriptions of the 15,
20 and 35 fundamentals are largely for bookmarking purposes and the mixing is extensive for the
11, 18, 21, and 35 fundamentals with contributions of more than 10% from 5 different modes. In
327
the Eq form the description of the 30′ fundamental was primarily for bookkeeping purposes. The
Eq conformer has extensive mixing for the 14′ and 30′ fundamentals with contributions of more
than 10% from 5 different modes. For the Ax form, 15 fundamental has been assigned as S15 (C-N
stretch), with 18% contribution by S15 whereas 20 fundamental has been assigned as S20 (-CH2
wag), with 11% contribution by S20 and 35 fundamental has been assigned as S35 (-CH2 wag),
with 17% contribution by S35. For the Eq form, 14′ fundamental has been assigned as S14 (ring
deformation), with 23% contribution by S14 and 30′ fundamental has been assigned as S30 (-CH2
wag), with 15% contribution by S30. Overall for both Ax and Eq forms, CH2 bends and ring
deformations are prominently mixed with each other which causes shift in some predicted
frequencies to lower wavenumbers. Hence observed frequencies of some CH2 bends and ring
deformations are higher than predicted frequencies.
Conformational Stability
For isocyanocyclopentane Ax, Eq, twist (Tw) and planar (Pl) forms are possible
conformers. For predictions of the most stable conformer, the MP2(full) and B3LYP calculations
with utilization of 18 basis sets from 6-31G(d) to aug-cc-p-VTZ. From MP2(full)/6-31G(d)
frequency calculations, the Ax and Eq conformers are predicted to have only positive frequencies
whereas the Tw and Pl forms are predicted to have one or more negative frequency. From these
calculations it was predicted that the Ax and Eq forms are stable conformers whereas the Tw and
Pl forms are not stable conformers. This prediction was confirmed by the lack of unassigned bands
that should be present if a Tw or Pl form is present in the sample.
For estimating the more stable conformer between the Ax and Eq forms, the electronic
energy was calculated (Table 87). From the MP2(full) basis sets calculations it is predicted that the
Ax form is the more stable conformer whereas calculations from the DFT method by the basic sets
B3LYP6-31G(d), 6-31+G(d), 6-311+G(2df,2pd) and aug-cc-pVTZ predicts that the Eq form is the
328
more stable conformer. Thus from these energy calculations it is difficult to predict which of the
two stable conformers is the more stable form.
To determine the more stable conformer and enthalpy differences between the Ax and Eq
forms of isocyanocyclopentane, the sample was dissolved in liquefied xenon and the mid-infrared
spectra were recorded as a function of temperature from -65 to -100°C. Very small interactions are
expected to occur between xenon and the sample though the sample can associate with itself
forming a dimer, trimer or higher order complex. However, due to the very small concentration of
the sample (~10-4 molar) self-association is greatly reduced. Therefore, only small frequency shifts
are anticipated for the xenon interactions when passing from the gas phase to the liquefied xenon
solution, which is confirmed with an average frequency shift of 1 cm-1. A significant advantage of
this study is that the conformer bands are better resolved in the xenon solution in comparison to
those observed in the infrared spectrum of the gas (Fig. 56). From ab initio calculations, the dipole
moments of the two conformers are predicted to have similar values and the molecular sizes of the
two conformers are nearly the same, so the ΔH value obtained from the temperature dependent FTIR study from the xenon solution is expected to be near to that for the gas [39-43].
Once confident assignments have been made for the fundamentals of the two observed
conformers the task was then to find pairs of bands from which the enthalpy difference could be
obtained. The bands should be sufficiently resolved for determining their intensities. These bands
should come from the region from 1200 to 400 cm-1, where there are a limited number of overtone
and combination bands possible. The bands at 407, 711, 803 and 1080 cm-1 were assigned to the
Ax conformer and 444, 749 and 1096 cm-1 were assigned to the Eq form (Fig. 54). These bands are
well resolved and believed to be relatively free from combination and overtone bands, thus they
were used for the enthalpy difference determinations. The intensities of these individual bands
were measured as a function of temperature (Fig. 57) and their ratios were determined (Table 91).
329
By application of the van’t Hoff equation lnK = H/(RT)  S/R, the enthalpy difference was
determined from a plot of lnK versus 1/T, where H/R is the slope of the line and K is substituted
with the appropriate intensity ratios, i.e. Iconf-1 / Iconf-2, etc. It was assumed that S and α are not
functions of temperature in this relatively small temperature range utilized.
These seven bands, with four from the Ax form and three from the Eq conformer, were
utilized for the determination of the enthalpy difference by combining them to form 12 band pairs.
By using these band pairs for the Ax and Eq conformers the individually determined enthalpy
differences ranged from the low value of 84  4 cm-1 to the highest value of 138  8 cm-1 (Table
91). However an average value was obtained by taking the data from all twelve band pairs as a
single data set. By this method the average value of 102  6 cm-1 was obtained. The error limit was
derived from the statistical standard deviation of two sigma. These error limits do not take into
account small associations with the liquid xenon or the interference of overtones and combination
bands in near coincidence with the measured fundamentals. The variations in the individual values
are undoubtly due to these types of interferences, but taking several pairs the effect of such
interferences should cancel. However, this statistical uncertainty is probably better than can be
expected from this technique and, therefore, an uncertainty of about 10% in the enthalpy difference
is probably more realistic i.e., 102  10 cm-1. From these enthalpy differences the abundance of the
Ax conformer at ambient temperature is estimated to be present at 62 % and 38  1 % for Eq form.
Structural Parameters
As there was no previous structural study reported on the c-C5H9NC we were interested in
determining the structural parameters for this molecule. The adjusted r0 structure can be
determined for the Ax conformer by utilizing the rotational constants reported herein from the
microwave spectra. There are three rotational constants available for the determination of the
structural parameter values of the Ax conformer of c-C5H9NC.
330
We have found that good structural parameters for hydrocarbons and many substituted ones
can be determined by adjusting the structural parameters obtained from the ab initio MP2(full)/6311+G(d,p) calculations to fit the rotational constants obtained from microwave experimental data
by using the computer program “A&M” (Ab initio and Microwave) developed [57] in our
laboratory.
We [44] have shown that ab initio MP2(full)/6-311+G(d,p) calculations predict the carbonhydrogen r0 structural parameters for more than fifty hydrocarbons to at least 0.002 Å compared to
the experimentally determined [45] values from isolated CH stretching frequencies which agree
with previously determined values from earlier microwave studies. Therefore, all of the carbonhydrogen parameters can be taken from the MP2(full)/6-311+G(d,p) predicted values for the Ax
conformer. However, in order to further reduce the number of independent variables, the structural
parameters are separated into sets according to their types where bond distances in the same set
keep their relative ratio, whereas bond angles and torsional angles in the same set keep their
difference in degrees. This assumption is based on the fact that errors from ab initio calculations
are systematic. Therefore, it should be possible to obtain “adjusted r0” structural parameters for the
twelve parameters of the seven heavy atoms by adjusting the C-C distances as a single set and
adjusting the CCC angles as another set leaving eight parameters to adjust. This structure,
however, must be evaluated using parameters from similar molecules since there are a limited
number of rotational constants available. By utilizing the experimentally determined rotational
constants obtained from the microwave spectra reported in this study, we have obtained the
complete structural parameters for the Ax form of c-C5H9NC.
The resulting adjusted r0 parameters are listed in Table 88, where the precisions are listed,
it is believed that the N≡C, C-N, and C-C distances should be accurate to ± 0.003 Å, the C-H
distances accurate to ± 0.002 Å, and the angles should be within ± 0.5. The fit of the three
331
determined rotational constants by the adjusted r0 structural parameters for the Ax conformer is
excellent with the differences being 0.10, 0.15 and 0.07 MHz for the A, B and C rotational
constants, respectively. Therefore, it is believed that the suggested uncertainties are realistic values
and the determined structural parameters are probably as accurate as can be obtained for the
molecule in the gas phase by either electron diffraction or microwave substitution methods.
Additionally, we have also shown that the differences in predicted distances and angles
from the ab initio calculations for different conformers of the same molecule can usually be used
as one parameter with the ab initio predicted differences except for some dihedral angles.
Therefore, we have predicted structural parameters for the Eq form of Isocyanocyclopentane by
applying the corresponding adjustments from the Ax conformer to the MP2(full)/6-311+G(d,p)
predicted parameters for the Eq form. These parameters should be close to the actual value except
for the dihedral angles.
Discussion
Average and percent errors have been calculated between predicted and observed
frequencies for the Ax and the Eq conformer of the isocyanocyclopentane. For both Ax and Eq
conformers, A′ mode has an average error of 6.64 and 6.19 cm-1, respectively, which represents
percent error of 0.38 and 0.40%, respectively, whereas A′′ mode has an average error of 4.46 and
4.13 cm-1, respectively, which represents percent error of 0.28 and 0.25%, respectively. Both the
average and percent errors are reasonable and show that predicted frequencies are meaningful with
respect to the vibrational assignments.
An interesting point to note is that we did not observed any A, B and C type band contours
in the IR spectra of gas of isocyanocyclopentane (Fig. 54), however theoretical calculations shows
that there should be A, B and C type band contours. Predicted A, B, and C type band contours for
both the Ax and Eq conformers are shown in Fig 58. Exact reasons for this unusual behavior
332
should be investigated. This has little effect on the experimental results in this study as the bands
are well resolved in the spectra of the xenon solutions.
The Ax fundamental 19 is assigned to the doublet at 711/661 cm-1 in the infrared spectra of
the xenon solutions and 683 cm-1 in the infrared spectra of the solid and predicted to be at 709 cm-1
from the ab initio predictions and 680 cm-1 with scaling factors. This is however a Fermi resonance
band of the Ax fundamental and difference band. This band is consistent with an Ax band in the
change in the band’s intensity due to variation of the temperature and therefore an appropriate
band for use in the enthalpy determination.
However, due to the nature of the Ax band at 711 cm-1 which was used for the enthalpy
determination the enthalpy value was also determined without the 711 cm-1 band. The
determination gives an enthalpy value of 95  2 cm-1 (1.13  0.03 kJ mol-1) from 9 band pairs. This
value is within the error limits of the enthalpy value including the band at 711 cm-1 and so the band
at 711 cm-1 does not significantly change the enthalpy value. Therefore the reported enthalpy value
is thought to be accurate within its error limits and as accurate as what could be obtained from a
dilute gas sample.
The natural population analysis (NPA) was carried out for the Ax and Eq conformers of
isocyanocyclopentane and for cyclopentane with the MP2(full) method at the 6-311+G(d,p) basis
set. The C atoms for cyclopentane all carry a -0.34 charge. However the –NC substitution of
cyclopentane gives a charge distribution for the Ax [Eq] form of Cα = 0.01 [0.00], Cβ = -0.33 [0.33], Cγ = -0.35 [-0.34], –NC* = 0.34 [0.35], N = -0.65 [-0.64]. As might be expected the –NC
group has a large effect on the charge density of the α Carbon with a quickly reduced effect on the
β and γ Carbons. However it is interesting to note that the Ax γ Carbon which is brought closer to
the –NC electron orbitals due to the conformational geometry is slightly more negative than the
corresponding Carbon in the Eq form.
333
To help account for the low number of rotational constants, the structural parameters of cC5H9NC must be evaluated for their accuracy by comparing them with similar molecules. As can
be seen from the data in Table 92, the N≡C distances and C-N≡C angles are consistent regardless
of the substituent whereas the C-N distances change depending on the substituent. The predicted
distances for the N≡C structural parameter values for the methyl- [144], vinyl- [145], Ethynyl[146] and cyclopropane- [147] isocyanide compounds has an average difference of 0.014 Å of the
structural parameter values obtained from the MP2(full)/6-311+G(d,p) calculations. The structures
in this study were determined for the Ax form of isocyanocyclopentane and the parameters were
allowed to change in fitting with the rotational constants. As can be seen from the information in
Table 88, the differences between the values from the MP2(full)/6-311+G(d,p) calculation and the
adjusted r0 structural parameter values for the N≡C bond distance is 0.010 Å which is similar to the
distances in the molecular structure for similar bond distances as can be seen in Table 92. Due to
the limited number of structural studies on molecules that contain the N≡C moiety, it would be of
interest to obtain more molecular structures of such molecules.
Additionally, it would be of interest to compare the structural parameters of c-C5H9NC
with the corresponding cyclopentanes with –C≡C or –C≡N moieties on them to see how the
substitutions would affect the values of the molecular structures. As it can be seen from the data in
Table 93, the ab initio MP2(full) calculation at the 6-311+G(d,p) basis set does a fairly good job
predicting the bond distances and angles for these molecules. There is no significant difference
between the cyano- and ethynl-cyclopentane rings parameters as the differences are within the
corresponding uncertainties. On the other hand, the Cα-Cβ,Cβ′ bond distances are ~0.008 Å shorter
for the ring that is substituted with –N≡C moiety as opposed to the rings that are substituted with –
C≡C and –C≡N moieties. The <Cβ-Cα-N is 1.1° smaller than both <Cβ-Cα-C and <Cβ-Cα-C angles.
These differences are expected and reflected in the ab initio calculations value. The rest of the ring
334
structural parameters of isocyanocyclopentane are within the errors associated with the parameters
when compared to the other substituted ring molecules.
Further, to study the effect of substituting isocyanide moiety, a comparison between the
unsubstituted cyclopentane and isocyanocyclopentane would be of interest. The ring parameters
are much more flexible where the ring distances and angles for five membered rings are often
relatively sensitive to substitution. The C-C distances of the cyclopentane were reported to be
1.546(1) Å from an electron diffraction study [148]. It can be seen that the Cα-Cβ bond distances
are ~0.012 Å shorter for the cyclopentane ring with –N≡C moiety on it as opposed to unsubstituted
cyclopentane. The other Cβ-Cγ bond distances are much longer and within the experimental error
to the value from the cyclopentane r0 structure. The Cγ-Cγ′ bond distance is ~0.008 Å longer for the
cyclopentane ring with –N≡C moiety on it when compared to unsubstituted cyclopentane.
The electronegative –NC group as compared to cyclopentane causes a significant
difference in the Cα charge which appears to cause a significant difference in the Cα-Cβ bond
distance where the –NC molecule is 0.012 Å shorter. This effect is reduced in the β Carbon and
the Cβ-Cγ distance is closer to the unsubstituted cyclopentane where the –NC molecule is only
0.004 Å shorter which is within the error limits. The Cα-Cβ bond is also 0.008 Å shorter than the
Cβ-Cγ distance in the same –NC molecule which reinforces the idea that the charge on the Cα is
the cause of the Cα-Cβ bond distance. There is a small 0.01 charge difference on the γ Carbon
charges between the Ax conformer of the –NC molecule and the cyclopentane Carbons.
However, this probably is not the cause of the difference in the Cγ-Cγ bond distance. Instead this
difference is a common occurrence in substituted cycplopentane rings and is probably related to
steric forces in the ring introduced by the substitution.
At the time this study was initiated there was some confusion regarding which conformer
was the most stable form of many mono substituted cyclopentanes. It should be noted that
335
cyanocyclopentane [135] which belongs to this group of mono-substituted cyclopentanes has the
Ax conformer as the more stable form by 109  37 cm-1 determined by the infrared spectra of the
xenon solutions. This is opposite to ethynylcyclopentane which has been reported [142] to have the
Eq form more stable by 94  9 cm-1. Thus from our current study of isocyanocyclopentane it was
necessary to be absolutely sure which conformer was more stable. Therefore in addition to the
enthalpy determinations from the infrared spectra of the xenon solution we obtained additional
scientific evidence of the conformational stabilities.
The first step in this process was to obtain the stable conformer in the solid state. By
utilizing ab initio predicted frequencies the spectra were predicted of the isolated molecule of Ax
and Eq forms and they are shown in Fig. 54E and 54F. The infrared spectrum of the solid was thus
obtained and it is shown in Fig. 54C. These spectra were then compared and as can be observed
from Fig. 54 this spectrum of the solid did not demonstrate a pure crystal of a single conformer. It
is believed that spectra is due to the sample being an amorphous mixture form instead of a pure
crystal. Nevertheless from the spectra of the solid it is clearly observed that the band intensities of
the Ax form is significantly higher compared to that of Eq form especially in the spectral regions
of beginning at 400 to 1100 cm-1. Thus it appears that the Ax form is the more stable conformer in
the solid. However it should be noted that Ax conformer need not be the more stable conformer in
spectra of the gas and xenon solutions.
The ab initio calculations were carried out for this study and the energy differences for the
four possible forms were obtained with the Ax form as the most stable conformer followed by the
Eq, Tw and Pl forms. From these ab initio calculations it was found that the other two possible
conformers (Tw and Pl forms) are not stable conformers but they are transition states. The
enthalpy differences obtained should be comparable to the ab initio predicted energy values and
also can be compared with other enthalpy differences obtained for other mono substituted
336
cyclopentanes. The ab initio predicted energy difference from the MP2(full)/aug-cc-pVTZ
calculations with 529 basis sets predicted the Ax conformer as the more stable form by 372 cm-1
(4.45 kJ/mol) than the Eq form. The same basis set but with B3LYP method gave instead the Eq
form as the most stable conformer. Thus the MP2(full) calculation gives better results for
conformational stability predictions in this study compared to the B3LYP method which was not
able to predict a specific conformer as the more stable form.
The jet propulsion FT-microwave study provided conclusive conformational stability
results. The rotational transitions that were assigned from the microwave study gave rotational
constants that fit those for the Ax conformer. Thus, it was demonstrated that between the Ax and
Eq forms, it is the Ax conformer that is the more stable form. Next to be determined was the
variable temperature spectrum of the sample in liquified xenon and the spectra was obtained and
also confidently assigned. The determined ΔH values conclusively showed that the Ax form was
the more stable form by 102  10 cm-1 compared to the Eq form. The lower wavenumber region is
confidently assigned for both Ax and Eq conformer and so by using 12 band pairs the value for
enthalpy difference is conclusively determined. The uncertainty of the value for each individual
band pair ranges from a high value of 12 cm-1 to a low value of 2 cm-1. This relatively small spread
in enthalpy differences indicates that there is little interference from overtone or combination
bands.
The quadratic centrifugal distortion constants have been determined from the fit of the
experimental rotational spectra and have been predicted by using the MP2(full) method with the 631G(d) and 6-311+G(d,p) basis sets. These data are given in Table 84 and as can be seen from the
values of the CDCs, there is reasonable agreement with the predicted values for both calculations.
Only the K and δK constants are relatively poorly predicted however this is probably due to the lack
of high Ka energy levels with which to fit these constants rather than an error in the predicted
337
results. We have found in the past that the ab initio calculations usually predict fairly good distortion
constants. The CDC’s were also predicted by using the B3LYP method with the 6-311+G(d,p) basis
set. However, only the ΔJ is meaningful as the remaining values are either the wrong sign or
multiples of magnitude in error (or both). This is interesting as this may be indicative of errors in the
calculation of the force constants for this molecule however that is out of the scope of this study.
Very few isocyano- molecules have been studied therefore wider conclusions on these
moieties’ effects on molecular structures and conformational stabilities are limited. It would
therefore be of interest to study more isocyano- molecules.
338
Table 83: Rotational transition frequencies (MHz) of the ground vibrational state of the Ax form of
Isocyanocyclopentane
c-C5H9NC
νobs
Δνa
Transitions
`
11050.297
414  322
11206.204
303  211
11356.812
211  101
12301.678
524  432
12719.040
413  321
13304.770
313  212
13527.017
303  202
13570.266
322  221
13613.488
321  220
13822.098
312  211
15107.086
515  423
15124.653
220  110
15286.347
221  111
15371.362
404  312
16142.913
312  202
17523.678
514  422
17727.813
414  313
17987.253
404  303
18085.072
423  322
18191.327
422  321
18415.617
404  312
19366.800
505  413
21031.511
413  303
a
 = obs calc in kHz.
339
14
-6
0
-2
-6
-3
-4
7
4
-1
0
4
0
-4
3
6
-2
-2
-1
-2
-4
-1
1
Table 84: Experimental rotational and centrifugal distortion constants of the Ax form of
Isocyanocyclopentane
MP2(full)/
6-31G(d)
MP2(full)/
6-311+G(d,p)
B3LYP/
6-311+G(d,p)
A (MHz)
4280.6003
4255.6016
4378.9869
4312.7954(7)
B (MHz)
2380.3796
2391.5263
2284.8714
2348.0136(2)
C (MHz)
2220.4767
2236.0916
2094.9175
2175.4439(2)
ΔJ (kHz)
1.034
1.097
0.924
1.197(7)
Experimental
ΔJK (kHz)
-0.36
-0.55
5.02
-0.59(8)
ΔK (kHz)
1.3
1.4
-3.5
2.0(1)
-0.091
-0.162(9)
δJ (kHz)
-0.109
-0.117
δK (kHz)
-0.4
-0.4
Na
fit (kHz)
a
3.5
-1.0(2)
23
c
3
Number of frequencies fitted.
340
Table 85: Observed and calculateda frequencies (cm-1) and potential energy distributions (P.E.D.s) for the Ax (Cs) conformer of
Isocyanocyclopentane.
Vib.
No.
Approx. description
ab
initio
Fixed
scaledb
IR
int.
Infrared
Raman
act.
Raman
39.7
0.3
46.1
114.7
2980
2980
Xe
soln.
2978
2978
2938
15.9
27.5
17.9
62.4
3.0
6.6
19.7
1.6
0.6
3.2
3.6
0.8
7.0
0.7
1.0
1.8
9.2
0.2
1.0
0.0
3.7
7.0
5.6
17.2
148.4
119.5
109.0
68.6
9.4
5.8
4.5
0.2
10.4
15.5
0.2
5.7
2.7
2.7
15.6
2.1
3.8
1.1
0.7
2.4
2.3
66.0
9.3
25.1
2961
2961
2931
2138
1482
1457
1349
1322
1287
1205
1181
1040
973
889
804
2980
2980
2931
2960
2960
2930
2137
1482
1457
1346
1321
1287
1205
1178
1038
970
929
887
803
711/661
407
2978
2978
2930
2966
2966
2934
2151/2138
1481
1457
1355
1318
1284
1208
1170
1041
968
924
885
839
794
683
409
2976
2976
2934
2924
1463
8.7
4.1
9.6
6.9
2931
1459
2930
1457
2934
1457
gas
341
A′ 1
2
-CH2 antisymmetric stretch
-CH2 antisymmetric stretch
3201
3003
3186
2989
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
A′′ 24
25
26
-CH stretch
-CH2 symmetric stretch
-CH2 symmetric stretch
N≡C stretch
-CH2 deformation
-CH2 deformation
-CH bend (in-plane)
-CH2 wag
-CH2 wag
-CH2 twist
-CH2 twist
Ring deformation
C-N stretch
Ring deformation
Ring breathing
Ring deformation
-CH2 rock
-CH2 rock
Ring-NC bend (in-plane)
Ring puckering
C-N≡C bend(in-plane)
-CH2 antisymmetric stretch
-CH2 antisymmetric stretch
3150
3138
3121
2148
1581
1553
1421
1395
1360
1272
1238
1103
1024
981
926
868
833
709
416
273
122
3191
3176
2956
2945
2927
2136
1487
1461
1350
1326
1291
1211
1180
1050
976
939
883
837
794
680
411
273
122
2994
2980
-CH2 symmetric stretch
3131
27 -CH2 symmetric stretch
28 -CH2 deformation
3118
1556
solid
2976
2976
P.E.D.c
liquid
Band
Contour
A
B
C
2973 74S1,26S2
2973 67S2,26S1
79
3
-
21
97
2959
2959
2930
2143
1480
1452
1350
1318
1287
1205
1178
1038
967
931
892
799
~711
415
271
2973
2973
2931
0 41 9 67 52 11 98 2 85 77 7 97 52 23 73 6 87 52 95 1 61 - 100
- 100
- 100
100
59
91
33
48
89
2
98
15
23
93
3
48
77
27
44
13
48
5
99
39
-
- 100
- 100
-
93S3
97S4
87S5
90S6,10S15
86S7,14S8
85S8,15S7
64S9,18S12
63S10,20S11
35S11,20S13,13S12,11S9,10S10
36S12,25S11
38S13,13S9,12S20
35S14,23S16,12S11,10S18
18S15,25S13,13S20,12S12
32S16,20S20,12S14,10S17
64S17,13S16,11S14
30S18,22S14,14S15,13S20,12S21
50S19,21S15,10S17
11S20,34S19,19S18,16S15
38S21,23S18,12S20,11S23,10S22
51S22, 47S23
39S23,32S22,30S21
73S24,22S25
77S25,18S24
99S26
2931 90S27
1457 90S28,10S29
342
3.1
18.8
1448
1448
1447 1449 90S29,10S28
29 -CH2 deformation
1538
1447
0.4
0.5
1322
1321
1315 1320 46S30,20S31,17S32
30 -CH2 wag
1396
1327
0.3
0.3
1282
1280
1280 1281 51S31,36S30
31 -CH2 wag
1347
1278
0.1
11.1
1266
1264
1260 1260 28S32,39S33,15S34
32 -CH bend (out-of-plane)
1335
1269
1.2
3.2
1214
1213 1212 47S33,15S35,14S34
33 -CH2 twist
1278
1215
1.4
4.9
1165
1163
1167 1167 38S34,19S36,10S38,10S32
34 -CH2 twist
1224
1167
4.4
7.4
1082
1080
1077 1084 17S35,20S34,18S38,16S32,12S36
35 -CH2 rock
1127
1074
Ring
deformation
0.9
2.9
1003
1003
999 1004 35S36,23S35,15S31
36
1054
1003
3.9
0.2
897
897
898
902 71S37,13S32
37 Ring deformation
941
905
0.0
0.7
810 44S38,17S35,16S39,13S36
38 -CH2 rock
848
810
1.8
0.9
650
650
645
655 57S39,13S40,11S35
39 Ring deformation
662
650
1.4
0.6
476
474
472
475 51S40,22S41,10S38
40 Ring-NC bend (out-of-plane)
485
480
2.6
3.4
182 73S41,23S40
41 C-N≡C bend(out-of-plane)
179
179
0.6
0.2
- 92S42
42 Ring twisting
64
64
a
4
MP2(full)/6-31G(d) ab initio calculations, scaled frequencies, infrared intensities (km/mol), Raman activities (Å /u) and potential energy distributions
(P.E.D.s).
b
Scaled ab initio calculations with factors of 0.88 for CH 2 stretches and CH2 deformations, 0.90 for all other modes except torsions, heavy atom bends
and NC stretch.
c
Symmetry coordinates with P.E.D. contribution less than 10% are omitted.
-
100
100
100
100
100
100
100
100
100
100
100
100
100
100
-
Table 86: Observed and calculateda frequencies (cm-1) and potential energy distributions (P.E.D.s) for the Eq (Cs) conformer of
Isocyanocyclopentane.
Infrared
Vib.
No.
343
A′ 1
2
4
5
3
6
7
8
9
19
20
12
13
14
15
10
16
17
11
18
21
22
23
A′′ 25
24
26
27
28
29
35
Approx. description
-CH2 antisymmetric stretch
-CH2 antisymmetric stretch
-CH2 symmetric stretch
-CH2 symmetric stretch
-CH stretch
N≡C stretch
-CH2 deformation
-CH2 deformation
-CH bend (in-plane)
-CH2 rock
-CH2 rock
-CH2 twist
-CH2 twist
Ring deformation
C-N stretch
-CH2 wag
Ring deformation
Ring breathing
-CH2 wag
Ring deformation
Ring-NC bend (in-plane)
Ring puckering
C-N≡C bend(in-plane)
-CH2 antisymmetric stretch
-CH2 antisymmetric stretch
-CH2 symmetric stretch
-CH2 symmetric stretch
-CH2 deformation
-CH2 deformation
-CH2 rock
ab
initio
3198
3184
3133
3126
3109
2156
1583
1560
1437
1375
1360
1279
1240
1106
1064
1014
931
909
788
505
500
267
137
3189
3172
3124
3121
1557
1545
1383
Fixed
scaledb
IR
int.
Raman
act.
3000
2987
2939
2932
2916
2144
1489
1468
1367
1307
1290
1219
1180
1055
1015
965
897
868
748
494
487
266
136
2992
2976
2931
2928
1465
1454
1315
41.3
1.3
24.8
23.4
3.3
69.6
1.3
7.5
18.2
0.5
0.9
1.8
1.8
6.2
9.0
1.5
3.0
0.7
3.1
0.1
4.4
0.1
3.8
10.6
7.1
19.3
8.0
3.8
0.9
0.2
49.0
117.7
166.7
46.8
120.1
95.2
8.5
7.3
7.2
6.7
3.2
7.0
8.4
6.4
2.7
3.0
4.0
17.1
0.3
1.3
3.0
2.6
2.1
68.0
13.3
25.8
9.3
4.9
21.4
6.1
gas
2980
2980
2931
2931
2918
2138
1485
1463
1358
1313
1287
1218
1180
1044
1003
970
896
869
749
480
474
2980
2980
2931
2931
1463
1455
1313
Xe
soln.
2976
2976
2930
2930
2917
2136
1482
1463
1358
1310
1287
1217
~1180
1042
1003
968
894
867
749
477
474
2976
2976
2930
2930
1463
1453
1310
Raman
solid
2976
2976
2934
2934
2920
2151/2138
1484
1465
1356
1310
1284
1221
1170
1036
999
961
898
877
755
484
476
2976
2976
2934
2934
1464
1452
1310
P.E.D.c
liquid
2973
2973
2931
2931
2916
2143
1482
1460
1359
1311
1287
1212
1184
1037
1003
965
893
866
746
481
475
271
2973
2973
2931
2931
1460
1452
1315
Band Contour
A
58S1,40S2
53S2,39S1
93S4
83S5,11S3
87S3,10S5
90S6,10S15
71S7,28S8
71S8,29S7
54S9,29S20
56S19,17S12,11S9
34S20,23S13,20S9,15S12
45S12,16S19,14S10,10S16
41S13,14S20
23S14,20S16,13S15,11S19,10S18
40S15,28S14,25S17
31S10,30S13
53S16,16S14
58S17,12S14,11S10
82S11
61S18,18S10
44S21,19S15,12S23,10S17
48S22,48S23
36S23,32S22,29S21
82S25,15S24
80S24,12S25
93S26
92S27
72S28,28S29
72S29,28S28
50S35,28S32
1
90
98
51
1
97
72
22
100
90
84
77
96
97
96
1
100
72
0
29
85
85
8
-
B
C
- 99
- 10
2
- 49
- 99
3
- 28
- 78
- 10
- 16
- 23
4
3
4
- 99
0
- 28
- 100
- 71
- 15
- 15
- 92
100
100
100
100
100
100
100
1378
1309
0.4
1.9
1310
1310
1310
1311 31S32,20S38,19S34,17S35
32 -CH bend (out-of-plane)
1333
1265
0.1
4.3
1266
1264
1260
1267 41S38,23S33,21S35,11S32
38 -CH2 rock
1297
1233
0.2
11.0
1233
1230
1221
1230 56S33,26S38
33 -CH2 twist
1233
1173
0.3
1.3
1179
1178
1170
1178 42S34,27S30,10S31
34 -CH2 twist
1155
1107
1.8
1.6
1096
1096
1097 42S36,16S39,16S37
36 Ring deformation
1010
966
1.3
2.7
970
968
961
967 33S37,26S36,16S34
37 Ring deformation
998
954
4.7
0.2
949
947
946
947 15S30,22S38,16S32,15S37,13S34
30 -CH2 wag
852
813
0.0
0.9
799 44S31,21S30,15S39,11S36
31 -CH2 wag
636
622
0.5
1.6
629
625
- 58S39,16S31
39 Ring deformation
457
454
2.2
0.5
444
444
442
443 51S40,32S41,11S37
40 Ring-NC bend (out-of-plane)
155
155
4.2
4.0
- 67S41,31S40
41 C-N≡C bend(out-of-plane)
43
43
0.1
0.0
- 93S42
42 Ring twisting
a
4
MP2(full)/6-31G(d) ab initio calculations, scaled frequencies, infrared intensities (km/mol), Raman activities (Å /u) and potential energy distributions
(P.E.D.s)
b
Scaled ab initio calculations with factors of 0.88 for CH 2 stretches and CH2 deformations, 0.90 for all other modes except torsions, heavy atom bends
and NC stretch.
c
Symmetry coordinates with P.E.D. contribution less than 10% are omitted.
-
100
100
100
100
100
100
100
100
100
100
100
100
-
344
Table 87: Calculated electronic energies (hartree) for the Ax (Cs) and energy differences (cm-1) for
Eq (Cs), Twist (C1) and Planar (Cs) forms of isocyanocyclopentane.
Twist (C1) c
Planar (Cs)
472
194
2632
0.841373
405
207
2579
180
1.092543
496
189
2657
MP2(full)/6-311+G(d,p)
208
1.099296
460
194
2612
MP2(full)/6-311G(2d,2p)
242
1.171390
421
223
2788
MP2(full)/6-311+G(2d,2p)
270
1.176539
430
223
2763
MP2(full)/6-311G(2df,2pd)
336
1.279727
398
229
2806
MP2(full)/6-311+G(2df,2pd)
364
1.284195
413
234
2784
MP2(full)/aug-cc-pVTZ
529
1.297561
372
186
2512
B3LYP/6-31G(d)
123
1.763988
-18
80
1760
B3LYP/6-31+G(d)
151
1.776886
-35
88
1678
B3LYP/6-311G(d,p)
180
1.840708
55
83
1748
B3LYP/6-311+G(d,p)
208
1.843920
17
90
1695
B3LYP/6-311G(2d,2p)
242
1.851564
16
81
1668
B3LYP/6-311+G(2d,2p)
270
1.854423
1
95
1646
B3LYP/6-311G(2df,2pd)
336
1.860152
1
81
1686
B3LYP/6-311+G(2df,2pd)
364
1.862786
-11
93
1654
B3LYP/aug-cc-pVTZ
529
1.871245
-15
95
1652
Method/Basis Set
# of Basis Sets
Ax (Cs)
MP2(full)/6-31G(d)
123
0.823376
MP2(full)/6-31+G(d)
151
MP2(full)/6-311G(d,p)
a
Eq (Cs)
Energy of conformer is given as – (E+287) H.
Difference is relative to Ax form and given in cm-1.
c
Ring parameters fixed due to optimization ending in an Ax conformer minim
b
345
Table 88: Structural Parameters (Å and Degree), Rotational Constants (MHz) and Dipole Moment
(Debye) for isocyanocyclopentane Ax and Eq (Cs) Forms.
Structural
Parameters
r N≡C
r Cα-N
r Cα-Cβ,Cβ′
r Cβ-Cγ, rCβ′-Cγ′
r Cγ-Cγ′
r Cα-H
r Cβ-H1, Cβ′-H1
r Cβ-H2, Cβ′-H2
r Cγ-H1, Cγ′-H1
r Cγ-H2, Cγ′-H2
Cα-N≡C
CβCα-N
CβCαCβ′
CαCβCγ
CβCγCγ′
HCα-N
HCαCβ
H1CβCα
H1CβCγ
H2CβCα
H2CβCγ
H1CβH2
H1CγCβ
H1CγCγ′
H2CγCβ
H2CγCγ′
H1CγH2
CβCαCβ′Cγ′
CβCγCγ′Cβ′
A(MHz)
B(MHz)
C(MHz)
|a|
|b|
|c|
|t|
Int.
coor.
R1
R2
R3
R4
R5
r1
r2
r3
r4
r5
1
2
1
2
3
ψ
δ
λ1
λ2
λ3
λ4
λ5
π1
π2
π3
π4
π5
1
2
MP2(full)/
6-311+G(d,p)
Ax
Eq
1.186
1.186
1.431
1.422
1.531
1.530
1.541
1.538
1.554
1.556
1.093
1.096
1.092
1.095
1.095
1.092
1.093
1.092
1.092
1.094
177.3
179.0
109.4
113.0
102.4
102.8
103.7
103.0
105.7
105.7
108.0
107.7
113.7
110.1
112.6
107.9
113.6
111.0
107.6
112.8
110.9
113.8
108.5
108.2
110.0
111.3
110.2
112.2
111.3
110.2
112.4
110.3
107.4
107.1
41.7
42.8
0.0
0.0
4255.6 6401.8
2391.5 1893.1
2236.1 1570.1
3.115
3.884
0.000
0.000
1.949
0.640
3.674
3.936
B3LYP/
6-311+G(d,p)
Ax
Eq
1.170
1.170
1.436
1.427
1.540
1.540
1.546
1.544
1.558
1.561
1.092
1.095
1.091
1.094
1.094
1.091
1.092
1.091
1.092
1.093
179.4
179.7
110.8
113.5
102.9
103.3
104.8
103.6
106.0
106.1
107.1
107.1
112.7
109.7
112.3
108.3
113.6
111.1
107.5
112.5
110.7
113.4
107.8
107.7
110.3
111.2
110.3
112.1
111.2
110.5
112.2
110.3
106.9
106.7
38.0
40.0
0.0
0.0
4379.0 6334.9
2284.9 1879.8
2094.9 1554.6
3.537
4.299
0.000
0.000
2.119
0.733
4.123
4.361
346
Adjusted r0
Predicted
Ax
1.176 (3)
1.432 (3)
1.534 (3)
1.542 (3)
1.554 (3)
1.092 (2)
1.092 (2)
1.095 (2)
1.093 (2)
1.092 (2)
177.8 (5)
110.4 (5)
102.9 (5)
103.6 (5)
105.9 (5)
108.0 (5)
112.5 (5)
112.6 (5)
115.9 (5)
107.5 (5)
108.4 (5)
108.4 (5)
110.0 (5)
109.6 (5)
111.3 (5)
112.6 (5)
107.4 (5)
40.7 (5)
0.0
4312.7954(7)
2348.0136(2)
2175.4439(2)
Eq
1.176
1.423
1.533
1.539
1.556
1.095
1.095
1.092
1.092
1.094
179.5
114.0
113.3
102.9
105.9
107.7
108.9
107.9
113.3
112.7
111.3
108.1
111.3
111.6
110.2
110.5
107.1
41.8
0.0
6426.9
1884.1
1562.0
Table 89: Symmetry Coordinates for Isocyanocyclopentane.
Description
A'
A"
-CH2 antisymmetric stretch
-CH2 antisymmetric stretch
-CH stretch
-CH2 symmetric stretch
-CH2 symmetric stretch
N≡C stretch
-CH2 deformation
-CH2 deformation
-CH bend (in-plane)
-CH2 wag
-CH2 wag
-CH2 twist
-CH2 twist
Ring deformation
C-N stretch
Ring deformation
Ring breathing
Ring deformation
-CH2 rock
-CH2 rock
Ring-NC bend (in-plane)
Ring puckering
C-N≡C linear bend (in-plane)
-CH2 antisymmetric stretch
-CH2 antisymmetric stretch
-CH2 symmetric stretch
-CH2 symmetric stretch
-CH2 deformation
-CH2 deformation
-CH2 wag
-CH2 wag
-CH bend (out-of-plane)
-CH2 twist
-CH2 twist
-CH2 rock
Ring deformation
Ring deformation
-CH2 rock
Ring deformation
Ring-NC bend (out-of-plane)
C-N≡C (out-of-plane)
Ring twisting
Symmetry Coordinate
S1
S2
S3
S4
S5
S6
S7
S8
S9
S10
S11
S12
S13
S14
S15
S16
S17
S18
S19
S20
S21
S22
S23
S24
S25
S26
S27
S28
S29
S30
S31
S32
S33
S34
S35
S36
S37
S38
S39
S40
S41
S42
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
347
r4 – r5 + r4′ – r5′
r2 – r3 + r2′ – r3′
r1
r4 + r5 + r4′ + r5′
r2 + r3 + r2′ + r3′
R1
π5 + π5′
λ5 + λ5′
δ + δ′ + 2 ψ
λ1 + λ2 – λ3 – λ4 + λ1′ + λ2′ – λ3′ – λ4′
π1 + π2 – π3 – π4 + π1′ + π2′ – π3′ – π4′
λ1 – λ2 – λ3 + λ4 + λ1′ – λ2′ – λ3′ + λ4′
π1 – π2 – π3 + π4 + π1′ – π2′ – π3′ + π4′
R3 + R4 + R3′ + R4′ – 4R5
R2
R3 – R4 + R3′ – R4′
R3 + R4 + R3′ + R4′ + R5
3θ1 – 2θ2 – 2θ3 + θ2′ + θ3′
π1 – π2 + π3 – π4 + π1′ – π2′ + π3′ – π4′
λ1 – λ2 + λ3 – λ4 + λ1′ – λ2′ + λ3′ – λ4′
2 + 2′
τ1 + τ1′
1
r2 – r3 – r2′ + r3′
r4 – r5 – r4′ + r5′
r4 + r5 – r4′ – r5′
r2 + r3 – r2′ – r3′
π5 – π5′
λ5 – λ5′
π1 + π2 – π3 – π4 – π1′ – π2′ + π3′ + π4′
λ1 + λ2 – λ3 – λ4 – λ1′ – λ2′ + λ3′ + λ4′
δ – δ′
π1 – π2 – π3 + π4 – π1′ + π2′ + π3′ – π4′
λ1 – λ2 – λ3 + λ4 – λ1′ + λ2′ + λ3′ – λ4′
π1 – π2 + π3 – π4 – π1′ + π2′ – π3′ + π4′
R3 – R4 – R3′ + R4′
R3 + R4 – R3′ – R4′
λ1 – λ2 + λ3 – λ4 – λ1′ + λ2′ – λ3′ + λ4′
θ2 – θ3 – θ2′ + θ3′
2 – 2′
τ4
τ1 – τ1′
Table 90: Comparison of frequencies (cm-1) of ring fundamentals for the Ax conformer of
molecules of the form c-C5H9-X.
A'
Ring deformation
Ring deformation
Ring breathing
Ring deformation
Ring puckering
A''
Ring deformation
Ring deformation
Ring deformation
Ring twisting
Isocyanocyclopentane
Predicted
This
Study
1050
1040
939
929
883
889
837
839
273
271
1003
905
650
64
1003
897
650
-
Cyanocyclopentane
Predicted Ref [135]
Ethynyl cyclopentane
Predicted Ref [142]
Bromocyclopentane
Predicted Ref [127]
Chlorocyclopentane
Predicted Ref [126]
1049
815
878
664
290
1038
812
884
298
1052
941
883
834
294
1041
937
893
803
282
1042
910
877
689
323
1038
910
884
690
310
1044
918
877
791
161
1030
914
889
805
185
1005
892
626
60
1004
893
633
-
1006
904
645
74
1002
895
629
-
1058
891
610
96
1069
890
613
115
1064
892
611
86
1067
903
617
87
348
Table 91: Temperature and intensity ratios of the Ax and Eq bands of isocyanocyclopentane.
Liquid
xenon
T(C)
1/T (10-3 K-1)
I407 / I444
I407 / I749
I407 / I1096
I711 / I444
I711 / I749
I711/ I1096
-65
-70
-75
-80
-85
-90
-95
-100
4.804
4.922
5.047
5.177
5.315
5.460
5.613
5.775
1.488
1.528
1.543
1.617
1.629
1.640
1.667
1.695
0.441
0.453
0.461
0.484
0.485
0.491
0.494
0.503
1.829
1.894
1.929
2.016
2.047
2.076
2.104
2.155
2.023
2.067
2.109
2.170
2.247
2.280
2.333
2.362
0.600
0.613
0.630
0.650
0.669
0.683
0.692
0.701
2.486
2.563
2.636
2.706
2.824
2.886
2.946
3.002
91  10
91  12
113  10
116  7
116  9
138  8
1/T (10-3 K-1)
I803 / I444
I803 / I749
I803 / I1096
I1080 / I444
I1080 / I749
I1080/ I1096
4.804
4.922
5.047
5.177
5.315
5.460
5.613
5.775
3.256
3.326
3.370
3.426
3.464
3.500
3.608
3.695
0.966
0.987
1.006
1.025
1.031
1.048
1.070
1.096
4.000
4.123
4.212
4.271
4.352
4.430
4.554
4.697
2.930
2.989
3.022
3.064
3.134
3.160
3.255
3.314
0.869
0.887
0.903
0.917
0.933
0.946
0.965
0.983
3.600
3.705
3.777
3.820
3.938
4.000
4.109
4.213

Ha
T(C)
Liquid
xenon
-65
-70
-75
-80
-85
-90
-95
-100
Ha
85  2
84  4
107  4
86  3
86  2
Average value ΔH = 102  6 cm-1 (1.22  0.07 kJ mol-1) with the Ax conformer the more
stable form and the statistical uncertainty (2σ) obtained by utilizing all of the data as a single
set.
a
349
109  3
Table 92: Comparison of select structural parameters (Å and Degree) of molecules of the form
CN-R.
CN-CH3
Structural
Pred.a
Ref[144] a
Parameters
1.183
1.1665
rNC
1.423
1.4266
rCα-NC
179.9
180.0
Cα-NC
a
MP2(full) 6-311+ G(d,p)
b
Electron Diffraction Structure
c
rs structural parameters
d
Adjusted r0 structural parameters
e
rs structural parameters
CN-C(H)=CH2
CN-CCH
a
Ref [145]
1.188
1.386
178.0
1.174(6)
1.379(6)
178.2(12)
Pred.
b
350
a
Ref[146]
1.192
1.317
180.0
1.176(1)
1.318(1)
Pred.
c-C3H5-NC
c
Pred.a
Ref [147]d
1.186
1.400
179.8
1.176(5)
1.377(8)
180.0
Table 93: Comparison of select structural parameters (Å and Degree) for the Ax conformer of
molecules of the form c-C5H9-XY.
c-C5H9-CC-H
Structural
MP2(full)/
Parameters
6-311+G(d,p)
rCα-Cβ,Cβ′
1.540
rCβ-Cγ, rCβ′-Cγ′
1.540
rCγ-Cγ′
1.553
111.0
CβCα-X
101.4
CβCαCβ′
103.8
CαCβCγ
105.6
CβCγCγ′
26.4
CαCβCγCγ′
0.0
CβCγCγ′Cβ′
a
Adjusted r0 parameters.
Ref [11]
a
1.542(3)
1.542(3)
1.555(3)
111.5(5)
102.1(5)
103.7(5)
105.9(5)
25.7(5)
0.0(5)
c-C5H9-CN
MP2(full)/
6-311+G(d,p)
1.539
1.540
1.553
110.5
101.8
103.8
105.7
25.9
0.0
351
Ref
[135]a
1.542(3)
1.542(3)
1.559(3)
111.5(5)
102.0(5)
103.6(5)
105.7(5)
25.8(5)
0.0
c-C5H9-NC
MP2(full)/
6-311+G(d,p)
1.531
1.541
1.554
109.4
102.4
103.7
105.7
25.6
0.0
This Studya
1.534 (3)
1.542 (3)
1.554 (3)
110.4 (5)
102.9 (5)
103.6 (5)
105.9 (5)
25.0 (5)
0.0 (5)
Figure 53: Conformers of isocyanocyclopentane (A) Eq (B) Ax.
352
Figure 54: Comparison of experimental and calculated infrared spectra of isocyanocyclopentane: (A)
observed spectrum of gas; (B) observed spectrum of Xe solution at -70°C; (C) observed spectrum of
solid; (D) simulated spectrum of a mixture of Ax and Eq conformers (ΔH = 102 cm-1) at 25°C; (E)
simulated spectrum of Eq conformer; (F) simulated spectrum of Ax conformer.
353
Figure 55: Comparison of experimental and calculated Raman spectra of isocyanocyclopentane: (A)
observed spectrum of liquid; (B) simulated spectrum of a mixture of Ax and Eq conformers (ΔH =
102 cm-1) at 25°C; (C) simulated spectrum of Eq conformer; (D) simulated spectrum of Ax
conformer.
354
Figure 56: Infrared spectra of isocyanocyclopentane (A) gas; (B) Xe solution at -70°C.
355
Figure 57: Temperature (-70 to -100°C) dependent infrared spectrum of isocyanocyclopentane
dissolved in liquid xenon solution.
356
Figure 58: Band contour predictions of Ax and Eq conformer of isocyanocyclopentane.
357
CHAPTER 14
MICROWAVE, INFRARED, AND RAMAN SPECTRA, STRUCTURAL PARAMETERS,
VIBRATIONAL ASSIGNMENTS AND THEORETICAL CALCULATIONS OF 1,3DISILACYCLOPENTANE
Introduction
Considerable interest in the structure of cyclopentane resulted when it was proposed that
the two low frequency bending modes could have nearly equal frequencies which resulted in a
vibration that was described as pseudorotation [149]. However the concept of this motion for
cyclopentane was not readily accepted particularly when the low frequency ring mode of this
molecule appeared normal [150]. Also there was further reluctance to accept the pseudorotation
motion of the puckering motion with the consequent indefiniteness of the mode [151, 152];
however after two decades evidence was reported [153] from the CH2 deformation of cyclopentane
that the ring was undergoing pseudorotation which was nearly barrier free.
With the acceptance of pseudorotation there was a number of investigations on
conformations of mono substituted cyclopentane molecules and the determination of the most
stable structure. Some of the earliest conformational determinations were the halides, i.e. bromine,
chlorine and fluorine. In the initial investigation [124, 128] of these three molecules it was reported
that all three had the envelope-equatorial conformer as the most stable forms in the fluid phases. A
second conformer was also identified in the fluid phases as the envelope-axial form. However from
later studies [129] it was demonstrated that there was only one conformer present in the fluid
phases of fluorocyclopentane and it was the envelope-equatorial form. Much later it was shown
that the one form was not the envelope conformer but the twisted form [131]. Also it should be
noted that incorrect conformational structures for five membered rings with another species, such
as nitrogen instead of CH2 have been initially reported [154, 155] as the envelope form. From a
358
later vibrational infrared study [156] it was shown that the conformer was the twisted form. Thus it
is not easy to predict what the conformational structure will be for many different types of five
membered rings. The conformational and structural studies of five membered rings continue to be
of scientific interest because of these uncertainties.
Of particular interest are heterocyclic five membered ring molecules many of which are
important biomolecules. The five membered heterocyclic rings of silicon and carbon have not been
significantly studied. Therefore, we recently began to investigate a number of mono-substituted 1silacyclopentane compounds [157-159] of the form c-C4H8SiH-X where X has been several
halogens. From these studies it was found that the twist conformer with no symmetry was the most
stable form. However, only for the X = F molecule were the adjusted r 0 structural parameters
determined. The 1,3- silacyclopentane compounds of the form c-C3H6(SiX2)2 where X= H and
perhaps other halogens has largely not been studied due to difficulties in the synthesis of these
compounds. The only previous publications of this compound are several synthesis studies [160163] with only basic characterizations of these molecules. These compounds are of significant
interest as the change from carbon to silicon significantly changes the physical and physiochemical
properties of these molecules where the carbon analogue is normally stable in the equatorial and
axial conformers. However, the single silicon heterocycle molecule is [160] the twist form with
only one stable conformer, whereas the structure of the double silicon molecule has not previously
been determined. Therefore, we have initiated microwave, infrared and Raman spectroscopy
studies to determine the stable conformer or conformers, the structural parameters, and vibrational
fundamental frequencies which we expect to compare to the corresponding carbon compound.
To support the experimental studies we have obtained harmonic force constants, infrared
intensities, Raman activities, depolarization ratios, and frequencies of the vibration fundamentals
from MP2(full)/6-31G(d) ab initio calculations. To obtain predictions on the conformational
359
stabilities we have carried out MP2(full) ab initio and density functional theory (DFT) calculations
by the B3LYP method by utilizing a variety of basis sets. The r0 structural parameters have been
obtained by combining the MP2(full)/6-311+G(d,p) ab initio predicted parameters with the
determined rotational constants obtained from this study. The results of these spectroscopic,
structural, and theoretical studies of 1,3-disilacyclopentane (c-C3H6Si2H4) are reported herein.
Experimental and Theoretical Methods
The c-C3H6Si2H4 compound was prepared according to the method of Laane (personal
communication) with modifications. The sample was first purified by trap-to-trap and was also
further purified by a low-temperature, low-pressure fractionation column. The purity of the sample
was checked by infrared and nuclear magnetic resonance spectroscopy.
The rotational spectrum of c-C3H6Si2H4 was obtained by using a CP-FTMW spectrometer
developed at the University of Virginia, operating in the 6.5 to 18 GHz range. The chirped pulse
methods used in this study have been described in detail previously [20], so only the brief details
relevant to this experiment are necessary.
The microwave source was a 24 GS/s arbitrary waveform generator, producing a 12-0.5 GHz
linear frequency sweep in 1 μs. The pulse was upconverted to 6.5-18 GHz by a 18.95 GHz phaselocked resonant dielectric oscillator (PDRO), and then amplified by a pulsed 300 W traveling wave
tube amplifier. The amplified pulse is then transmitted through free space between two standardgain microwave horns, where it interacts with a molecular beam generated by five pulsed nozzles
(General Valve Series 9) operating perpendicular to the propagation direction of the microwave
pulse. On the detection end, the receiver is protected from the high power pulse by a combination
of a PIN diode limiter and single-pole microwave switch. The resulting molecular free induction
decay (FID) was then amplified and digitized directly on a 100 GS/s oscilloscope with 33 GHz of
hardware bandwidth, with a 20 μs detection time per FID. Due to the speed of this excitation and
360
detection process, a sequence of 10 excitation/detection cycles is possible per gas pulse, and all ten
detected FIDs are collected and averaged together before the next valve injection cycle begins.
Phase stability of this experiment over the course of many valve injection cycles is enabled by
locking all the frequency sources and the oscilloscope to a 10 MHz Rb-disciplined quartz
oscillator. For this experiment, approximately 78 000 valve injection cycles of the sample gas were
completed at 3.3 Hz to create a time-averaged spectrum of 780 000 molecular FIDs (approximately
6.5 hours of averaging). Additionally, the time domain resolution afforded by a 20 μs FID
generates an average Doppler broadened linewidth of approximately 130 kHz at FWHM.
The sample for spectral investigation was prepared by balancing c-C3H6Si2H4 vapor with
approximately 3.4 atm of Ne gas (GTS Welco) for a total sample concentration of approximately
0.1%. This afforded a frequency-domain dynamic range of approximately 4000:1 at 780 000
averages, which enabled assignment of all common heavy atom single isotopologues (13C,
30
Si) in natural abundance as well as a double isotopologue (29Si /
30
29
Si,
Si). These assignments are
listed in Tables 94-96 with an experimental uncertainty of approximately 20 kHz (line centers
determined to ±10 kHz). The experimental analysis of c-C3H6Si2H4 was supplemented with ab
initio electronic structure calculations for rotational constant and centrifugal distortion predictions
(Table 97). These calculations were performed with the Gaussian 09 suite of program [164].
The infrared spectrum of the gas (Fig. 59) was obtained from 4000 to 220 cm-1 on a PerkinElmer model 2000 Fourier transform spectrometer equipped with a Ge/CsI beamsplitter and a DTGS
detector. Atmospheric water vapor was removed from the spectrometer housing by purging with dry
nitrogen. The spectrum of the gas was obtained with a theoretical resolution of 0.5 cm-1 with 128
interferograms added and truncated.
Raman spectrum (Fig. 60) of the liquid was collected in back-scattering geometry using the
514.532 nm line of an Argon ion laser as the excitation source, with ~ 22 mW incident on the
361
diamond cell. A Semrock 514 nm edge filter was used to separate the laser line from the Raman
scattered light. The scattered light was dispersed in a Spectra-pro 500i spectrograph and detected
with a Spec-10 liquid nitrogen cooled CCD detector. Raman spectra were collected by using a
2400 g/mm grating with a slit width of 100 μm, which gives spectral resolution of ~ 0.2 cm-1. The
observed bands in the infrared spectrum of the gas and Raman spectrum of the liquid along with
their proposed assignments are listed in Table 98.
The ab initio calculations were performed with the Gaussian 03 program [21] by using
Gaussian-type basis functions. The energy minima with respect to nuclear coordinates were
obtained by the simultaneous relaxation of all geometric parameters by the gradient method of
Pulay [22]. A variety of basis sets as well as the corresponding ones with diffuse functions were
employed with the Møller-Plesset perturbation method [23] to the second order MP2 with full
electron correlation as well as with density functional theory by the B3LYP method. The predicted
conformational energy differences are listed in Table 99.
The predicted scaled frequencies were used together with a Lorentzian function to obtain
the simulated spectra. Infrared intensities were obtained based on the dipole moment derivatives
with respect to Cartesian coordinates. The derivatives were transformed with respect to normal

coordinates by (u/Qi) = j (u/Xj)Lij, where Qi is the ith normal coordinate, Xj is the jth
Cartesian displacement coordinate, and Lij is the transformation matrix between the Cartesian
displacement coordinates and the normal coordinates. The infrared intensities were then calculated
by (N)/(3c2) [(x/Qi)2 + (y/Qi)2 + (z/Qi)2]. A comparison of the experimental infrared
spectrum of the gas and simulated infrared spectrum of the isolated twist conformer is shown in
Fig. 59.
362
Additional support for the vibrational assignments was obtained from the simulated Raman
spectra. The evaluation of Raman activity by using the analytical gradient methods has been
developed [25-27, 37] and the activity Sj can be expressed as: Sj = gj(45αj2 + 7βj2), where gj is the
degeneracy of the vibrational mode j, αj is the derivative of the isotropic polarizability, and βj is the
anisotropic polarizability. To obtain the Raman scattering cross sections, the polarizabilities are
incorporated into Sj by multiplying Sj with (1-ρj)/ (1+ρj) where ρj is the depolarization ratio of the
jth normal mode. The Raman scattering cross sections and calculated wavenumbers obtained from
the Gaussian 03 program were used together with a Lorentzian function to obtain the simulated
Raman spectra. A comparison of the experimental Raman spectrum of the liquid and predicted
Raman spectrum of the isolated twist form are shown in Fig. 60. This comparison is reasonable as
the average difference in band center due to going from gas to liquid is 3 cm-1, and thus, there is
little interaction between molecules in the liquid.
Microwave Results
An overview of the fit rotational parameters for c-C3H6Si2H4 and the comparison to ab
initio results can be found in Table 97. The spectrum was satisfactorily fit (Tables 94-96) using a
standard Watson semi-rigid rotor Hamiltonian of the A-reduction type in the Ir representation
[165], with 3 kHz RMS error, better than the experimental uncertainty of approximately 20 kHz
(line centers determined to ±10 kHz). As mentioned previously, the high dynamic range afforded
in this experiment enabled assignment of all common heavy atom single isotopologue. Due to
symmetry considerations, the silicons are spectroscopically equivalent, as well as the carbons in
the Si-C-C-Si backbone. This equivalence leads to doubling in intensity in the spectra of these
isotopologue. A summary of these isotopologue fits can be found in Table 97.
This kind of isotopic determination enables direct structure determination of the heavy
atom structure via Kraitchman’s equations [166]. The Kraitchman rs substitution structure,
363
calculated using the freely available KRA program [167] and it is in reasonable agreement with the
MP2/6-311++G(d,p) results.
Structural Parameters
With accurate rotational constants the adjusted r0 structure can be determined for the twist
conformer by utilizing the rotational constants reported herein from the microwave spectra. The
rotational constants for five isotopic species were determined in the current study and are listed in
Table 97. Therefore, 15 rotational constants are available for the determination of the structural
parameter values of the twist conformer of c-C3H6Si2H4.
We [44] have shown that ab initio MP2(full)/6-311+G(d,p) calculations predict the carbonhydrogen r0 structural parameters for more than fifty hydrocarbons to at least 0.002 Å compared to
the experimentally determined [45] values from isolated CH stretching frequencies which agree to
previously determined values from earlier microwave studies. Therefore, all of the carbonhydrogen parameters can be taken from the MP2(full)/6-311+G(d,p) predicted values for the twist
conformer of c-C3H6Si2H4. The silicon-hydrogen r0 structural parameter values can be
experimentally determined [168] from isolated SiH stretching frequencies. These values are listed
in Table 100 as determined from the assignments in Tables 98.
We have found that good structural parameters for hydrocarbons and many substituted ones
can be determined by adjusting the structural parameters obtained from the ab initio MP2(full)/6311+G(d,p) calculations to fit the rotational constants obtained from microwave experimental data
by using a computer program “A&M” (Ab initio and Microwave) developed [57] in our
laboratory. Therefore, it should be possible to obtain “adjusted r0” structural parameters for the
fourteen parameters, taking the C-H angles as a single set, by utilizing the experimentally
determined fifteen rotational constants obtained from the microwave spectra reported in this study.
Therefore we have obtained the complete structural parameters for the twist form of c-C3H6Si2H4.
364
The resulting adjusted r0 parameters are listed in Table 100, where the precisions are listed.
It is believed that these precisions are probably smaller than the accuracy this method can achieve,
and therefore, for the C-Si and C-C distances the value listed should be accurate to ± 0.002 Å, the
C-H and Si-H distances accurate to ± 0.002 Å, and the angles should be within ± 0.4. The fit of
the fifteen determined rotational constants (Table 101) by the adjusted r0 structural parameters for
the twist conformer is excellent with the largest difference of 0.18 MHz and an average of 0.06
MHz for the fits. Therefore, it is believed that the suggested uncertainties are realistic values and
the determined structural parameters are probably as accurate as can be obtained for the molecule
in the gas phase by either electron diffraction or microwave substitution methods.
Vibrational Assignment
In order to obtain descriptions of the molecular motions involved of the fundamental modes
of c-C3H6Si2H4, a normal coordinate analysis was carried out. The force field in Cartesian
coordinates was obtained with the Gaussian 03 program at the MP2(full) level with the 6-31G(d)
basis set. The internal coordinates used to calculate the G and B matrices are given for the twist
conformer in Table 100 with the atomic numbering shown in Fig. 61. By using the B matrix [24],
the force field in Cartesian coordinates was converted to a force field in internal coordinates.
Subsequently, 0.88 was used as the scaling factor for the CH stretches, the SiH stretches, and the
CH2 deformations and 0.90 was used for all other modes to obtain the fixed scaled force constants
and the resulting wavenumbers. A set of symmetry coordinates was used (Table 102) to determine
the corresponding potential energy distributions (P.E.D.s). A comparison between the observed
and calculated wavenumbers, along with the calculated infrared intensities, Raman activities,
depolarization ratios and P.E.D.s for the twist conformer of c-C3H6Si2H4 are given in Table 98.
The major reason for carrying out a vibrational assignment of c-C3H6Si2H4 is to determine
the frequencies of the fundamentals of the heavy atoms of the ring which should aid in future
365
studies of the corresponding five membered rings with more complex additions. The vibrations of
the CH2 group are expected to be very similar to those found in the usual five membered rings with
just carbon atoms so a discussion of their assignments is not necessary. Therefore, the assignments
of the nine fundamentals of the ring will be provided first and this will be followed by the
vibration assignments involving the SiH2 modes.
The heavy atom modes of the five membered rings result in a total of nine fundamentals
with five of those in the ‘A’ block and the remaining four in the ‘B’ block. Five membered rings
containing carbons have ring deformations occurring around 1000 cm-1 and the second ring
deformation is usually around 200 cm-1 lower. For the molecule with the two silicon atoms
these modes are at 963 cm-1 and significant lower at 660 cm-1 in the Raman spectra of the liquid.
The ring breathing mode is observed at about the same frequency as the second deformation when
the ring has carbon atoms and for the ring with the silicon atoms this mode is nearly the same
frequency as the second deformation but both of them are about 158 cm-1 lower than those for the
molecule with carbon.
For the vibrations in ‘B’ block the two ring deformations are fairly close in frequency at
728 cm-1 and 663 cm-1 with the third ring deformation at 334 cm-1 in the infrared spectra of the gas.
The corresponding mode in the ‘A’ block is predicted at nearly the same frequency i.e. 4 cm -1
difference so it is assumed that they are accidently degenerate. It should be noted that for this
mode in the ‘A’ block it has a predicted IR intensity of 0.0 and a Raman activity of 3.2 whereas for
this mode in the ‘B’ block the predicted IR intensity is 3.7 and the Raman activity is 0.4. The
twisting mode and ring puckering modes are predicted at 189 cm-1 and 55 cm-1 and are assigned at
184 and 50 cm-1, respectively, from the Raman spectra with both 0.0 predicted Raman intensity.
The four SiH2 stretch modes are observed in the expected frequency region with those in
the ‘A’ and ‘B’ blocks with the same frequencies. Also the two SiH2 deformations are predicted to
366
have similar frequencies of 947 ‘A’ and 936 ‘B’ cm-1 and they are relatively pure modes.
However, the predicted intensities of these two modes are significantly different with infrared
intensities of 6.3 ‘A’ and 213.1 ‘B’ km/mol and Raman activities of 28.1 ‘A’ and 2.4 ‘B’ Å4/u.
These relative Raman activities make assignment of the ‘A’ mode very simple where the Raman
band at 955 cm-1 in the liquid is clearly the ν12 ‘A’ block SiH2 symmetric deformation
fundamental. The ‘B’ block fundamental is significantly more complex as there are two clear Atype fundamentals that appear in this region at 996 and 945 cm-1 in the infrared spectra of the gas,
both of which have the same intensity and band contour. The band at 996 cm-1 is clearly not a band
from impurities and is one of the three most intense bands in the spectra. These bands at 996 and
945 cm-1 appear to be due to a Fermi resonance of the ν30 ‘B’ SiH2 antisymmetric deformation
fundamental with a combination band in near coincidence. The two SiH2 wags have significantly
different frequencies of 899 ‘A’ and 825 ‘B’ cm-1 where their P.E.D.s are 50 and 59% but the
remaining contributions are different. The two SiH2 rocks are assigned at 503 and 491 cm-1,
respectively, in the infrared spectra of the gas. However, the two SiH2 twists are predicted to have
bands separated by more than 100 cm-1 and they are observed at 609 and 715 cm-1 with the 715
cm-1 ‘B’ fundamental with only 36% contributions to this mode. This large difference is likely due
to this significantly different mixing of these two fundamentals. With these assignments the
remaining vibration modes are CH2 modes which are easily assigned so the assignments for this
molecule are concluded.
Discussion
The ab initio energy differences of the possible forms have been determined by a variety of
basis sets (Table 99). The MP2(full) method consistently predicts the twist form as the most stable
conformer and the planar and envelope forms as not being stable species with all the basis sets and
with only relatively small changes due to basis set selection. The B3LYP method agrees with the
367
MP2(full) method as the identity of the most stable form but the magnitude of the energy differences
are approximately two-thirds that of the MP2(full) method. However the B3LYP calculations still
show the twist conformer as being the only stable form.
The vibrational assignments reported herein are based on a significant amount of
information with the infrared and Raman spectrum of the xenon solution and predictions of the
fundamental frequencies from the scaled ab initio MP2(full)/6-31G(d) calculations as well as the
predicted infrared band contours and their intensities. For the twist conformer the fixed scaled ab
initio predicted frequencies for the ‘A’ block fundamentals are in error with an average value of 11
cm-1 which represent an error of 0.80% and for the ‘B’ block modes it is 10 cm-1 which is 0.77%
error. The largest errors are in the ring deformations, which includes the ring breathing mode, and
the SiH2 bending modes. It should also be noted that these fundamentals are associated with the
structural parameters which show the largest changes when comparing the ab initio MP2(full)/6311+G(d,p) predicted structural parameters to the corresponding r0 structures. It appears that ab
initio calculations have some difficulties with predicting these parameters. Those errors are clearly
observable but not necessarily so significant that the predictions lose value. Thus, the relatively
small basis set of 6-31G(d) by the MP2(full) calculations with two scaling factors provides
excellent predicted frequencies for aiding the vibrational assignments.
The identity of the combination band which interacts with the ν30 fundamental and
produces the Fermi resonance at 996 and 945 cm-1 were determined by a few logical deductions.
First the combination band must be of ‘B’ symmetry and in near coincidence to the ν30
fundamental energy level. Therefore, the combination band would have to be an ‘A’ + ‘B’
combination which gives a ‘B’ symmetry band. The energy of the ν30 fundamental would be 940
cm-1 determined from the predicted difference from the ab initio calculation applied to the 952 cm 1
assignment of the ν12 fundamental. This energy would be in near coincidence with the 943 ν38
368
(334) + ν17 (609) cm-1 combination band, which matches all the necessary criteria to cause the
afore mentioned Fermi resonance band. The remaining possible combination bands are much less
likely with energies above 990 or below 900 cm-1. Therefore, this is the most logical candidate for
the Fermi resonance bands at 996 and 945 cm-1.
This Fermi resonance account for the two A-type bands at 996 and 945 cm-1 which
otherwise could not be accounted for and are very good examples of A-type band contours.
Furthermore, the band at 899 cm-1 is an excellent example of a B-type band which aided
considerably in the assignment of the spectra. The remaining bands in the spectra are somewhat
nondescript Q-branches or broad bands indicating B-type but without the characteristic dip
frequently associated with this band type. The Si-H fundamentals at 2152 and 2141 cm-1 are
,however, interesting as they are accompanied by the difference and combination bands at 2099
and 2089 cm-1 and 2212 and 2200 cm-1, respectively. These differences and combination bands
may be used to determine a frequency of 53 cm-1 for the ring puckering mode, which is observed
in the Raman spectra of the liquid at 50 cm-1. These data can be used along with a reduced mass
calculation to determine ring puckering potential function which is part of the conformational
interchange of the molecule.
It is of interest to compare the vibrational fundamentals and their assignments to similar
molecules with corresponding fundamentals. This comparison is very limited in scope though for
c-C3H6Si2H4 as there are no other 1,3-disilacyclopentanes vibrational assignments in the literature.
The closest molecules to c-C3H6Si2H4 where vibrational data has been published are 1,3disilacyclopent-4-ene (c-CH=CHSi2H4CH2) and 1,1,3,3-tetrachloro-1,3-disilacyclopent-4-ene (cCH=CHSi2Cl4CH2) which have the planar ring conformation (C2v) due to the double bond
connecting the back carbons. These molecules were studied [163] with the Raman spectra of the
liquid for both samples and the infrared spectra of the vapor for the 1,3-disilacyclopent-4-ene
369
sample. It is of interest to compare and contrast the SiH2 modes of c-C3H6Si2H4 with the
corresponding modes in c-CH=CHSi2H4CH2 where the SiH2 stretching modes in both assignments
are only ~3 cm-1 up-shifted by the proximity to the double bond indicating there is little change in
the Si-H bond distance between these two molecules. The SiH2 deformation (o.p.) and SiH2
deformation (i.p.) modes are assigned at 999 and 948 cm-1, respectively, for c-CH=CHSi2H4CH2.
The SiH2 deformation (i.p.) mode is similar in description and energy to the SiH2 symmetric
deformation assigned at 952 cm-1 for c-C3H6Si2H4. However, the SiH2 deformation (o.p.) mode is
similar in description to the SiH2 antisymmetric deformation but is considerably different in energy
even when the Fermi resonance of the SiH2 antisymmetric deformation band is taken into account.
This difference is probably largely due to the change from planar to twist conformation and the
resulting change from C2v to C2 symmetry.
Next, the SiH2 wag (A1) for c-CH=CHSi2H4CH2 is similar in description to the ν13
fundamental described as a SiH2 wag (A) for c-C3H6Si2H4 molecule and the bands for each are
assigned at 868 and 899 cm-1, respectively. This 31 cm-1 difference is probably due to the
significant mixing of the ν13 fundamental which is only 50% SiH2 wag (A) and 30% β-CH2 rock.
This mixing is actually related to going from a planar (C2v) form to a twist (C2) conformation as
this mixing is not possible in the planar (C2v) form. This is due to the symmetry constraints of the
planar conformation where the two planes of symmetry divide these modes between two different
symmetry blocks. No further SiH2 fundamentals can be compared as the c-CH=CHSi2H4CH2 study
[163] did not assign the remaining 3 SiH2 bending modes along with several other A2 symmetry
block modes. This is most likely due to the A2 symmetry block not being infrared active. Finally, it
is interesting to note the ring puckering modes for both molecules are very similar in energy at 48
and 53 cm-1 for c-CH=CHSi2H4CH2 and c-C3H6Si2H4, respectively. This is somewhat unexpected
since the puckering angle for the twist form of c-C3H6Si2H4 is only 11.5. It would be of interest to
370
compare the vibrational studies of other 1,3-disilacyclopentane molecules to evaluate how
substitution affects the vibrational modes and to emphasize more ring modes which can be
compared for these unique heterocyclic five-membered rings.
The rs structural parameters determined by using the Kraitchman method can be compared
with the corresponding parameters from the adjusted r0 structural parameters, and the values are
listed in Table 100. The rs Si-C bond distance parameters match the r0 structural parameters within
error limits, but it is somewhat meaningless as the rs parameters have very large uncertainties. The
rs angles SiCαSi and SiCβCβ′, are within their respective error limits as compared to the
corresponding adjusted r0 structural parameters and these parameters are well determined by both
methods. The determined dihedral angles CβSiCαSi and SiCβCβ′Si are within the stated error
limits to the adjusted r0 structural parameters, but it is again somewhat meaningless as the rs
parameters again have very large uncertainties. These large uncertainties are due to the problem of
the Kraitchman method when one or more of the atoms lie on a rotational axis. The remaining C βCβ parameter which can be determined has a small uncertainty and is within the combined error
limits of the rs and adjusted r0 parameters. When the rs structural parameters have small
uncertainties they agree well with the corresponding adjusted r0 structural parameters.
The natural population analysis was carried out for the twist conformer of c-C3H6Si2H4
with the MP2(full)6-311+G(d,p) calculation. The charge of the heavy atoms are; Cα = -1.36, Si =
+1.30, Cβ = -0.82, and
Si-H
= -0.22. The large charge on the silicon atom is of interest and could
counteract some of the larger radius of the silicon atom than a carbon atom. These relatively large
charges could also explain the larger than normal difference between the ab initio frequencies for
the heavy atom bends and the experimental values, where normally no scaling factor is used a
scaling factor of 0.9 had to be used for this molecule. Additionally, the atoms with charges of
greater than one have significant differences between the predicted and adjusted r0 structural
371
parameters and the major structural changes for the adjusted r0 structure are the Si-C and Si-H
parameters. These quantum calculation problems with the silicon atoms are due to their large
charges while being a neutral atom and are well known.
There is little information known about substituted cyclopentanes with silicons in the ring
and no previously published structures for the disilacyclopentane molecules have been reported.
Further study of these types of molecules would be of interest with several different substituents on
the silicons so comparisons of these molecules structures, vibrational assignments, and
conformational stabilities may be carried out.
372
Table 94: Rotational transition frequencies (MHz) of the ground vibrational state of c-C3H6Si2H4.
Transitions
νobs
31 2 ← 30 3
6029.5499
11 1 ← 00 0
6355.8855
53 2 ← 52 3
7271.7928
22 1 ← 21 2
7438.3389
63 3 ← 62 4
7609.4435
52 3 ← 51 4
7697.0649
20 2 ← 11 1
7796.2244
43 1 ← 42 2
7883.8794
42 3 ← 33 0
8765.6491
33 0 ← 32 1
8847.0955
32 2 ← 31 3
9038.6740
41 3 ← 40 4
9163.4355
84 4 ← 83 5
9613.0200
74 3 ← 73 4
9839.0175
33 1 ← 32 2
10202.2062
21 2 ← 10 1
10232.3159
62 4 ← 61 5
10780.1754
43 2 ← 42 3
10945.4142
64 2 ← 63 3
10961.8345
53 3 ← 44 0
11040.7921
31 2 ← 22 1
11056.0749
42 3 ← 41 4
11161.2729
53 3 ← 52 4
12218.9057
54 1 ← 53 2
12263.4777
42 2 ← 33 1
12380.1696
30 3 ← 21 2
12464.8663
51 4 ← 50 5
12642.8522
44 0 ← 43 1
13167.9785
44 1 ← 43 2
13633.0508
31 3 ← 20 2
13780.9194
54 2 ← 53 3
13783.4088
63 4 ← 62 5
14050.0729
64 3 ← 63 4
14280.1935
72 5 ← 71 6
14437.3442
22 1 ← 11 0
15191.2031
22 0 ← 11 1
16463.7296
40 4 ← 31 3
16765.1093
41 3 ← 32 2
16889.8728
65 2 ← 64 3
17262.7051
41 4 ← 30 3
17329.8483
a
 = obs calc in MHz.
373
Δνa
0.002
-0.002
0.001
-0.003
-0.005
-0.001
0.004
-0.004
0.002
-0.001
-0.002
0.000
0.000
-0.002
-0.001
0.000
-0.001
0.000
-0.002
-0.001
0.004
-0.003
0.000
0.004
0.007
0.002
0.000
0.009
0.008
-0.001
0.010
0.007
0.007
-0.003
-0.003
-0.006
-0.010
-0.006
-0.013
0.006
Table 95: Rotational transition frequencies (MHz) of the ground vibrational state of c-C3H6Si2H4.
29
30
Si
Transitions
a
Si
νobs
Δνa
Transitions
νobs
Δνa
11 1  00 0
6335.7803
-1
11 1  00 0
6316.6031
-1
53 2  52 3
7392.4986
4
52 3  51 4
7473.4399
-7
22 1  21 2
7477.9138
2
22 1  21 2
7516.4809
0
52 3  51 4
7580.1960
2
20 2  11 1
7558.8852
-1
63 3  62 4
7626.8853
-5
43 1  42 2
8210.8749
4
20 2  11 1
7675.3853
1
32 2  31 3
9046.1478
2
43 1  42 2
8049.7378
2
33 0  32 1
9160.1439
9
33 0  32 1
9006.9139
-3
21 2  10 1
10127.7041
0
41 3  40 4
9024.6961
2
33 1  32 2
10409.0807
5
32 2  31 3
9041.9601
-3
42 3  41 4
11081.4784
-8
21 2  10 1
10178.9173
3
43 2  42 3
11089.4188
6
33 1  32 2
10307.2952
1
30 3  21 2
12175.8957
-4
31 2 22 1
10789.0408
1
31 3  20 2
13614.9723
2
43 2  42 3
11017.9372
6
22 1  11 0
15138.6816
-5
42 3  41 4
11119.9144
0
41 3  32 2
16284.8693
27
30 3  21 2
12318.0859
4
22 0  11 1
16344.5281
-10
54 1  53 2
12513.7769
-3
40 4  31 3
16439.3333
-20
44 0  43 1
13375.2004
1
41 4  30 3
17086.2790
2
31 3  20 2
13696.1474
2
22 1  11 0
15164.1798
-5
22 0  11 1
16402.3518
-4
41 3  32 2
16582.3751
-4
40 4  31 3
16599.6488
1
41 4  30 3
17205.3214
1
 = obs calc in kHz.
374
Table 96: Rotational transition frequencies (MHz) of the ground vibrational state of c-C3H6Si2H4.
13
13
Cβ
Transitions
a
Cα
νobs
Δνa
Transitions
νobs
Δνa
11 1 ← 00 0
6264.2017
4
11 1 ← 00 0
6260.1739
15
20 2 ← 11 1
7791.5602
2
20 2 ← 11 1
7823.7598
6
21 2 ← 10 1
10105.9171
1
21 2 ← 10 1
10105.5467
8
30 3 ← 21 2
12402.3824
1
54 1 ← 53 2
11626.1524
-2
31 3 ← 20 2
13623.2458
-10
30 3 ← 21 2
12433.5927
5
22 1 ← 11 0
14950.8533
5
31 3 ← 20 2
13626.1160
10
22 0 ← 11 1
16245.0166
-4
22 0 ← 11 1
16245.0166
-4
41 4 ← 30 3
17151.2461
4
40 4 ← 31 3
16670.7003
-20
41 4 ← 30 3
17161.3089
-1
 = obs calc in kHz.
375
Table 97: Experimental rotational and centrifugal distortion constants of the ground vibrational
state of c-C3H6Si2H4 isotopologues.
MP2(full)/
29
30 a
6c-C3H6Si2H4
Si
Si
311++G(d,p)
A (MHz)
4427.8194
4417.6710(7)
4414.2128(7)
4411.0525(10)
B (MHz)
2860.4216
2887.0548(6)
2851.5540(8)
2817.5636(8)
C (MHz)
1927.4735
1938.2171(6)
1921.5697(6)
1905.5525(6)
ΔJ (kHz)
0.40
0.41(2)
0.43(2)
[0.41]
ΔJK (kHz)
-0.16
-0.17(3)
-0.11(6)
[-0.17]
ΔK (kHz)
0.89
1.04(3)
0.87(6)
[1.04]
0.110
0.118(4)
0.13(1)
[0.118]
J (kHz)
0.41
0.44(3)
0.38(6)
[0.44]
K (kHz)
Nb
40
24
18
3
2
7
fit (kHz)c
a
Distortion constants for all isotopologue are held fix to the normal species values.
b
Number of frequencies fitted.
c
Total RMS error of the observed-calculated frequencies for all N lines in fit.
376
13
Cβa
4343.3367(14)
2877.0556(19)
1920.8618(6)
[0.41]
[-0.17]
[1.04]
[0.118]
[0.44]
8
3
13
Cαa
4337.4673(25)
2887.1237(25)
1922.6925(11)
[0.41]
[-0.17]
[1.04]
[0.118]
[0.44]
9
6
Table 98: Observed and calculateda frequencies (cm-1) and potential energy distributions (P.E.D.s) for the twist (C2) conformer of cC3H6Si2H4.
Sym.
Block
A
377
B
Vib.
No.
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
ab
initio
3155
-CH2 antisymmetric stretch
α-CH2 symmetric stretch
3134
3095
-CH2 symmetric stretch
SiH2 antisymmetric stretch
2289
SiH2 symmetric stretch
2281
1525
-CH2 symmetric deformation
α-CH2 deformation
1479
1318
-CH2 twist
1147
-CH2 wag
α-CH2 twist
1045
Ring deformation
1013
SiH2 symmetric deformation
998
SiH2 wag
949
884
-CH2 rock
Ring deformation
681
Ring breathing
668
SiH2 twist
633
SiH2 rock
519
Ring deformation
346
Ring twist
190
α-CH2 antisymmetric stretch
3195
3161
-CH2 antisymmetric stretch
3097
-CH2 symmetric stretch
SiH2 antisymmetric stretch
2289
SiH2 symmetric stretch
2277
-CH2 antisymmetric deformation 1515
1303
-CH2 antisymmetric wag
α-CH2 wag
1101
1090
-CH2 twist
SiH2 antisymmetric deformation
986
SiH2 antisymmetric wag
872
α-CH2 rock
831
Approximate Description
fixed
scaledb
2960
2940
2903
2172
2163
1434
1390
1251
1090
992
963
947
901
841
649
638
602
494
340
189
2997
2965
2905
2172
2160
1424
1238
1045
1035
936
831
791
IR
int.
11.1
0.5
5.4
23.4
0.7
6.4
10.0
2.5
0.1
6.6
0.3
6.3
268.8
109.4
0.3
0.6
0.1
0.1
0.0
0.2
0.9
14.7
18.6
358.4
317.7
8.3
3.3
74.1
44.1
213.1
63.5
62.6
Raman
act.
112.4
85.7
145.0
219.8
226.2
8.2
11.9
3.8
3.6
3.8
5.8
28.1
1.5
3.6
5.9
26.8
3.2
5.1
3.2
0.0
66.0
74.5
34.8
14.6
7.0
19.1
0.3
3.4
3.5
2.4
11.4
9.8
IR
Gas
2940
2935
2905
2152
2142
1424
1364
1234
1000
952
899
833
655
609
503
334
2969
2946
2905
2152
2142
1416
1221
1038
1027
996/945
825
781
Raman
liquid
2933
2924
2893
2155
2144
1426
1361
1235
1070
1000
963
955
~899
835
660
656
613
503
332
184
2964
2939
2895
2155
2144
1411
1038
1027
993/945
823
777
contour
A
B
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
2
42
38
23
79
2
100
87
78
96
97
5
-
C
98
58
62
77
21
98
13
22
4
3
95
P.E.D.c
90S1,10S3
100S2
90S3,10S1
64S4,36S5
64S5,36S4
100S6
100S7
53S8,32S9,13S14
49S9,28S8,15S11
84S10
63S11,10S16,10S12,11S9
88S12,10S11
50S13,30S14
23S14,25S13,17S17,13S15,10S19,10S8
46S15,17S16,13S13,12S14,10S17
62S16,13S19,11S11
60S17,17S15,10S14,10S10
70S18,10S19,10S16,10S14
72S19,11S18,10S15
93S20
100S21
98S22
98S23
94S24
94S25
100S26
80S27, 10S38, 10S29
53S28,31S29
51S29,35S28
98S30
60S31,20S36,14S38
33S32,28S34,17S37,10S39,10S36
758
721
17.9
12.2
728
727 100
42S33,23S35,12S31,10S34,10S28
33 Ring deformation
741
704
35.1
8.6
715
714
13
87
36S34,25S36,15S33,10S38,10S37
34 SiH2 twist
685
658
19.1
5.6
663
668
80
20
42S35,25S38,19S31
35 Ring deformation
587
560
8.9
3.5
558
556
54
46
19S36,26S33,21S35,15S34,13S32,11S37
36 -CH2 rock
SiH
rock
515
491
15.0
0.5
491
494
14
86
39S37,42S32,10S38,10S34
37
2
353
344
3.7
0.4
334
332
32
68
39S38,15S37,15S36,12S39
38 Ring deformation
55
55
2.4
0.0
56*
50
7
93
79S39,14S38
39 Ring puckering
a
MP2(full)/6-31G(d) ab initio calculations, scaled frequencies, infrared intensities (km/mol), Raman activities (Å 4/u) and potential energy distributions
(P.E.D.s)
b
Scaled ab initio calculations with factors of 0.88 for CH 2 stretches and CH2 deformations, 0.90 for all other modes.
*
Ref [163].
c
Symmetry coordinates with P.E.D. contribution less than 10% are omitted.
378
Table 99: Calculated energies in Hartrees (H) and energy differences (cm-1) for the three
conformers of c-C3H6Si2H4.
Method/Basis Set
MP2(full)/6-31G(d)
MP2(full)/6-31+G(d)
MP2(full)/6-31G(d,p)
MP2(full)/6-31+G(d,p)
MP2(full)/6-311G(d,p)
MP2(full)/6-311+G(d,p)
MP2(full)/6-311G(2d,2p)
MP2(full)/6-311+G(2d,2p)
MP2(full)/6-311G(2df,2pd)
MP2(full)/6-311+G(2df,2pd)
MP2(full)/aug-cc-pVTZ
B3LYP/6-31G(d)
B3LYP/6-31+G(d)
B3LYP/6-31G(d,p)
B3LYP/6-31+G(d,p)
B3LYP/6-311G(d,p)
B3LYP/6-311+G(d,p)
B3LYP/6-311G(2d,2p)
B3LYP/6-311+G(2d,2p)
B3LYP/6-311G(2df,2pd)
B3LYP/6-311+G(2df,2pd)
B3LYP/aug-cc-pVTZ
a
Energya, E
Twist
0.858570
0.869926
0.942927
0.953429
1.309829
1.313460
1.387107
1.389146
1.471870
1.473414
1.353673
2.340691
2.347209
2.354676
2.360873
2.428704
2.429736
2.439413
2.439994
2.446767
2.447263
2.451245
Energy Differences, ∆
Envelopeb
Planarb
941
1391
859
1292
950
1381
877
1294
921
1267
889
1260
927
1423
916
1393
924
1508
912
1492
935
1579
689
940
699
932
671
926
683
923
685
913
687
905
672
923
667
914
673
928
670
925
679
936
Energy of conformer is given as –(E+697) H.
Difference is relative to twist form and given in cm-1.
b
379
Table 100: Structural parameters (Å and degrees), rotational constants (MHz) and dipole moments
(Debye) for twist conformer c-C3H6Si2H4.
Structural
Int.
6-311+G(d,p)
6-311++G(d,p)
rs
Adjusted
Parameters
coor.
r0a
MP2(full)
B3LYP
MP2
B3LYP
rCα-Si
R1
1.891
1.902
1.893
1.902
1.886(15)
1.8856(1)
rSi-Cβ,Cβ′
R2
1.892
1.905
1.895
1.905
1.8847(66)
1.8884(4)
rCβ-Cβ′
R3
1.550
1.554
1.552
1.554
1.5481(40)
1.5515(4)
rSi-H1
R4
1.482
1.490
1.482
1.490
1.4880(1)*
rSi-H2
R5
1.480
1.489
1.481
1.489
1.4860(1)*
rCα-H
r1
1.094
1.093
1.094
1.093
1.0938(1)
rCβ-H1, Cβ′-H1
r2
1.098
1.097
1.098
1.097
1.0977(1)
rCβ-H2, Cβ′-H2
r3
1.095
1.094
1.095
1.094
1.0946(1)
104.5
104.5
104.6
104.5
103.84(20)
103.87(6)
SiCαSi
1
101.6
102.1
101.6
102.1
102.20(6)
CαSiCβ
2
106.7
107.6
106.8
107.6
106.51(37)
106.37(6)
SiCβCβ′
3
110.2
110.3
110.3
110.3
110.343(1)
CαSiH1
1
113.5
113.3
113.3
113.3
113.053(1)
CαSiH2
2
109.7
110.3
109.8
110.4
109.451(1)
CβSiH1
3
113.1
112.7
113.1
112.6
113.143(1)
CβSiH2
4
108.5
108.1
108.6
108.1
108.519(1)
 H1SiH2
5
λ1
111.2
111.5
111.2
111.1
111.46(2)
HCαSi
λ2
107.1
107.0
107.0
107.1
107.19(2)
HCαH
λ
108.8
108.5
108.8
108.4
108.818(4)
H1CβSi
3
λ4
113.0
112.6
113.0
112.5
113.037(4)
H2CβSi
λ5
109.7
109.9
109.7
109.9
110.016(4)
H1CβCβ′
λ6
111.9
111.9
111.8
111.9
111.935(4)
H2CβCβ′
λ
106.7
106.4
106.6
106.4
106.645(4)
H1CβH2
7
11.4
10.5
11.3
10.5
13.8(17)
11.46(3)
CβSiCαSi
1
45.7
42.4
45.4
42.4
45.25(86)
45.60(3)
SiCβCβ′Si
2
A(MHz)
4429.38
4361.66
4416.85
4361.12
4417.76
B(MHz)
2859.11
2824.45
2848.85
2824.79
2887.12
C(MHz)
1926.63
1891.26
1919.11
1891.43
1938.27
0.000
0.000
0.000
0.000
|a|
0.146
0.247
0.146
0.243
|b|
0.000
0.000
0.000
0.000
|c|
a
Adjusted parameters using the microwave data reported in this study; Si-H distance determined from the isolated
stretching frequency.
380
Table 101: Comparison of rotational constants (MHz) obtained from experimental values from
microwave spectra, and from the adjusted r0 structural parameters for c-C3H6Si2H4.
Isotopomer
Normal Species
29
Si
30
Si
13
Cα
13
Cβ
Rotational
constant
A
B
C
A
B
C
A
B
C
A
B
C
A
B
C
Experimental
4417.6710(7)
2887.0548(6)
1938.2171(6)
4414.2128(7)
2851.5540(8)
1921.5697(6)
4411.0525(10)
2817.5636(8)
1905.5525(6)
4337.4673(25)
2887.1237(25)
1922.6925(11)
4343.3367(14)
2877.0556(19)
1920.8618(6)
381
Adjusted
r0
4417.7765
2887.1194
1938.2759
4414.3074
2851.5317
1921.5868
4411.1363
2817.4596
1905.5304
4337.4049
2887.1194
1922.6450
4343.1530
2877.0777
1920.8820
||
0.1055
0.0646
0.0588
0.0946
-0.0223
0.0171
0.0838
-0.1040
-0.0221
-0.0624
-0.0043
-0.0475
-0.1837
0.0221
0.0202
Table 102: Symmetry coordinates of c-C3H6Si2H4.
Description
-CH2 antisymmetric stretch
α-CH2 symmetric stretch
-CH2 symmetric stretch
SiH2 asymmetric stretch
SiH2 symmetric stretch
-CH2 deformation
α-CH2 symmetric deformation
-CH2 twist
-CH2 wag
α-CH2 twist
ring deformation
SiH2 symmetric deformation
SiH2 wag
-CH2 rock
ring deformation
ring breathing
SiH2 twist
SiH2 rock
ring deformation
ring twist
B
α-CH2 antisymmetric stretch
-CH2 antisymmetric stretch
-CH2 symmetric stretch
SiH2 asymmetric stretch
SiH2 symmetric stretch
-CH2 asymmetric deformation
-CH2 wag
α-CH2 wag
-CH2 twist
SiH2 asymmetric deformation
SiH2 wag
α-CH2 rock
ring deformation
SiH2 twist
ring deformation
-CH2 rock
SiH2 rock
ring deformation
ring puckering
a
Not normalized.
A
S1
S2
S3
S4
S5
S6
S7
S8
S9
S10
S11
S12
S13
S14
S15
S16
S17
S18
S19
S20
S21
S22
S23
S24
S25
S26
S27
S28
S29
S30
S31
S32
S33
S34
S35
S36
S37
S38
S39
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
Symmetry Coordinatea
r2′ - r3′ + r2 - r3
r1 + r1′
r2′ + r3′ + r2 + r3
R4′ - R5′ + R4 - R5′
R4′ + R5′ + R4 + R5′
λ7 + λ7′
λ2
λ4 - λ3 – λ6 + λ5 + λ4′ - λ3′ - λ6′ + λ5′
λ3′ + λ4′ – λ5′ - λ6′ + λ4 + λ3 - λ6 - λ5
λ2 – λ1 – λ2′ + λ1′
R1 + R1′ - R2 – R2′
5′ + 5
2′ + 1′ - 4′ - 3′ + 1 + 2 - 3- 4
λ3′ + λ5′ - λ4′ - λ6′ - λ6 - λ4 + λ5 + λ3
4θ1 - θ2 - θ2′ - θ3 – θ3′
R1 + R2 + R3 + R2′ + R1′
1 - 2 - 3 + 4 + 1′ - 2′ - 3' + 4′
2′ + 4′ - 1′ - 3′ - 1 - 3 + 2 + 4
4R3 – R1 - R1′ - R2 – R2′
1+ 2
r1 - r1′
r2′ - r3′ - r2 + r3
r2′ + r3′ - r2 - r3
R4′ - R5′ - R4 + R5′
R4′ + R5′ - R4 - R5′
λ7 - λ7′
λ3′ + λ4′ - λ5′ - λ6′ - λ4 - λ3 + λ6 + λ5
λ2 + λ1 – λ2′ - λ1′
λ4 - λ3 - λ6 + λ5 - λ4′ + λ3′ + λ6′ - λ5′
5′ - 5
2′ + 1′ - 4′ - 3′ - 1 - 2 + 3+ 4
λ2 - λ1 + λ2′ - λ1′
θ2 - θ2′ + θ3 - θ3′
1 - 2 - 3 + 4 - 1′ + 2′ + 3' - 4′
R1 - R1′ + R2 - R2′
λ3′ + λ5′ - λ4′ - λ6′ + λ6 + λ4 - λ5 - λ3
2′ + 4′ - 1′ - 3′ + 1 + 3 - 2 - 4
R1 - R1′ - R2 + R2′
1- 2
382
Figure 59: Comparison of experimental and calculated infrared spectra of c-C3H6Si2H4: (A) observed
spectrum of gas; (B) simulated spectrum of twist conformer.
383
Figure 60: Comparison of experimental and calculated Raman spectra of c-C3H6Si2H4: (A) observed
spectrum of liquid; (B) simulated spectrum of twist conformer.
384
Figure 61: Model of c-C3H6Si2H4 showing atomic numbering
385
CHAPTER 15
MICROWAVE, INFRARED, AND RAMAN SPECTRA, STRUCTURAL PARAMETERS,
VIBRATIONAL ASSIGNMENTS AND THEORETICAL CALCULATIONS OF 1,1,3,3TETRAFLUORO-1,3-DISILACYCLOPENTANE
Introduction
The saturated five-membered cyclic hydrocarbon ring molecule (cyclopentane) has two
“out-of-plane” vibrational modes which are usually described as ring-puckering and ring-twisting
modes. When the frequencies of these two modes are nearly equal, the cross terms in the potential
function give rise to a vibrational motion which was initially described by Pitzer and colleagues
as pseudorotation [122]. This motion was treated by these scientists for the two degenerate out-ofplane ring bending coordinates in terms of an amplitude coordinate q and a phase angle ø.
However this concept for cyclopentane was questioned [150] when the low frequency ring mode
of this molecule appeared normal but the fundamentals could not be assigned on the basis that
cyclopentane had D5h symmetry. The authors [150] stated that the spectral data were consistent
with a rigid structure with Cs, C2, or C1 symmetry. Nevertheless these scientists [150] concluded
that a decision among these three rigid models as well as pseudorotation could not be made. There
was further reluctance [151, 152] to accept the pseudorotation of the puckering motion and the
consequent indefiniteness of the cyclopentane conformation persisted. However, a later infrared
study [153] of the CH2 deformation of cyclopentane clearly showed that the ring was undergoing
pseudorotation which was nearly barrier free. Also from this study an estimate of the value of the
pseudorotational moment [169] of inertia was obtained from the spectral data.
With the acceptance of the pseudorotation motion there was a number of investigations on
the possible conformations of mono substituted cyclopentane molecules and the determination of
the most stable structures. Even before the acceptance of pseudorotation motion was reported in
386
the scientific literature [123] Pitzer and his coworkers reported how one could predict the most
stable structure of such molecules. Some of the earliest conformational determinations of the
mono substituted cyclopentane molecules was for the halides i.e. bromine, chlorine and fluorine.
In the initial studies [124, 128] of these molecules it was concluded that all three had two
conformers present in the fluid phases with the equatorial form the most stable. Later studies
[129] of fluorocyclopentane demonstrated that there was only one form in the fluid phases and it
was the envelope-equatorial forms. However several years later [131] it was shown correctly that
there was only one form but it was not the envelope conformer but was the twisted form. There
has been other reported incorrect conformational structures for five membered rings so it is not
easy to predict what the conformational structure will be for many different types of five
membered rings.
Recently some investigations of five membered rings [157-159] were initiated where
instead of all five ring atoms being carbon one of them has been replaced by a silicon atom. There
has been some limited studies of these molecules whether they are in the equatorial, axial or
twisted forms or even possibly a planar form. As a continuation of these studies we have
expanded the atoms in the ring instead of a single silicon atom to two silicon atoms. For the first
molecule we studied [170] with two silicon atoms in the ring, it included an oxygen atom
between them instead of the carbon atoms since this molecule was available. For this study we
were interested in the determination of the stable conformer which was found to be the twisted
form as well as interested in the vibrational assignment of the fundamentals of the ring atoms. As
a continuation of this initial study of the five membered rings with two silicon atoms we initiated
a study with a carbon atom between the silicon atoms along with fluorine atoms on them i.e.
1,1,3,3-tetrafluoro-1,3-disilacyclopentane (c-C3H6Si2F4). With the fluorine atoms on the silicon
atoms having largest electronegativity it could have a significant effect on the conformational
387
stability of the heavy atom ring and the structural parameters. Additionally a very small fluorine
atom should not have any steric effect. From this study we expected to determine the
conformational form of the heavy atoms as well as all of the structural parameters of the
molecule, and the frequencies of the fundamental vibrations. Additionally we carried out ab initio
calculations to evaluate how well relatively small basis sets can predict the conformational
stability and structural parameters of the five membered ring. The results of this microwave,
infrared and Raman spectroscopic study along with the ab initio predicted values are reported
herein.
Experimental and Theoretical Methods
The c-C3H6Si2F4 compound was prepared by the fluorination of 1,1,3,3-tetrachloro-1,3dilacyclopentane [163] by freshly sublimed antimony trifluoride without solvent at room
temperature for three hours. The sample was first purified by trap-to-trap distillation and finally by
low pressure, low temperature sublimation. The purity of the sample was checked by infrared and
nuclear magnetic resonance spectroscopy.
The rotational spectrum of c-C3H6Si2F4 was studied by using a CP-FTMW spectrometer
developed at the University of Virginia, operating in the 6.5 to 18 GHz range. The chirped pulse
methods used in this study have been described in detail previously [20], so only the brief details
relevant to this experiment are necessary.
The microwave source was a 24 GS/s arbitrary waveform generator, producing a 12-0.5 GHz
linear frequency sweep in 1 μs. The pulse was upconverted to 6.5-18 GHz by a 18.95 GHz phaselocked resonant dielectric oscillator (PDRO), and then amplified by a pulsed 300 W traveling wave
tube amplifier. The amplified pulse is then transmitted through free space between two standardgain microwave horns, where it interacts with a molecular beam generated by five pulsed nozzles
(General Valve Series 9) operating perpendicular to the propagation direction of the microwave
388
pulse. On the detection end, the receiver is protected from the high power pulse by a combination
of a PIN diode limiter and single-pole microwave switch. The resulting molecular free induction
decay (FID) was then amplified and digitized directly on a 100 GS/s oscilloscope with 33 GHz of
hardware bandwidth, with a 20 μs detection time per FID. Due to the speed of this excitation and
detection process, a sequence of 10 excitation/detection cycles is possible per gas pulse, and all ten
detected FIDs are collected and averaged together before the next valve injection cycle begins.
Phase stability of this experiment over the course of many valve injection cycles is enabled by
locking all the frequency sources and the oscilloscope to a 10 MHz Rb-disciplined quartz
oscillator. For this experiment, approximately 78 000 valve injection cycles of the sample gas were
completed at 3.3 Hz to create a time-averaged spectrum of 780 000 molecular FIDs (approximately
6.5 hours of averaging). Additionally, the time domain resolution afforded by a 20 μs FID
generates an average Doppler broadened linewidth of approximately 130 kHz at FWHM.
The sample for spectral investigation was prepared by balancing c-C3H6Si2F4 vapor with
approximately 3.4 atm of Ne gas (GTS Welco) for a total sample concentration of approximately
0.1%. This afforded a frequency-domain dynamic range of approximately 4000:1 at 780 000
averages, which enabled assignment of all common heavy atom single isotopologues (13C,
30
Si) in natural abundance as well as a double isotopologue (29Si /
30
29
Si,
Si). These assignments are
listed in Tables 103-105 and the experimental analysis of c-C3H6Si2F4 was supplemented with ab
initio electronic structure calculations for rotational constant and centrifugal distortion predictions
(Table 106). These calculations were performed with the Gaussian 09 suite of programs [164].
The infrared spectrum of the gas (Fig. 62) was obtained from 4000 to 220 cm-1 on a PerkinElmer model 2000 Fourier transform spectrometer equipped with a Ge/CsI beamsplitter and a DTGS
detector. Atmospheric water vapor was removed from the spectrometer housing by purging with dry
389
nitrogen. The spectrum of the gas was obtained with a theoretical resolution of 0.5 cm-1 with 128
interferograms added and truncated.
Raman spectrum (Fig. 63) of the liquid was collected in back-scattering geometry using the
514.532 nm line of an Argon ion laser as the excitation source, with ~ 22 mW incident on the
diamond cell. A Semrock 514 nm edge filter was used to separate the laser line from the Raman
scattered light. The scattered light was dispersed in a Spectra-pro 500i spectrograph and detected
with a Spec-10 liquid nitrogen cooled CCD detector. Raman spectra were collected using a 2400
g/mm grating with a slit width of 100 μm, which gives spectral resolution of ~ 0.2 cm-1. The
observed bands in the infrared spectrum of the gas and Raman spectrum of the liquid along with
their proposed assignments are listed in Table 107.
Additional ab initio calculations were performed with the Gaussian 03 program [21] by using
Gaussian-type basis functions. The energy minima with respect to nuclear coordinates were
obtained by the simultaneous relaxation of all geometric parameters by the gradient method of
Pulay [22]. A variety of basis sets as well as the corresponding ones with diffuse functions were
employed with the Møller-Plesset perturbation method [23] to the second order MP2 with full
electron correlation as well as with density functional theory by the B3LYP method. The predicted
conformational energy differences are listed in Table 108.
In order to obtain descriptions of the molecular motions involved of the fundamental modes
of c-C3H6Si2F4, a normal coordinate analysis was carried out. The force field in Cartesian
coordinates was obtained with the Gaussian 03 program at the MP2(full) level with the 6-31G(d)
basis set. The internal coordinates used to calculate the G and B matrices are given for the twist
conformer in Table 108 with the atomic numbering shown in Fig. 64. By using the B matrix [24],
the force field in Cartesian coordinates was converted to a force field in internal coordinates.
Subsequently, 0.88 was used as the scaling factor for the CH stretches, the SiH stretches, and the
390
CH2 deformations and 0.90 was used for all other modes to obtain the fixed scaled force constants
and the resulting wavenumbers. A set of symmetry coordinates was used (Table 109) to determine
the corresponding potential energy distributions (P.E.Ds). A comparison between the observed and
calculated wavenumbers, along with the calculated infrared intensities, Raman activities,
depolarization ratios and P.E.D.s for the twist conformer of c-C3H6Si2F4 are given in Table 107.
The predicted scaled frequencies were used together with a Lorentzian function to obtain
the simulated spectra. Infrared intensities were obtained based on the dipole moment derivatives
with respect to Cartesian coordinates. The derivatives were transformed with respect to normal
coordinates by (u/Qi) =
 ( /X )L , where Q
j
u
j
ij
i
is the ith normal coordinate, Xj is the jth
Cartesian displacement coordinate, and Lij is the transformation matrix between the Cartesian
displacement coordinates and the normal coordinates. The infrared intensities were then calculated
by (N)/(3c2) [(x/Qi)2 + (y/Qi)2 + (z/Qi)2]. A comparison of the experimental infrared
spectrum of the gas and simulated infrared spectrum of the isolated twist conformer is shown in
Fig. 62.
Additional support for the vibrational assignments was obtained from the simulated Raman
spectra. The evaluation of Raman activity by using the analytical gradient methods has been
developed[25-27, 37] and the activity Sj can be expressed as: Sj = gj(45αj2 + 7βj2), where gj is the
degeneracy of the vibrational mode j, αj is the derivative of the isotropic polarizability, and βj is the
anisotropic polarizability. To obtain the Raman scattering cross sections, the polarizabilities are
incorporated into Sj by multiplying Sj with (1-ρj)/ (1+ρj) where ρj is the depolarization ratio of the
jth normal mode. The Raman scattering cross sections and calculated wavenumbers obtained from
the Gaussian 03 program were used together with a Lorentzian function to obtain the simulated
Raman spectra. A comparison of the experimental Raman spectrum of the liquid and predicted
391
Raman spectrum of the isolated twist form are shown in Fig. 63. This comparison is reasonable as
the average difference in band center due to going from gas to liquid is 3 cm -1, and thus, there is
little interaction between molecules in the liquid.
Microwave Results
An overview of the fitted rotational parameters for c-C3H6Si2F4 and the comparison to ab
initio results can be found in Table 106. The spectrum was fit by using a standard Watson semirigid rotor Hamiltonian of the A-reduction type in the Ir representation with the individual
transitions fit as shown in Tables 103-105. As mentioned previously, the high dynamic range
afforded in this experiment enabled assignment of all common heavy atom single isotopologues, as
well as a double silicon isotopologue. Due to symmetry considerations, the silicons are equivalent,
as well as the carbons in the Si-C-C-Si backbone. This equivalence leads to doubling in intensity in
the spectra of these isotopologues, as well as with the double 29Si – 30Si isotopologue. A summary
of these isotopologue fits can be found in Table 106. The fit also includes centrifugal distortion
constants shown in Table 106. The fit is reasonable and they are in satisfactory agreement with the
ab initio predicted centrifugal distortion constants. Utilization of a larger basis set would be
beneficial.
The Kraitchman rs substitution structure, calculated via Kraitchman’s equations [166] by
using the freely available KRA program [167] are good agreement with the MP2/6-311++g(d,p)
results. Since the ring is nearly planar in the ac plane, the determination of a nonzero b coordinate
of the Si-C-C-
two types of carbons, confirms
the twisted conformation over the planar form. Although some rs coordinates could not be
determined due to their close proximity to one of the principal axes, the derived rs structural
parameters are in general in good agreement with the ab initio structure. A full summary of these
parameters, for all of the bond lengths and angles, can be found in Table 110.
392
Structural Parameters
The adjusted r0 structural parameters can be determined for the twist conformer by utilizing
the rotational constants reported herein from the microwave spectra. The rotational constants for
six isotopic species were determined in the current study and they are listed in Table 106 with their
respective fits in Tables 103-105; therefore, 18 rotational constants are available for the
determination of the structural parameter values of the twist conformer of c-C3H6Si2F4.
We [44] have shown that ab initio MP2(full)/6-311+G(d,p) calculations predict the carbonhydrogen r0 structural parameters for more than fifty hydrocarbons to at least 0.002 Å compared to
the experimentally determined [45] values from isolated CH stretching frequencies which agree to
previously determined values from earlier microwave studies. Therefore, all of the carbonhydrogen parameters can be taken from the MP2(full)/6-311+G(d,p) predicted values for the twist
conformer of c-C3H6Si2F4.
We have found that good structural parameters for hydrocarbons and many substituted ones
can be determined by adjusting the structural parameters obtained from the ab initio MP2(full)/6311+G(d,p) calculations to fit the rotational constants obtained from microwave experimental data
by using a computer program “A&M” (Ab initio and Microwave) developed [57] in our
laboratory. In order to reduce the number of independent variables, the structural parameters are
separated into sets according to their types where bond distances in the same set keep their relative
ratio, and bond angles and torsional angles in the same set keep their difference in degrees. This
assumption is based on the fact that errors from ab initio calculations are systematic. It also should
be noted that the C2 symmetry reduces the number of independent parameters. Therefore, it should
be possible to obtain “adjusted r0” structural parameters for the 16 independent parameters, taking
the C-H distances as a single set, by utilizing the 18 experimentally determined rotational constants
393
listed in Table 106. Therefore we have obtained the complete structural parameters for the twist
form of c-C3H6Si2F4.
The resulting adjusted r0 parameters are listed in Table 110, where the precisions are listed.
It is believed that these precisions are probably smaller than the accuracy this method can achieve,
and therefore, for the C-Si and Si-F distances the value listed should be accurate to ± 0.002 Å, the
C-C distance accurate to ± 0.003 Å, the C-H distances accurate to ± 0.002 Å, and the angles should
be within ± 0.3. The fit of the 18 determined rotational constants (Table 111) by the adjusted r0
structural parameters for the twist conformer is excellent with the differences being less than 0.04
MHz. Thus, it is believed that the suggested uncertainties are realistic values and the determined
structural parameters are probably as accurate as can be obtained for the molecule in the gas phase
by either electron diffraction or microwave substitution methods where all of the atoms are
substituted.
Vibrational Assignment
The major reason for carrying out a vibrational assignment of c-C3H6Si2F4 is to determine
the effect of the fluorine atoms on the fundamentals. It is expected that the fluorine atoms will
have the largest effect on the vibrations involving silicon atoms of the ring which is expected to
provide modes that are strongly mixed with the ring vibrations. Additionally the ring parameters
are expected to be affected since they would be quite different from those found in silicon atoms
compared to the ones involving non polar groups [170]. The vibrations of the CH2 group are
expected to be very similar to those found in the usual five membered rings and so a discussion of
their assignments is not necessary. Therefore the assignments of nine fundamentals of the ring will
be provided first and this will be followed by the vibrations involving the SiF2 modes.
The heavy atom modes of the five membered rings result in a total of nine fundamentals
with five of those in ‘A’ block and the remaining four in the ‘B’ block. Five membered rings
394
containing carbons have ring deformations occurring around 1000 cm-1. The second ring
deformation is usually around 200 cm-1 lower and for our molecule we have frequencies (800 cm-1)
which are consistent with the one observed in silicon. Ring breathing is observed at 650 cm-1 with
deformations at 214 cm-1 and the twist at 92 cm-1. For the vibrations in ‘B’ block it has the two
ring deformations fairly close in frequency at 696 cm-1 and 657 cm-1 with the third ring
deformation at 416 cm-1. The ring puckering mode is predicted at 33 cm-1 but this vibration is not
observed. It is rather interesting that the frequencies for most of the ring modes of c-C3H6Si2F4 are
very similar to those of the silicon containing oxygen atom molecule [170] substituted with non
polar methyl groups.
With the assignments of ring vibrations completed the other modes of interest are the ones
of the SiF2 group. In the ‘A’ block the antisymmetric stretch is observed at 970 cm-1 and the
symmetric one at 897 cm-1 and the corresponding vibration in ‘B’ block are slightly lower in
frequency at 936 cm-1 and 848 cm-1, respectively. The SiF2 deformations in the ‘A’ block is
observed at 801 cm-1 and the other deformation at 282 cm-1 which is considerably lower. The other
SiF2 bending modes are mainly the wag, rock and the twist which are observed at 298 cm-1, 257
cm-1 and 151 cm-1 and in the the ‘B’ block the corresponding vibrations have similar frequencies
of 319 cm-1, 254 cm-1 and 197 cm-1 respectively.
These assignments are relatively easy to make since the predicted frequencies from ab
initio calculations in most cases, have little questions concerning their frequencies even though
P.E.D.s are somewhat different. This effect due to presence of SiF 2 groups will be further
addressed in the discussion.
Discussion
The ab initio energy differences of the possible forms have been determined by a variety of
basis sets (Table 107). The MP2(full) method consistently predicts the twist form as the most stable
395
conformer and the planar and envelope forms as not being stable conformers with all the basis sets
and with only relatively small changes due to basis set selection. The B3LYP method agrees with
the MP2(full) method as to the identity of the most stable form but the magnitude of the energy
differences are approximately half that of the MP2(full) method. However the B3LYP calculations
still shows the twist conformer as being the only stable form.
The vibrational assignments reported herein are based on a significant amount of
information with the infrared and Raman spectrum of the xenon solution and predictions of the
fundamental frequencies from the scaled ab initio MP2(full)/6-31G(d) calculations as well as the
predicted infrared bands contours and their intensities. For the twist conformer the fixed scaled ab
inito predicted frequencies for the ‘A’ block fundamentals are in error with an average value of 9
cm-1 which represent 0.87% error and for the ‘B’ block modes it is 5 cm-1 which is 0.43% error.
The largest errors are in the SiF2 deformations and several of the ring deformations which includes
the ring breathing mode. This is interesting as these modes show significant mixing with each
other which is likely due to the large masses of the SiF2 groups. Thus, the relatively small basis set
of 6-31G(d) by the MP2(full) calculations with two scaling factors provides excellent predicted
frequencies for aiding the vibrational assignments.
In the ‘A’ symmetry block there are only two modes which have less than 40%
contribution from the vibrational description used. These are the low frequency ν17 and ν18. For ν17
the largest contribution is 33% of S17 (SiF2 rock) with the other two significant contributions from
the SiF2 wag and deformation. However for ν18 (ring deformation) there is only 23% S18 with 41%
S15 for the SiF2 deformation which is assigned at 420 cm-1 with 40% S15 and 37% S18 the ring
deformation. Thus there is significant mixing of the SiF2 vibrational modes with ring vibrations as
might be expected with the similar mass values.
396
In the ‘B’ block there are five modes (ν30 through ν34) which have less than 40% for the
main contribution and only one of them (ν33) which has 29% S33 has a greater contribution from
another mode (36% S30). These five modes are three ring deformations and two CH2 rocks. This is
not surprising that these fundamentals are extensively mixed. Nevertheless the vibrational
assignments were relatively easy to make with confidence.
The rs structural parameters determined using the Kraitchman method can be compared
with the corresponding parameters from the adjusted r0 structural parameters, and both of these
structures can be found in Table 110. The rs Si-C bond distance parameters match the r0 structural
parameters within error limits, but it is somewhat meaningless as the rs parameters have very large
uncertainties. These large uncertainties are clearly due to the problem the Kraitchman method has
determining parameters in which one or more atoms lie on a rotational axis. The rs angle structural
parameters i.e. SiCαSi, CαSiCβ, and SiCβCβ′, are within their respective error limits as
compared to the corresponding adjusted r0 structural parameters and these parameters are well
determined by both methods. The two remaining parameters which can be determined have good
uncertainties, but they are significantly different than the corresponding r0 structural parameters. It
is believed that the r0 parameters are the more accurate determined as the back Cβ-Cβ distance is
normally [131] predicted by the MP2(full) calculations as smaller than the experimental value as
determined from the adjusted r0 structure. The predicted structural parameters were also obtained
from the B3LYP calculations with the same 6-311+G(d,p) basis set as those obtained from the
MP2 calculations. The Cα-Si predicted distance is 0.108 Å longer from the MP2 and the Cβ-Cβ
distance is predicted longer by 0.018 Å and hence the structure predictions with B3LYP
calculations are not very satisfactory. From such predicted distances it is not possible to obtain a fit
of the experimentally determined rotational constants. However the predicted angles obtained from
the B3LYP calculations are reasonably similar to those obtained from the MP2 calculation.
397
The natural population analysis was carried out for the twist conformer of c-C3H6Si2F4 with
the MP2(full) method at the 6-311+G(d,p) basis set. The charge of the heavy atoms are; Cα = 1.50, Si = +2.30, Cβ = -0.90, and F = -0.68. The large charge on the silicon atom is of interest and
could counteract some of the larger radius of the silicon atom than a carbon atom. Comparison of
this structure with 1,3-disilacyclopentane or tetrafluorocyclopentane would be of interest to
determine the effects these different substituents on their corresponding ring structures.
It would be of interest to compare c-C3H6Si2F4 with similar molecules to elucidate the
underlying physical causes for the molecular geometry observed in this study. However, there are
currently
no
previously
reported
experimentally determined
structures
for
the
1,3-
disilacyclopentane moiety.
The closest structure in terms of chemical properties and molecular geometry is 1-fluoro-1silacyclopentane (c-C4H9SiF) which has been previously studied [157] where an adjusted r0
structure was reported. The first difference which is to be expected is the difference between a
singly fluorinated sila-alkane and a difluorinated sila-alkane where the Si-F distances in the
difluorinated sila-alkane are ~0.02 Å shorter. This difference is due to the increasing positive
charge on the silane which means a smaller electron cloud which corresponds with the fluorines
needing to be closer to bond with the silane. Similarly this explains the significantly shorter Si-C
distances, which are ~0.01 Å shorter in the difluorinated sila-alkane, where the more positive
silane causes the shorter bonds. These shorter bonds in this case are actually counterintuitive where
the silane groups are normally larger than carbon groups. As c-C4H9SiF has only one silane group
in the ring and c-C3H5Si2F4 by this logic a trend toward an increasing ring size and thereby longer
bonds in the ring might be expected. The in ring bond angles are fairly similar with differences of
less than 1-2 degrees and there appears to be a common trend for overall smaller angles in the cC3H5Si2F4 ring which seems counterintuitive if the normal silane group versus alkane group size
398
trend is considered. The back Cβ-Cβ distance in the ring is ~0.02 Å shorter for c-C3H5Si2F4 than the
corresponding parameter in c-C4H9SiF. This difference stems from the more negatively charged
carbons in the c-C3H5Si2F4 ring rather than a steric effect, due to the Cβ atoms in this molecule
which have a nearly full electron unit charge as discussed above. Finally, the ring torsional angles
are significantly smaller in the c-C3H5Si2F4 ring than the corresponding torsional angles in the cC4H9SiF where difference in the SiCβCβ′Si angle is ~9° and the other torsional angles range from
1 to 4° smaller for c-C3H5Si2F4. This difference causes the c-C3H5Si2F4 ring to be much “flatter”
than the c-C4H9SiF ring which is clearly reflected in the predicted energy differences between the
planar and twist conformations where the MP2(full) ab initio calculation performed with the augcc-pVTZ basis set predicts the c-C4H9SiF energy difference as ~600 cm-1 larger in amplitude than
the same prediction for the c-C3H5Si2F4 molecule. As can be seen there are many significant
differences between the c-C4H9SiF and c-C3H5Si2F4 molecules, and therefore, it would be of
interest to study more 1,3-disilacyclopentane type molecules where the molecular geometries,
vibrational fundamentals, and conformational stabilities for a better understanding of the observed
differences.
399
Table 103: Rotational transition frequencies (MHz) of the ground vibrational state of c-C3H6Si2F4.
Transitions
71 6 ← 62 4
92 7 ← 83 5
133 11 ←132 11
123 10 ←122 10
113 9 ←112 9
103 8 ←102 8
93 7 ←92 7
83 6 ←82 6
73 5 ←72 5
63 4 ←62 4
53 3 ←52 3
43 2 ←42 2
33 1 ←32 1
33 0 ←32 2
43 1 ←42 3
53 2 ←52 4
63 3 ←62 5
73 4 ←72 6
83 5 ←82 7
93 6 ←92 8
103 7 ←102 9
113 8 ←112 10
123 9 ←122 11
133 10 ←132 12
113 9 ←104 7
113 8 ←104 6
153 12 ←152 14
163 13 ←162 15
22 0 ←11 0
22 1 ←11 1
134 10 ←125 8
41 3 ←30 3
60 6 ←51 4
81 8 ←72 6
176 11 ←167 9
102 9 ←93 7
81 7 ←72 5
102 8 ←93 6
123 10 ←114 8
123 9 ←114 7
32 1 ←21 1
32 2 ←21 2
70 7 ←61 5
51 4 ←40 4
91 9 ←82 7
194 16 ←193 16
184 15 ←183 15
174 14 ←173 14
164 13 ←163 13
154 1 ←153 12
144 1 ←143 11
164 1 ←163 14
134 1 ←133 10
154 1 ←153 13
144 1 ←143 12
124 9 ←123 9
134 9 ←133 11
114 8 ←113 8
124 8 ←123 10
114 7 ←113 9
104 7 ←103 7
104 7 ←103 7
104 6 ←103 8
94 6 ←93 6
94 5 ←93 7
84 5 ←83 5
c-C3H6Si2F4
νobs
6535.1069
6626.8707
6677.9774
6708.3201
6732.4364
6751.1888
6765.3359
6775.6965
6782.9645
6787.8277
6790.8723
6792.6222
6793.5023
6794.1100
6794.4469
6795.1378
6796.3666
6798.3599
6801.3714
6805.7262
6811.7705
6819.9203
6830.6024
6844.3702
6860.9769
6862.8502
6883.4665
6910.0946
7044.8446
7059.5546
7119.1018
7384.6694
7449.4537
7555.5886
7634.9756
8070.2200
8073.9295
8129.1638
8350.7954
8353.8904
8518.3533
8562.2389
8887.0220
8910.2708
8974.8062
9444.7055
9459.3982
9471.3280
9480.8782
9488.4987
9494.4804
9497.9125
9499.1396
9500.1345
9502.2529
9502.7047
9504.1552
9505.3919
9505.8380
9507.2569
9507.3872
9507.3872
9508.4516
9508.8200
9509.3986
9509.8327
Δνa
0.0064
0.0105
-0.0123
-0.0006
-0.0161
-0.0058
-0.0158
-0.0044
-0.0076
-0.0059
-0.0082
-0.0061
-0.0065
-0.0064
-0.0065
-0.0065
-0.0061
-0.0021
-0.0061
-0.0025
-0.0016
0.0070
-0.0076
-0.0060
-0.0207
-0.0014
-0.0034
-0.0382
-0.0063
-0.0049
0.0027
0.0000
-0.0039
-0.0003
-0.0320
0.0026
0.0056
0.0073
-0.0071
0.0047
-0.0073
-0.0048
-0.0045
0.0012
-0.0001
-0.0073
-0.0052
0.0113
-0.0098
-0.0020
-0.0084
0.0108
-0.0017
-0.0073
0.0054
-0.0013
-0.0039
-0.0018
-0.0019
-0.0157
0.0046
0.0046
-0.0036
-0.0016
0.0000
-0.0023
Transitions
74 4 ←73 4
74 3 ←73 5
64 3 ←63 3
54 2 ←53 2
44 1 ←43 1
112 10 ←103 8
91 8 ←82 6
112 9 ←103 7
133 11 ←1249
133 10 ←124 8
42 2 ←31 2
42 3 ←31 3
80 8 ←71 6
101 10 ←92 8
61 5 ←50 5
122 11 ←113 9
122 10 ←113 8
101 9 ←92 7
33 0 ←22 0
33 1 ←22 1
143 12 ←134 10
143 11 ←134 9
52 3 ←41 3
52 4 ←41 4
54 2 ←61 6
90 9 ←81 7
111 11 ←102 9
71 6 ←60 6
145 10 ←144 10
145 9 ←144 11
135 9 ←134 9
115 7 ←114 7
105 6 ←104 6
95 5 ←94 5
85 4 ←84 4
75 3 ←74 3
65 2 ←64 2
55 0 ←54 2
132 12 ←123 10
132 11 ←123 9
111 10 ←102 8
43 1 ←32 1
43 2 ←32 2
62 4 ←51 4
62 5 ←51 5
100 10 ←91 8
121 12 ←112 10
81 7 ←70 7
216 16 ←207 14
142 13 ←133 11
53 2 ←42 2
53 3 ←42 3
72 5 ←61 5
110 11 ←101 9
72 6 ←61 6
156 10 ←155 10
146 8 ←145 10
136 8 ←135 8
126 7 ←125 7
116 6 ←115 6
106 4 ←105 6
96 3 ←95 5
86 2 ←85 4
66 0 ←65 2
91 8 ←80 8
44 0 ←33 0
c-C3H6Si2F4
νobs
9510.5566
9510.6425
9510.9970
9511.2667
9511.3859
9551.3928
9616.1991
9635.9327
9840.9429
9845.8772
9985.1545
10072.3176
10314.4550
10386.0539
10444.0441
11030.5403
11147.8743
11160.8351
11257.5551
11257.6783
11331.4508
11339.0022
11445.7429
11589.7965
11612.8264
11731.0570
11789.2550
11986.3947
12223.3226
12223.4900
12224.7772
12226.8317
12227.5115
12228.0248
12228.4189
12228.6817
12228.8390
12228.9894
12507.3288
12665.6940
12706.6446
12745.1683
12745.7720
12900.7440
13114.6949
13136.0844
13184.3540
13537.8178
13592.5927
13981.3797
14232.3182
14234.1177
14350.9095
14528.7478
14647.0325
14942.6553
14943.6304
14944.3313
14944.8909
14945.3672
14945.7256
14946.0129
14946.2350
14946.4569
15098.8363
15463.0775
400
Δνa
0.0315
-0.0134
0.0220
0.0144
-0.0243
-0.0008
0.0045
0.0049
-0.0112
0.0033
-0.0079
-0.0037
-0.0063
0.0009
0.0026
-0.0078
-0.0012
-0.0014
-0.0088
-0.0064
0.0017
-0.0260
-0.0078
-0.0037
-0.0232
-0.0080
-0.0006
0.0011
-0.0206
0.0304
0.0026
0.0196
0.0097
0.0044
0.0193
0.0145
-0.0090
0.0258
-0.0087
0.0235
-0.0184
-0.0049
-0.0040
-0.0067
-0.0019
-0.0089
0.0076
0.0052
0.0312
-0.0229
-0.0062
-0.0101
-0.0057
-0.0256
-0.0019
-0.0015
0.0581
0.0145
-0.0247
-0.0215
-0.0297
-0.0200
-0.0024
-0.0259
-0.0180
0.0033
Transitions
63 4 ←52 4
82 6 ←71 6
82 7 ←71 7
101 9 ←90 9
54 1 ←43 1
73 4 ←62 4
73 5 ←62 5
92 7 ←81 7
92 8 ←81 8
c-C3H6Si2F4
νobs
15722.8977
15797.1394
16186.8760
16670.0780
16950.9792
17203.9313
17212.2902
17240.4083
17734.2041
Δνa
0.0076
0.0182
0.0339
-0.0550
0.0351
0.0443
0.0467
0.0466
0.0516
a
Transitions
84 4 ←83 6
c-C3H6Si2F4
νobs
9510.1223
Δνa
-0.0010
Transitions
63 3 ←52 3
c-C3H6Si2F4
νobs
15718.6738
 = obs calc in mHz.
401
Δνa
-0.0218
Transitions
c-C3H6Si2F4
νobs
Δνa
Table 104: Rotational transition frequencies (MHz) of the ground vibrational state of c-C3H6Si2F4.
29
Transitions
83 6  82 6
63 4  62 4
53 3  52 3
43 2  42 2
33 1  32 1
33 0  32 2
43 1  42 3
53 2  52 4
63 3  62 5
73 4  72 6
83 5  82 7
93 6  92 8
113 8 112 10
22 0  11 0
22 1  11 1
41 3  30 3
60 6  51 4
81 8  72 6
81 7  72 5
32 1  21 1
32 2  21 2
70 7  61 5
51 4  40 4
91 9  82 7
114 7  113 9
104 7  103 7
104 6  103 8
94 6  93 6
94 5  93 7
84 5  83 5
84 4  83 6
74 3  73 5
64 2  63 4
54 1  53 3
44 0  43 2
42 2  31 2
42 3  31 3
80 8  71 6
61 5  50 5
33 0  22 0
33 1  22 1
52 3  41 3
52 4  41 4
90 9  81 7
111 11  102 9
71 6  60 6
105 5  104 7
95 5  94 5
85 4  84 4
75 2  74 4
65 1  64 3
43 1  32 1
43 2  32 2
62 4  51 4
100 10  91 8
62 5  51 5
81 7  70 7
53 2  42 2
53 3  42 3
72 5  61 5
72 6  61 6
91 8 80 8
Si
νobs
6787.1087
6799.0530
6802.0549
6803.7724
6804.6434
6805.2385
6805.5735
6806.2541
6807.4613
6809.4267
6812.3889
6816.6805
6830.6015
7042.4581
7057.0701
7368.1723
7420.7400
7514.2952
8029.3238
8511.5030
8555.1003
8854.1369
8888.9613
8929.4253
9522.9108
9523.0314
9524.0735
9524.4387
9525.0211
9525.4485
9525.7237
9526.2316
9526.5990
9526.8667
9526.9938
9973.8781
10060.4755
10277.4821
10417.8618
11255.0150
11255.1441
11430.0721
11573.2027
11690.0975
11735.8591
11955.2754
12247.5813
12248.0739
12248.4486
12248.7171
12248.8932
12738.0816
12738.6762
12880.7007
13091.2487
13093.2980
13501.6898
14220.6843
14222.4573
14326.5038
14620.7833
15057.6363
30
a
Transitions
9.6
0.4
0.6
-3.8
-0.3
-3.7
-0.7
-0.6
-3.4
2.6
-5.2
0.7
-47
-3.3
-3.4
-0.6
-3.9
7.8
9.9
-3
-2.7
0.1
0.4
7.6
3.9
4.1
-1.8
-4.3
14.2
8
1.6
-16
-15
13.9
-1.8
-2.9
-1.3
-1.9
1.8
-7.2
2.9
-2
-0.4
-0.7
-3.9
2
18.4
7.6
8.7
12.9
11.1
2.9
3.7
-0.1
4.1
0.2
9.5
0.6
-2.9
0.6
-0.4
7.1
63 4  62 4
53 3 52 3
43 2  42 2
33 1 32 1
33 0  32 2
43 1  42 3
53 2  52 4
63 3  62 5
83 5  82 7
22 0  11 0
22 1  11 1
41 3  30 3
60 6  51 4
81 8  72 6
81 7  72 5
32 1  21 1
32 2  21 2
70 7  61 5
51 4  40 4
112 10  103 8
112 9  103 7
91 8  82 6
94 6  93 6
94 5  93 7
84 5  83 5
74 4  73 4
74 3  73 5
64 3  63 3
54 2  53 2
44 1  43 1
42 2  31 2
42 3  31 3
80 8  71 6
61 5  50 5
33 0  22 0
33 1  22 1
52 3  41 3
52 4  41 4
90 9  81 7
71 6  60 6
95 4  94 6
105 5  104 7
85 4  84 4
75 2  74 4
65 1  64 3
55 1  54 1
43 1  32 1
43 2  32 2
62 4  51 4
62 5  51 5
81 7  70 7
53 2  42 2
53 3  42 3
72 5  61 5
72 6  61 6
44 1  33 1
63 3  52 3
63 4  52 4
Δν
Si
νobs
6810.0547
6813.0094
6814.7074
6815.5651
6816.1554
6816.4797
6817.1537
6818.3473
6823.2033
7040.1189
7054.6343
7352.0001
7392.5881
7473.7892
7985.5776
8504.7848
8548.1017
8821.8846
8868.0719
9430.6744
9512.8399
9518.2710
9539.7768
9540.3195
9540.7461
9541.4291
9541.5399
9541.8715
9542.1412
9542.2648
9962.8200
10048.8654
10241.2198
10392.1951
11252.5247
11252.6556
11414.7030
11556.9320
11649.9281
11924.7668
12267.7443
12267.2486
12268.1285
12268.3701
12268.5249
12268.6806
12731.1236
12731.7152
12861.0443
13072.3198
13466.2604
14209.2793
14211.0196
14302.5703
14595.0490
15457.8457
15686.6441
15690.7440
402
29
a
Transitions
-5.3
-8.5
-7.4
-4.6
-4.2
-6.9
-3.5
-2.1
-2.6
-4.1
-6.5
-0.9
5.7
13.4
7.3
-3.9
-4.6
6.7
0.7
-6.1
38.8
-29.9
5.6
-2.7
-6.8
7.3
-6.7
13.4
14.1
-15.7
-2.8
-3.2
5.9
1
-12.9
0.7
-2.6
-3.3
15.7
0.3
5
8.7
24
5.1
-15.6
27.9
-6
0.4
0.4
-2.6
-0.6
2.5
-8
1.6
-3.5
5.3
-22
5
22 0  11 0
22 1  11 1
41 3  30 3
32 1  21 1
32 2  21 2
51 4  40 4
42 2  31 2
42 3  31 3
61 5  50 5
33 0  22 0
33 1  22 1
52 3  41 3
43 1  32 1
43 2  32 2
53 2  42 2
53 3  42 3
11 0  61 6
Δν
Si/30Si
νobs
7037.7001
7052.1253
7335.3357
8497.8651
8540.8970
8846.5483
9951.4234
10036.9146
10365.7674
11249.9686
11250.1030
11398.8753
12723.9606
12724.5416
14197.5039
14199.2849
14568.5471
Δνa
-11.8
-7.6
-8
-1.9
-0.7
-7.1
-6
4.4
7.9
-5.9
12.9
2.9
-7.3
-2.7
-19
37.2
-1.7
29
Transitions
a
44 0  33 0
63 3  52 3
63 4  52 4
82 7  71 7
54 1  43 1
Si
νobs
15460.4318
15702.5043
15706.6499
16155.6801
16943.7174
30
Δν
a
Transitions
Si
νobs
5.9
-16
-2.7
-9.8
-21
 = obs calc in kHz.
403
29
Δν
a
Transitions
Si/30Si
νobs
Δνa
Table 105: Rotational transition frequencies (MHz) of the ground vibrational state of c-C3H6Si2F4.
13
13
Transitions
C-/′
νobs
22 0 ← 11 0
22 1 ← 11 1
41 3 ← 30 3
60 6 ← 51 4
12 8 ← 93 6
32 1 ← 21 1
32 2 ← 21 2
51 4 ← 40 4
70 7 ← 61 5
54 2 ← 53 2
42 2 ← 31 2
42 3 ← 31 3
80 8 ← 71 6
61 5 ← 50 5
33 1 ← 22 1
52 3 ← 41 3
52 4 ← 41 4
71 6 ← 60 6
43 1 ← 32 1
43 2 ← 32 2
62 4 ← 51 4
62 5 ← 51 5
53 2 ← 42 2
53 3 ← 42 3
72 5 ← 61 5
44 0 ← 33 0
63 3 ← 52 3
54 1 ← 43 1
6976.4177
6988.5518
7335.0759
7468.2632
8178.7685
8448.4280
8484.6601
8849.9715
8911.4741
9367.2192
9914.8230
9986.8607
10346.7302
10371.4973
11142.7882
11375.9305
11495.1632
11899.9354
12626.4538
12626.8729
12832.1895
13009.5686
14109.8586
14111.1132
14284.1405
15303.0869
15592.6958
16786.9472
a
Δν
a
Transitions
0.8
2
-0.1
7.7
8.1
-0.3
0.5
-0.6
-20.7
3.8
5.1
-1.3
-9.5
-1.1
-21.7
4.2
0.9
-2.8
4.1
7.3
5
-3.1
3.2
13.1
27.7
18
-24.7
-22.9
22 0 ← 11 0
22 1 ← 11 1
41 3 ← 30 3
60 6 ← 51 4
32 1 ← 21 1
32 2 ← 21 2
51 4 ← 40 4
70 7 ← 61 5
42 2 ← 31 2
42 3 ← 31 3
61 5 ← 50 5
33 0 ← 22 0
33 1 ← 22 1
52 3 ← 41 3
52 4 ← 41 4
71 6 ← 60 6
43 1 ← 32 1
43 2 ← 32 2
62 4 ← 51 4
62 5 ← 51 5
81 7 ← 70 7
53 2 ← 42 2
53 3 ← 42 3
72 5 ← 61 5
72 6 ← 61 6
44 0 ← 33 0
 = obs calc in kHz.
404
C-α
νobs
7004.2435
7017.2495
7356.6206
7466.7058
8477.7036
8516.5446
8876.0991
8909.0007
9945.1779
10022.3748
10402.7330
11189.0574
11189.1404
11407.0357
11534.7471
11936.8419
12675.0517
12675.5218
12863.7961
13053.6749
13478.8260
14160.6738
14162.1113
14316.0182
14579.1945
15367.5008
Δνa
-0.2
-4.9
-3.5
0.1
-5.1
-4.7
-8
3.1
0.3
-4.9
-7.3
0.8
-11.4
-4.1
-4.9
-5.5
0.6
-4.8
7.1
-4
11
-9.1
4.9
-1.2
15
20.9
Table 106: Experimental rotational and centrifugal distortion constants of c-C3H6Si2F4
isotopologues
c-C3H6Si2F4
MP2(full)/
Experimental
6-31G(d)
2058.3427 2102.74026(68)
A (MHz)
29
Si b
30
Si b
29
Si/30Si b
13C-αb
13
C-/′b
2102.68742(51)
2102.63717(55)
2102.5846(20)
2080.16608(98)
2089.2035(12)
B (MHz)
744.6153
751.34319(32)
749.01578(33)
746.73383(32)
744.3835(12)
748.05599(53)
749.64852(57)
C (MHz)
725.8189
736.51478(31)
734.28573(27)
732.09979(25)
729.8480(25)
735.84108(49)
736.54375(70)
DJ (kHz)
0.02379
0.03236(77)
DJK (kHz)
0.3310
0.0638(39)
DK (kHz)
0.423
0.190(14)
dJ (kHz)
0
[0]
dK (kHz)
-0.17
-0.41(9)
142
67
59
17
28
26
16.6
9.14
10.8
12.0
10.9
7.43
N
a
 (kHz)
a
Number of frequencies fitted.
Distortion constants are fixed to that of the normal species for all isotopologues.
b
405
Table 107: Calculateda and observed frequencies (cm-1) for the twist form of c-C3H6Si2F4 (C2).
Vib.
No.
406
A 1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
B 21
22
23
24
25
26
27
28
29
30
31
Approx. description
-CH2 antisymmetric stretch
α-CH2 symmetric stretch
-CH2 symmetric stretch
-CH2 deformation
α-CH2 symmetric
deformation
-CH2 twist
-CH2 wag
SiF2 asymmetric stretch
Ring deformation
SiF2 symmetric stretch
α-CH2 twist
Ring deformation
-CH2 rock
Ring breathing
SiF2 symmetric deformation
SiF2 wag
SiF2 rock
Ring deformation
SiF2 twist
Ring twist
α-CH2 antisymmetric stretch
-CH2 antisymmetric stretch
-CH2 symmetric stretch
-CH2 asymmetric
deformation
-CH2 wag
α-CH2 wag
-CH2 twist
SiF2 asymmetric stretch
SiF2 symmetric stretch
-CH2 rock
Ring deformation
ab initio
3171
3134
3105
1521
1458
1321
1153
1032
1003
938
908
844
816
665
418
301
267
214
166
97
3198
3177
3107
1511
1321
1098
1087
979
892
824
734
fixed
scaledb
IR
int.
2975
2940
2912
1430
1371
1254
1095
980
954
891
861
805
778
634
405
289
256
205
158
95
3000
2980
2915
1421
1255
1044
1032
929
848
789
698
3.2
4.1
0.0
17.8
27.2
9.8
9.2
2.8
6.1
21.0
9.1
248.3
19.3
0.1
0.0
27.4
16.8
1.9
0.5
0.7
1.1
5.6
10.4
8.8
26.4
262.7
91.2
237.1
257.4
32.0
6.9
Raman
act.
100.8
95.4
140.3
8.0
9.5
5.0
3.7
0.4
7.1
1.8
3.8
0.7
1.8
23.5
2.2
0.2
0.5
1.0
0.3
0.1
65.6
71.7
24.3
17.0
0.4
0.1
1.5
0.8
0.5
0.4
1.4
dp
ratio
0.42
0.11
0.08
0.61
0.74
0.73
0.72
0.74
0.48
0.63
0.74
0.66
0.58
0.04
0.24
0.68
0.52
0.42
0.73
0.73
0.75
0.75
0.75
0.75
0.75
0.75
0.75
0.75
0.75
0.75
0.75
IR
gas
Raman
liquid
2979
2958
2917
1435
1353
1240
1080
970
2980
2958
2920
1438
1349
1241
1079
897
~861
801
789
648
420
2989
2981
2913
1418
1244
1036
1009
936
865
789
698
944
897
861
800
778
650
421
298
257
214
151
92
2993
2979
2920
1412
1241
1008
938
867
696
P.E.D.c
88S1, 12S3
100S2
89S3, 12S1
100S4
100S5
52S6, 32S7, 13S13
52S7, 30S6, 13S9
41S8, 41S11
74S9, 13S7
60S10,15S8, 11S14
44S11, 35S8, 17S10
47S12, 22S13, 16S16
44S13, 16S12, 12S6
64S14, 12S18
40S15, 37S18
50S16, 18S14
33S17, 23S16, 13S15
23S18, 41S15, 10S16
68S19, 21S17
56S20, 28S17, 16S19
100S21
95S22
95S23
100S24
79S25, 29S28
73S26, 17S32
73S27
79S28
77S29, 10S26, 10S32
23S30, 28S31, 26S34
32S31, 19S33, 19S30
Band Contour
A
B
100
58
42
47
53
3
97
99
1
100
14
86
100
98
2
98
2
3
97
C
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
-
34S32, 22S33, 15S34
Ring deformation
688
658
5.4
0.4
0.75
660
657
17
83
32
29S33, 36S30, 13S32,
α-CH2 rock
619
590
1.3
1.1
0.75
597
593
99
1
33
35S
3134, 17S31, 15S32,
Ring deformation
427
416
15.9
1.7
0.75
416
416 11S
96
4
34
90S
3635
SiF2 wag
333
317
11.6
1.3
0.75
319 11S
97
3
35
75S36, 12S30
SiF2 asymmetric deformation
303
288
25.9
0.3
0.75
282
86
14
36
38S37, 20S33, 20S38,
SiF2 rock
266
254
2.5
0.2
0.75
254
83
17
37
68S
3938, 12S37
SiF2 twist
208
199
0.6
0.7
0.75
197 12S
64
36
38
71S
,
13S
,
13S
Ring
puckering
33
33
3.6
0.0
0.75
4
96
39
39
34
37
a
4
MP2(full)/6-31G(d) ab initio calculations, scaled frequencies, infrared intensities (km/mol), Raman activities (Å /u), depolarization ratios (dp) and potential energy
distributions (P.E.D.s).
b
Scaled frequencies with scaling factors of 0.88 for CH2 stretches and CH2 deformations and 0.90 for all other modes.
c
Symmetry coordinates with P.E.D. contribution less than 10% are omitted.
*
Bend perpendicular and parallel to the ring, respectively.
-
407
Table 108: Calculated energies in Hartrees (H) and energy differences (cm-1) for the two
conformers and transition state of c-C3H6Si2F4.
Method/Basis Set
RHF/3-21G
MP2(full)/6-31G(d)
MP2(full)/6-31+G(d)
MP2(full)/6-31G(d,p)
MP2(full)/6-31+G(d,p)
MP2(full)/6-311G(d,p)
MP2(full)/6-311+G(d,p)
MP2(full)/6-311G(2d,2p)
MP2(full)/6-311+G(2d,2p)
MP2(full)/6-311++G(2d,2p)
MP2(full)/6-311G(2df,2pd)
MP2(full)/6-311+G(2df,2pd)
MP2(full)/6311++G(2df,2pd)
MP2(full)/6-311G(3df,3pd)
MP2(full)/6-311+G(3df,3pd)
MP2(full)/6311++G(3df,3pd)
MP2(full)/aug-cc-pVTZ
B3LYP/6-31G(d)
B3LYP/6-31+G(d)
B3LYP/6-31G(d,p)
B3LYP/6-31+G(d,p)
B3LYP/6-311G(d,p)
B3LYP/6-311+G(d,p)
B3LYP/6-311G(2d,2p)
B3LYP/6-311+G(2d,2p)
B3LYP/6-311++G(2d,2p)
B3LYP/6-311G(2df,2pd)
B3LYP/6-311+G(2df,2pd)
B3LYP/6-311++G(2df,2pd)
B3LYP/aug-cc-pVTZ
a
Energya, E
Twist
0.007559
1.297743
1.344176
1.347648
1.392907
1.993979
2.016998
2.202762
2.218807
2.219499
2.382396
2.397520
2.398050
2.486937
2.497203
2.497466
2.311046
3.584387
3.617723
3.594313
3.627284
3.787429
3.800665
3.828197
3.837634
3.837866
3.845567
3.855744
3.855938
3.877661
Energy of conformer is given as –(E+1093) H.
Difference is relative to twist form and given in cm-1.
b
408
Energy Differences, ∆
Envelopeb
Planarb
670
670
912
956
785
820
917
974
799
845
850
960
792
891
848
896
861
950
868
957
899
1069
899
1062
897
1058
938
1159
946
1180
941
1183
893
1167
619
510
608
503
570
524
472
517
522
546
552
550
565
625
512
615
507
586
535
476
528
534
558
567
565
567
Table 109: Symmetry coordinates of c-C3H6Si2F4.
Description
Symmetry Coordinatea
S1 = r2′ - r3′ + r2 - r3
CH2 antisymmetric stretch
S2 = r1 + r1′
′CH2 symmetric stretch
S3 = r2′ + r3′ + r2 + r3
CH2 symmetric stretch
S4 = λ7 + λ7′
CH2 deformation
′CH2 symmetric deformation S5 = λ2
S6 = λ4 - λ3 – λ6 + λ5 + λ4′ - λ3′ - λ6′ + λ5′
CH2 twist
S7 = λ3′ + λ4′ – λ5′ - λ6′ + λ4 + λ3 - λ6 - λ5
CH2 wag
S8 = R4′ - R5′ + R4 - R5′
SiF2 asymmetric stretch
S9 = R1 + R1′ - R2 – R2′
ring deformation
S10 = R4′ + R5′ + R4 + R5′
SiF2 symmetric stretch
S11 = λ2 – λ1 – λ2′ + λ1′
′CH2 twist
S12 = 4θ1 - θ2 - θ2′ - θ3 – θ3′
ring deformation
S13 = λ3′ + λ5′ - λ4′ - λ6′ - λ6 - λ4 + λ5 + λ3
CH2 rock
S14 = R1 + R2 + R3 + R2′ + R1′
ring breathing
SiF2 symmetric deformation S15 = 5′ + 5
S16 = 2′ + 1′ - 4′ - 3′ + 1 + 2 - 3- 4
SiF2 wag
S17 = 2′ + 4′ - 1′ - 3′ - 1 - 3 + 2 + 4
SiF2 rock
S18 = 4R3 – R1 - R1′ - R2 – R2′
ring deformation
S19 = 1 - 2 - 3 + 4 + 1′ - 2′ - 3' + 4′
SiF2 twist
S20 = 1+ 2
ring twist
S21 = r1 - r1′
B ′CH2 antisymmetric stretch
S22 = r2′ - r3′ - r2 + r3
CH2 antisymmetric stretch
S23 = r2′ + r3′ - r2 - r3
CH2 symmetric stretch
CH2 asymmetric deformation S24 = λ7 - λ7′
S25 = λ3′ + λ4′ - λ5′ - λ6′ - λ4 - λ3 + λ6 + λ5
CH2 wag
S26 = λ2 + λ1 – λ2′ - λ1′
′CH2 wag
S27 = λ4 - λ3 - λ6 + λ5 - λ4′ + λ3′ + λ6′ - λ5′
CH2 twist
S28 = R4′ - R5′ - R4 + R5′
SiF2 asymmetric stretch
S29 = R4′ + R5′ - R4 - R5′
SiF2 symmetric stretch
S30 = λ3′ + λ5′ - λ4′ - λ6′ + λ6 + λ4 - λ5 - λ3
CH2 rock
S31 = θ2 - θ2′ + θ3 - θ3′
ring deformation
S32 = R1 - R1′ + R2 - R2′
ring deformation
S33 = λ2 - λ1 + λ2′ - λ1′
‘CH2 rock
S34 = R1 - R1′ - R2 + R2′
ring deformation
S35 = 2′ + 1′ - 4′ - 3′ - 1 - 2 + 3+ 4
SiF2 wag
SiF2 asymmetric deformation S36 = 5′ - 5
S37 = 2′ + 4′ - 1′ - 3′ + 1 + 3 - 2 - 4
SiF2 rock
S38 = 1 - 2 - 3 + 4 - 1′ + 2′ + 3' - 4′
SiF2 twist
S39 = 1- 2
ring puckering
a
Not normalized.
*Bend perpendicular and parallel to the ring, respectivel.
A
409
Table 110: Structural parameters (Å and degrees), rotational constants (MHz) and dipole moments
(Debye) for twist conformer c-C3H6Si2F4.
Structural
Parameters
rCα-Si
rSi-Cβ,Cβ′
rCβ-Cβ′
rSi-F1
rSi-F2
rCα-H
rCβ-H1, Cβ′-H1
rCβ-H2, Cβ′-H2
SiCαSi
CαSiCβ
SiCβCβ′
CαSiF1
CαSiF2
CβSiF1
CβSiF2
 F1SiF2
HCαSi
HCαH
H1CβSi
H2CβSi
H1CβCβ′
H2CβCβ′
H1CβH2
CβSiCαSi
SiCβCβ′Si
A(MHz)
B(MHz)
C(MHz)
|a|
|b|
Int.
coor.
R1
R2
R3
R4
R5
r1
r2
r3
1
2
3
1
2
3
4
5
λ1
λ2
λ3
λ4
λ5
λ6
λ7
1
2
6-311+G(d,p)
MP2(full)
1.8600
1.8633
1.5582
1.6142
1.6100
1.0940
1.0972
1.0932
102.75
104.22
106.60
109.58
113.37
110.17
113.04
106.48
111.30
108.01
108.37
113.34
109.75
111.86
106.86
10.71
41.05
2061.4092
742.8442
724.1462
0.0
0.0
B3LYP
1.8708
1.8743
1.5640
1.6209
1.6175
1.0935
1.0961
1.0926
102.77
104.86
107.64
109.80
113.04
110.64
112.49
106.08
111.39
107.70
108.00
112.91
109.96
111.72
106.53
9.52
36.57
2035.6232
731.0688
714.6206
0.0
0.0
6-311++G(d, p)
MP2
B3LYP
1.8625
1.8708
1.8659
1.8742
1.5607
1.5641
1.6147
1.6210
1.6104
1.6174
1.0946
1.0935
1.0979
1.0961
1.0939
1.0926
102.77
102.74
104.22
104.85
106.60
107.60
109.58
109.77
113.37
113.07
110.15
110.57
113.05
112.55
106.48
106.10
111.30
111.36
107.98
107.73
108.38
108.05
113.33
112.88
109.76
109.98
111.87
111.72
106.86
106.56
10.71
9.57
41.04
36.74
2058.2277
2035.8908
741.2055
731.3494
722.8103
714.8816
0.0
0.0
0.0
0.0
1.279
1.322
1.271
|c|
a
Determined using the microwave data reported in this study.
410
1.315
rsa
1.860(13)
1.856(17)
1.5510(39)
102.21(63)
104.68(28)
106.47(22)
41.71(42)
Adjusted
r0a
1.8589(1)
1.8636(1)
1.5592(1)
1.5826(1)
1.5781(1)
1.0940(1)
1.0972(1)
1.0932(1)
102.78(2)
104.28(2)
106.60(2)
109.55(2)
113.41(2)
110.28(2)
112.72(2)
106.63(2)
111.29(2)
108.01(2)
108.37(2)
113.34(2)
109.63(2)
111.98(2)
106.86(2)
10.67(2)
40.85(2)
2102.7784
751.3680
736.5361
Table 111: Comparison of rotational constants (MHz) obtained from experimental values from
microwave spectra, and from the adjusted r0 structural parameters for c-C3H6Si2F4.
Isotopomer
c-C3H6Si2F4
29
Si
30
Si
29
30
Si / Si
13
Cα
13
Cβ
Rotational
constant
A
Experimental
2102.74026(68)
Adjusted
r0
2102.7784
0.0382
B
751.34319(32)
751.3680
0.0248
C
736.51478(31)
736.5361
0.0213
A
2102.68742(51)
2102.7030
0.0155
B
749.01578(33)
749.0241
0.0083
C
734.28573(27)
734.2919
0.0061
A
2102.63717(55)
2102.6292
0.0080
B
746.73383(32)
746.7263
0.0075
C
732.09979(25)
732.0914
0.0084
||
A
2102.5846(20)
2102.5540
0.0306
B
744.3835(12)
744.3588
0.0247
C
729.8480(25)
729.8237
0.0243
A
2089.2035(12)
2089.2114
0.0079
B
749.64852(57)
749.6286
0.0199
C
736.54375(70)
736.5361
0.0076
A
2080.16608(98)
2080.1431
0.0230
B
748.05599(53)
748.0751
0.0191
C
735.84108(49)
735.8545
0.0135
411
Figure 62: Comparison of experimental (path length = 9.5 cm & gas pressure = 400 mTorr) and
calculated infrared spectra of c-C3H6Si2F4: (A) observed spectrum of gas; (B) simulated spectrum of
twist conformer.
412
Figure 63: Comparison of experimental and calculated Raman spectra of c-C3H6Si2F4: (A) observed
spectrum of liquid; (B) simulated spectrum of twist conformer.
413
Figure 64: Model of c-C3H6Si2F4 showing atomic numbering.
414
CHAPTER 16
RAMAN AND INFRARED, MICROWAVE SPECTRA, ADJUSTED R0 STRUCTURAL
PARAMETERS , CONFORMATIONAL STABILITY, AND VIBRATIONAL ASSIGNMENTS OF CYCLOPENTYLAMINE
Introduction
Cyclopentane and many of its monosubstituted derivatives have been of interest for many
years since they have two “out-of-plane” vibrational modes which are usually described as ring
puckering and ring twisting modes. When the wavenumbers of these two modes are nearly equal,
the cross terms in the potential function resulted in a vibrational motion that was described as
pseudorotation [122]. The motion was first treated [122] for cyclopentane for the two degenerate
out of plane ring bending coordinates in terms as an amplitude coordinate q and a phase angle ø.
The concept was questioned [150] when the low frequency ring modes of cyclopentane appeared
“normal” but the fundamentals could not be assigned since the cyclopentane had D5h symmetry.
The scientists [150] reported that the spectral data for structures with Cs, C2 or C1 symmetry were
consistent; however, they could not conclude a decision among the pseudorotation structure.
Interestingly, there were two studies [151, 152]
that indicated the refusal to accept the
pseudorotation of the puckering motion. A latter infrared study of the CH2 deformation of
cyclopentane indicated that the ring clearly undergos pseudorotation which was nearly barrier free.
From this study [153] the authors also estimated the value of the pseudorotational moment of
inertia [169] which was obtained from the spectral data. Several other monohalocycolpentanes i.e.
fluoride, chloride, and bromide were initially carried out by utilizing the infrared and Raman
spectra and it was concluded that all of the compounds had doublets for carbon-halogen stretches
arising from the envelope-axial and envelope-equatorial forms of the substituted cyclopentanes.
From an investigation of the Raman spectrum [129] of the liquid of fluorocyclopentane, it was
415
concluded that only the envelope-equatorial conformer was preferred which was supported by the
theoretical predictions [130] from the CNDO model. A later study [171] showed from the infrared
spectra of the gas and variable temperature xenon solutions that there was one favored conformer
which is the twist form which was supported by the structural parameters obtained from the
rotational constants obtained from previous microwave studies. On the other hand, both the
chlorocyclopentane [171] and bromocyclopentane [172] molecules showed that the envelope-axial
and envelope-equatorial conformers as the two stable forms of these two ring compounds. Other
studies of the variable-temperature infrared spectra of rare gas solutions of silacyclopentane [173]
and germacyclopentane [174], showed that only one stable twist (C2) conformer was present in all
physical states. Therefore, it is expected that the conformational conclusions of the structural
stabilities of some other substituted five membered rings may be in error.
Another saturated five membered ring molecule of considerable conformational interest is
pyrrolidine [175]. From the infrared spectra of gaseous and variable temperature liquid xenon
solutions, an enthalpy difference has been determined to be 109 ± 11 cm-1 with the envelopeequatorial conformer more stable than the twist form with 37 ± 3% present at ambient temperature.
It is rather surprising that the second most stable conformer for pyrrolidine is the twisted form
rather than the envelope-axial form which has been found for most of the previously studied five
membered rings which have two stable conformers. From the first conventional microwave study,
Caminati et al. [154] reported that the pyrrolidine molecule exists in an envelope-axial form but
there was no spectroscopic evidence for the envelope-equatorial form. In a subsequent electron
diffraction study [176] which was supported by HF/4-21N* ab initio calculations, again the
envelope-axial form was reported to be the more stable conformer with the envelope-equatorial
conformer higher in energy by 339 cm-1 and the barrier to pseudorotation was predicted to be 580
cm-1. However, from a more recent microwave study, Caminati et al. [155] discovered transitions
416
of the equatorial conformer during the investigation of rotational free jet spectrum of pyrrolidinewater adduct. In this microwave investigation, it was observed that the axial conformer relaxes to
the equatorial conformer when the cooling conditions were increased, which clearly shows that the
equatorial form is the more stable conformer. Therefore, it is of interest to investigate whether
there are other five membered rings with two conformers where the equatorial form is the more
stable conformer and the second most stable form the twisted conformer!
Also, as a continuation of our studies of the conformational stabilities of mono-substituted
cyclopentanes, we also investigated the Raman spectra of the xenon solutions and the infrared
spectra of the gas of ethynylcyclopentane [142] where the enthalpy difference between the more
stable envelope-equatorial conformer and the envelope-axial form has been determined to be 94 ±
9 cm-1. Also from a microwave study [132] the spectra of both the equatorial and axial conformers
were identified and the enthalpy difference was determined to be 94 ± 24 cm -1 with the Eq being
the more stable form. Also it should be noted that the isocyanocyclopentane which is a very
similar molecule to ethynylcyclopentane was reported [177] to have the Ax conformer more stable
form by 102  10 cm-1 than the Eq conformer from the infrared and Raman spectra of the gas,
liquid, and solid. Therefore, as a continuation of these studies we have turned our attention to
another mono-substituted cyclopentane, i.e. cyclopentylamine, c-C5H9NH2.
In this investigation, the conformers are designated where the axial and equatorial
orientations refer to the ring atoms whereas the trans and gauche orientations refer to the position
of the NH2 group. These combinations result in four possible conformers i.e. the trans-Axial (tAx), trans-Equatorial (t-Eq), gauche-Axial (g-Ax) and gauche-Equatorial (g-Eq) conformers as
shown in Fig. 65.
In the current study, we reported the microwave spectra for the most abundant conformer
of the cyclopentylamine. Also, we initiated variable temperature (-60 to -100 °C) studies of xenon
417
solutions of cyclopentylamine by recording Raman spectra from 3800 to 50 cm-1. To predict the
spectroscopic studies we have carried out more extensive ab initio calculations by utilizing a
variety of basis sets. To predict the experimental studies we have also obtained the harmonic force
constants, infrared intensities, Raman activities, depolarization ratios, and vibrational
wavenumbers from MP2(full)/6-31G(d) ab initio calculations. To obtain predictions on the
conformational stabilities we have carried out MP2(full) ab initio and density functional theory
(DFT) calculations by the B3LYP method by utilizing a variety of basis sets. The results of these
spectroscopic, structural and theoretical studies of cyclopentylamine are reported herein.
Experimental
The sample of cyclopentylamine was purchased from Sigma-Aldrich Chemical Co., with
stated purity of ≥99%. The sample was used without any further purification and the purity of the
sample was verified by comparing the infrared spectrum with that previously reported [178].
Microwave spectra of the sample were recorded by a “mini-cavity” Fourier-transform
microwave spectrometer [17], [18] at Kent State University. The Fabry-Perot resonant cavity is
established by two 7.5-inch diameter diamond-tip finished aluminum mirrors with a 30.5-cm
spherical radius. The Fabry-Perot cavity resides inside a vacuum chamber formed by a 6-way cross
and a 15-inch long, 8-inch diameter extension tube. The two cavity mirrors are nominally
separated by 30 cm.
The sample was entrained in a 70:30 Ne-He carrier gas mixture at 2 atm and expanded into
the cavity to attain 4K by using a reservoir nozzle [18] made from a modified Series-9 General
Valve. The reservoir nozzle is mounted in a recessed region of the mirror flange, external to the
vacuum chamber, and the expansion passes through a 0.182-inch diameter hole into the resonant
cavity. The center of the expansion is offset from the center of the mirror by 1 inch.
The sample is irradiated by microwave radiation generated by an Agilent Technologies
418
E8247C PSG CW synthesizer; details of the irradiation and heterodyne detection circuitry can be
found in Ref. [19]. The vacuum system can accommodate pulse repetition rates of up to 15 s-1
while maintaining a pressure below 10-4 torr, and the instrument can scan 450 MHz in 6 hours
while averaging 100 shots per scan segment. The wavenumbers for the measured transitions in the
region of 10,500 to 22,000 MHz are listed in Table 112 along with their assignments. Also listed
are the frequency differences between the measured values and the values obtained from the
determined rotational constants (Table 113).
The mid-infrared spectrum of the gas was obtained from 4000 to 220 cm-1 on a Perkin-Elmer
model 2000 Fourier transform spectrometer equipped with a Ge/CsI beamsplitter and a DTGS
detector. Atmospheric water vapor was removed from the spectrometer housing by purging with dry
nitrogen. The spectrum of the gas was obtained with a theoretical resolution of 0.5 cm-1 for the gas
with 128 interferograms added and truncated Fig. 66.
The Raman spectra (3800 to 50 cm-1) of the sample dissolved in liquefied xenon (Fig. 67) at
five different temperatures (-60 to -100oC) were recorded on a Trivista 557 spectrometer consisting
of a double f = 50 cm monochromator equipped with a 2000 lines mm-1 grating, a f = 70 cm
spectrograph equipped with a 2400 lines mm-1 grating, and a back-illuminated LN2-cooled PI
Acton Spec-10:2 kB/LN 2048 x 512 pixel CCD detector. For all experiments, the 514.5 nm line of
a 2017-Ar S/N 1665 Spectra-Physics argon ion laser was used for Raman excitation, with the
power set to 0.8 Watt. Signals related to the plasma lines were removed by using an interference
filter. The wavenumbers were calibrated by using Neon emission lines, and depending on the setup
used, are expected to be accurate within 0.4 cm-1. The experimental set-up used to investigate the
solutions has been described before [15, 16]. A home-built liquid cell equipped with four quartz
windows at right angles was used to record the spectra. A comparison between the observed and
calculated wavenumbers, along with the calculated infrared intensities, Raman activities,
419
depolarization ratios and potential energy distributions for the t-Ax, t-Eq, g-Eq and g-Ax conformers
of the cyclopentylamine are listed in Tables 114, 115, 116 and 117, respectively.
Ab Initio Calculations
The LCAO-MO-SCF restricted Hartree-Fock calculations were performed with the
Gaussian-03 program [21] with Gaussian-type basis functions. The energy minima with respect to
nuclear coordinates were obtained by the simultaneous relaxation of all geometric parameters by
using the gradient method of Pulay [22]. Several basis sets as well as the corresponding ones with
diffuse functions were employed with the Møller-Plesset perturbation method [23] to second order
(MP2(full)) as well as with the density functional theory by the B3LYP method. The predicted
conformational energy differences are listed in Table 118.
In order to obtain a complete description of the molecular motions involved in the
fundamental modes of cyclopentylamine, a normal coordinate analysis has been carried out. The
force field in Cartesian coordinates was obtained with the Gaussian 03 program [21] at the
MP2(full) level with the 6-31G(d) basis set. The internal coordinates used to calculate the G and B
matrices are given in Table 119. By using the B matrix [24], the force field in Cartesian
coordinates was converted to a force field in internal coordinates. Subsequently, scaling factor of
0.88 for CH stretches and CH2 deformation, 0.90 for all other modes excluding the heavy atom
bends were applied, along with the geometric average of the scaling factors for the interaction
force constants, to obtain the fixed scaled force field and resultant wavenumbers. A set of
symmetry coordinates was used (Table 120) to determine the corresponding potential energy
distributions (P.E.D.s).
The vibrational spectra were predicted from the MP2(full)/6-31G(d) calculations. The
predicted scaled wavenumbers were used together with a Lorentzian function to obtain the
simulated spectra. Infrared intensities were obtained based on the dipole moment derivatives with
420
respect to Cartesian coordinates. The derivatives were transformed with respect to normal
coordinates by (u/Qi) = j (u/Xj)Lij, where Qi is the ith normal coordinate, Xj is the jth
Cartesian displacement coordinate, and Lij is the transformation matrix between the Cartesian
displacement coordinates and the normal coordinates. The infrared intensities were then calculated
by (N)/(3c2) [(x/Qi)2 + (y/Qi)2 + (z/Qi)2].
Additional information on the vibrational assignments was obtained from the simulated
Raman spectra. The evaluation of Raman activity by using the analytical gradient methods has
been developed [25-27, 37] and the activity Sj can be expressed as: Sj = gj(45αj2 + 7βj2), where gj is
the degeneracy of the vibrational mode j, αj is the derivative of the isotropic polarizability, and βj is
the anisotropic polarizability. To obtain the Raman scattering cross sections, the polarizabilities are
incorporated into Sj by multiplying Sj with (1-ρj)/ (1+ρj) where ρj is the depolarization ratio of the
jth normal mode. The Raman scattering cross sections and calculated wavenumbers obtained from
the Gaussian 03 program were used together with a Lorentzian function to obtain the simulated
Raman spectra.
Microwave Results
There are no previous microwave investigations of cyclopentylamine and there is no
previously reported molecular structure. We are therefore interested in determining the adjusted r 0
structural [15] parameters and, thus, the rotational constants. In order to determine the rotational
constants the rotational transitions were assigned and the wavenumbers for these bands are listed
in Table 112. These transitions correspond to the normal species of t-Ax conformer of
cyclopentylamine, which indicates that this conformer is the most stable conformer in the vapor
state. As can be seen from the data the largest difference is 0.4 MHz and the standard deviation is
0.2 MHz. The N atom in cyclopentylamine contains a nucleus with a spin quantum number I = 1
421
and, therefore, a nuclear quadrapole moment which interacts with the electric field gradient created
by the electrons of the rotating molecule. However, the magnitude of this interaction is relatively
small and can be largely ignored in assigning the transition wavenumbers and fitting the spectra as
the differences in the rotational constants will not meaningfully affect the adjusted r0 structural
parameters. The experimentally determined rotational constants are listed in Table 113. The values
of the centrifugal distortion constants were set to zero as the predicted values from the ab initio
calculation were small and disagreed significantly with those predicted by the DFT calculation.
Vibrational Assignment
In order to determine the enthalpy difference between the four stable conformers it was
essential to have a confident assignment for all of the fundamentals of the most stable conformer
and identify vibrations which cannot be assigned to this form. These additional modes must be
overtones, combination bands, or fundamentals of the other conformers. In the initial vibrational
assignment [178] of cyclopentylamine was made for the t-Ax conformer with most of the observed
vibrational bands assigned to this conformer as fundamentals in addition to another NH2 wag
fundamental for conformer II. The previous vibrational assignment for cyclopentylamine [178]
was made by utilizing the Raman spectra of the gas, liquid, and solid as well as the infrared spectra
of the gas and solid which made it possible to identify the modes involving mainly the amines.
Vibrational assignments were proposed and the structural features of cyclopentylamine were
discussed. The vibrational spectra of cyclopentylamine exhibit no features that can be conveniently
assigned but the axial form was listed as the more preferred form.
The lower frequency region is the most desirable spectral region for the enthalpy
determinations since it will have fewer overtones and combination bands for interference with the
measured intensities of the conformer pairs which are selected. Also, in this lower frequency
region the greatest separation is expected for the vibrational fundamentals of the various
422
conformers. Thus, the spectral region from 600-1200 cm-1 was selected and the infrared spectra of
the gas and xenon solution are shown in Fig. 68.
In the current study, the region from 1700 to 400 cm-1 is fairly complex with twenty nine
predicted fundamentals for the t-Ax conformer. Most of the assignments of these bands agree with
the previously reported from the infrared and Raman spectra previously reported [178] except for
seventeen
fundamentals where the assignments of these fundamentals were not previously
reported.
It is interesting to note that the Raman spectra of the gas has been obtained which usually
provides mainly the A′ bands for molecules which have a plane of symmetry and the data are more
definitive than the depolarization values. In previous vibrational assignments [178] the Raman
spectra of the gas was used to assign some of the fundamentals in the A'' region!
As can be noted, the breadth is believed, in many cases, to be due to association of the
molecules although the presence of significant amounts of four conformers can give rise to the
breadth since the corresponding fundamentals have similar wavenumbers for many cases. In the
current study the complete vibrational assignments for all the four stable conformers were made by
utilizing the wavenumbers predicted from the ab initio calculations along with the predicted band
contours, the infrared intensities, Raman activities and the well-known group wavenumbers for the
amine group. In the previous investigation [178] of the infrared spectra of the gas, the resolution
utilized did not permit the observation of the multiple fundamentals of the four conformers. For
example, in the 1000 - 550 cm-1 region we observed bands which could be assigned for the four
stable conformers fundamentals whereas in the previous study [178] only a few fundamentals were
reported in this spectral region. In Tables (114-117) complete assignments are proposed for the
fundamentals of all four stable conformers along with their approximate descriptions. With the
423
proposed assignments we then proceeded to utilize bands, which we believed to be relatively pure
fundamentals of the indicated conformers, for the enthalpy determinations.
Conformational Stability
To determine the enthalpy differences among the four observed conformers of
cyclopentylamine, the sample was dissolved in liquefied xenon and the Raman spectra was
recorded as a function of temperature from -60 to -100°C (Fig. 69). Relatively small interactions
are expected to occur between xenon and the sample though the sample can associate with itself
forming a dimmer, trimer or higher order complex. However, due to the very low concentration of
sample self-association is greatly reduced. Therefore, only small wavenumber shifts are anticipated
for the xenon interactions when passing from the gas phase to the liquefied xenon solutions except
for the NH2 modes (Fig. 68). A significant advantage of this study is that the conformer bands are
better resolved in comparison with those in the Raman spectrum of the liquid or far infrared
spectra of the gas. From ab initio calculations, the dipole moments of the four conformers are
predicted to have similar values and the molecular sizes of the four conformers are nearly the
same, so the ΔH value obtained from the temperature dependent Raman study is expected to be
near to that for the gas [39-43].
Once confident assignments have been made for the fundamentals of the observed
conformers the task was then to find a pair or pairs of bands from which the enthalpy
determination could be obtained. The bands should be sufficiently resolved as to determine their
intensities. The selection of the bands to use in the enthalpy determination was complicated due to
the presence of four conformers. The fundamentals at 1051, 1010, and 835 cm-1 were initially
selected for the t-Ax conformer. Also, the 1027, 950 and 806 cm-1 bands were also assigned for the
t-Eq, g-Eq and g-Ax conformers, respectively, as they are confidently assigned, satisfactory
resolved and a limited number of overtone and combination bands are possible. These bands were
424
selected in the Raman spectra as it was found that the band intensities were not significantly
affected by the underlying bending modes.
The intensities of the individual bands were measured as a function of temperature and their
ratios were determined. By application of the van’t Hoff equation, lnK = H/(RT)  S/R, the
enthalpy differences were determined from a plot of lnK versus 1/T, where H/R is the slope of
the line and K is substituted with the appropriate intensity ratios, i.e. Iconf-1 / Iconf-2, etc. It was
assumed that H, S, and the ratio of the molar absorption coefficients εconf-1 / εconf-2 are not
functions of temperature in the spectral range studied.
The conformational enthalpy differences in Table 121 were determined to be 195 ± 10 cm-1
and 203 ± 22 cm-1 from the xenon solution for the 1051/1027 and 1010/1027 band pairs,
respectively. Another band at 835 cm-1 was combined with the 1027 cm-1 band from which an
additional H value of 236 ± 22 cm-1 was obtained. An average value was obtained by utilizing all
the data as a single set which gave a H value of 211 ± 11 cm-1 with the t-Ax the most stable form.
This error limit is derived from the statistical standard deviation of one sigma of the measured
intensity data taken as a single data set, but it does not take into account small associations with the
liquid xenon or the possible presence of overtones and combination bands in near coincidence of
the measured fundamentals.
For the enthalpy determination between t-Ax the most stable conformer and the third most
stable conformer the g-Eq two conformers pairs of 1051/950 and 1010/950 cm-1 provided a
consistent plot of the intensity ratios from -60 to -100°C. These pairs gave enthalpy values of 231
± 35 cm-1 and 224 ± 25 cm-1, respectively. An average value was obtained by utilizing the data as a
single set which gave a H value of 227 ± 20 cm-1. Similarly, two conformer pairs of 1051/806
and 1010/806 cm-1 were found for the determination of the enthalpy difference between the most
stable conformer t-Ax and the least stable conformer g-Ax and H values of 259 ± 18 cm-1 and 251
425
± 14 cm-1 were obtained, respectively. An average value of H was obtained by combining all the
data as a single set which gave a H value of 255 ± 11 cm-1.
The error limits are derived from the statistical standard deviation of one sigma of the
measured intensity data taken as a single data set, but it does not take into account small
associations with the liquid xenon or the possible presence of overtones and combination bands in
near coincidence of the measured fundamentals. The variations in the individual values are
undoubtedly due to these types of interferences, but by taking several pairs, the effect of such
interferences should cancel. However, this statistical uncertainty is probably better than can be
expected from this experimental technique and, therefore, an uncertainty of about 10% in the
enthalpy difference is probably more realistic i.e. the enthalpy difference in relative to t-Ax of 211
± 21 cm-1 for t-Eq ≥ 227 ± 22 cm-1 for g-Eq ≥ 255 ± 25 cm-1 for g-Ax. From these enthalpy
differences the abundance of the t-Ax conformer present at ambient temperature is estimated to be
53%, with the remaining values of 11 ± 1 % for the t-Eq conformer, 20 ± 2 % for the g-Eq
conformer and 16 ± 2% for the g-Ax form.
Structural Parameters
The adjusted r0 structural parameters can be determined for the t-Ax conformer by utilizing
the rotational constants reported herein from the microwave spectra. There are three rotational
constants available for the determination of the structural parameter values of the t-Ax conformer
of cyclopentylamine.
We [44] have shown that ab initio MP2(full)/6-311+G(d,p) calculations predict the carbonhydrogen r0 structural parameters for more than fifty hydrocarbons to at least 0.002 Å compared to
the experimentally determined [45] values from isolated CH stretching wavenumbers which agree
to previously determined values from earlier microwave studies. Therefore, all of the carbon-
426
hydrogen parameters can be taken from the MP2(full)/6-311+G(d,p) predicted values for the t-Ax
form of cyclopentylamine.
We have found that good structural parameters for hydrocarbons and many substituted ones
can be determined by adjusting the structural parameters obtained from the ab initio MP2(full)/6311+G(d,p) calculations to fit the rotational constants obtained from microwave experimental data
by using a computer program “A&M” (Ab initio and Microwave) developed [57] in our
laboratory. In order to reduce the number of independent variables, the structural parameters are
separated into sets according to their types where bond distances in the same set keep their relative
ratio, and bond angles and torsional angles in the same set keep their difference in degrees. This
assumption is based on the fact that errors from ab initio calculations are systematic. It also should
be noted that the plane of symmetry reduces the number of independent parameters. With these
limitations of the independent parameters there are 9 independent parameters left to be determined.
Therefore the final structure determined must be evaluated based upon previously determined
molecular structures of similar molecules.
The resulting adjusted r0 parameters are listed in Table 119, where it is believed that the CC, C-N and N-H distances should be accurate to ± 0.003 Å and the C-H distances accurate to ±
0.002 Å, and the uncertainties of the angles should be within ± 0.5. The fit of the three rotational
constants by the adjusted r0 structural parameters are good with the differences being less than 0.09
MHz. Therefore, it is believed that the suggested uncertainties are realistic values and the
determined structural parameters are probably as accurate as can be obtained for the molecule in
the gas phase by either electron diffraction or microwave substitution methods. However with
more rotational constants from further isotopic substitutions the accuracy of the adjusted r0
parameter values could be improved.
427
Discussion
The vibrational assignments reported herein are based on a significant amount of
information with the Raman spectra of the xenon solution, infrared spectra of the solid, and
predictions from scaled force constant(s) by ab initio MP2(full)/6-31G(d) calculations as well as
the predicted infrared band contours. The ab initio predicted Raman and infrared band intensities
were also used but they seemed to be the least reliable of the data utilized. One of the possible
reasons for significant difference in predicted and observed intensities could be the results of the
association of the amine portion of the molecule with xenon. The evidence for the van der Waals
molecules was the significant decreases in the NH stretching wavenumbers from the gas to the
solutions i.e. 12 cm-1 lower for the A' mode for the t-Ax form as well as 14 cm-1 for the A'' mode
for this same conformer. However, for the other fundamentals the difference between the
wavenumbers in the gas and xenon solution averages five wavenumbers except for the NH2 modes.
Finally, it should be noted that there is a very, very small amount of the dimer present in the xenon
solution as evidence by the very low intensity of the bands which are present at approximately
3072 cm-1 in xenon solutions.
For the t-Ax form the fixed scaled ab initio predicted wavenumbers was an average of 4
cm-1 in error for the A′ fundamentals, except for the NH2 wag, which represent 0.17% error. The
predicted wavenumber for the NH2 wag is too high with 0.9 scaling but it can be corrected with a
scaling factor of 0.7. However, if this value is used it usually results in two or three unacceptable
P.E.D. values. Since the NH2 wag gives rise to a very intense infrared band with well-known group
wavenumbers, it is usually better to use 0.9 as the scaling factor so the other fundamentals have
“normal” P.E.D.s. The predicted errors for the A′′ modes have an average 2 cm-1 which represent
0.09% error. Thus the relatively small basis set of 6-31G(d) by the MP2(full) calculations with two
428
or three scaling factors for the force constants provides good predicted wavenumbers for aiding the
vibrational assignments.
For the t-Ax form the mixing is relatively small for the A′ modes except for 15 which has
only 18%S15 (Ring deformation) and four other symmetry coordinates with 20%S19 (Ring
deformation) being the largest value of this group. For the A′′ modes only 33 (CH2 rock) and 37
(NH2 twist) have relatively small percentage values where 33 has 26%S33 with 25%S34 (CH2 rock)
and 12% of three other symmetry coordinates. The NH2 twist 22%S37 with 35%S32 (CH out of
plane bend) and the two other coordinates of 14% and less. Therefore, the approximate
descriptions for the normal modes provide reasonable information on the molecular motions of the
vibrations.
To help evaluate the accuracy of the adjusted r0 structural parameters determined in Table
119 it is of interest to compare the structural parameters of cyclopentylamine with the
corresponding parameters of other monosubstituted-cyclopentanes. The adjusted r0 structure has
been determined for the Ax form of ethynylcyclopentane [142] with the distances (Å) rCα-Cβ =
1.542(3), rCβ-Cγ = 1.542(3), rCγ-Cγ′ = 1.555(3) and dihedral angle (°) CβCαCβ'Cγ′ = 40.8(5). The
Cα-Cβ bond distance is significantly different between these two molecules which is to be expected
based on the difference in the substituent but the other ring parameters are within their respective
error limits. The adjusted r0 structure has also been determined for the Ax conformers of
bromocyclopentane [127] with the distances (Å) rCα-Cβ = 1.526(5), rCβ-Cγ = 1.543(5), rCγ-Cγ′ =
1.555(5) and dihedral angle (°) CβCαCβ′Cγ′ = 40.2(5). The the CβCαCβ′Cγ′ dihedral angle is
slightly larger in cyclopentylamine but this result seems to be the reasonable result of the
difference in substituent size on the five membered ring. The remaining parameters are within their
respective uncertainties. These cases show that the five membered ring matches well with those
previously determined.
429
In the amine substituted alkanes previously studied [62], [179, 180] the amine groups are
usually within 0.003 Å of the values predicted by the ab initio MP2 calculation with full electron
correlation at the 6-311+G(d,p) basis set. The largest difference reported is for cyclobutylamine
[181] where the difference between the experimental and predicted C-N bonding distance are
0.005 Å. This difference in bond distance is similar to that determined in this study. This may be a
trend with rings but there are no other primary amine substituted rings for comparison. It would
therefore be of interest to determine the structural parameters for further primary amine substituted
rings.
To support the experimental values the ab initio energy differences have been determined by
a variety of basis sets (Table 118) which can be compared to the experimentally determined
enthalpy differences listed in Table 121. The order of stability is not well predicted by the ab initio
MP2(full) calculations as the order of stability changes from one basis set to another. The DFT
calculations by the B3LYP method does not predict the order of stability as well since the stability
changes for the conformers depending on the size of the basis set except for g-Ax.
It is interesting to note that the conformational stability study never been reported on the
cyclopentyl amine molecule. However, in the current study the Raman spectra of Xenon solution
was used to predict the conformational stability where all the four conformers were observed in the
experimental spectra and their order of stability was determined. These results indicated the t-Ax
conformer as the most stable form from both the Raman and microwave spectra studies and the
remaining three conformers are nearly equal in energy.
It would be of interest to obtain the enthalpy differences for the cyclohexylamine molecule
since it has never been studied before and a spectroscopy investigation would be of interest to see
how different amine substituents affect the conformational stability of five and six membered ring
430
molecules and whether there will be a nearly equal amount of all four conformers of the
cyclohexylamine molecule.
431
Table 112: Microwave spectrum for the t-Ax form of c-C5H9NH2. Observed wavenumbers of
rotational transitions and deviations of calculated values (MHz)
Transition
533
432
212
202
10 3 7
651
211
← 514
← 413
← 111
← 101
← 10 3 8
← 643
← 101
obs (MHz)
 (MHz)
10823.263
10897.743
12397.684
12867.411
13563.192
14301.194
15706.476
-0.448
-0.155
-0.110
0.122
0.016
0.440
0.013
432
Table 113: Rotational Constants (MHz), Quadratic centrifugal distortion constants (kHz) and
quadrupole coupling constants (MHz) for the 14N isotopomer of the t-Ax conformer of
Cyclpentylamine.
A
B
C
∆J
∆JK
∆K
δJ
δK
χaa
χac
χbb
χca
χcc

n
σ
MP2(full)/
6-311+G(d,p)
4919.79
3603.90
2920.81
1.01
-0.93
1.17
0.02
0.04
-0.70
-3.74
2.05
-3.74
-1.35
-0.32
a
B3LYP/
6-311+G(d,p)
4960.23
3496.76
2806.33
1.04
7.31
-6.88
0.02
2.45
-0.42
-3.98
2.29
-3.98
-1.88
-0.36
Fita
4909.46(5)
3599.01(4)
2932.94(4)
[0]
[0]
[0]
[0]
[0]
Distortion constants fixed to 0
433
-0.33
7
0.17
Table 114: Calculateda and Observed Wavenumbers (cm-1) for cyclopentylamine t-Ax Form.
Vib.
No.
A'
434
A"
Approx. description
ab
initio
Fixed
scaledb
IR
int.
1
NH2 symmetric stretch
3497
3318
0.4
2
γ CH2 antisymmetric stretch
3198
3000
3
β CH2 antisymmetric stretch
3156
2961
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
γ CH2 symmetric stretch
β CH2 symmetric stretch
α CH stretch
NH2 deformation
γ CH2 deformation
β CH2 deformation
α CH in plane bend
β CH2 rock
γ CH2 rock
β CH2 twist
γ CH2 twist
Ring deformation
CN stretch
β CH2 wag
NH2 wag
Ring deformation
Ring breathing
γ CH2 wag
Ring deformation
CN in plane bend
Ring puckering
NH2 antisymmetric stretch
3135
3106
3034
1716
1579
1554
1442
1399
1362
1278
1243
1119
1069
1017
944
926
888
821
705
405
204
3599
2941
2913
2847
1628
1485
1462
1371
1329
1293
1215
1186
1063
1017
969
902
881
845
784
677
400
203
3414
26
γ CH2 antisymmetric stretch
3181
27
28
29
30
31
β CH2 antisymmetric stretch
γ CH2 symmetric stretch
β CH2 symmetric stretch
γ CH2 deformation
β CH2 deformation
3152
3125
3100
1553
1541
Raman
act.
Infrared
gasc
Raman
solid Ac solid Bc
gasc
liquidc
solid Bc
IR
gas
Xe
soln.
P.E.D.d
Band
Contour*
A
C
108.9
3339
3235
3259
3336
3310
3173
3334
3322 100S1
57
43
48.4
52.5
-
-
-
-
-
-
3002
3000
91S2
35
65
19.9
100.0
-
-
-
-
-
-
2959
2959
94S3
28
72
34.9
41.3
67.6
33.8
3.6
5.0
18.6
0.5
0.6
1.2
5.7
2.0
1.3
23.5
52.3
18.4
6.5
39.0
10.7
2.6
3.2
0.1
175.9
123.7
83.3
8.4
9.6
7.0
2.4
0.3
13.2
16.8
2.2
5.7
7.6
3.5
13.2
6.2
13.5
0.7
0.8
0.2
0.2
67.1
1624
1461
1184
1056
1012
398
3400
2915
1602
1453
1180
1051
970
-
2915
1623
1453
1175
1064
1028
958
-
2935
1443
1048
1020
-
2930
1445
1055
1025
-
2921
1607
1445
1067
1028
961
-
418
3348
425
3355
400
3410
836
402
3360
410
3344
2944
2912
2843
1621
1491
1460
1372
1333
1293
1215
1188
1055
1018
967
782
887
835
774
677
398
3400
2941 93S4
2912 94S5
2845 98S6
1617 77S7,21S18
1498 75S8,24S9
1463 75S9,25S8
1377 60S10,15S13
1330 64S11,16S12
1291 40S12,19S14,16S13
1210 31S13,24S12,12S10,10S15
1185 39S14,10S23,10S17,10S10
1051 18S15,20S19,15S13,12S16,11S12
1018 31S16,28S19,11S13
965 26S17,18S14,15S18,10S23
796 22S18,30S15,13S20,10S17
888 24S19,29S20,13S17
835 43S20,17S16,10S15
- 42S21,24S22,13S18
- 25S22,36S21,10S23,10S17,10S16
393 44S23,20S22,17S24,15S17
- 75S24,22S23
3386 100S25
2984
2.0
56.0
-
-
-
-
-
-
2986
2986
86S26,10S27
-
-
2957
2932
2908
1461
1449
16.7
30.8
28.2
3.7
1.0
19.0
27.8
13.3
4.0
22.8
1461
-
1453
-
1453
-
1443
-
1445
-
2959
2935
2885
1460
1450
2959
2938
2886
1463
1446
90S27,10S26
92S28
97S29
62S30,38S31
62S31,39S30
-
-
1445
-
97
3
18 82
32 68
98
2
100
7 93
86 14
44 56
74 26
38 62
20 80
95
5
66 44
2 98
2 98
- 100
9 91
3 97
6 100
6 94
36 54
-
32
33
34
35
36
37
38
39
40
41
42
43
44
45
435
α CH out of plane bend
1429
1360
0.2
0.6
1356
1369
1390 1350
1348
1350
1353
1349 33S32,23S37,14S40,13S34
β CH2 rock
1382
1313
0.6
0.2
1308
1311
1312 26S33,25S34,12S38,12S36,12S35
γ CH2 rock
1348
1279
0.3
0.3
1272
1290
1278 1278
1287
1277
1272
1272 49S34,39S33,10S32
γ CH2 twist
1305
1239
1.0
14.2
1236
1239 77S35,10S37
442S36,2S36
β CH2 twist
1238
1178
0.6
0.6
1176
1173
1177
1177 42S36,25S39,10S41
NH2 twist
1204
1146
0.2
14.5
1142
1150
- 22S37,35S32,14S36,12S33
Ring deformation
1065
1014
0.0
5.5
1012
1028 1020
1025
1028
1010
1010 42S38,19S37,13S33,10S36
γ CH2 wag
1057
1005
1.7
0.8
1007
1002 33S39,18S41,15S36,10S38
Ring deformation
944
909
1.7
0.4
908
908 75S40,10S32
β CH2 wag
846
809
0.0
0.7
- 41S41,20S39,16S42,12S38,10S40
Ring deformation
654
641
1.6
0.6
640
- 62S42,11S41,10S39
CN out of plane bend
393
388
3.1
0.3
385
383 79S43,10S41
NH2 torsion
305
289 41.7
1.8
291 96S44
Ring twist
85
85
0.0
0.0
84 94S45
a
MP2(full)/6-31G(d) ab initio calculations, scaled wavenumbers, infrared intensities (km/mol), Raman activities (Å4/u), depolarization ratios (dp) and potential energy distributions
(P.E.D.s).
b
Scaled wavenumbers with scaling factors of 0.88 for CH stretches and CH2 deformations, 1.0 for heavy atom bends, and 0.90 for all other modes.
c
Ref [13]
d
Symmetry coordinates with P.E.D. contribution less than 10% are omitted.
*
A and C values in the last two columns are percentage infrared band contours.
-
Table 115: Calculateda and Observed Wavenumbers (cm-1) for cyclopentylamine t-Eq Form.
Vib.
No.
NH2 symmetric stretch
3497
3317
0.7
117.8
3339
3235
3259
3336
γ CH2 antisymmetric stretch
3185
2988
61.8
54.4
-
-
-
-
-
β CH2 antisymmetric stretch
3163
2967
8.2
101.6
2968
2949
2959
2970
γ CH2 symmetric stretch
β CH2 symmetric stretch
α CH stretch
NH2 deformation
γ CH2 deformation
β CH2 deformation
α CH in plane bend
γ CH2 rock
β CH2 rock
β CH2 twist
γ CH2 twist
CN stretch
Ring deformation
β CH2 wag
Ring deformation
NH2 wag
Ring breathing
γ CH2 wag
Ring deformation
CN in plane bend
Ring puckering
NH2 antisymmetric stretch
3126
3108
2997
1719
1585
1562
1457
1376
1352
1286
1238
1159
1094
1025
949
926
906
788
553
480
193
3596
2932
2916
2811
1631
1490
1469
1386
1308
1283
1222
1180
1102
1040
975
909
885
864
748
539
464
193
3412
31.8
21.7
60.2
32.3
1.9
5.1
20.8
0.3
0.1
2.3
3.3
5.8
0.1
20.8
39.8
45.0
39.7
1.4
3.8
11.2
1.5
0.1
178.3
102.1
73.8
10.5
10.6
5.7
5.7
8.0
3.1
12.9
10.3
7.9
3.7
5.1
2.1
6.7
13.2
0.9
4.1
0.7
0.1
70.6
1624
1184
1107
537
460
3400
1602
1473
1180
1104
984
-
1623
1472
1175
1113
978
-
-
860
536
465
3348
γ CH2 antisymmetric stretch
3169
2973
5.6
79.9
-
27
β CH2 antisymmetric stretch
3155
2960
20.9
0.1
28
γ CH2 symmetric stretch
3115
2922
29.6
29
β CH2 symmetric stretch
3105
2913
24.0
A 1
' 
2
3
436
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
A 25
"
26
Raman
act.
Raman
Fixed
scaledb
Approx. description
IR
int.
Infrared
ab
initio
gasc
solid
Bc
3310 3173
solid Ac solid Bc gasc liquidc
IR Xe
gas soln.
P.E.D.
d
Band
Contour*
A
3334
3322 100S1
-
2991
2990
2950
2950
2965
2967
1472
1380
1110
950
-
1473
1382
1102
947
-
1607
1478
1388
1308
1109
973
-
858
542
468
3355
834
538
460
3410
850
536
457
3360
861
540
466
3344
2935
2912
2811
1621
1491
1472
1388
1311
1285
1222
1188
1108
1035
972
908
764
868
748
535
461
3400
2938 93S4
2912 96S5
2815 99S6
1617 78S7,21S19
1498 67S8,32S9
1475 67S9,33S8
1388 50S10,25S12
1312 51S11,17S13,13S10,10S16,10S12
1282 31S12,26S14,15S10,13S13
1220 42S13,18S11,13S17,10S18
1185 37S14,12S12,10S23,10S17,10S11,10S10
1103 56S15,11S12
1035 47S16,18S18,11S11,10S17,10S15
971 19S17,32S14,10S23,10S19,10S13
908 39S18,17S19,13S17,10S22
784 24S19,26S16,14S20,10S17,10S18
867 53S20,23S19
- 81S21
536 40S22,19S15,18S23,10S20,10S16
466 35S23,23S22,18S17,15S24
185 73S24,24S23
3386 100S25
-
-
-
-
-
2978
2979
-
-
-
-
2959
22.2
-
-
-
-
-
-
16.6
-
-
-
-
-
-
C
100
-
84S2,15S3
4
96
83S3,14S2
10
90
97
75
5
67
91
6
100
84
72
12
46
97
97
18
46
31
36
37
87
75
92
-
3
25
95
33
9
94
16
18
88
54
3
3
82
54
69
64
63
13
25
8
-
51S26,48S27
-
-
2959
51S27,44S26
-
-
2927
2926
92S28
-
-
2912
2912
96S29
-
-
437
1558
1465
2.4
4.6
1461
1453 1453 1443 1445 1445 1460 1463 69S30,31S31
30 γ CH2 deformation
1547
1455
0.5
23.4
- 1450 1446 69S31,32S30
31 β CH2 deformation
1444
1373
1.5
1.2
- 1372 1377 41S32,23S36,10S40,10S43
32 CH out of plane bend
1374
1306
0.5
0.0
- 1303 1303 1302 57S33,12S34,10S38,10S37,10S35
33 γ CH2 rock
1334
1266
0.1
1.8
- 1268
- 1268 1266 1267 51S34,23S33,19S35
34 β CH2 rock
1317
1254
1.5
17.0
- 1254 1254 39S35,18S36,17S32,12S34
35 γ CH2 twist
1261
1200
0.2
4.6
1204
1207 1193 1188 1174 1193 1204 1199 442S36,2S36
20S36,26S35,14S32,11S38,10S39
36 NH2 twist
442S36,2S36
1234
1174
0.6
2.8
1176
1173
- 1177 1177 49S37,24S39,10S41
37 β CH2 twist
442S36,2S36
1078
1027
0.1
4.5
- 1027 1027 41S38,26S36,14S34,10S42
38 Ring deformation
1008
960
0.2
1.5
957
961 17S39,27S37,17S40,11S38,10S32
39 γ CH2 wag
983
943
3.3
0.1
948
- 44S40,17S41,10S39
40 Ring deformation
851
813
0.0
1.0
- 43S41,20S39,15S42,11S38,10S40
41 β CH2 wag
638
624
0.1
1.7
623
624 60S42,16S41,10S39,10S38
42 Ring deformation
363
357
13.9
0.6
356
334
338
355
355
368
355
347 67S43,14S44
43 CN out of plane bend
264
252
38.5
1.5
252 82S44
44 NH2 torsion
46
46
0.1
0.0
50 93S45
45 Ring twist
a
MP2(full)/6-31G(d) ab initio calculations, scaled wavenumbers, infrared intensities (km/mol), Raman activities (Å4/u), depolarization ratios (dp) and potential energy distributions
(P.E.D.s).
b
Scaled wavenumbers with scaling factors of 0.88 for CH stretches and CH2 deformations, 1.0 for heavy atom bends, and 0.90 for all other modes.
c
Ref [178]
d
Symmetry coordinates with P.E.D. contribution less than 10% are omitted.
*
A, B and C values in the last two columns are percentage infrared band contours.
Table 116: Calculateda and Observed Wavenumbers (cm-1) for cyclopentylamine g-Eq Form.
Vib.
No.
Fixed
scaledb
NH2 antisymmetric
stretch
NH symmetric stretch
3593
3409
0.1
58.9
3400
3348
3490
3311
0.4
92.5
3339
3235
γ CH2 antisymmetric
stretch
β CH2 antisymmetric
stretch
γ CH2 antisymmetric
stretch
β CH2 antisymmetric
stretch
γ CH symmetric stretch
3186
2988
59.2
56.2
-
γ CH2 symmetric stretch
β CH2 symmetric stretch
β CH2 symmetric stretch
α CH stretch
NH2 deformation
γ CH2 deformation
β CH2 deformation
γ CH2 deformation
β CH2 deformation
α CH in plane bend
CH out of plane bend
β CH2 rock
γ CH2 rock
γ CH2 rock
β CH2 rock
γ CH2 twist
γ CH2 twist
β CH2 twist
3175
3163
3152
3125
3115
3098
3086
3081
1717
1583
1558
1556
1543
1467
1401
1386
1378
1370
1334
1311
1296
1260
2979
2967
2957
2932
2922
2906
2895
2891
1629
1489
1465
1464
1452
1395
1331
1317
1310
1302
1266
1248
1231
1201
10.2
2.8
32.8
37.3
19.4
52.0
15.4
6.8
32.2
2.2
5.5
2.6
0.7
8.4
6.3
2.4
0.4
0.7
0.0
0.3
2.0
4.1
109.5
57.1
51.4
128.3
35.7
29.3
19.9
130.5
7.8
10.3
4.9
7.2
22.0
5.4
4.6
3.4
3.6
6.0
3.5
7.4
8.6
16.0
26
γ CH2 wag
1235
1176
0.6
27
Ring deformation
1186
1135
28
29
30
CN stretch
NH2 twist
Ring deformation
1173
1110
1093
1114
1056
1040
1
2
3
438
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
2
2
IR
int.
Raman
act.
Infrared
ab
initio
Approx. description
c
Raman
c
Xe
soln.
P.E.D.d
c
3355
3410
3360
3344 3400
3259
3336
3310
3173 3334
-
-
-
-
- 2991
2990
67S3,21S6
2968
2875
2875
1624
1461
1461
1322
1226
1204
2949
2862
2862
1602
1453
1453
1302
1225
1207
2959
2875
2875
1623
1453
1453
1325
1268
1231
1193
2970
2882
2882
1443
1443
1188
2950
2875
2875
1445
1445
1174
- 2978
2950 2965
2959
- 2935
- 2927
- 2885
2870 2874
2870 2874
1607 1621
- 1491
1445 1460
1445 1460
- 1450
- 1398
- 1333
1324 1322
1308 1311
1303 1303
1268 1266
1245 1248
- 1226
1193 1204
2979
2967
2959
2938
2926
2886
2871
2871
1617
1498
1463
1463
1446
1394
1330
1319
1312
1302
1267
1245
1226
1199
32S4,29S3,18S6
76S5,17S6
28S6,43S4,12S5
90S7
90S8
54S9,34S11
66S10,21S11,11S4
43S11,33S9,14S10,10S6
78S12,21S35
75S13,23S14
41S14,32S15,17S13,11S16
48S15,35S14,10S16,10S13
79S16,21S15
1.6
1176
1173
-
-
-
- 1177
1177
12.9
2.2
-
-
1134
-
-
- 1135
1138
6.6
5.1
0.0
6.0
3.0
3.7
1111
-
-
-
-
-
- 1122
- 1046
- 1035
1123 41S28,20S19,12S23
1044
,14S
25,14S
27,10S28,10S23,10S17
1035 26S
46S2930
,19S
34,11S21
solid A solid B
gas liquid
solid B
c
IR
gas
c
gas
c
Band Contour*
A
B
3386 100S1
75
23
2
3317 100S2
49
3
48
3
4
93
10
17
2
92
10
18
61
21
92
4
95
40
4
15
19
71
12
92
100
76
4
76
8
86
80
39
32
26
37
89
45
76
31
78
88
5
-
14
79
22
4
82
20
47
8
70
63
11
5
15
20
54
3
29
3
-
26S26,43S33,10S37
4
87
9
25S27,10S39,10S31
25
69
6
98
77
-
23
45
2
55
51S17,17S29,11S19
37S18,13S20,12S19
34S19,17S18,10S29,10S17
39S20,13S21,11S25,10S24,10S22
43S21,13S25,13S20
35S22,23S24,20S20
29S23,14S32,12S24,11S29
41S24,31S22
26S25,17S21,11S2310S34
C
439
1013
968 14.9
3.7
970
958
961 967
965 24S31,26S27,10S23,10S33
46 51 3
31 Ring deformation
997
951
4.9
1.5
942
925
- 952
950 17S32,17S31,12S37,10S26,10S23
6 91 3
32 β CH2 wag
994
946
4.4
2.0
- 948
945 17S33,19S26,14S32,10S23,10S18
46 40 14
33 β CH2 twist
939
898 61.9
1.0
893
892
895
992 898
898 33S34,27S35,10S31,10S31,10S12
78 22
34 Ring deformation
24S
,18S
,17S
,10S
,10S
924
884 73.7
4.3
764
784
82 18
35 NH2 wag
35
36
30
34
12
902
861 26.5
17.2
860
858
834
850
861 868
867 442S36,2S36
48S36,10S40,16S35
83 17
36 Ring breathing
843
805
4.0
1.0
- 800
800 41S37,18S26,14S39,11S27
75 25
37 β CH2 wag
γ
CH
wag
787
748
1.9
1.5
748
59S
,16S
23
77
38
2
38
37
632
619
1.1
2.2
609
601
611 605
604 40S40,18S41,17S28
22 78
39 Ring deformation
550
537
1.5
4.9
537
536
542
538
536
540 535
536 40S40,18S41,17S28,10S36
20
2 78
40 Ring deformation
472
455
2.1
2.5
459
- 452
455 34S41,22S40,18S32
7 27 66
41 CN in plane bend
CN
out
of
plane
bend
355
350
15.8
1.5
352
347
78S
,10S
86
4 10
42
42
31
289
275 49.2
1.8
272 93S43
10 24 66
43 NH2 torsion
195
194
0.7
0.4
185 69S44,25S41
63
2 35
44 Ring puckering
43
41
0.3
0.0
50 91S45
75
9 81
45 Ring twist
a
MP2(full)/6-31G(d) ab initio calculations, scaled wavenumbers, infrared intensities (km/mol), Raman activities (Å4/u), depolarization ratios (dp) and potential energy distributions
(P.E.D.s).
b
Scaled wavenumbers with scaling factors of 0.88 for CH stretches and CH2 deformations, 1.0 for heavy atom bends, and 0.90 for all other modes.
c
Ref [178]
d
Symmetry coordinates with P.E.D. contribution less than 10% are omitted.
*
A, B and C values in the last two columns are percentage infrared band contours.
Table 117: Calculateda and Observed Wavenumbers (cm-1) for cyclopentylamine g-Ax Form.
Vib.
No.
Approx. description
ab
initio
Fixed
scaledb
IR
int.
Raman
act.
Infrared
Raman
gasc solid Ac solid Bc gasc liquidc solid Bc
IR
gas
P.E.D.d
Xe
soln.
Band
Contour*
A
B
C
71
8
1
NH2 antisymmetric stretch
3598
3414
0.0
44.3
3400
3348
3355 3410
3360
3344
3400
3386 100S1
21
2
NH2 symmetric stretch
β CH2 antisymmetric
stretch
γ CH2 antisymmetric
stretch
γ CH2 antisymmetric
stretch
β CH2 antisymmetric
stretch
α CH stretch
3496
3317
0.3
84.0
3339
3235
3259 3336
3310
3173
3334
3322 100S2
2
27 72
3188
2991
45.6
50.7
-
-
-
-
-
-
2991
2990
39S3,34S4
38
6 56
2981
2967
2958
2937
2930
2922
2921
2904
1631
1491
1469
1462
1450
1386
1330
1321
1300
1284
1273
1236
1228
1199
18.6
1.4
26.9
20.3
65.4
25.1
3.6
22.3
29.5
2.6
3.8
4.7
3.7
6.3
2.1
0.7
0.6
5.1
0.7
1.7
3.0
2.7
98.9
59.8
45.2
214.5
47.3
19.1
45.6
61.3
6.9
8.6
8.6
5.8
19.0
4.1
1.8
0.7
13.1
3.6
0.9
3.4
11.2
9.4
2968
1624
1461
1322
1226
1204
2949
2915
1602
1473
1453
1302
1272
1225
1207
2959
2915
1623
1742
1453
1325
1290
1231
1193
2970
2935
-
2950
2950
γ CH2 symmetric stretch
β CH2 symmetric stretch
γ CH2 symmetric stretch
β CH2 symmetric stretch
NH2 deformation
γ CH2 deformation
γ CH2 deformation
β CH2 deformation
β CH2 deformation
α CH in plane bend
CH out of plane bend
β CH2 rock
γ CH2 rock
β CH2 rock
γ CH2 rock
β CH2 wag
γ CH2 twist
γ CH2 twist
3178
3163
3153
3130
3124
3115
3114
3096
1719
1586
1562
1555
1541
1459
1399
1391
1368
1350
1342
1296
1292
1261
1472
1443
1380
1278
1188
2930
1473
1445
1382
1278
1174
2921
1607
1478
1445
1388
1324
1303
1287
1193
2986
2965
2959
2944
2935
2927
2927
2885
1621
1491
1472
1460
1450
1388
1333
1322
1303
1285
1277
1236
1226
1204
2986
2967
2959
2941
2938
2926
2926
2886
1617
1498
1475
1463
1446
1388
1330
1319
1302
1282
1272
1239
1226
1199
58S4,24S6
73S5,18S3
44S6,31S3,14S5
57S7,20S11,14S8
75S8,18S7
24S9,31S10,19S11,13S7
60S10,14S9,14S11
41S11,52S9
78S12,21S33
86S13,13S15
91S14
84S15,13S13
92S16
54S17,20S37
18S18,22S19,21S22
41S19,15S22,13S32
37S20,30S29,10S37
19S21,24S24,16S26
48S22,31S21,15S18
15S23,22S25,11S27,10S30,10S24
38S24,13S29,10S20
13S25,18S24,14S2010S29,10S28,10S26
58
58
4
12
45
2
8
74
2
2
2
87
77
79
58
67
76
13
2
43
11
9
67
9
2
88
92
65
26
3
68
98
7
19
9
29
10
6
81
35
31
33
28
79
54
10
35
73
24
31
98
13
16
2
32
4
14
81
18
21
26
β CH2 twist
1228
1170
4.7
4.5
-
1173
-
-
-
-
1175
1175
35S26,17S31,11S37,11S28
24
76
-
27
NH2 twist
1174
1116
1.9
7.5
1111
-
-
-
-
-
1122
1123
22S27,21S18,15S17
36
43 21
28
29
30
γ CH2 wag
β CH2 twist
CN stretch
1111
1097
1070
1059
1042
1018
17.6
1.1
1.3
4.3
4.1
6.0
1012
-
1028 1020
1025
1028
1046
1035
1018
1044
1035
1018
10S28,11S37,11S26
20S29,18S35, ,15S25,12S32,10S30
23S30,20S35,13S31,10S29,10S25
44
2
9
46 9
94 4
2 90
3
440
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
441
1050
1000
2.0
3.8
- 1007 1002 26S31,21S28,11S21,10S37
31 Ring deformation
974
929
0.6
0.6
- 28S32,37S28,10S25
32 Ring deformation
964
919 91.2
2.3
799
806 31S33,22S34,10S26,10S12
33 NH2 wag
932
893 42.3
1.6
893
- 892
895
992
898
898 43S34,21S33,10S35
34 Ring deformation
923
882
7.4
12.1
887
888 15S35,46S37,13S32
35 Ring deformation
866
824 20.5
12.6
823
823 442S36,2S36
32S36,23S30,16S39,13S32
36 Ring breathing
846
808 13.1
7.3
803
803
804
800
800 23S37,10S28,10S40,10S31
37 β CH2 wag
815
779
5.7
3.6
774
776 20S38,26S39,15S37
38 Ring deformation
710
683
4.0
2.5
695
693 31S39,27S38,10S42,10S32,10S23
39 γ CH2 wag
645
630
6.3
1.4
623
624 57S40,10S37,10S28
40 Ring deformation
408
402 11.1
0.7
398
418
425 400
402
410
398
393 66S41,10S42,10S37
41 CN out of plane bend
404
399
2.6
0.6
398
418
425 400
402
410
398
393 38S42,15S38,14S44,11S41
42 CN in plane bend
257
246 47.0
1.7
238
240
240
238
246 80S43,12S44
43 NH2 torsion
198
196
5.1
0.3
185 60S44,21S42,13S43
44 Ring puckering
68
68
0.6
0.0
58 88S45,10S44
45 Ring twist
a
MP2(full)/6-31G(d) ab initio calculations, scaled wavenumbers, infrared intensities (km/mol), Raman activities (Å4/u), depolarization ratios (dp) and potential energy
(P.E.D.s).
b
Scaled wavenumbers with scaling factors of 0.88 for CH stretches and CH2 deformations, 1.0 for heavy atom bends, and 0.90 for all other modes.
c
Ref [178]
d
Symmetry coordinates with P.E.D. contribution less than 10% are omitted.
*A, B and C values in the last two columns are percentage infrared band contours.
15 63 22
40 21 39
53 29 18
68 8 23
88 5 7
69 11 21
54 12 35
47 26 27
26 17 57
65 2 33
88 9 3
47 2 52
50 27 23
63 34 3
46 48 6
distributions
Table 118: Calculated energies in (H) and energy differences (cm-1) for the conformers of
cyclopentylamine.
Energya, E
Method/Basis Set
MP2(full)/6-31G(d)
MP2(full)/6-31+G(d)
MP2(full)/6-31G(d,p)
MP2(full)/6-31+G(d,p)
MP2(full)/6-311G(d,p)
MP2(full)/6-311+G(d,p)
MP2(full)/6-311G(2d,2p)
MP2(full)/6-311+G(2d,2p)
MP2(full)/6-311G(2df,2pd)
MP2(full)/6-311+G(2df,2pd)
MP2(full)/aug-cc-pVTZ
# basis set
112
136
145
169
174
198
237
261
334
358
529
B3LYP/6-31G(d)
112
B3LYP/6-31+G(d)
136
B3LYP/6-31G(d,p)
145
B3LYP/6-31+G(d,p)
169
B3LYP/6-311G(d,p)
174
B3LYP/6-311+G(d,p)
198
B3LYP/6-311G(2d,2p)
237
B3LYP/6-311+G(2d,2p)
261
B3LYP/6-311G(2df,2pd)
334
B3LYP/6-311+G(2df,2pd)
358
B3LYP/aug-cc-pVTZ
529
a
Energy of conformer is given as –(E+251) H.
-1
b
Difference is relative to t-Ax form and given in cm
442
Energy Differences, ∆b
t-Ax
t-Eq
g-Eq
g-Ax
0.0312748
0.0471294
0.1256127
0.1398061
0.2988582
0.3064130
0.3726749
0.3785483
0.4731256
0.4784292
448
299
448
333
479
350
428
341
411
313
475
420
459
441
502
416
485
429
443
379
520
514
507
523
474
455
447
458
417
434
0.4916858
347
402
0.9004192
0.9117836
0.9194864
0.9303775
0.9749910
0.9798241
0.9858919
0.9903203
0.9919779
0.9962351
1.0033749
48
18
47
27
81
44
58
28
60
23
33
-3
46
-12
45
69
59
58
60
52
47
57
205
374
198
372
248
378
230
395
230
396
401
Table 119: Structural parameters (Å and degrees), rotational constants (MHz) and dipole moments
(Debye) for t-Ax form of Cyclopentylamine.
Structural
Parameters
Int.
coor.
MP2(full)/
B3LYP/
Adjusted
66r0a
311+G(d,p)
311+G(d,p)
rC-N
R1
1.466
1.472
1.470(3)
rCα-Cβ
R2
1.528
1.537
1.529(3)
rCβ-Cγ
R3
1.542
1.547
1.544(3)
rCγ-Cγ′
R4
1.552
1.556
1.550(3)
rN-H
R5
1.016
1.015
1.016(3)
rCα-H
r1
1.100
1.100
1.100(2)
rCβ-H1
r2
1.095
1.094
1.095(2)
rCβ-H2
r3
1.096
1.095
1.096(2)
rCγ-H1
r4
1.092
1.091
1.092(2)
rCγ-H2
r5
1.094
1.093
1.094(2)
109.6
δ
108.8
108.7(5)
CβCαN
1
102.4
102.0
101.4(5)
CβCαCβ
1
105.1
104.2
104.3(5)
CγCβCα
2
105.8
105.5
105.3(5)
CβCγCγ′
3
110.9
110.2
110.3(5)
CαNH
1
107.2
107.0
106.9(5)
HNH′
2
112.6
λ1
113.2
113.2(5)
 HCαN
111.1
λ2
111.7
112.0(5)
 HCαCβ
112.4
λ
112.6
112.6(5)
 H1CβCα
3
112.9
λ4
112.9
111.9(5)
 H1CβCγ
108.7
λ5
108.7
108.7(5)
 H2CβCα
110.6
λ6
110.7
111.6(5)
 H2CβCγ
107.1
λ
107.7
107.7(5)
 H1CβH2
7
110.4
π1
110.1
110.1(5)
 H1CγCβ
110.2
π3
110.0
110.6(5)
 H1CγCγ′
111.3
π4
111.4
111.3(5)
 H2CγCβ
112.2
π
112.1
111.7(5)
 H2CγCγ′
5
107.1
π6
107.7
107.7(5)
 H1CγH2
38.2
41.3
42.0(5)
CβCαCβ′Cγ′ 1
64.6
65.9
66.3(5)
HNCαCβ
2
59.5
58.9
58.9(5)
HNCαH
3
A(MHz)
4919.79
4960.23 4909.55(9)
B(MHz)
3603.90
3496.76 3599.09(8)
C(MHz)
2920.81
2806.33 2932.87(7)
0.972
0.850
|a|
0.000
0.000
|b|
0.887
0.866
|c|
1.316
1.213
|t|
a
Adjusted parameters using the microwave data from Table 112 for the given ground states.
Values in parentheses are error limits for structural parameters and for the rotational
constants they are the absolute difference from the experimentally determined value.
443
Table 120: Symmetry Coordinates for cyclopentylamine with respect to the t-Ax.
Symmetry Coordinatea
Description
A'
A"
NH2 symmetric stretch
γ CH2 antisymmetric stretch
β CH2 antisymmetric stretch
γ CH2 symmetric stretch
β CH2 symmetric stretch
α CH stretch
NH2 deformation
γ CH2 deformation
β CH2 deformation
α CH in plane bend
β CH2 rock
γ CH2 rock
β CH2 twist
γ CH2 twist
Ring deformation
CN stretch
β CH2 wag
NH2 wag
Ring deformation
Ring breathing
γ CH2 wag
Ring deformation
CN in plane bend
Ring puckering
NH2 antisymmetric stretch
γ CH2 antisymmetric stretch
β CH2 antisymmetric stretch
γ CH2 symmetric stretch
β CH2 symmetric stretch
γ CH2 deformation
β CH2 deformation
α CH out of plane bend
β CH2 rock
γ CH2 rock
γ CH2 twist
β CH2 twist
NH2 twist
Ring deformation
γ CH2 wag
Ring deformation
β CH2 wag
Ring deformation
S1
S2
S3
S4
S5
S6
S7
S8
S9
S10
S11
S12
S13
S14
S15
S16
S17
S18
S19
S20
S21
S22
S23
S24
S25
S26
S27
S28
S29
S30
S31
S32
S33
S34
S35
S36
S37
S38
S39
S40
S41
S42
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
444
R5 + R5'
r4 – r4′ + r5 – r5′
r2 – r2′ + r3 – r3′
r4 + r4′ + r5 + r5′
r2 + r2′ + r3 + r3′
r1
2
π 6 + π 6′
λ7 + λ7′
2 – λ3 – λ3'
λ3 + λ3′ + λ5 + λ5′ – λ4 – λ4′ – λ6 – λ6′
π1 + π1′ + π4 + π4′ – π3 – π3′ – π5 – π5′
λ3 – λ3′ + λ5 – λ5′ – λ4 + λ4′ – λ6 + λ6′
π1 – π1′ + π4 – π4′ – π3 + π3′ – π5 + π5′
R2 + R2′+ R3 + R3′ – 4R4
R1
λ3 –λ3′ + λ5 – λ5′ + λ4 – λ4′ + λ6 – λ6′
1 + 1′
R2 + R2′ – R3 – R3′
R2 + R2′+ R3 + R3′ + R4
π1 – π1′ + π4 – π4′ + π3 – π3′ + π5 – π5′
3θ1 – 2θ2 – 2θ2′ + θ3 + θ3′
δ1 + δ1′
θ2 – θ2′
R5 – R5'
r4 – r4′ – r5 + r5′
r2 – r2′ – r3 + r3′
r4 + r4′ – r5 – r5′
r2 + r2′ – r3 – r3′
π6 – π6′
λ7 – λ7′
λ3 – λ3'
λ3 + λ3′ – λ5 – λ5′ + λ4 + λ4′ – λ6 – λ6′
π1 + π1′ – π4 – π4′ + π3 + π3′ – π5 – π5′
π1 – π1′ – π4 + π4′ – π3 + π3′ + π5 – π5′
λ3 – λ3′ – λ5 + λ5′ – λ4 + λ4′ + λ6 – λ6′
1 – 1′
R2 – R2′– R3 + R3′
π1 – π1′ – π4 + π4′ + π3 – π3′ – π5 + π5′
R2 – R2′+ R3 – R3′
λ3 – λ3′ – λ5 + λ5′ + λ4 – λ4′ – λ6 + λ6′
θ2 – θ2′ + θ3 – θ3′
CN out of plane bend
NH2 torsion
Ring twist
a
Not normalized.
S43
S44
S45
=
=
=
445
δ1 – δ1′
3
θ2 + θ2′
Table 121: Temperature and intensity ratios of the t-Ax, t-Eq, g-Eq and g-Ax bands of
cyclopentylamine.
Liquid
xenon
H (cm-1)
H (cm-1)a
a
T(C)
1/T (10-3 K-1)
60.0
70.0
80.0
90.0
100.0
4.692
4.923
5.177
4.460
5.775
t-Ax → t-Eq
t-Ax → g-Eq
t-Ax → g-Ax
I1051 / I1027
I1010 / I1027
I835/ I1027
I1051/ I950
I1010 / I950
I1051 / I806
I1010 / I806
4.891
4.588
4.313
3.879
3.636
7.578
6.688
6.275
5.808
5.455
6.172
5.363
5.000
4.636
4.182
2.204
2.640
2.666
2.954
3.279
3.415
3.849
3.879
4.423
4.918
1.944
2.198
2.413
2.685
2.920
3.012
3.204
3.510
4.021
4.380
195  10
203  22
211  11
236 ± 22
231  35 224  25
227  20
Average value H and statistical uncertainty (1σ) obtained by utilizing all of the data as a single set.
446
259  18
251  14
255  11
Figure 65: Atomic numbering of cyclopentylamine with the t-Ax form shown.
447
Figure 66: Comparison of experimental and calculated infrared spectra of cyclopentylamine: (A)
observed spectrum of gas; (B) simulated spectrum of a mixture of the four stable conformers of
cyclopentylamine at 25°C; (C) simulated spectrum of g-Ax conformer; (D) simulated spectrum of
g-Eq conformer; (E) simulated spectrum of t-Eq conformer; (F) simulated spectrum of conformer
t-Ax.
448
Figure 67: Comparison of experimental and calculated Raman spectra of cyclopentylamine: (A) xenon
solution at -100°C; (B) simulated spectrum of a mixture of the four stable conformers of
cyclopentylamine at -100°C; (C) simulated spectrum of g-Ax conformer; (D) simulated spectrum of gEq
conformer;
(E) simulated
spectrum of t-Eq
conformer;Raman
(F) simulated
of conformer t-Ax.
Figure
68: Comparison
of experimental
and calculated
spectraspectrum
of cyclopentylamine:
(A)
xenon solution at -100°C; (B) simulated spectrum of a mixture of the four stable conformers of
cyclopentylamine at -100°C; (C) simulated spectrum of g-Ax conformer; (D) simulated spectrum
of g-Eq conformer; (E) simulated spectrum of t-Eq conformer; (F) simulated spectrum of
conformer t-Ax.
449
Figure 69: Spectra of cyclopentylamine (A) mid-infrared gas; (B) Raman Xe. solution at -100°C.
450
Figure 70: Raman spectra of cyclopentylamine in xenon solution at different temperatures.
451
CHAPTER 17
MICROWAVE AND INFRARED SPECTRA, ADJUSTED R0 STRUCTURAL PARAMETERS,
CONFORMATIONAL STABILITIES, VIBRATIONAL ASSIGNMENTS AND THEORETICAL
CALCULATIONS OF CYCLOBUTYLCARBOXYLIC ACID CHLORIDE
Introduction
Our interest in the conformational stabilities and structural parameters of small ring
compounds, particularly monosubstituted four and five membered ring molecules, has led us to
reinvestigate some of these molecules where there is considerably controversy of the most stable
conformers. One such molecule is the monosubstituted four-membered ring cyclobutylcarboxylic
acid chloride where there have been four different conformers/structure studies reported.These
four possible conformers are the gauche-equatorial (g-Eq), gauche-axial (g-Ax), trans-equatorial
(t-Eq), and trans-axial (t-Ax) where the three stable conformers (g-Eq, g-Ax and t-Eq) are shown
in Fig. 70.
The earlier structural determination of this molecule in the vapor state was obtained by the
electron diffraction technique by Adam and Bartell [182] in 1971. These scientists were interested
in the structure and conformations of the series R-COX and concluded that the cyclobutyl
derivative exists almost entirely as the gauche conformations. From this study it was concluded
that there was coupling between the ring bending and torsional displacements based on the
electron diffraction intensities. From these couplings it was observed that the gauche conformation
aligned toward the cis side favored the equatorial puckered ring and those on the trans side favored
the axial puckered ring. Bond lengths and bond angles were determined and it was concluded that
they were “normal” which indicates that they were similar to those obtained in “unstrained” alkyl
carboxaldhydes [183-185] which is rather surprising since cyclobutyl compounds are moderately
strained.
452
Following this initial structural investigation [182] there were three [186-188] additional
structural investigations with two of them vibrational studies[186, 187] and the other a microwave
and vibrational study [188]. In the first vibrational study[186] the spectra were recorded of the
condensed phases (liquid and solid) and it was concluded that there was a single conformer present
in both phases and it was the same form.
A subsequent vibration investigation [187] was reported which included the infrared
spectrum of the gas as well as the spectra of the liquid and solid and the Raman spectrum of the
liquid. From this study there was clear evidence from the temperature dependence of the spectrum
of the liquid that multiple conformers exist in the fluid state. For example, some bands increased
where others decreased as the temperature was decreased and one band disappeared in the
spectrum of the crystal. From these studies it was assumed that the proposed three conformers
from the electron diffraction investigation were probably nearly equal concentrations.
From a later microwave and vibrational investigation [188] only the microwave spectrum
of the
35
Cl isotope of the g-Eq form was assigned and there was no microwave data for the
37
Cl
species or other conformers. From this microwave data a partial structure was reported for the gEq conformer. Also, in this study4 an investigation of the Raman spectrum of the liquid was
reported and the energy difference between the preferred g-Eq form and high energy (assumed to
be t-Eq) conformer was determined to be 1.4 kcal mol-1 (490 cm-1). Therefore from the four
reported studies to date, there is at least two and possibly three conformers present in the fluid
phases but there is still a question which conformer is the most stable form and what are the energy
differences, as well as, to assign the microwave spectral transitions of the 37Cl isotopologue for the
g-Eq form and determine a complete set of structural parameters from the microwave data. Thus, a
microwave and vibration investigation has been initiated to answer these questions.
453
To aid in identifying the fundamental vibrations for the additional conformers and to assign
fundamentals for the less stable conformers for use in determining the enthalpy differences ab
initio calculations have been utilized. The harmonic force fields, infrared intensities, Raman
activities, depolarization ratios, and vibrational frequencies were obtained from MP2/6-31G(d) ab
initio calculations with full electron correlation. Additionally both MP2(full) ab initio and Density
Functional Theory (DFT) calculations by the B3LYP method have been carried out with a variety
of basis sets up to aug-cc-pVTZ as well as with diffuse functions to predict the conformational
stabilities. The results of these spectroscopic, structural, and theoretical studies of
cyclobutylcarboxylic acid chloride are reported herein.
Experimental and Theoretical Methods
The sample of cyclobutylcarboxylic acid chloride was purchased from Sigma-Aldrich
Chemical Co., with stated purity of 98%. The sample was further purified by low-temperature,
low-pressure fractionation column and the purity of the sample was verified by comparing the
infrared spectrum with that previously reported4.
Microwave spectra were recorded by using a “mini-cavity” Fourier-transform microwave
spectrometer [17, 18] at Kent State University. The Fabry-Perot resonant cavity is established by
two 7.5-inch diameter diamond-tip finished aluminum mirrors with a 30.5-cm spherical radius.
The Fabry-Perot cavity resides inside a vacuum chamber formed by a 6-way cross and a 15-inch
long, 8-inch diameter extension tube. The two cavity mirrors are nominally separated by 30 cm.
The sample was entrained in a 70:30 Ne-He carrier gas mixture at 2 atm and expanded into
the cavity to attain 4K by using a reservoir nozzle [18] made from a modified Series-9 General
Valve. The reservoir nozzle is mounted in a recessed region of the mirror flange, external to the
vacuum chamber, and the expansion passes through a 0.182-inch diameter hole into the resonant
cavity. The center of the expansion is offset from the center of the mirror by 1 inch.
454
The sample was irradiated by microwave radiation generated by an Agilent Technologies
E8247C PSG CW synthesizer; details of the irradiation and heterodyne detection circuitry can be
found in Ref. [19] The vacuum system can accommodate pulse repetition rates of up to 15 s-1
while maintaining a pressure below 10-4 torr, and the instrument can scan 450 MHz in 6 hours
while averaging 100 shots per scan segment. The frequencies for the measured transitions in the
region of 10,500 to 22,000 MHz are listed in Tables 122 and 123 along with their assignments.
The infrared spectrum of the gas (Fig. 71A) was obtained from 4000 to 220 cm-1 on a PerkinElmer model 2000 Fourier transform spectrometer equipped with a Ge/CsI beamsplitter and a DTGS
detector. Atmospheric water vapor was removed from the spectrometer housing by purging with dry
nitrogen. The spectrum of the gas was obtained with a theoretical resolution of 0.5 cm-1 for the gas
with 128 interferograms added and truncated. The mid-infrared spectra (4000 to 400 cm-1) of the
sample dissolved in liquid xenon (Fig. 71B) were recorded on a Bruker model IFS-66 Fourier
transform spectrometer equipped with a globar source, a Ge/KBr beamsplitter and a DTGS
detector. In all cases, 100 interferograms were collected at 1.0 cm-1 resolution, averaged and
transformed with a boxcar truncation function. For these studies, a specially designed cryostat cell
was used. It consists of a copper cell with a path length of 4 cm with wedged silicon windows
sealed to the cell with indium gaskets. The copper cell was enclosed in an evacuated chamber
fitted with KBr windows. The temperature was maintained with boiling liquid nitrogen and
monitored by two Pt thermo resistors. After cooling to the designated temperature, a small amount
of the sample was condensed into the cell and the system was then pressurized with the noble gas,
which condensed in the cell, allowing the compound to dissolve. All of the observed bands in the
infrared spectra of the gas along with their proposed assignments of the g-Eq, g-Ax and t-Eq
conformers are listed in Tables 124, 125 and 126, respectively.
455
The LCAO-MO-SCF restricted Hartree-Fock calculations were performed with the
Gaussian-03 program [21] with Gaussian-type basis functions. The energy minima with respect to
nuclear coordinates were obtained by the simultaneous relaxation of all geometric parameters by
using the gradient method of Pulay [22]. Several basis sets as well as the corresponding ones with
diffuse functions were employed with the Møller-Plesset perturbation method [23] to second order
(MP2(full)) as well as with the density functional theory by the B3LYP method. The predicted
conformational energy differences are listed in Table 127. In order to obtain a complete description
of the molecular motions involved in the fundamental modes of cyclobutylcarboxylic acid
chloride, a normal coordinate analysis has been carried out. The force field in Cartesian
coordinates was obtained with the Gaussian 03 program [21] at the MP2(full) level with the 631G(d) basis set. The internal coordinates used to calculate the G and B matrices are given in
Table 128 with the atomic numbering shown in Fig. 70. By using the B matrix [24], the force field
in Cartesian coordinates was converted to a force field in internal coordinates. Subsequently,
scaling factor of 0.88 for the CH stretches, β-CH2 and γ-CH2 deformations and 0.90 for all other
modes excluding the heavy atom bending modes were used, along with the geometric average of
the scaling factors for the interaction force constants, to obtain the fixed scaled force field and
resultant wavenumbers. A set of symmetry coordinates was used for the g-Eq and g-Ax forms
(Table 129) and another set for the symmetric t-Eq conformer (Table 130) to determine the
corresponding potential energy distributions (P.E.D.s).
The vibrational spectra were predicted from the MP2(full)/6-31G(d) calculations. The
predicted scaled wavenumbers were used together with a Lorentzian function to obtain the
simulated spectra. Infrared intensities were obtained based on the dipole moment derivatives with
respect to Cartesian coordinates. The derivatives were transformed with respect to normal
coordinates by (u/Qi) = j (u/Xj)Lij, where Qi is the ith normal coordinate, Xj is the jth
456
Cartesian displacement coordinate, and Lij is the transformation matrix between the Cartesian
displacement coordinates and the normal coordinates. The infrared intensities were then calculated
by (N)/(3c2) [(x/Qi)2 + (y/Qi)2 + (z/Qi)2]. The resulting simulated spectra are shown in
Figure 72 and are compared to the infrared spectra of the xenon solutions at three different
temperatures where the simulated spectrum of the mixture of conformers should be comparable to
the experimental infrared spectra. It is interesting to note that whereas the band positions are
reasonably predicted the band intensities are relatively poorly predicted.
Microwave Results
The Cl atom in Cyclobutylcarboxylic acid chloride contains a nucleus with a spin quantum
number I = 3/2 and, therefore, a nuclear quadrupole moment which interacts with the electric field
gradient created by the electrons of the rotating molecule. The quadrupole moments of
35
Cl and
37
Cl are not equal. The angular momentum J of the rotating molecule couples with the angular
momentum I of the nuclear spin to create an overall angular momentum F whose quantum number
F can assume any of the values F = J + I, J + I - 1, … |J - I|. Only F is an exact quantum number
whereas J is only an approximate value. Therefore, a rotational level with quantum number J in the
absence of quadrupole coupling splits into sublevels that can be labeled by the exact quantum
number F when a nuclear quadrupole is present. Each rotational level with J > 1 splits into 4
components (1 and 3 components for J = 0 and 1, respectively). The selection rules for rotational
transitions are ΔF = 0, ± 1 and the strongest components are those given by ΔF = ΔJ.
The frequencies of the components of the split rotational transitions were used in a leastsquares fit to determine rotational, centrifugal distortional and quadrupole coupling constants. For
that purpose, a computer program developed for molecules with one or two quadrupole nuclei in a
plane of symmetry [189, 190] was used. Of course, g-Eq cyclobutylcarboxylic acid chloride does
not have a molecular symmetry plane. Because ab initio calculations performed by using the
457
MP2(full) method with the 6-311+G(d,p) basis set predicted the full nuclear quadrupole coupling
tensor with rather small values for the off-diagonal elements χac and χbc (a few MHz) but a
significant χab (tens of MHz), fitting the spectrum with this program seemed worth a trial. The
rotational and centrifugal distortion constants reported earlier4 from conventional microwave
spectroscopy for the Cl isotopologue together with quadrupole coupling constants from ab initio
calculations were used to predict the spectrum for initial assignments. The final results of the leastsquares fits (Table 131) justified the use of this program. The Fit 1 for the 35Cl isotopologue of 69
components of 18 rotational transitions ended with a standard deviation of 2.2 kHz and for the Fit
2 included transitions from Ref.4 and transition from this study where the hyperfine components
could not be resolved. For the fit of the
37
Cl substituted species, 84 components of 19 transitions
led to a standard deviation of 2.6 kHz. The frequencies of the assigned components of the observed
transitions with their residuals in the least-squares fit are listed in Tables 122 and 123 and are
excellent with errors equal to or less than 4 kHz for
35
Cl and 7 kHz for the
37
Cl. With accurately
determined rotational constants the adjusted r0 structure may also be determined. It should,
however, be noted that the fit of these rotational constants is probably better than the experimental
data can achieve where the digital precision for instrument used is 2.5 kHz. Therefore, the
rotational constants are reported for the 35Cl [37Cl] isotope; A = 4349.8429(25) [4322.0555(56)], B
= 1414.8032(25) [1384.5058(25)], and C = 1148.2411(25) [1126.3546(25)] MHz.
Vibrational Assignment
An earlier vibrational assignment [188] for cyclobutylcarboxylic acid chloride was made by
utilizing the Raman spectra of liquid and solid and the infrared spectra of the gas and solid for a
complete assignment of the g-Eq conformer fundamentals and to assign two fundamentals for high
energy conformer which could not be determined but it was assumed to be t-Eq. By the utilization
of MP2(full)/6-31G(d) fundamental wavenumber predictions along with ab initio predicted
458
intensities, and infrared data from xenon solutions it has been possible to assign nearly all of the
fundamentals for the g-Eq conformer in the current study. Additionally, a nearly complete
assignment of the fundamentals of the g-Ax form was made and a significant number of
fundamentals for the t-Eq conformer were assigned. These assignments are important for obtaining
the enthalpy differences since the fundamentals which are used in the variable temperature study
need to be identified correctly for a single conformer. Therefore, an attempt was made to assign
the fundamentals in the region from 1200 to 400 cm-1 for the three most stable conformers, where
the overtone and combination bands are greatly reduced in this region compared to those possible
in the higher wavenumber region. It is also interesting to determine what effect the acid chloride
group will have on the fundamental frequencies particularly those which involve the ring modes.
The vibrations of the CH2 groups are expected to be very similar to those found in the usual four
membered-ring molecules and a discussion of their assignments in detail is usually not necessary.
The most interesting assignments will be for the heavy atom fundamentals but these are in
the region where the lower frequency CH2 bending modes will undoubtedly interact with these
heavy atom vibrations. Therefore, these modes are expected to have contributions of three or four
motions. For example, in the case of the -CH2 twist for the g-Eq form the band at 946 cm-1 in the
infrared spectra of the gas has contributions from two of the heavy atom ring deformation modes.
Four membered carbon atom rings have well known ring deformations usually occurring in
a pattern of one near 1000 cm-1 and three near 900 cm-1. This pattern makes the assignment of
these modes relatively simple for the g-Eq conformer where they are predicted at 1032, 932, and
911 cm-1 and observed at 1027, 933, and 889 cm-1 in the infrared spectra of the gas. The ring
breathing mode usually occurs in this region as well and it is predicted at 970 cm -1 for the g-Eq
form and assigned at 960 cm-1 in the infrared spectra of the gas. This mode was predicted to have a
459
‘A’ type band contour and the band in the spectra of the gas corresponds clearly to a ‘A’ type band
contour and it was assigned based on this prediction.
For the g-Eq conformer there are some assignments necessary for bands in the infrared
spectra of the gas since from the previous study4 there were four fundamentals in the finger print
region that were not assigned in the infrared spectra of the gas. Three of these fundamentals are
CH2 bending modes and they are now assigned based on frequencies observed in the spectra of the
xenon solutions and are in agreement with the CH2 bending modes of the usual four memberedring molecules and the previous assignment from the infrared and/or Raman spectra of the liquid
and solid. The ring deformation fundamental is predicted at 932 cm-1 and is now assigned at 933
cm-1 in the infrared spectra of the gas. This band has a predicted Raman activity of 10.1 and was
previously assigned at 927 cm-1 in the Raman spectra of the liquid. This assignment is further away
than expected and the previous assignment is for a shoulder of a much stronger band, therefore, we
have reassigned the band center to 930 cm-1 by using the previously reported4 Raman spectra of
the liquid.
It should be noted that most of the bands which are reassigned in the current study in the
infrared spectra of the gas are within a few wavenumber differences from the previously reported
assignments except for ν17, the β-CH2 twist. This fundamental is predicted at 1205 cm-1 but it was
previously assigned at 1143 cm-1 in the infrared spectra of gas but it is now reassigned to the band
at 1204 cm-1. The band at 1143 cm-1 is now assigned as ν16′ for the g-Ax conformer based on the
predicted band position and the temperature dependence in the infrared spectra of the xenon
solutions.
The vibrational assignments were also made for the g-Ax conformer which is the second
most abundant conformer. It should be noted that this conformer had not been mentioned in any of
the previous reported vibrational studies[186-188]. Most of the fundamental frequencies assigned
460
for this conformer are based on the spectra obtained from the xenon solutions. The C-C stretch
from the g-Ax conformer is predicted with high intensity and assigned to the strong band at 957
cm-1 from the spectra of the gas and it is mainly attributed to this heavy atom stretch. The COCl
scissoring motion for this conformer is predicted at 872 cm-1 which is 49 cm-1 higher frequency
with respect to the corresponding mode of g-Eq conformer with approximately one-third the
intensity and is therefore assigned at 875 cm-1.
There are only a few of the vibrational assignments which could be made for the t-Eq
conformer. This is not unexpected since the intensity predictions are low and this conformer is
present at ambient temperature as only 12 ± 1% of the sample. Again from the spectra of the xenon
solutions, it was possible to assign the ring breathing mode and three of the ring deformational
fundamentals and these fundamentals were observed at 964, 933, 919 and 672 cm-1, respectively.
Once these vibrations were observed in the xenon solutions, it was possible to assign them in the
infrared spectra of the gas. The mode predicted at 975 cm-1 is observed at lower frequency than the
corresponding modes of the two most abundant conformers and is now assigned to the band at 964
cm-1. The ring deformation assignments at 933 and 919 cm-1 are in a good agreement with the
corresponding vibrations of the other two conformers. However, the band at 666 cm-1 is assigned
at a higher frequency compared to the band assignments of these fundamentals of the other two
conformers. The fifth ring deformation, ν31′′, was not observed but it is believed to be at 946 cm-1
which is the same frequency for the corresponding mode for the g-Eq conformer.
Conformational Stability
To determine the enthalpy differences among the three observed conformers of
cyclobutylcarboxylic acid chloride, the sample was dissolved in liquefied xenon and the infrared
spectra were recorded as a function of temperature from -70 to -100°C. Relatively small
interactions are expected to occur between xenon and the sample. Therefore, only small
461
wavenumber shifts are anticipated for the xenon interactions when passing from the gas phase to
the liquefied xenon which is confirmed with an average shift of 1 cm-1 for the fundamentals for the
three observed conformers indicating this should not be a major influence on the enthalpy
difference between the forms. A significant advantage of this study is that the conformer bands are
better resolved in comparison with those in the infrared spectrum of the vapor. From ab initio
calculations, the dipole moments of the three conformers are predicted to have similar values and
the molecular sizes of the three conformers are nearly the same, and thus, the ΔH value obtained
from the temperature dependent infrared study is expected to be near that for the gas [40-43].
Once confident assignments have been made for the fundamentals of three observed
conformers the task was then to find pairs of bands from which the enthalpy difference could be
obtained. The bands should be sufficiently resolved for determining their intensities. The selection
of the bands to use in the enthalpy determination was complicated due to the presence of three
conformers with each having several broad intense bands however it was significantly simplified
by the exceptionally good predictions of the relative band positions for the g-Eq, g-Ax, and t-Eq
fundamentals in the 1200 to 400 cm-1 region of the spectra (Fig. 72) where there are a limited
number of overtone and combination bands possible. Examples of this are the fundamentals at
1177 and 1021 cm-1 for the g-Eq and g-Ax forms where the bands are predicted at 1179 and 1022
cm-1, respectively. The g-Eq band at 1177 cm-1 was found to not be significantly affected by the
underlying t-Eq fundamental predicted at 1181 cm-1 with a predicted intensity of 0.6 km/mol. The
bands at 1137 and 1143 cm-1 were assigned for the t-Eq and g-Ax fundamentals, respectively,
where these assignments are based on their relative band positions and intensities. The g-Ax band
at 1020 cm-1 predicted at 1022 cm-1 was found to not be significantly affected by the underlying tEq fundamental at 1029 cm-1 which has a predicted intensity of 0.6 km/mol. Finally the band at
1108 cm-1 was assigned to the g-Eq fundamental (-CH2 rock) predicted at 1120 cm-1 which was
462
found to not be significantly affected by the broad band of the same conformer at 1066 cm-1 nor
the two fundamentals located beneath the 1065 cm-1 band predicted at 1105 cm-1 for the g-Ax
conformer and 1101 cm-1 for the t-Eq form and assigned at 1089 and 1079 cm-1, respectively.
The fundamentals at 569 cm-1 for the g-Eq conformer and 488 cm-1 for the t-Eq form are
predicted to be free of any bands in near coincidence. The bands at 645, 660, and 666 cm -1 were
selected for the g-Eq, g-Ax, and t-Eq conformers, respectively, which can be confidently assigned
based on their relative frequencies from the ab initio calculations. These four g-Eq, three g-Ax, and
three t-Eq bands were used for the enthalpy determination as they were each found to not be
significantly affected by any nearby predicted fundamentals. The assignment of these bands is
shown in Figure 71, where the conformer assigned is labeled above the band.
The intensities of the individual bands were measured as a function of temperature and their
ratios were determined. An example set of assigned fundamental bands with one from the g-Eq
(645 cm-1), g-Ax (873 cm-1), and t-Eq (1137 cm-1) conformers each have been provided in Figure
73. The intensities of the g-Eq and t-Eq bands in the figure have been doubled to allow for better
comparison of the bands, but this doubling in no way affects the enthalpy determination. With the
measured intensity values of the conformer fundamentals the enthalpy values were obtained by
application of the van’t Hoff equation lnK = H/(RT)  S/R. The enthalpy difference was
determined from a plot of lnK versus 1/T, where H/R is the slope of the line and K is substituted
with the appropriate intensity ratios, i.e. Iconf-1 / Iconf-2, etc. It was assumed that S, and α (thermal
expansion constant) are not functions of temperature in the range studied.
These bands were utilized for the determination of the enthalpy difference by combining
them to form 12 independent band pairs each for the g-Eq to g-Ax and for the g-Eq to t-Eq
independent enthalpy values and 9 band pairs for the g-Ax to t-Eq independent enthalpy values.
These values are listed for each band pair in Table 132 and are within a relatively small range. The
463
intensity data from each band pair is combined with the other band pairs to form a single data set
for each conformer pair. From this single data set for each pair of conformers the enthalpy
differences and error limits were determined to be 91  4 cm-1 from the g-Eq to the g-Ax form, 173
 4 cm-1 from the g-Eq to the t-Eq conformer, and 82  3 cm-1 from the g-Ax to the t-Eq conformer
as listed in Table 132. The error limit is derived from the statistical standard deviation of one
sigma of the measured intensity data taken as a single data set, but it does not take into account
small associations with the liquid xenon or the possible presence of overtones and combination
bands in near coincidence of the measured fundamentals. The variations in the individual values
are undoubtedly due to these types of interferences, but by taking several pairs, the effect of such
interferences should cancel. However, this statistical uncertainty is probably better than can be
expected from this technique and, therefore, an uncertainty of about 10% in the enthalpy difference
is probably more realistic i.e. 91  9 cm-1 from the g-Eq to the g-Ax conformer, 173  17 cm-1
from the g-Eq to the t-Eq form, and 82  8 cm-1 from the g-Ax to the t-Eq conformer. From the
enthalpy differences the conformer abundance is estimated to be 54% g-Eq, 35 ± 1% g-Ax and 12
± 1% t-Eq at ambient temperature.
Structural Parameters
An electron diffraction (ED) study [182] was the first study performed on
cyclobutylcarboxylic acid chloride where the structure was determined for the g-Eq conformer
with a mean C-C distance of 1.540(3) Å for all the carbons in the ring and +0.05 (1.590) Å for the
C-C distance outside the ring. The remaining structural parameters have very large errors which
makes them somewhat meaningless for comparison with other structures. This was followed by a
combined microwave and vibrational study [188] where the 35Cl isotopologue of g-Eq conformer
was observed and rotational constants were determined. In this microwave study the diagnostic
least-squares adjusted structural parameters were determined by utilizing a combination of
464
assumed parameters and parameters taken from the structure reported in the ED study [182]. The
resulting extremely large uncertainties in the heavy atom angles determined are most likely a result
of these assumed parameters and the parameters taken from the ED study. Therefore, we have
determined the rotational constants for two isotopic species for the g-Eq conformer giving six
constants available for the structural determination.
We have found that good structural parameters for hydrocarbons and many substituted ones
can be determined by adjusting the structural parameters obtained from the ab initio MP2(full)/6311+G(d,p) calculations to fit the rotational constants obtained from microwave experimental data
by using a computer program “A&M” (Ab initio and Microwave) developed [44] in our
laboratory.
We [45] have shown that ab initio MP2(full)/6-311+G(d,p) calculations predict the carbonhydrogen r0 structural parameters for more than fifty hydrocarbons to at least 0.002 Å compared to
the experimentally determined [191] values from isolated CH stretching frequencies which agree to
previously determined values from earlier microwave studies. Therefore, all of the carbonhydrogen parameters can be taken from the MP2(full)/6-311+G(d,p) predicted values for the g-Eq
conformer of cyclobutylcarboxylic acid chloride. In order to further reduce the number of
independent variables, the structural parameters are separated into sets according to their types
where bond distances in the same set keep their relative ratio, and bond angles and torsional angles
in the same set keep their difference in degrees. This assumption is based on the fact that errors
from ab initio calculations are systematic. Therefore, it should be possible to obtain “adjusted r 0”
structural parameters for the eleven parameters of the seven heavy atoms by adjusting the C-C
distances as a single set and adjusting the CCC angles as another set leaving seven sets of
parameters to adjust. This structure however must be evaluated using parameters from similar
molecules to help account for the relatively low number of rotational constants available and, thus,
465
the –C(O)Cl parameters have a limited range that they change from the ab initio predicted values
as has been reported in the literature. This is particularly true for the C=O bond distance which
changes very little from its molecular environment.
The resulting adjusted r0 parameters are listed in Table 128, where it is believed that the ClC and O=C distances should be accurate to ± 0.003 Å, the C-C distances accurate to ± 0.004 Å, the
C-H distances accurate to ± 0.002 Å, and the uncertainties of the angles should be within ± 0.5.
The fit of the six determined rotational constants (Table 133) by the adjusted r0 structural
parameters are good with variations of the differences with values being less than 0.9 MHz.
Therefore, it is believed that the suggested uncertainties are realistic values and the determined
structural parameters are probably as accurate as can be obtained for the molecule in the gas phase
by either electron diffraction or microwave substitution methods. However with more rotational
constants from further isotopic substitutions the accuracy of the adjusted r 0 parameter values could
be improved.
Discussion
The vibrational assignments reported herein are based on a significant amount of
information with the infrared spectrum of the xenon solutions, band contours in the infrared
spectra of the gas, and predictions of the fundamental frequencies from the scaled ab initio
MP2(full)/6-31G(d) calculations as well as the predicted intensities. For the g-Eq conformer the ab
initio predicted frequencies differ by an average of 8 cm-1 which represents 0.6% error. The
percent error for the predictions for the g-Ax conformer is 4 cm-1 or 0.3% error. Thus the relatively
small basis set of 6-31G(d) by the MP2(full) calculations with two scaling factors provides
excellent predicted frequencies for aiding the vibrational assignments.
There are three repeated t-Eq frequencies which were assigned based on the most stable gEq conformer. These fundamentals are the C-C stretch which is now assigned at 1110 cm-1 in the
466
infrared spectra of the gas and the second fundamental is the ring deformation which is assigned at
946 cm-1. The third fundamental is the β-CH2 rock which is assigned to the band at 776 cm-1 in the
infrared spectra of the xenon solutions. The t-Eq –COCl wag motion is an interesting fundamental
where this motion is predicted at 654 cm-1 with a low infrared intensity of 0.7 km/mol and was not
observed in the infrared spectra of the gas unlike the other two forms where this motion was
observed at 645 and 662 cm-1 for the g-Eq and g-Ax conformers, respectively. This is due to the
low intensity of this fundamental as compared to the g-Eq fundamental with an intensity of 8.2
km/mol. These are several examples where the fundamentals of the g-Eq and g-Ax form have
significantly different intensities than the same mode from the t-Eq conformer.
In general for the g-Eq conformer the mixing was extensive for the fundamentals starting at
1232 cm-1 and lower frequencies. Most of the fundamentals have extensive contributions from
three or four symmetry coordinates and several of the descriptions are more for bookkeeping than
an accurate description of the molecular motions. The g-Ax conformer is similar, with extensive
mixing starting from the vibration at 1209 cm-1 and below. Similarly, several of the approximate
descriptions for the fundamentals of the g-Ax form are again more for bookkeeping than providing
descriptions of the molecular motions. Of significant note is the 20′ fundamental where it has been
assigned as S20 (C-C stretch) despite only being 10% contribution and it has been described as
such as any other placement results in several bookkeeping descriptions. However, for the t-Eq
conformer, the descriptions are much more realistic than the other two conformers except for the
ν18′′ fundamental described as the COCl rock which have only 17%S 18(COCl rock) with
contributions of 34%S22, 21%S20 and 10%S12 modes. The A′′ modes are reasonably well described
with no specific arbitrary descriptions necessary. This reduction of the mixing is largely due to the
symmetry constraints due to the change from C1 to Cv symmetry going from gauche to trans.
Therefore, the approximate descriptions for the normal modes provide reasonable information on
467
the molecular motions of vibrations with several modes described more for bookkeeping purposes
than as a complete description. It should be noted that this large change in mixing due to the
symmetry constraints may be at least partially responsible for the significant differences in the
fundamental intensities noted above.
Because the measured information was just barely sufficient to determine all quartic
centrifugal distortion constants, another fit was made for the
35
Cl isotopologue that included 13
frequencies involving higher rotational constants from the earlier microwave investigation [39].
Each of these represented a blend of unresolved hyperfine components; each blend’s frequency
was assigned to the two innermost components of the predicted hyperfine quartet. (Four transitions
from the previous study [39] were omitted because their hyperfine pattern consisted of a widely
spaced pair of narrow doublets.) Of course this fit (Fit 2 in Table 131) had a much larger standard
deviation of 64 kHz, but the root mean square of the residuals for the FT data was only 8.0 kHz
whereas the rms of the low-resolution measurements was 114 kHz. The difference of any constant
[39] between Fit 1 and Fit 2 was smaller than the combined error limits except for ΔJ where the
difference was only slightly larger.
The derived centrifugal distortion constants generally agree between the isotopic species
and the ab initio predictions (Table 131). The same can be said for the quadrupole coupling
constants, taking into account that the theoretical ratio χ(35Cl)/χ(37Cl) for any coupling constant χ
should be 1.269 [192] provided there is no rotation of the principal inertial axes between the
isotopologues.
The natural population analysis (NPA) was carried out for the g-Eq, g-Ax, and t-Eq
conformers of cyclobutylcarboxylic acid chloride and for cyclobutane with the MP2(full) method
at the 6-311+G(d,p) basis set. The C atoms for cyclobutane all carry a -0.34 charge. However the –
C(O)Cl substitution of cyclobutane gives a charge distribution for the g-Eq form of Cα = -0.32, Cβ
468
= -0.31, Cβ′ = -0.32, Cγ = -0.33, Cl = -0.12, O = -0.59, and C = 0.69. The difference in NPA
predicted for the other two stable conformers are less than 0.01 from those listed for the g-Eq form.
The majority of the ring has similar charges as those of cyclobutane indicating that –C(O)Cl has
relatively little influence on the electron charge density in the ring.
Due to the relatively low number of rotational constants available for the g-Eq form of
cyclobutylcarboxylic acid chloride the structural parameters must be evaluated for their accuracy
and the number of parameters to change must be decreased. As stated in the structural parameters
section the C-C distances were taken as a single set and the –C(O)Cl parameters were constrained
within the limits determined from similar molecules. As can be seen in Table 134A the ab initio
MP2(full) calculation at the 6-311+G(d,p) basis set does an excellent job predicting the bond
distances and angles for this moiety. The predicted structural parameter values for the methyl [193]
and ethyl compounds [194] for the distances are usually within 0.001 Å and within 0.5° for the
angles of the r0 structural parameter values. The isopropyl molecule [195] shows larger differences
between the predicted structural parameter values and the values from the r0 structural parameters.
The structure was determined for the g-Eq form of cyclobutylcarboxylic acid chloride and the
parameters where allowed to change in fitting of the rotational constants. As can be seen from
Table 128 the differences between the MP2(full)/6-311+G(d,p) and the adjusted r0 structural
parameter values for the –C(O)Cl bond distances and angles are less than the proposed
experimental errors and are very close to the values for similar bond distances and angles in Table
134A.
The ring parameters are much more variable where the ring distances and angles for fourmembered rings are often relatively sensitive to substitution as shown in Table 134B. As the
charges in the ring are fairly consistent with those for the cyclobutane molecule the ring structural
parameters should be comparable between the two. It can be seen that the Cγ-Cβ bond distances are
469
~0.008 Å shorter for the –C(O)Cl molecule as opposed to unsubstituted cyclobutane. These are
probably not due to the electronic effects in the rings as these effects are predicted to be relatively
small but rather due to the steric effects of the larger –C(O)Cl molecule’s ring puckering angle.
The Cα-Cβ bond distances in the –C(O)Cl molecule are different from each other due to the oxygen
group overlapping one of the C-C bonds, this bond distance is drastically reduced due to the steric
effects of this overlap. The other Cα-Cβ bond distance is much longer and is within the
experimental error to the value from the cyclobutane r0 structure. The ring structure of
cyclobutylcarboxylic acid chloride is similar to that of cyclobutane as expected but with some
logical departures brought on by the substitution of a –H with a –C(O)Cl group.
In this current study, the adjusted r0 structural parameters have been determined for the g-Eq
conformer these should be comparable to the previously reported ED study [182]. In the ED study
the structural parameters were obtained for the g-Eq form from a least squares analysis of the
anharmonic radial distribution function [196] in the interval г ≈ 0.0 Å to г ≈ 1.95 Å. All of the
parameters have very large errors except the mean ring C-C, out of ring C-C, and the C-Cl distances
which equal to 1.540(3), 1.590(3), and 1.793(6), respectively. These values are in excellent
agreement with the adjusted r0 structural parameters obtained in this study. The remaining
parameters with large uncertainties are not meaningful though due to these large uncertainties for
the most part they agree within the stated errors to the adjusted r0 structural parameters.
A decade later, a microwave and vibrational study followed4 where one isotopomer of the gEq conformer was observed and the rotational constants were determined. The structural parameters
were determined by fixing all but five parameters where these assumptions were in part informed by
using the ED study structural parameters. The five parameters were then determined with a
diagnostic least squares adjustment as described by Curl [196]. The structural parameters obtained
have very large errors, which is partially due to the assumed parameters being off from the actual
470
structure but also due to fitting three rotational constants with five parameters. The error is so large
on these parameters that the ones that are far off from the adjusted r0 structural parameters are
probably not meaningful.
One of the major goals of this current study was the determination of the enthalpy
difference between the conformers of the cyclobutylcarboxylic acid chloride where there had been
different results reported. Fabregue [186] reported that the cyclobutylcarboxylic acid chloride exist
in both the solid and liquid phases as the same nonsymmetrical isomer in which COCl group is in
the equatorial position with respect to the ring. From the ED study [182] it was indicated that the
molecule exist as a conformational equilibrium mixture of the gauche-like axial and a gauche-like
equatorial conformers. A decade later, the vibrational spectra was reinvestigated [187] and it was
found that the sample exists in a conformational equilibrium in the liquid and vapor phases. A few
years later, the variable temperature Raman spectra of the liquid were studied4 from which the
enthalpy difference between the g-Eq and high energy (assumed t-Eq) conformations was
determined to be 1.4 kcal mol-1 (490 cm-1). The enthalpy value obtained in this study from the
Raman spectra of the liquid would give a population of 4% t-Eq at room temperature. This
population is extremely small though with the assignment they propose it is not beyond reason.
When compared to the vibrational assignment as determined in the current study it can be seen that
this enthalpy value is far too large. In comparison the enthalpy determination between the g-Eq to
t-Eq conformers is determined to be 173  17 cm-1 in the current study where it was determined
from 12 band pairs. This is nearly one-third of the value determined in the liquid state. However,
enthalpy values determined from spectra of the liquid state are normally significantly higher than
those determined for samples in the vapor state or in xenon solutions, and therefore, the difference
in the determined enthalpy values may be largely due to the difference from the samples from
which they were obtained.
471
The experimental values determined from the variable temperature infrared spectra of the
xenon solutions should be near to those of the gas which should be similar to those of the isolated
molecule. Therefore it can be useful to compare the ab initio and DFT energy differences shown in
Table 127 with the experimental enthalpy differences. The order of stability is well predicted by the
ab initio and DFT calculations where the correct order of stability is predicted by both the MP2 and
B3LYP methods for all basis sets. The energy differences determined by the calculations
approximately doubles for the g-Eq to g-Ax difference with addition of diffuse functions for the
MP2 method, where the largest basis set without diffuse functions is very close to the experimental
value. The B3LYP method for the same energy difference predicts the energy difference
significantly higher and over three times the experimental value. The g-Eq to t-Eq energy difference
is predicted significantly too large for all basis sets and both calculation methods. This is an
interesting phenomenon where the addition of more functions into the ab initio basis sets actually
causes poorer results. However, as is previously stated the order of stability is predicted correctly for
all the calculations attempted.
Conclusions
There are few molecular geometries of the carboxylic acid chloride derivatives determined in
the literature, which lead to a scarcity of molecular structural parameters and conformational
determinations with which the current molecule could be compared to. This would be an interesting
topic to research and the five membered ring and open chain derivatives would be of particular
interest.
The microwave spectra for the two isotopologues of g-Eq have been assigned and the
rotational constants have been reported. From these rotational constants a complete structure has
been determined which is made possible due to the reduction in the number of independent
472
parameters as the C-H distances can be taken from the MP2(full)/6-311+G(d,p) structural
parameters for substituted hydrocarbons.
The ring parameters appear to be largely determined due to steric effects rather than
electrostatic effects. This is supported by the NPA predictions where the ring for
cyclobutylcarboxylic acid chloride is nearly the same as cyclobutane in contrast to the significant
difference in the ring puckering angle and Cα-Cβ bond distances.
The observed fundamentals for the g-Eq, g-Ax, and t-Eq conformers have been assigned
based upon the variable temperature infrared spectra of the xenon solutions. From the band
intensities of several of these assigned fundamentals the enthalpy differences have been
determined. The relative amounts at ambient temperature are 54% g-Eq, 35 ± 1% g-Ax and 12 ±
1% t-Eq forms. This relatively small difference in the amount of each conformer is in direct
contrast to the results from previous studies [186-188] where the g-Ax and t-Eq conformers went
largely unassigned and were assumed to be either high energy or transition states.
473
Table 122: Microwave spectrum for the g-Eq form of c-C4H7C(O)35Cl. Observed frequencies of
hyperfine components of rotational transitions (MHz) and deviations of calculated values (kHz).
Transition
2F' 2F"
obs (MHz)  (kHz)
5 0 5 ← 4 0 4 11
7
13
9
5
11
12488.0928
12488.4513
12488.8314
0.0
0.0
0.0
423 ← 404
7
5
11
9
5
5
11
7
12510.4984
12512.2725
12513.7552
12515.6503
7
13
9
11
7
13
9
11
5 4 2 ← 4 4 1 11
9
13
7
Transition
2F' 2F"
obs (MHz)  (kHz)
523 ←
422
-1.0
-3.0
2.0
2.0
11
11
9
13
7
7
11
9
7
11
5
7
13102.2947
13102.8295
13102.9112
13103.2076
13103.2512
13103.7903
-9.0
0.0
1.0
0.0
1.0
-3.0
606 ←
515
12801.0148
12801.9124
12804.5374
12805.6276
0.0
1.0
0.0
-3.0
9
15
11
13
7
13
9
11
13298.0792
13298.7292
13299.1049
13299.7042
-2.0
1.0
-2.0
-1.0
514 ←
413
9
7
11
5
12851.4369
12852.8816
12854.9682
12856.3981
1.0
-1.0
0.0
1.0
11
9
13
7
9
7
11
5
13397.6336
13397.7486
13398.1012
13398.1832
-1.0
2.0
0.0
1.0
835 ←
826
5 4 1 ← 4 4 0 11
9
13
7
9
7
11
5
12851.6285
12853.0725
12855.1571
12856.5838
1.0
-1.0
-1.0
-3.0
17
15
19
13
17
15
19
13
13543.8893
13544.1742
13545.0002
13545.0375
-1.0
-1.0
-1.0
0.0
633 ←
624
5 3 3 ← 4 3 2 11
9
11
13
7
9
7
11
11
5
12866.4754
12867.4525
12868.2870
12868.6320
12869.0708
1.0
-2.0
0.0
2.0
1.0
13
11
9
15
13
11
9
15
14581.4210
14581.4906
14581.5304
14581.5809
-4.0
-3.0
-1.0
2.0
606 ←
505
11
13
9
11
14837.8797
14838.0792
1.0
1.0
5 3 2 ← 4 3 1 11
9
13
7
9
7
11
5
12881.2035
12881.7769
12883.0272
12883.7434
3.0
5.0
4.0
-2.0
431 ←
422
5
11
7
9
5
11
7
9
15145.9291
15146.7867
15148.3560
15149.1610
3.0
0.0
2.0
1.0
9 3 6 ← 9 2 7 19
17
21
15
19
17
21
15
12938.1931
12938.3418
12939.1964
12939.3744
2.0
1.0
1.0
-2.0
10 2 9 ← 10 1 10 23
19
21
23
19
21
16847.3301
16850.9893
16851.6156
0.0
0.0
0.0
7 2 6 ← 7 1 7 11
13
15
11
13
15
13089.0202
13093.7375
13094.7280
-1.0
2.0
0.0
11
13
9
17144.2289
17144.3366
17144.8521
1.0
2.0
-2.0
524 ← 505
707 ←
474
606
13
15
11
Table 123: Microwave spectrum for the g-Eq form of c-C4H7C(O)37Cl. Observed frequencies of
hyperfine components of rotational transitions (MHz) and deviations of calculated values (kHz).
2F' 2F"
obs (MHz)  (kHz)
9
7
11
13
9
7
7
5
9
11
9
7
11843.7145
11844.1191
11844.1191
11844.3502
11845.9516
11850.3009
0.0
-2.0
6.0
0.0
-1.0
0.0
9
9
7
11
5
9
7
5
9
3
11854.2267
11858.5569
11858.7653
11860.5299
11860.6000
-3.0
0.0
-2.0
-1.0
5.0
9
11
7
13
9
7
7
9
5
11
9
7
12246.1823
12246.3667
12246.6679
12246.8701
12247.8543
12251.4504
0.0
1.0
-1.0
2.0
-2.0
-2.0
524 ← 423
11
9
13
7
7
9
7
11
5
7
12517.5782
12517.7214
12518.4264
12518.5449
12520.1200
-1.0
1.0
-1.0
2.0
-2.0
542 ← 441
11
9
13
7
9
7
11
5
12588.5521
12589.7455
12591.4810
12592.6693
1.0
-2.0
-1.0
0.0
541 ← 440
11
9
13
7
9
7
11
5
12588.7204
12589.9163
12591.6493
12592.8426
1.0
0.0
0.0
6.0
533 ← 432
11
9
13
7
9
7
11
5
12602.7191
12603.3464
12604.3872
12604.8893
-1.0
1.0
1.0
-1.0
532 ← 431
11
9
13
7
9
7
11
5
12616.1242
12616.5958
12617.6440
12618.2493
-1.0
-2.0
-2.0
0.0
523 ← 422
11
11
9
13
7
7
11
9
7
11
5
7
12824.7289
12825.2798
12825.3557
12825.6322
12825.6801
12826.2405
-7.0
-2.0
6.0
-1.0
0.0
2.0
Transition
515 ← 414
414 ← 303
505 ← 404
475
Transition
2F' 2F"
obs (MHz)  (kHz)
7 2 6 ← 7 1 7 11
17
13
15
11
17
13
15
12959.0087
12959.7415
12962.6624
12963.4274
-1.0
2.0
0.0
-1.0
606 ← 515
9
15
11
13
7
13
9
11
12981.6414
12982.1523
12982.4399
12982.9145
-1.0
1.0
-1.0
0.0
5 1 4 ← 4 1 3 11
9
13
7
9
7
11
5
13121.1218
13121.2000
13121.4873
13121.5366
-2.0
6.0
1.0
0.0
6 0 6 ← 5 0 5 11
13
9
15
9
11
7
13
14554.8534
14554.9830
14555.3695
14555.4882
-2.0
3.0
-2.0
2.0
6 5 2 ← 5 5 1 13
11
15
9
11
9
13
7
15104.3045
15105.1831
15106.9214
15107.7958
7.0
3.0
2.0
0.0
6 5 1 ← 5 5 0 13
11
15
9
11
9
13
7
15104.3045
15105.1831
15106.9214
15107.7958
1.0
-4.0
-4.0
-6.0
6 4 3 ← 5 4 2 13
11
15
9
11
9
13
7
15119.1757
15119.6815
15120.8317
15121.3266
0.0
0.0
1.0
2.0
6 4 2 ← 5 4 1 13
11
15
9
11
9
13
7
15119.9299
15120.4393
15121.5822
15122.0754
-2.0
2.0
-1.0
-1.0
6 3 4 ← 5 3 3 13
11
15
9
11
9
13
7
15137.0321
15137.1476
15137.8741
15138.1516
-2.0
0.0
0.0
-3.0
6 3 3 ← 5 3 2 13
11
15
9
11
9
13
7
15172.4394
15172.6830
15173.3002
15173.4499
1.0
-2.0
1.0
3.0
Table 124: Observeda and predicted fundamentalb frequencies for the g-Eq conformer of cyclobutylcarboxylic acid chloride.
Fundamental
476
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
β-CH2 antisymmetric stretch
β-CH2 antisymmetric stretch
γ-CH2 antisymmetric stretch
-CH stretch
β-CH2 symmetric stretch
γ-CH2 symmetric stretch
β-CH2 symmetric stretch
C=O stretch
β-CH2 deformation
γ-CH2 deformation
β-CH2 deformation
-CH in plane bend
γ-CH2 wag
β-CH2 wag
β-CH2 wag
-CH out of plane bend
β-CH2 twist
γ-CH2 twist
β-CH2 rock
C-C stretch
Ring deformation
Ring breathing
β-CH2 twist
Ring deformation
Ring deformation
COCl scissors
β-CH2 rock
γ-CH2 rock
COCl wag
Ring deformation
C-Cl stretch
COCl rock
Ring-COCl bend
Ring-COCl bend
Ring puckering
COCl Assymetric torsion
ab
initio
3213
3207
3194
3147
3141
3134
3133
1853
1575
1550
1542
1404
1320
1315
1294
1282
1268
1239
1170
1127
1078
1013
994
975
942
847
818
750
661
591
446
345
304
163
145
47
fixed
scaledc
3014
3008
2996
2952
2946
2940
2939
1760
1483
1459
1452
1337
1254
1251
1229
1217
1205
1179
1120
1075
1032
970
946
932
911
823
779
728
651
578
432
339
298
163
144
47
IR
int.
25.2
13.2
7.7
2.1
12.9
25.6
14.4
205.2
2.0
5.2
2.3
17.9
5.0
11.4
0.9
1.4
4.8
3.4
23.9
62.6
3.8
52.9
0.9
1.7
7.7
152.8
7.3
36.5
8.2
6.9
24.0
19.1
2.6
1.1
1.7
0.0
Raman
act.
45.8
62.4
78.1
30.7
122.0
98.8
54.1
14.2
5.3
17.8
5.6
8.9
5.5
0.6
4.3
6.6
5.9
8.7
4.2
4.4
5.2
9.7
2.5
10.1
2.1
2.4
0.9
4.6
2.4
3.7
12.0
5.8
0.5
0.6
0.4
1.0
IR
gas
3003
2996
2986
2961
2945
2943
2926
1807
1470
1453
1442
1324
1247
1245
1232
1213
1204
1177
1110
1069
1027
960
946
933
889
809
778
721
645
569
433
340
-
Xe
soln.
3000
2994
2984
2959
2944
2943
2926
1807
1469
1453
1442
1322
1246
1244
1232
1213
1201
1176
1108
1067
1026
958
946
933
887
807
776
720
645
569
431
-
gasd
3004
2995
solidd
Raman
Liquidd
3002
2995
2980
2952
3004
2879
1805
1466
1452
1110
1071
1030
955
2866
1792
1467
1446
1446e
1319
1246
1242e
1228e
1215
1198
1176
1104
1070
1017
954
1328
1250
1250
1232e
1217
1208
1190
1109
1071
1025
962
888
810
775
721
646
570
437
338
278
168
-
923
888
799
779
722
640
566
433
338
282
176
153
83
930e
891
804
784
721
651
572
434
343
290
181
-
2964
2887
1808
1451
1451e
1328
1252
1252e
1209
2984
2956
P.E.D.f
73S1,25S2
97S2
73S3,23S1
88S4,11S5
48S5,42S7
93S6
51S7,38S5
92S8
62S9,34S10
64S10,34S9
98S11
56S12,11S20,10S14
63S13,10S16,10S21
72S14,10S25
43S15,22S18,14S17,10S23
37S16,14S17,14S13,10S18,10S15
43S17,18S16,12S28,10S23
31S18,39S15,10S21,10S23
37S19,14S22,11S28,11S35
25S20,21S12,11S22
22S21,21S24,17S23,14S22,10S20
45S22,18S25,11S20
37S23,23S16,16S18,10S24,10S21
51S24,29S21,10S13
33S25,24S30,13S28,11S19,10S14
19S26,14S30,13S25,12S28
68S27,11S18
30S28,13S29,12S33,10S17,10S32
48S29,11S30,11S19
31S30,16S32,13S19,11S20
61S31,35S26
11S32,22S26,19S31,19S34,11S20
26S33,35S35,13S32,12S29
55S34,33S32
48S35,40S33
100S36
Band Contour
A
B
C
6
70
45
23
76
27
3
23
8
29
94
6
78
13
25
87
53
98
78
96
90
1
46
83
90
86
69
43
41
78
82
21
39
3
16
75
16
65
24
31
93
36
42
42
5
82
19
87
75
4
45
2
22
2
10
68
83
51
17
14
2
52
56
22
45
60
84
19
30
39
12
42
4
41
50
29
1
12
3
9
2
2
31
17
3
10
29
5
3
18
34
61
37
a
Observed spectra: gas, Xe, and solid are IR while liquid is Raman.
MP2(full)/6-31G(d) ab initio calculations, scaled frequencies, infrared intensities (km/mol), Raman activities (Å4/u), depolarization ratios (dp) and potential energy distributions
(P.E.D.s).
c
Scaled frequencies with scaling factors of 0.88 for the CH stretches, β-CH2 and γ-CH2 deformations and 0.90 for all other modes except the heavy atom bending modes.
d
Reference [188].
e
Our Assignment.
f
Symmetry coordinates with P.E.D. contribution less than 10% are omitted.
b
477
Table 125: Observeda and predicted fundamentalb frequencies for the g-Ax conformer of cyclobutylcarboxylic acid chloride.
Fundamental
478
1
2
3
4
6
5
7
8
10
9
11
12
14
13
15
18
17
16
19
23
22
20
24
21
25
26
27
28
29
30
31
32
33
34
35
36
β-CH2 antisymmetric stretch
β-CH2 antisymmetric stretch
γ-CH2 antisymmetric stretch
-CH stretch
γ-CH2 symmetric stretch
β-CH2 symmetric stretch
β-CH2 symmetric stretch
C=O stretch
γ-CH2 scissors
β-CH2 scissors
β-CH2 scissors
-CH in plane bend
β-CH2 wag
γ-CH2 wag
β-CH2 wag
γ-CH2 twist
β-CH2 twist
-CH out of plane bend
β-CH2 rock
β-CH2 twist
Ring breathing
C-C stretch
Ring deformation
Ring deformation
Ring deformation
COCl scissors
β-CH2 rock
γ-CH2 rock
COCl wag
Ring deformation
C-Cl stretch
COCl rock
Ring-COCl bend
Ring-COCl bend
Ring puckering
COCl Assymetric torsion
ab
initio
3214
3205
3195
3165
3142
3138
3134
1847
1572
1550
1535
1402
1331
1322
1293
1282
1265
1207
1155
1117
1075
999
985
966
940
896
804
746
669
635
452
354
300
200
153
55
fixed
scaledc
3015
3007
2997
2969
2947
2944
2940
1754
1480
1459
1445
1334
1266
1257
1227
1219
1203
1147
1105
1069
1022
956
938
930
907
872
767
723
654
623
439
347
297
200
150
55
IR
int.
23.6
8.9
4.8
4.6
21.4
14.7
18.8
189.8
2.8
3.7
5.2
24.9
2.8
3.3
0.0
3.6
3.9
1.4
5.6
52.7
31.0
88.8
22.9
2.2
47.9
47.5
0.9
58.0
18.7
14.5
18.3
20.7
1.2
0.7
1.5
0.0
Raman
act.
49.7
62.8
62.4
79.4
112.8
84.8
41.9
10.1
7.6
15.7
7.2
3.8
0.4
1.2
0.3
15.3
9.8
3.7
3.2
6.5
14.0
5.9
11.1
0.3
2.3
1.4
0.3
3.3
4.7
2.1
10.9
5.8
0.8
0.7
0.2
1.2
IR
gas
3012
3008
3002
2963
2945
2944
2943
1760
1468
1458
1446
1339
1260
1251
1209
1200
1145
1090
1065
1021
957
940
933d
903
875
773
721d
662
625
439
345
293
IR
Xe
soln.
3012
3007
2999
2963
2944
2943
2943
1759
1468
1457
1443
1337
1259
1248
1208
1199
1143
1089
1063
1020
956
938
932
902
873
773
719
660
624
437
P.E.D.e
48S1,42S3,10S2
76S2,21S3
35S3,49S1,13S2
96S4
96S6
58S5,35S7
63S7,34S6
91S8
53S10,44S9
47S10,44S10
90S11,10S9
54S12,15S17,11S22
60S14,17S13,10S25
49S13,20S14
70S15,14S13,10S16
39S18,24S16,14S17,10S21
43S17,17S28,11S12,10S19
27S16,18S23,16S15,13S24,11S18
30S19,17S12,17S33,10S20,10S28
37S23,10S27,10S22
51S22,14S23,10S20
10S20,26S25,23S22
69S24
52S21,12S16,11S23,10S13,10S27
32S25,16S28,15S30,10S14
10S26,40S30,17S19,10S25
58S27,17S24,13S18
31S28,11S17,11S32,10S33
35S29,12S34,11S20,10S32,10S27
33S30,20S29,19S19,10S28
52S31,44S26
11S32,25S31,22S34,14S26
26S33,32S35,23S32,10S29
51S34,26S32,12S29
52S35,38S33
100S36
Band Contour
A
B
C
47
43
10
74
6
20
18
2
80
38
5
57
61
39
23
6
71
5
86
9
15
82
3
39
33
28
3
54
43
21
43
36
94
6
1
99
1
99
44
15
41
5
89
6
41
59
17
83
22
61
17
92
8
100
98
2
100
18
54
28
87
8
5
92
7
1
100
95
5
71
23
6
67
25
8
54
43
3
86
11
3
90
10
4
34
62
78
22
31
69
a
Observed spectra: gas and Xe are IR.
MP2(full)/6-31G(d) ab initio calculations, scaled frequencies, infrared intensities (km/mol), Raman activities (Å4/u), depolarization ratios (dp) and potential energy
distributions (P.E.D.s).
c
Scaled frequencies with scaling factors of 0.88 for the CH stretches, β-CH2 and γ-CH2 deformations and 0.90 for all other modes except the heavy atom bending modes.
d
This band is also assigned as a g-Eq conformer fundamental.
e
Symmetry coordinates with P.E.D. contribution less than 10% are omitted.
b
479
Table 126: Observeda and predicted fundamentalb frequencies for the t-Eq conformer of cyclobutylcarboxylic acid chloride.
A'
480
A"
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
Fundamental
ab initio
Fixed
scaledc
IR
int.
Raman
act.
IR
gas
β-CH2 antisymmetric stretch
γ-CH2 antisymmetric stretch
α CH stretch
β-CH2 symmetric stretch
γ-CH2 symmetric stretch
C=O stretch
β-CH2 deformation
γ-CH2 deformation
α CH in plane bend
β-CH2 wag
β-CH2 twist
β-CH2 rock
C-C stretch
Ring breathing
Ring deformation 1
γ-CH2 rock
Ring deformation 2
COCl rock
C-Cl stretch
COCl scissors
Ring puckering
Ring-COCl bend
β′-CH2 antisymmetric stretch
β-CH2 symmetric stretch
β-CH2 deformation
γ-CH2 wag
α CH out of plane bend
β-CH2 wag
γ-CH2 twist
β-CH2 twist
Ring deformation 2
Ring deformation 1
β-CH2 rock
COCl wag
Ring-COCl bend
COCl torsion
3217
3195
3148
3144
3135
1841
1574
1550
1389
1312
1282
1198
1155
1018
948
835
688
527
502
414
264
137
3211
3142
1539
1324
1298
1289
1241
1072
988
977
824
660
217
54
3018
2997
2953
2949
2941
1748
1482
1459
1320
1248
1220
1149
1101
975
917
805
670
517
481
403
261
136
3012
2948
1449
1259
1233
1224
1181
1029
944
931
785
654
217
54
22.2
8.6
3.3
1.8
25.6
229.8
1.0
5.0
1.0
0.8
3.3
84.7
50.1
27.8
11.0
67.0
67.1
0.2
47.7
9.8
0.8
0.7
12.0
32.8
2.3
3.9
0.0
0.0
0.6
0.6
0.8
1.5
2.3
0.7
2.1
0.6
33.3
70.6
54.2
198.7
66.3
21.1
4.1
23.3
6.6
1.1
10.7
2.5
9.4
12.2
2.0
1.1
2.9
4.0
12.1
6.0
1.1
0.6
56.4
5.5
5.1
6.8
2.0
2.9
7.9
0.9
0.7
13.7
1.1
0.1
0.6
1.0
3014
2997
2961
2945
2924
1750
1468
1453
1321
1248
1222
1140
1081
964
919
803
666
~500
490
389
IR
Xe
soln.
3014
2995
2960
2944
2922
1750
1465
1451
1320
1245
1221
1137
1079
962
919
801
666
500
488
3014
2945
3014
2945
1031
946d
933
778
1029
946d
932
776d
P.E.D.e
Band Contour
A
B
79S1,19S2
79S2,19S1
93S3
91S4
94S5
92S6
52S7,46S8
53S8,47S7
66S9,14S10,10S15
70S10,12S9
73S11,13S16
25S12,22S13,19S22,10S18,10S16
24S13,37S14,13S12
46S14,19S15,10S12,10S10
31S15,26S17,15S12,12S16,10S10
30S16,16S15,13S11,11S20,10S17
32S17,23S16,14S12,13S20
17S18,34S22,21S20,10S12
47S19,17S17,16S13,10S12
29S20,37S19,10S18,10S13
39S21,33S18,11S20
42S22,38S21,16S18
100S23
100S24
100S25
53S26,23S27,10S33,10S29
34S27,23S30,21S28,11S26,10S29
40S28,21S29,19S26,14S27
33S29,33S28,14S30,10S31
35S30,22S31,21S32,10S34,10S35
32S31,22S30,19S27,16S29
65S32,13S31,10S26
72S33,12S29
61S34,14S31,13S35
64S35,28S34
90S36,10S35
1
12
39
2
99
98
10
8
30
99
56
8
42
80
69
19
46
35
46
24
2
48
-
99
88
61
98
1
2
90
92
70
1
44
92
58
20
31
81
54
65
54
76
98
52
-
a
Observed spectra: gas and Xe are IR
MP2(full)/6-31G(d) ab initio calculations, scaled frequencies, infrared intensities (km/mol), Raman activities (Å4/u), depolarization ratios (dp) and potential energy distributions
(P.E.D.s).
c
Scaled frequencies with scaling factors of 0.88 for the CH stretches, β-CH2 and γ-CH2 deformations and 0.90 for all other modes except the heavy atom bending modes.
d
This band is also assigned as a g-Eq conformer fundamental.
e
Symmetry coordinates with P.E.D. contribution less than 10% are omitted.
b
481
Table 127: Calculated energiesa in (H) and energy differences (cm-1) for the
four possible conformers of cyclobutylcarboxylic acid chloride
Method/Basis Set
RHF/6-31G(d)
MP2(full)/6-31G(d)
MP2(full)/6-31+G(d)
MP2(full)/6-31G(d,p)
MP2(full)/6-31+G(d,p)
MP2(full)/6-311G(d,p)
MP2(full)/6-311+G(d,p)
MP2(full)/6-311G(2d,2p)
MP2(full)/6-311+G(2d,2p)
MP2(full)/6-311G(2df,2pd)
MP2(full)/6-311+G(2df,2pd)
MP2(full)/aug-cc-pVTZ
g-Eq
-0.739087
-1.729837
-1.748407
-1.787077
-1.804584
-2.046919
-2.058889
-2.151748
-2.160468
-2.282393
-2.290185
-2.293038
Energy Differences, ∆b
g-Ax t-Eq t-Axc
287
551
952
125
454
954
293
468
918
153
433
1005
302
438
947
120
509
1001
216
377
869
83
425
834
171
310
702
82
383
818
168
279
693
149
213
550
B3LYP/6-31G(d)
-3.147955
288
396
775
B3LYP/6-31+G(d)
-3.159073
335
363
B3LYP/6-311G(d,p)
-3.249426
280
525
863
B3LYP/6-311+G(d,p)
-3.255032
345
400
B3LYP/6-311G(2d,2p)
-3.261525
284
513
866
B3LYP/6-311+G(2d,2p)
-3.266090
334
417
B3LYP/6-311G(2df,2pd)
-3.270451
294
522
B3LYP/6-311+G(2df,2pd)
-3.274864
326
416
B3LYP/aug-cc-pVTZ
-3.288942
320
410
a
Energy of conformer is given as –(E+727) H.
b
Difference is relative to g-Eq form and given in cm-1.
c
Blank energy differences optimize to t-Eq conformer when using that method/basis set.
482
Table 128: Structural parameters (Å and degrees), rotational constants (MHz) and dipole moments
(Debye) for g-Eq form of cyclobutylcarboxylic acid chloride.
Structural Int.
Parameters coor.
rCα-C
R1
rC=O
R2
rCα-Cβ
R3
rCα-Cβ′
R4
rCγ-Cβ
R5
rCγ-Cβ′
R6
rC-Cl
R7
rCα-H
r1
rCβ-H1
r2
rCβ′-H1
r3
rCβ-H2
r4
rCβ′-H2
r5
rCγ-H1
r6
rCγ-H2
r7
CαCO
1
 ClCCα
2
 ClCO
3
CβCαC
4
Cβ′CαC
5
Cβ′CαCβ
1
CγCβCα
2
CγCβ′Cα
3
Cβ′CγCβ
4
 HCαCβ
1
 HCαCβ′
2
 HCαC
3
 H1CβCα λ1
 H1Cβ′Cα λ2
λ3
 H1CβCγ
 H1Cβ′Cγ λ4
 H2CβCα λ5
 H2Cβ′Cα λ6
λ7
 H2CβCγ
 H2Cβ′Cγ λ8
 H1CβH2 λ9
 H1Cβ′H2 λ10
π1
 H1CγCβ
 H1CγCβ′ π2
π3
 H2CγCβ
 H2CγCβ′ π4
 H1CγH2 π5
CγCβCβ′Cα 1
HCαCO
2
HCαCCl
3
ClC2CαO 4
A(MHz)
MP2(full)/
6-311+G(d,p)
1.492
1.193
1.555
1.542
1.548
1.548
1.798
1.094
1.094
1.093
1.091
1.091
1.091
1.093
128.0
111.1
120.8
116.6
118.7
88.4
87.1
87.6
88.4
109.9
112.6
109.2
110.3
110.1
111.1
111.4
117.5
117.7
119.3
118.8
109.7
109.6
117.7
118.0
110.7
111.1
109.5
31.3
127.5
55.0
4368.30
Microwavea
1.509b
1.191b
1.535b
1.535b
1.55b
1.55b
1.790b
1.10b
1.085b
1.085b
1.085b
1.085b
123.2(29)
112.7(15)
110.07b
20.0(24)
61.1(24)
4349.86(17)
483
EDc
1.490(3)d
1.195(13)
1.540(3)d
1.540(3)d
1.540(3)d
1.540(3)d
1.793(6)d
1.128(14)d
1.128(14)d
1.128(14)d
1.128(14)d
1.128(14)d
1.128(14)d
1.128(14)d
127.0(15)
111.0(20)
122.0
112.8
112.8
90.9
90.9
90.9
90.9
116.2
116.2
107.5
116.2
116.2
116.2
116.2
116.2
116.2
116.2
116.2
102.0
102.0
116.2
116.2
116.2
116.2
102.0
21(5)
123
57(5)
Adjusted
r0e
1.491(4)
1.193(3)
1.553(4)
1.540(4)
1.547(4)
1.546(4)
1.801(3)
1.094(2)
1.094(2)
1.093(2)
1.091(2)
1.091(2)
1.091(2)
1.093(2)
127.8(5)
110.8(5)
121.2(5)
117.5(5)
119.6(5)
87.4(5)
88.0(5)
88.6(5)
87.5(5)
107.6(5)
113.7(5)
109.2(5)
110.3(5)
110.1(5)
111.4(5)
111.7(5)
117.3(5)
117.9(5)
118.4(5)
117.9(5)
109.7(5)
109.6(5)
118.1(5)
118.3(5)
110.8(5)
111.1(5)
109.5(5)
30.9(5)
130.0(5)
55.0(5)
4350.60
B(MHz)
1411.29
1414.78(1)
1414.66
C(MHz)
1150.70
1148.24(1)
1148.28
|a|
3.049
1.743
|b|
0.848
|c|
|t|
3.613
a
Proposed structural parameters, rotational constants, and dipole
moments from reference [188].
b
Assumed values.
c
Proposed structural parameters from the electron diffraction study
[182]
d
C-C bond length values and all C-H distances are assumed to be the
same.
e
Adjusted parameters using the microwave data from Table 1 for the
given ground states.
484
Table 129: Symmetry coordinatesa for cyclobutylcarboxylic acid chloride.
Description
β-CH2 antisymmetric stretch
β-CH2 antisymmetric stretch
γ-CH2 antisymmetric stretch
-CH stretch
β-CH2 symmetric stretch
γ-CH2 symmetric stretch
β-CH2 symmetric stretch
C=O stretch
β-CH2 deformation
γ-CH2 deformation
β-CH2 deformation
-CH in plane bend
γ-CH2 wag
β-CH2 wag
β-CH2 wag
-CH out of plane bend
β-CH2 twist
γ-CH2 twist
β-CH2 rock
C-C stretch
Ring deformation
Ring breathing
β-CH2 twist
Ring deformation
Ring deformation
COCl scissors
β-CH2 rock
γ-CH2 rock
COCl wag
Ring deformation
C-Cl stretch
COCl rock
Ring-COCl bend
Ring-COCl bend
Ring puckering
COCl Assymetric torsion
aNot normalized.
S1
S2
S3
S4
S5
S6
S7
S8
S9
S10
S11
S12
S13
S14
S15
S16
S17
S18
S19
S20
S21
S22
S23
S24
S25
S26
S27
S28
S29
S30
S31
S32
S33
S34
S35
S36
Symmetry Coordinatea
r 2 – r4 + r3 – r5
r 2 – r4 – r3 + r5
r 6 – r7
r1
r2 +r4 + r3 + r5
r6 + r7
r 2 + r4 – r3 – r5
R2
λ9 + λ10
π5
λ9 – λ10
1 + 2
π 1 - π2 + π 3 - π4
λ1 – λ3 + λ5 – λ7 + λ2 – λ4 + λ6 – λ8
λ1 – λ3 + λ5 – λ7 – λ2 + λ4 – λ6 + λ8
1 - 2
λ1 – λ3 – λ5 + λ7 + λ2 – λ4 – λ6 + λ8
π 1- π 2 - π 3 + π 4
λ1 + λ3 – λ5 – λ7 + λ2 + λ4 – λ6 – λ8
R1
R3 – R4 + R5 – R6
R3 + R4 + R5 + R6
λ1 – λ3 – λ5 + λ7 – λ2 + λ4 + λ6 – λ8
R3 – R4 – R5 + R6
R3 + R4 – R5 – R6
3
λ1 + λ3 – λ5 – λ7 – λ2 – λ4 + λ6 + λ8
π 1 + π 2 – π3 – π4
4
1 – 2 – 3 + 4
R7
1 – 2
4 + 5
4 – 5
1 + 2 + 3 + 4
2
485
Table 130: Symmetry coordinatesa for cyclobutylcarboxylic acid chloride.
A′
A′′
Description
β-CH2 antisymmetric stretch
γ-CH2 antisymmetric stretch
-CH stretch
β-CH2 symmetric stretch
γ-CH2 symmetric stretch
C=O stretch
β-CH2 deformation
γ-CH2 deformation
-CH in plane bend
β-CH2 wag
β-CH2 twist
β-CH2 rock
C-C stretch
Ring breathing
Ring deformation1
γ-CH2 rock
Ring deformation
COCl rock
C-Cl stretch
COCl scissors
Ring puckering
Ring-COCl bend
β-CH2 antisymmetric stretch
β-CH2 symmetric stretch
β-CH2 deformation
γ-CH2 wag
-CH out of plane bend
β-CH2 wag
γ-CH2 twist
β-CH2 twist
Ring deformation
Ring deformation
β-CH2 rock
COCl wag
Ring-COCl bend
COCl Assymetric torsion
aNot normalized.
S1
S2
S3
S4
S5
S6
S7
S8
S9
S10
S11
S12
S13
S14
S15
S16
S17
S18
S19
S20
S21
S22
S23
S24
S25
S26
S27
S28
S29
S30
S31
S32
S33
S34
S35
S36
486
Symmetry Coordinatea
r2 – r4 + r3 – r5
r6 – r7
r1
r2 +r4 + r3 + r5
r6 + r7
R2
λ9 + λ10
π5
1 + 2
λ1 – λ3 + λ5 – λ7 + λ2 – λ4 + λ6 – λ8
λ1 – λ3 – λ5 + λ7 + λ2 – λ4 – λ6 + λ8
λ1 + λ3 – λ5 – λ7 + λ2 + λ4 – λ6 – λ8
R1
R3 + R4 + R5 + R6
R3 – R4 + R5 – R6
π1 + π 2 – π3 – π4
R3 – R4 – R5 + R6
1 – 2
R7
3
1 + 2 + 3 + 4
4 + 5
r2 – r4 – r3 + r5
r2 + r4 – r3 – r5
λ9 – λ10
π1 - π2 + π 3 - π4
1 - 2
λ1 – λ3 + λ5 – λ7 – λ2 + λ4 – λ6 + λ8
π1- π2 - π3 + π4
λ1 – λ3 – λ5 + λ7 – λ2 + λ4 + λ6 – λ8
R3 + R4 – R5 – R6
1 – 2 – 3 + 4
λ1 + λ3 – λ5 – λ7 – λ2 – λ4 + λ6 + λ8
4
4 – 5
2
Table 131: Rotational Constants (MHz), Quadratic centrifugal distortion constants (kHz) and
quadrupole coupling constants (MHz) for the 35Cl and 37Cl isotopomers of the g-Eq conformer of
cyclobutylcarboxylic acid chloride.
Isotopomer
A
B
C
∆J
∆JK
∆K
δJ
δK
χaa
χab
χbb
χcc
n
s
rsm(FT)
rsm(MW)
MP2(full)/
6-311+G(d,p)
4368.2065
1411.2866
1150.6863
0.29993
-1.12561
5.04755
0.11171
1.64174
-16.9372
-44.6267
-6.2552
23.1924
c-C4H7C(O)35Cl
Ref. [4]
Fit 1
4349.86(17)
1414.78(1)
1148.24(1)
0.25(2)
-2.44(49)
0.088(16)
17
64
8
114
4349.84294(48)
1414.80319(36)
1148.24114(18)
0.2786(40)
-0.9523(32)
4.741(37)
0.09685(96)
1.401(55)
-18.2005(43)
-46.21(11)
-5.9667(49)
24.1672(56)
69
2.2
487
Fit 2
4349.8383(103)
1414.8038(18)
1148.2389(15)
0.2811(85)
-0.988(83)
4.52(97)
0.1034(32)
1.61(26)
-18.17(13)
-46.3(32)
-5.88(14)
24.06(16)
95
64
8
114
c-C4H7C(O)37Cl
MP2(full)/
Fit
6-311+G(d,p)
4339.5884
4322.0555(56)
1381.0680
1384.50581(105)
1128.6613
1126.35465(101)
0.29247
0.2730(46)
-1.16673
-1.001(21)
5.16867
5.8(1)
0.10923
0.0994(77)
1.59286
1.44(19)
-14.0482
-15.0614(42)
-35.0851
-36.01(18)
-4.2349
-3.9980(59)
18.2831
19.0594(52)
84
2.6
Table 132A: Temperature and intensity ratios of the g-Eq and g-Ax bands of cyclobutylcarboxylic acid
chloride.
Liquid
xenon
T(C)
-70
-75
-80
-85
-90
-95
-100
1/T (10-3 K-1)
4.922
5.047
5.177
5.315
5.460
5.613
5.775
Ha
Liquid
xenon
T(C)
-70
-75
-80
-85
-90
-95
-100
1/T (10-3 K-1)
4.922
5.047
5.177
5.315
5.460
5.613
5.775
I569 / I660
1.250
1.302
1.323
1.328
1.379
1.414
1.433
I569 / I1020
0.816
0.848
0.847
0.845
0.875
0.895
0.906
I569 / I1143
1.860
1.909
1.955
1.933
1.978
2.000
2.043
I645 / I660
1.406
1.426
1.462
1.496
1.530
1.564
1.597
I645 / I1020
0.918
0.929
0.936
0.951
0.971
0.990
1.009
I645 / I1143
2.093
2.091
2.159
2.178
2.196
2.213
2.277
107  9
80  11
67  9
107  3
78  4
67  7
I1108 / I1143
1.977
2.023
2.045
2.067
2.087
2.064
2.234
I1176 / I660
1.359
1.395
1.431
1.511
1.530
1.519
1.597
I1176 / I1020
0.888
0.909
0.916
0.961
0.971
0.962
1.009
I1176 / I1143
2.023
2.045
2.114
2.200
2.196
2.149
2.277
I1108 / I660
1.328
1.380
1.385
1.420
1.455
1.459
1.567
I1108 / I1020
0.867
0.899
0.887
0.903
0.923
0.924
0.991
Ha
115  14
89  17
75  18
125  16
97  13
85  19
Average value H = 91  4 cm-1 (1.09  0.05 kJ mol-1) with the g-Eq conformer the more stable form and the statistical
uncertainty (1σ) obtained by utilizing all of the data as a single set.
a
488
Table 132B. Temperature and intensity ratios of the g-Eq and t-Eq bands of cyclobutanecarbonyl chloride.
Liquid
xenon
T(C)
-70
-75
-80
-85
-90
-95
-100
1/T (10-3 K-1)
4.922
5.047
5.177
5.315
5.460
5.613
5.775
Ha
Liquid
xenon
T(C)
-70
-75
-80
-85
-90
-95
-100
1/T (10-3 K-1)
4.922
5.047
5.177
5.315
5.460
5.613
5.775
I569 / I488
0.842
0.884
0.915
0.926
0.989
1.022
1.043
I569 / I666
0.833
0.878
0.901
0.915
0.959
0.994
1.018
I569 / I1137
0.792
0.840
0.860
0.870
0.919
0.949
0.980
I645 / I488
0.947
0.968
1.011
1.043
1.098
1.130
1.163
I645 / I666
0.938
0.961
0.995
1.030
1.065
1.099
1.134
I645 / I1137
0.891
0.920
0.950
0.980
1.020
1.051
1.092
176  12
159  10
166  11
176  8
158  4
166  3
I1176 / I666
0.906
0.940
0.974
1.040
1.065
1.068
1.134
I1176 / I1137
0.861
0.900
0.930
0.990
1.020
1.020
1.092
I1108 / I488
0.895
0.937
0.957
0.989
1.043
1.054
1.141
I1108 / I666
0.885
0.930
0.943
0.977
1.012
1.025
1.113
I1108 / I1137
0.842
0.890
0.900
0.930
0.970
0.980
1.071
I1176 / I488
0.916
0.947
0.989
1.053
1.098
1.098
1.163
Ha
184  12 167  14 174  16 194  16 177  16 184  16
a
Average value H = 173  4 cm-1 (2.07  0.04 kJ mol-1) with the g-Eq conformer the more stable form and the statistical
uncertainty (1σ) obtained by utilizing all of the data as a single set.
489
Table 132C. Temperature and intensity ratios of the g-Ax and t-Eq bands of cyclobutanecarbonyl chloride.
Liquid
xenon
T(C)
1/T (10-3 K-1)
-70
-75
-80
-85
-90
-95
-100
4.922
5.047
5.177
5.315
5.460
5.613
5.775
I660 /
I488
0.674
0.679
0.691
0.697
0.717
0.723
0.728
I660 /
I666
0.667
0.674
0.681
0.688
0.696
0.703
0.710
I660 /
I1137
0.634
0.645
0.650
0.655
0.667
0.672
0.684
I1020 /
I488
1.032
1.042
1.080
1.096
1.130
1.141
1.152
I1020 /
I666
1.021
1.034
1.063
1.082
1.096
1.110
1.124
I1020 /
I1137
0.970
0.990
1.015
1.030
1.051
1.061
1.082
I1143 /
I488
0.453
0.463
0.468
0.479
0.500
0.511
0.511
I1143 /
I666
0.448
0.460
0.461
0.473
0.485
0.497
0.498
109 
Ha
69  5
51  1
59  3
97  9
80  6
87  6
91  8
a
-1
-1
Average value H = 82  3 cm (0.98  0.04 kJ mol ) with the g-Ax conformer the more stable form 10
and the statistical
uncertainty (1σ) obtained by utilizing all of the data as a single set.
490
I1143 /
I1137
0.426
0.440
0.440
0.450
0.465
0.475
0.480
99  8
Table 133: Comparison of rotational constants (MHz) obtained from modified ab initio MP2(full)/6-311+G(d,p)
structural parameters and the experimental values from the microwave spectra of g-Eq conformer of
cyclobutylcarboxylic acid chloride.
Isotopomers
c-C4H7C(O)35Cl
c-C4H7C(O)37Cl
Rotational
constant
A
B
C
A
B
C
Fit 1 from
Table 8
4349.8429(25)
1414.8032(25)
1148.2411(25)
4322.0555(56)
1384.5058(25)
1126.3546(25)
491
Adjusted
r0
4350.60
1414.66
1148.28
4321.22
1384.54
1126.43
||
0.76
0.14
0.04
0.84
0.04
0.08
Table 134A: Structural parametersa of a few acetyl chloride molecules of the form R-C(O)Cl (Å and degree).
Structural
R = CH3b
R = CH2CH3c
R = CH(CH3)2d
Parameters
Cis
Cis
rC=O
1.192 [1.189(3)] 1.192 [1.192(3)] 1.193 [1.186(3)]
rC-Cl
1.795 [1.794(3)] 1.795 [1.796(4)] 1.798 [1.804(4)]
rC-Cα
1.500 [1.499(3)] 1.505 [1.506(3)] 1.510 [1.511(3)]
rCα-Cβ
1.522 [1.523(3)] 1.525 [1.534(3)]
rCα-Cβ′
1.535 [1.540(3)]
120.6 [120.4(5)] 120.6 [120.7(6)] 120.3 [119.1(6)]
OCCl
127.6 [127.2(5)] 127.4 [127.2(7)] 127.2 [127.3(7)]
OCC
111.9 [112.4(5)] 111.9 [112.1(5)] 112.4 [113.6(5)]
ClCC
112.3 [112.4(8)] 110.1 [109.7(8)]
CCαCβ
108.9 [109.9(8)]
CCαCβ′
111.7 [113.8(27)]
CβCαCβ′
a
MP2(full)/6-311+G(d,p) [Experimental]
b
Adjusted r0 parameters determined from rotational constants taken from
reference [192]
c
Ref[193] ; adjusted r0 parameters.
d
Ref [194]; adjusted r0 parameters.
492
Table 134B.
degree).
Structural parametersa of a few four-membered ring molecules of the form c-C4H7X (Å and
X = OHc
t-Eq
1.5478
1.542
rCα-Cβ
[1.5555(2)]
[1.547(3)]
1.5478
1.548
rCβ-Cγ
[1.5555(2)]
[1.556(3)]
Puckering
28.5
32.34 [28.58(9)]
angle
[31.3(10)]
a
MP2(full)/6-311+G(d,p) [Experimental]
b
Ref [195]; r0 parameters.
c
Ref [197]; adjusted r0 parameters.
d
Ref [198]; adjusted r0 parameters.
e
Ref [198]; adjusted r0 parameters.
f
Ref[199] ; adjusted r0 parameters.
Structural
Parameters
X = Hb
X = Brd
Eq
1.535
[1.541(3)]
1.551
[1.552(3)]
X = Fe
Eq
1.530
[1.543(3)]
1.551
[1.554(3)]
Ax
1.534
[1.546(3)]
1.552
[1.554(3)]
Eq
1.559
[1.562(3)]
1.546
[1.551(3)]
Ax
1.559
[1.561(3)]
1.547
[1.553(3)]
34.4 [29.8(5)]
34.0 [37.4(5)]
29.2 [20.7(5)]
31.6 [29.0(5)]
29.0 [23.5(5)]
493
X = SiH3f
Figure 71: Labeled conformers of cyclobutylcarboxylic acid chloride with atomic numbering.
494
Figure 72: Mid-infrared spectra of cyclobutylcarboxylic acid chloride (A) spectrum of gas; (B) spectrum of
xenon solution at -80°C with bands used in the enthalpy determination assigned on spectra.
495
Figure 73: Experimental and predicted mid-infrared spectra of cyclobutylcarboxylic acid chloride: (A) observed
spectra of xenon solutions at -80, -90, and -100°C; (B) simulated spectrum of mixture of g-Eq, g-Ax (ΔH = 91
cm-1), and t-Eq (ΔH = 173 cm-1) conformers at -80°C; (C) simulated g-Eq conformer; (D) simulated g-Ax
conformer; (E) simulated t-Eq conformer.
496
Figure 74: Temperature (-70 to -100°C) dependent mid-infrared spectrum of cyclobutylcarboxylic
chloride dissolved in liquid xenon
497
CHAPTER 18
VIBRATIONAL ASSIGNMENTS, THEORETICAL CALCULATIONS, STRUCTURAL PARAMETERS
AND CONFORMER STABILITY DETERMINATIONS OF CYCLOBUTYLDICHLOROSILANE
Introduction
The structure of the cyclobutane molecule generated controversy whether it was a puckered or planar
molecule. Several different methods were used to try to obtain conclusive evidence of the structure of the
molecule. This controversy continued for several years until finally a vibrational study showed conclusively that
the molecule was puckered [200].
There was no controversy concerning the structure of the ring for the cyclobutyl molecules which had
monosubstituted additions but there was a different controversy. There was an interesting controversy that
developed when studies were reported from the infrared vibrations investigation of cyclopropyl chloride and
bromide. From the study of these two molecules it was reported that both of these molecules had both axial and
equatorial molecules present from infrared vibrational investigations [201, 202]. However one of these
investigators carried out a microwave investigation of cyclobutylbromide [203] but the equatorial conformer
was the only form identified. Additionally, a subsequent microwave investigation of cyclobutyl chloride which
was very extensive with several isomers from a well known scientist (Prof. W. D. Gwinn) and it was clearly
shown that there was only the axial conformer present [204]. Continuing microwave studies did not provide
evidence of a second conformer whereas the vibration spectrum clearly showed [204] the presence of the
second conformer [205, 206]. This difference persisted until the 80s when cyclobutyl halides and silane
reported to have both the axial and equatorial conformers present in the monosubstituted cyclobutyl molecules.
The utilization of microwave spectroscopy for the determination of structural parameters of significant
organic molecules became in use in the 60’s. Of particular interest was the excellent study of the microwave
investigation of chlorocyclobutane. The structural parameters were reported for this chloride where in addition
to the rotational constants for the 35Cl species there were rotational constants from eight of the isotopes of this
molecule. By using these data the r0 parameters were obtained for this molecule. However, the CH distance
498
ranged from 1.090 to 1.110 Å which is a very large difference. Therefore the utilization of a significant number
of hydrogen isotopes to obtain molecular distances results for many molecules with unusual structural
parameters.
An attempt was made to obtain more realistic parameters by combining ab initio predicted values and
the experimentally determined rotational constants to determine the r0 structural parameters more accurately by
utilization of diagnostic least square [207]. However, many of the A rotational constants of the isotopmers could
not be fitted but from a later microwave study [208] the spectrum of both the axial and equatorial conformers
were assigned and the A rotational constants for the 35Cl and 37Cl isotopomers were significantly different from
those previously reported . Therefore, by using these new values of the rotational constants and by combining
them with ab initio calculations again parameters were obtained which seemed more reasonable.
The data of greatest interest in connection with the geometrical structure are the internuclear distances.
Initially the rs method was derived in 1953 directly from rotational-vibrational spectroscopy. When the
rotational constants for various isotopic species are observed then the rs structure can be determined by
Kraitchman’s method [166]. The major problem with the rs structure is that it assumes the geometry to be
isotope invariant. This would be true if the harmonic approximation were exactly obeyed but it is not and an
uncertainty in the rs structure arises from anharmonicity. Additionally if the atom lies on the axis then there is
no way to obtain the rs structural parameters for that species. Thus it is reasonable to imply that there is no real
way to relate rs to other kinds of experimental structures.
Another method for estimating an equilibrium structure from ground state rotational constants was
developed by Watson [209] in 1973 who proposed a mass dependent method (rm) and applied it to a number of
diatomic and triatomic molecules. The advantage of this mass dependent method over the conventional r e
method is insensitive to the perubations and resonances that affect excited vibrational states. This method was
utilized by Nakata and coworkers [210] for determination of the rm structure of phosgene. It was observed in the
analysis that the rm structure derived from different combination of isotopic species were different from one
another and exceeded their experimental error. This systematic uncertainty can be reduced or practically
499
cancelled [211, 212] by taking proper average of rm parameters obtained from different isotopic species.
Nevertheless the need for significant isotopic species has lead to few determinations of rm parameters. Thus the
r0 parameters are the ones mainly used in the last decade. Therefore we initiated some adjustments to improve
the ability to obtain the heavy atom parameters of molecule with two or more conformers by the use of ab initio
predictions.
We have found that good structural parameters for hydrocarbons and many substituted ones can be
determined by adjusting the structural parameters obtained from the ab initio MP2(full)/6-311+G(d,p)
calculations to fit the rotational constants obtained from microwave experimental data by using the computer
program “A&M” (Ab initio and Microwave) developed [57] in our laboratory. To reduce the number of
parameters that are necessary for completed structural data we have shown that the C-H distance can be
obtained by theory predictions. We [44] have shown that ab initio MP2(full)/6-311+G(d,p) calculations predict
the carbon-hydrogen r0 structural parameters for more than fifty hydrocarbons to at least 0.002 Å compared to
the experimentally determined [45] values from isolated CH stretching frequencies which agree with previously
determined values from earlier microwave studies. Therefore, all of the carbon-hydrogen parameters can be
taken from the MP2(full)/6-311+G(d,p) predicted values for the respective conformer. However, in order to
further reduce the number of independent variables, the structural parameters are separated into sets according
to their types where bond distances in the same set keep their relative ratio, whereas bond angles and torsional
angles in the same set keep their difference in degrees. This assumption is based on the fact that errors from ab
initio calculations are systematic.
Additionally, we have also shown that the differences in predicted distances and angles from the ab
initio calculations for different conformers of the same molecule can usually be used as one parameter with the
ab initio predicted differences except for some dihedral angles. Therefore if we have complete set of rotational
constants then the adjusted ro structural parameters for a molecule can be obtained. The germane-hydrogen and
silicon-hydrogen r0 structural parameter values can be experimentally determined [45] from isolated SiH and
GeH stretching frequencies.
500
Experimental and Computational Methods
The sample of cyclobutyldichlorosilane was prepared by coupling trichlorosilane to the Grignard reagent
of cyclobutyl magnesiumbrome in dry diethyl ether under dry nitrogen. After stirring over night at room
temperature the sample was filtered under nitrogen and the ether was distilled off. The product was originally
purified by trap-to-trap distillation twice and the final purification was obtained at low pressure and low
temperature by a sublimation column. The sample was further purified by a fractionation column and the purity
of the sample was verified from the infrared spectra of the gas and NMR spectrum of the liquid.
The midinfrared spectrum of the gas (Fig. 75) was obtained from 4000 to 250 cm-1 on a Perkin-Elmer
model 2000 Fourier transform spectrometer equipped with a Ge/CsI beamsplitter and a DTGS detector.
Atmospheric water vapor was removed from the spectrometer housing by purging with dry nitrogen. The
theoretical resolution used to obtain the spectrum of the gas was 0.5 cm-1. One hundred twenty eight
interferograms were added and transformed with a boxcar truncation function.
The Raman spectra (Fig. 76) were recorded on a Spex model 1403 spectrometer equipped with a SpectraPhysics model 2017 argon laser operating on the 514.5 nm line. The laser power used was 0.5 W with a spectral
bandpass of 3 cm-1. The spectrum of the liquid was recorded with the sample sealed in a Pyrex glass capillary.
The measurements of the Raman frequencies are expected to be accurate to  2 cm-1.
The LCAO-MO-SCF restricted Hartree-Fock calculations were performed with the Gaussian-03
program [21] with Gaussian-type basis functions. The energy minima with respect to nuclear coordinates were
obtained by the simultaneous relaxation of all geometric parameters by using the gradient method of Pulay [22].
Several basis sets as well as the corresponding ones with diffuse functions were employed with the MøllerPlesset perturbation method [23] to second order (MP2(full)) as well as with the density functional theory by
the B3LYP method.
The infrared spectra were predicted from the MP2(full)/6-31G(d) calculations. The predicted scaled
frequencies were used together with a Lorentzian function to obtain the calculated spectra. Infrared intensities
determined from MP2(full)/6-31G(d) calculations were obtained based on the dipole moment derivatives with
501
respect to Cartesian coordinates. The derivatives were transformed with respect to normal coordinates by
(u/Qi) =

j
(u/Xj)Lij, where Qi is the ith normal coordinate, Xj is the jth Cartesian displacement
coordinate, and Lij is the transformation matrix between the Cartesian displacement coordinates and the normal
coordinates. The infrared intensities were then calculated by [(N)/(3c2)] [(x/Qi)2 + (y/Qi)2 + (z/Qi)2].
The predicted spectrum of the mixture of conformers should be compared to the infrared spectra of the gas
(Fig.76).
Results and Discussion
The ab initio calculations were carried out for this molecule and the energy differences [Table 135] for
the four possible forms were obtained with the t-Eq form as the most stable conformer followed by the g-Eq, tAx and g-Ax forms. The ab initio predicted energy difference from the MP2(full)/6-311G(d,p) calculations with
198 basis sets gives the t-Eq conformer as the more stable form by 57 cm-1 (0.68 kJ/mol) than the g-Eq form.
The B3LYP method with all the basis sets used in this study consistently predicts t-Eq as the more stable
conformer followed by g-Eq, t-Ax and g-Ax forms, respectively. From the band intensities of the t-Eq and g-Eq
SiCl2 symmetric stretch fundamentals assigned at 519 and 481 cm-1, respectively, an initial H can be
determined with a value of 271 cm-1. The accuracy of this value is likely to be very poor as there are underlying
fundamentals from the t-Ax and g-Ax fundamentals in near coincidence with these two bands but this can give
an idea of which conformer is more stable and it gives a value to compare the predicted energy differences.
It is interesting to note that when we proceed to a higher basis set like MP2(full)/6-311+G(2d,2p) it
predicts the g-Eq form to be more stable than the t-Eq conformer by 82 cm-1. This is an interesting phenomenon
where the addition of more functions into the ab initio basis sets actually causes poorer results. The same basis
set but with B3LYP method gave instead the t-Eq form as the most stable conformer from the four possible
ones. These values are both far off in magnitude and the ab initio predicted MP2(full) values even predicts the
Eq-g conformer as being the more stable form. Thus the B3LYP method gives more consistent results for
conformational stability predictions in this study compared to the MP2(full) calculation which was not able to
502
predict a specific conformer as the more stable form. The experimentally determined enthalpy differences if
obtained later should be comparable to the ab initio predicted energy values and also can be compared with
other enthalpy differences obtained for other molecules of the form c-C4H7SiHX2. The B3LYP predictions
appear to correspond much better with the experimental result and so are thought to be more reliable for the
determination of the energy differences.
There has been no vibrational investigation previously reported on the cyclobutyldichlorosilane. In the
current study, with the aid of MP2(full)/6-31G(d) predicted vibrational wavenumbers, ab initio predicted
intensities along with infrared band contours it has been possible to assign a significantly large number of the
fundamentals for the most stable conformers of c-C4H7SiHCl2 Tables (136-139). The vibrational assignments
reported herein are based on a significant amount of information with the mid infrared spectrum of the gas and
predictions from the scaled ab initio MP2(full)/6-31G(d) calculations. The ab initio infrared band intensities
were also used but they seemed to be the least reliable of the data utilized.
For the t-Eq conformer the average error for the fixed scaled predicted frequencies for the A′ block
fundamentals was 9 cm-1 which represent 0.6% error. The average error for the A′′ modes for the t-Eq form was
7 cm-1 which represent 0.55% error. Thus, the relatively small basis set of 6-31G(d) by the MP2(full)
calculations with two scaling factors provides excellent predicted frequencies for aiding the vibrational
assignments.
The assignment of the fundamentals for the four conformers of cyclobutyldichlorosilane was
complicated due to their close proximity and similar intensities. In the infrared spectra of the gas the
fundamentals could not be sufficiently resolved to assign them to different bands, which leads to the assignment
of multiple fundamentals to single bands. However, if the Raman spectra of the liquid (Fig. 76) is compared to
the infrared spectra of the gas (Fig. 75) it can be seen that additional bands can be resolved. For example, in the
region of 1300 – 1100 cm-1 in the infrared spectra of the gas (Fig. 75) one can see two bands, whereas, in the
spectra of the Raman liquid one can see at least five well resolved bands in the same region. For a more
503
complete assignment of the fundamentals of the conformers of cyclobutyldichlorosilane further Raman spectra
of the liquid and infrared spectra of additional phases and solutions should be obtained.
It would be of interest to obtain the enthalpy differences for the c-C4H8SiCl2 molecule since it has never
been studied and a spectroscopy investigation would be of interest to see how different substituents affect the
conformational stability of four membered ring molecules and whether the experimental work will agree with the
ab initio calculation results. Therefore, a continuation of this study would be excellent in order to obtain a more
reliable experimental value for the conformational enthalpy difference by using a variable temperature spectra of
the sample dissolved in the liquefied xenon. Some of the advantages behind carrying such an experiment is that a
very little interaction of the solute with the solvent molecules so the frequencies observed in the solution were
shifted very little from those observed in the gas. Also, the bands will be relatively narrow particularly when the
data was recorded at relatively low temperatures. Thus, it will be possible to resolve relatively close frequency
fundamentals so individual bands could be assigned for very complex gas phase contours. Also, relative
intensities of bands which are extensively overlapped in the gas will be obtainable when they are recorded in low
temperature rare gas solutions. Therefore, such spectra are very good for making vibrational assignments
particular for molecules where more than one conformer is present and many fundamentals are closely spaced.
It would be interesting to obtain the complete adjusted r0 structural parameters for the stable conformers
of cyclobutyldichlorosilane. To do so we must predict the rotational constants to be fit from the microwave
spectra using a suitable predicted structures (Table 140). We [44] have shown that ab initio MP2(full)/6311+G(d,p) calculations predict the carbon-hydrogen r0 structural parameters for more than fifty hydrocarbons to
at least 0.002 Å compared to the experimentally determined [45] values from isolated CH stretching frequencies
which agree to previously determined values from earlier microwave studies. Therefore, all of the carbonhydrogen parameters can be taken from the MP2(full)/6-311+G(d,p) predicted values for the conformers of
cyclobutyldichlorosilane. The silane-hydrogen r0 structural parameter value was experimentally determined [45]
for the t-Eq conformer to be 1.477(2) Å from isolated SiH stretching frequency listed in Table 136. This leaves 9
independent structural parameters to be determined for a completed adjusted r0 structural parameters. Therefore a
504
minimum of 9 rotational constants would be desirable to obtain such an experimental structure. Additionally, we
have also shown that the differences in predicted distances and angles from the ab initio calculations for different
conformers of the same molecule can usually be used as one parameter with the ab initio predicted differences
except for some dihedral angles. This would mean that each additional conformer would only increase the
minimum number of rotational constants by three. Such a microwave study would be desirable as there are few
cyclobutylsilane structures determined, but the study of the microwave spectra of cyclobutyldichlorosilane will
be a challenging investigation with a high degree of difficulty. This difficulty is due to the Cl atoms on the Si
substituent where each chlorine nucleus has a spin quantum number I = 3/2 and, therefore, a nuclear quadrupole
moment which interacts with the electric field gradient created by the electrons of the rotating molecule.
505
Table 135: Calculated Electronic Energies (Hartree) for the t-Eq and Energy Differences (cm-1) for g-Eq, t-Ax,
and g-Ax Forms of cyclobutyldichlorosilane
Energy Differenceb
t-Eqa
g-Eq
t-Ax
g-Ax
MP2(full)/6-31G(d)
0.000684
12
164
286
MP2(full)/6-31+G(d)
0.0175112
-6
258
226
MP2(full)/6-31G(d,p)
0.0669021
14
182
331
MP2(full)/6-31+G(d,p)
0.0827558
6
267
259
MP2(full)/6-311G(d,p)
0.456566
57
112
306
MP2(full)/6-311+G(d,p)
0.467003
-35
214
185
MP2(full)/6-311G(2d,2p)
0.5953155
-19
48
90
MP2(full)/6-311+G(2d,2p)
0.6013542
-82
69
26
B3LYP/6-31G(d)
2.2093142
64
266
336
B3LYP/6-31+G(d)
2.2164985
75
280
343
B3LYP/6-31G(d,p)
2.2203848
61
258
331
B3LYP/6-31+G(d,p)
2.2272266
71
271
331
B3LYP/6-311G(d,p)
2.3313051
92
226
308
B3LYP/6-311+G(d,p)
2.3346166
70
235
292
B3LYP/6-311G(2d,2p)
2.3489741
84
207
266
B3LYP/6-311+G(2d,2p)
2.3509751
65
212
251
Method/Basis Set
a
b
Energy of conformer is given as -(E + 1365) H.
Energy difference related to the t-Eq conformer.
506
Table 136: Observed and calculateda frequencies (cm -1) for Eq-t cyclobutyldichlorosilane.
Band
Contoursd
A
C
β-CH2 antisymmetric stretch
3198
3000
36.3
68.2 0.67 2986
100
A 1
γ-CH2 antisymmetric stretch
3182
2985
8.1
81.0 0.35 2975 100
2
CH stretch
3129
2935
11.4
32.3 0.63 2943
16
84
3
γ-CH2 symmetric stretch
3126
2932
33.8
123.3 0.14 2938 100
4
β-CH
symmetric
stretch
3113
2921
6.7
148.0
0.14
2926
8
92
5
2
SiH stretch
2343
2222 107.6
95.2 0.12 2202
100
6
β-CH2 deformation
1575
1483
1.7
6.3 0.67
34
66
7
γ-CH2 deformation
1549
1458
4.9
20.0 0.73 1451
9
91
8
β-CH2 wag
1351
1284
1.5
5.9 0.60 1268 100
9
CH in-plane bend
1301
1237
10.2
4.6 0.75 1248
75
25
10
β-CH2 twist
1266
1205
4.4
5.7 0.75 1189
93
7
11
β-CH2 rock
1128
1076
26.8
2.6 0.01 1056
98
2
12
Ring breathing
1055
1002
3.4
14.7 0.16
998
2
98
13
Ring deformation 1
959
924
8.6
2.1 0.27
910
81
19
14
Ring deformation 2
912
884
2.0
3.8 0.06
856
100
15
SiH in-plane bend
828
787 160.3
9.5 0.73
800 100
16
γ-CH2 rock
751
718
26.5
3.3 0.10
717
98
2
17
Si-C stretch
588
569
60.1
1.4 0.61
578 100
18
SiCl2 symmetric stretch
537
514
31.0
8.7 0.08
519
60
40
19
SiHCl2 Ring bending
306
296
2.5
3.3 0.12
295
34
66
20
SiCl2 deformation
239
234
4.1
3.1 0.31
95
5
21
β-CH2 antisymmetric stretch
161
161
6.5
3.9 0.72
4
96
22
β-CH2 symmetric stretch
122
120
10.1
0.4 0.71
100
23
β-CH2 deformation
3190
2992
18.6
62.4 0.75 2980
A 24
γ-CH2 wag
3113
2920
27.0
3.6 0.75 2926
25
β-CH2 wag
1544
1453
0.9
5.4 0.75 1445
26
CH out-of-plane bend
1321
1255
4.4
3.5 0.75 1248
27
γ-CH2 twist
1305
1238
1.3
0.4 0.75 1248
28
β-CH2 twist
1279
1214
0.2
7.4 0.75 1208
29
Ring deformation 1
1235
1173
2.9
6.6 0.75 1168
30
Ring deformation 2
1041
994
11.8
1.0 0.75
993
31
SiH out-of-plane bend
985
936
1.2
10.8 0.75
935
32
β-CH2 rock
972
933
4.6
3.4 0.75
912
33
SiCl2 antisymmetric stretch
853
810 168.0
9.8 0.75
800
34
SiHCl2 Ring bending
812
772
0.0
0.0 0.75
770
35
Ring puckering
607
578 140.0
4.7 0.75
578
36
SiCl2 wag
293
290
0.0
3.4 0.75
295
37
SiCl2 twist
85
84
0.2
0.7 0.75
38
SiCl2 rock
47
45
0.2
0.3 0.75
39
a
MP2(full)/6-31G(d) ab initio calculations, scaled frequencies, infrared intensities (km/mol), Raman activities (Å 4/amu),
depolarization ratios and potential energy distributions (P.E.D.s)
b
MP2(full)/6-31G(d) fixed scaled frequencies with factors of 0.88 for CH and SiH stretches and CH2 deformations, 1.0 for heavy
atom bends, and 0.90 for all other modes.
c
Contributions less than 10% are omitted.
d
A and C values in the last two columns are percentage infrared band contours.
Vib.
No.
Approximate Descriptions
ab
initio
fixed
scaledb
507
IR
int.
Raman
act.
dp
ratio
IR
Gas
Table 137: Observed and calculateda frequencies (cm -1) for Eq-g cyclobutyldichlorosilane.
Vib.
No.
Approximate Descriptions
ab
initio
fixed
scaledb
1
β-CH2 antisymmetric stretch
3201
3003
2
β-CH2 antisymmetric stretch
IR
int.
33.8
Raman
act.
dp
ratio
IR
Gas
53.0
0.68
2986
Band
Contoursd
A
B C
- 84 16
2980
5
3 92
3193
2995
17.0
64.0 0.75
γ-CH2 antisymmetric stretch
95
2
3
3
3182
2985
8.0
75.5 0.45 2975
β-CH2 symmetric stretch
36
2 62
4
3128
2934
9.4
147.8 0.04 2943
β-CH2 symmetric stretch
66
7 27
3126
2932
35.0
45.9 0.12 2938
5
γ-CH2 symmetric stretch
30 31 39
6
3125
2931
26.7
45.0 0.66 2938
CH stretch
13 53 34
7
3107 2914
11.6
84.0 0.31 2907
SiH
stretch
2202
9
2 89
2343
2223 136.5
138.5 0.16
8
β-CH2 deformation
36 51 13
9
1575
1483
0.7
5.5 0.70
γ-CH2 deformation
15 70 15
10
1549
1458
3.9
21.4 0.72 1451
β-CH2 deformation
7 26 67
1544
1454
1.2
5.2 0.75 1445
11
β-CH2 wag
6 80 14
12
1354
1287
0.9
4.9 0.70 1268
γ-CH
wag
1248
14
20 66
13
1322
1255
4.9
3.2 0.75
2
β-CH2 wag
45 53
2
1306
1239
1.9
1.2 0.70 1248
14
CH in-plane bend
26 41 33
15
1302
1238
7.5
3.4 0.75 1248
CH out-of-plane bend
32 12 56
16
1279
1214
0.3
7.9 0.75 1208
β-CH2 twist
68 17 15
1270
1209
3.2
7.3 0.73 1189
17
γ-CH
twist
1168
22 18 60
18
1235
1173
3.1
6.7 0.75
2
β-CH2 rock
97
3
19
1132
1080
10.7
2.6 0.18 1056
β-CH2 twist
998
58
2 40
1055
1002
19.3
6.7 0.24
20
Ring breathing
998
3 94
3
21
1049
1001
10.8
9.5 0.16
Ring deformation 2
935
- 14 86
22
986
937
1.3
7.0 0.75
Ring deformation 1
935
56
7 37
975
936
4.7
6.9 0.75
23
Ring deformation 1
910
67 32
1
24
961
926
29.2
1.4 0.11
Ring deformation 2
856
20 72
8
25
917
887
23.0
3.6 0.14
SiH out of plane bend
800
22 78
846
803 139.4
9.2 0.75
26
SiH in of plane bend
800
82 12
6
27
831
790
91.7
4.8 0.75
β-CH2 rock
765 100
28
800
761
65.4
4.0 0.72
γ-CH2 rock
693
33 55 12
726
696
6.4
6.9 0.40
29
SiCl2 antisymmetric stretch
578
10 90
30
605
577 115.1
4.1 0.75
Si-C stretch
557
79
5 16
31
573
549
50.4
5.1 0.58
SiCl2 symmetric stretch
481
2 21 77
497
476
34.5
9.8 0.02
32
SiHCl
Ring
bending
387
35
52 13
33
412
402
17.4
1.9 0.61
2
SiHCl2 Ring bending
295
55
2 43
34
297
291
2.4
4.4 0.35
Ring puckering
- 75 25
210
208
0.6
1.4 0.71
35
SiCl2 deformation
35 20 45
36
181
180
5.0
3.0 0.70
SiCl2 wag
50
- 50
37
116
115
3.1
0.8 0.63
SiCl
twist
14
66
20
97
96
1.8
1.0 0.73
38
2
SiCl2 rock
33
4 63
39
46
44
0.4
0.4 0.74
a
MP2(full)/6-31G(d) ab initio calculations, scaled frequencies, infrared intensities (km/mol), Raman activities (Å4/amu),
depolarization ratios and potential energy distributions (P.E.D.s)
b
MP2(full)/6-31G(d) fixed scaled frequencies with factors of 0.88 for CH and SiH stretches and CH2 deformations, 1.0
for heavy atom bends, and 0.90 for all other modes.
c
Contributions less than 10% are omitted.
d
A, B and C values in the last two columns are percentage infrared band contours.
508
Table 138: Observed and calculateda frequencies (cm -1) for Ax-t cyclobutyldichlorosilane.
Band
Contoursd
A
C
γ-CH
antisymmetric
stretch
3198
3000
40.0
71.1
0.52
2986
77
23
A 1
2
β-CH2 antisymmetric stretch
3173
2977
13.2
60.4
0.75 2971
100
2
γ-CH2 symmetric stretch
3132
2938
10.4
167.9
0.06 2943
54
46
3
CH stretch
3127
2933
24.4
48.6
0.23 2938
41
59
4
β-CH
symmetric
stretch
3117
2924
4.4
114.5
0.26
2926
96
4
5
2
SiH stretch
2345
2225 104.2
92.8
0.12 2207
5
95
6
β-CH2 deformation
1580
1488
0.5
8.0
0.64
53
47
7
γ-CH2 deformation
1554
1463
4.6
16.8
0.75
100
8
β-CH2 wag
1341
1274
3.8
1.8
0.57 1248
72
28
9
CH in-plane bend
1316
1254
5.7
1.9
0.44 1248
60
40
10
β-CH2 twist
1250
1191
6.2
8.9
0.74 1189
100
11
β-CH2 rock
1087
1035
13.5
4.6
0.48 1125
100
12
Ring
breathing
1063
1009
8.4
20.4
0.12
1009
62
38
13
Ring deformation 1
966
931
24.4
0.1
0.34
912
100
14
Ring deformation 2
909
880
24.7
3.3
0.56
856
90
10
15
SiH in-plane bend
816
777 114.8
9.7
0.75
770
100
16
Si-C
stretch
718
688
59.7
3.7
0.16
681
97
3
17
γ-CH2 rock
668
644
11.9
2.3
0.40
631
78
22
18
SiCl2 symmetric stretch
540
516
56.3
10.1
0.09
519
81
19
19
SiHCl2 Ring bending
289
282
0.2
4.1
0.14
100
20
SiCl
wag
224
220
5.7
1.4
0.67
100
21
2
β-CH2 antisymmetric stretch
165
165
6.1
3.4
0.74
81
19
22
β-CH2 symmetric stretch
141
139
5.0
0.1
0.46
21
79
23
β-CH2 deformation
3178
2981
3.1
66.8
0.75 2971
77
23
A 24
γ-CH
wag
3118
2925
44.9
2.8
0.75
2926
100
25
2
β-CH2 wag
1550
1459
2.2
7.4
0.75
54
46
26
γ-CH2 twist
1324
1258
3.6
0.6
0.75 1248
41
59
27
CH out-of-plane bend
1291
1226
1.8
3.9
0.75 1233
96
4
28
β-CH
twist
1273
1210
2.5
8.7
0.75
1189
5
95
29
2
Ring deformation 1
1209
1148
1.0
4.7
0.75 1140
53
47
30
Ring deformation 2
1070
1018
12.2
0.5
0.75 1019
100
31
SiH out-of-plane bend
991
941
4.4
13.2
0.75
935
72
28
32
β-CH
rock
963
925
0.4
0.6
0.75
910
60
40
33
2
SiCl2 antisymmetric stretch
844
801 158.8
8.4
0.75
800
100
34
SiHCl2 Ring bending
811
773
0.0
0.0
0.75
770
100
35
SiCl2 deformation
608
578 138.0
5.0
0.75
578
62
38
36
Ring puckering
297
295
0.0
2.6
0.75
295
100
37
SiCl2 twist
97
97
0.2
1.0
0.75
90
10
38
SiCl2 rock
39
37
0.1
0.1
0.75
100
39
a
MP2(full)/6-31G(d) ab initio calculations, scaled frequencies, infrared intensities (km/mol), Raman activities (Å 4/amu),
depolarization ratios and potential energy distributions (P.E.D.s)
b
MP2(full)/6-31G(d) fixed scaled frequencies with factors of 0.88 for CH and SiH stretches and CH2 deformations, 1.0 for heavy
atom bends, and 0.90 for all other modes.
c
Contributions less than 10% are omitted.
d
A and C values in the last two columns are percentage infrared band contours.
Vib.
No.
Approximate Descriptions
ab
initio
fixed
scaledb
509
IR
int.
Raman
act.
dp
ratio
IR
Gas
Table 139: Observed and calculateda frequencies (cm -1) for Ax-g cyclobutyldichlorosilane.
Band
Contoursd
A
B C
γ-CH
antisymmetric
stretch
3210
3011
26.6
46.5
0.66
2996
63
37 1
2
β-CH2 antisymmetric stretch
3180 2984
4.1
67.2
0.75 2975 22
25 53
2
β-CH2 antisymmetric stretch
3175 2978
14.5
65.6
0.67 2971 13
85 2
3
γ-CH2 symmetric stretch
3143 2948
26.0 124.7
0.11 2955 70
9
21
4
β-CH2 symmetric stretch
3123 2930
16.2
105.7
0.11 2938 5
58 37
5
β-CH2 symmetric stretch
3116 2924
35.8
59.5
0.10 2926 10
5
85
6
CH stretch
3102 2910
19.7
51.0
0.56 2907 53
26 21
7
SiH stretch
2344 2223
131.5 130.7
0.16 2202 7
2
91
8
β-CH2 deformation
1578 1486
0.4
10.0
0.68 79
0
21
9
γ-CH2 deformation
1550 1459
3.5
16.4
0.74 1451 4
88 8
10
β-CH2 deformation
1548 1457
2.9
6.9
0.74 1451 6
3
91
11
CH in-plane bend
1340 1272
3.4
2.0
0.60 1248 27
50 23
12
γ-CH2 wag
1328 1262
3.2
0.6
0.75 1248 20
12 68
13
β-CH2 wag
1321 1258
3.1
2.0
0.42 1248 79
21 0
14
β-CH2 wag
1294 1228
1.8
3.9
0.75 1233 22
11 67
15
γ-CH
twist
1274
1211
2.6
8.8
0.75
1189
13
9
78
16
2
β-CH2 twist
1249 1190
3.5
7.9
0.73 1189 87
5
8
17
CH out-of-plane bend
1221 1158
1.4
4.1
0.75 44
6
50
18
β-CH2 rock
1084 1032
7.5
5.3
0.44 1025 11
20 69
19
β-CH2 twist
1075 1024
11.7
1.5
0.49 1025 31
55 14
20
Ring
breathing
1067
1013
15.2
19.7
0.11
1019
51
46 3
21
Ring deformation 1
1002 951
3.9
13.4
0.75 935
19
4
77
22
Ring deformation 1
973
937
3.4
0.4
0.23 935
95
1
4
23
Ring deformation 2
968
932
6.0
0.4
0.69 912
100 24
Ring deformation 2
889
862
1.7
6.0
0.63 22
74 4
25
SiH out of plane bend
844
801
139.6
7.6
0.72 800
6
94 26
SiH in of plane bend
827
787
177.8
7.2
0.72 800
92
5
3
27
β-CH2 rock
805
765
18.9
0.9
0.67 765
93
3
4
28
γ-CH2 rock
685
664
9.7
1.3
0.69 663
53
45 2
29
Si-C stretch
662
631
14.0
6.3
0.44 631
24
74 2
30
SiCl
antisymmetric
stretch
598
569
134.2
5.6
0.74
557
41
59 31
2
SiCl2 symmetric stretch
516
492
46.6
11.4
0.02 490
7
24 69
32
SiHCl2 Ring bending
383
376
16.3
1.9
0.73 369
39
36 25
33
SiHCl2 Ring bending
287
282
0.7
3.6
0.34 39
6
55
34
SiCl2 deformation
190
189
3.3
3.0
0.69 31
15 54
35
Ring puckering
165
163
0.9
0.8
0.54 3
63 34
36
SiCl2 wag
131
129
2.9
1.0
0.70 64
- 36
37
SiCl2 twist
111
110
0.5
1.2
0.75 3
71 26
38
SiCl2 rock
41
39
0.3
0.3
0.75 45
9
46
39
a
MP2(full)/6-31G(d) ab initio calculations, scaled frequencies, infrared intensities (km/mol), Raman activities (Å 4/amu),
depolarization ratios and potential energy distributions (P.E.D.s)
b
MP2(full)/6-31G(d) fixed scaled frequencies with factors of 0.88 for CH and SiH stretches and CH2 deformations, 1.0 for heavy atom
bends, and 0.90 for all other modes.
c
Contributions less than 10% are omitted.
d
A, B and C values in the last two columns are percentage infrared band contours.
Vib.
No.
Approximate Descriptions
ab
initio
fixed
scaledb
510
IR
int.
Raman
act.
dp
ratio
IR
Gas
Table 140: Structural parametersa (Å and degrees) and rotational constants (MHz) of cyclobutyldichlorosilane.
Structural
Parameters
rCα-Si
rCα-Cβ
rCα-Cβ′
rCγ-Cβ
rCγ-Cβ′
rSi-Cl1
rSi-Cl2
rCα-H
rCβ-H1
rCβ′-H1
rCβ-H2
rCβ′-H2
rCγ-H1
rCγ-H2
CαSiCl1
CαSiCl2
 Cl1SiCl2
CβCαSi
Cβ′CαSi
Cβ′CαCβ
CγCβCα
CγCβ′Cα
Cβ′CγCβ
 HCαCβ
 HCαCβ′
 HCαSi
 HSiCα
 H1CβCα
 H1Cβ′Cα
 H1CβCγ
 H1Cβ′Cγ
 H2CβCα
 H2Cβ′Cα
 H2CβCγ
 H2Cβ′Cγ
 H1CβH2
 H1Cβ′H2
 H1CγCβ
 H1CγCβ′
 H2CγCβ
 H2CγCβ′
 H1CγH2
CγCβCβ′Cα
HCαSiCl
t- Eq
g-Eq
t-Ax
g-Ax
1.845
1.559
1.559
1.546
1.546
2.050
2.050
1.096
1.091
1.091
1.095
1.095
1.093
1.092
109.9
109.9
109.3
118.6
118.6
87.3
87.8
87.8
88.2
109.7
109.7
110.7
111.9
111.2
111.2
110.7
110.7
117.5
117.5
118.8
118.8
109.2
109.2
118.2
118.2
110.4
110.4
109.5
148.5
60.2
1.849
1.558
1.558
1.546
1.546
2.050
2.051
1.098
1.092
1.092
1.094
1.094
1.094
1.093
108.9
108.8
108.7
119.8
120.7
87.4
87.8
87.8
88.2
109.0
109.2
108.9
114.9
117.4
117.6
118.7
118.6
111.2
111.2
110.8
110.7
109.4
109.4
110.6
110.6
118.2
118.2
109.5
148.3
63.3
1.849
1.559
1.559
1.546
1.546
2.049
2.049
1.096
1.094
1.094
1.094
1.094
1.093
1.092
109.1
109.1
109.4
114.2
114.2
87.5
88.3
88.3
88.4
116.0
116.0
107.9
114.0
118.8
118.8
118.3
118.3
110.1
110.1
111.3
111.3
108.7
108.7
111.1
111.1
117.9
117.9
109.0
150.9
59.7
1.849
1.560
1.556
1.547
1.547
2.052
2.050
1.098
1.094
1.094
1.093
1.093
1.091
1.092
111.1
108.8
108.4
116.5
116.5
88.0
88.7
88.7
88.9
114.9
114.8
105.8
113.3
118.6
118.3
117.6
117.6
110.4
110.4
111.7
111.8
108.7
108.7
111.4
111.3
117.3
117.4
109.2
154.6
61.0
A(MHz)
1991.28
1986.82
1925.35 1997.63
B(MHz)
996.60
1030.34
1087.77 1115.48
C(MHz)
698.58
794.54
751.43
862.16
a
Predicted structural parameters obtained from MP2(full)/6-311+G(d,p) calculation.
511
Figure 75: Conformers of cyclobutyldichlorosilane
512
Figure 76: Comparison of experimental and calculated infrared spectra of cyclobutyldichlorosilane: (A) observed
spectrum of gas; (B) simulated spectrum of a mixture of the four stable conformers of cyclobutyldichlorosilane at
25°C; (C) simulated spectrum of g-Ax conformer; (D) simulated spectrum of t-Ax conformer; (E) simulated
spectrum of g-Eq conformer; (F) simulated spectrum of conformer t-Eq
513
Figure 77: Raman spectra of the Liquid from 1500 – 1100 cm-1
514
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525
VITA
Ikhlas Darkhalil is an American-Palestinian who was born in Ramallah, Palestine on August 3 to Hasna
and Daoud Darkhalil. She spent a large part of her childhood in Palestine where she attended her middle School
and high School there. She relocated to Kansas City, Missouri in 2005. She completed her Bachelor degree in
Chemistry at the University of Missouri-Kansas City in 2008.
Ikhlas had worked as a scientist for Interstate Brand Company located in Kansas City for two years.
During her undergraduate study, she carried out research in the fields of molecular spectroscopy and
conformational analysis under the supervision the Curators' Professor of Chemistry and Geosciences at the
Department of Chemistry Professor James R. Durig. With strong desire for higher education and encouragement
from her husband, family and friends, she was convinced to pursue a Ph.D. in Physical Chemistry. In the pursuit
of a Ph.D., Ikhlas joined the Department of Chemistry at the University of Missouri-Kansas City, where she
officially started her graduate career in Professor Durig’s Spectroscopy Laboratory in 2012. Ikhlas has been
investigating molecules that have considerable biological interest since they contain alcohol and/or amine
groups and have relationships to a large number of important biological molecules. Ikhlas has been carrying out
infrared and Raman spectral studies as well as microwave investigations from which she obtains structural
parameters, conformational stabilities, and other physiochemical scientific information. During her graduate
studies, she managed to teach Analytical Chemistry laboratory for Chemistry and Pharmaceutical sciences
undergraduate senior students. She also managed to hold a faculty position at the Metropolitan College where
she taught General Chemistry lecture and laboratory. She received an outstanding teaching award in 2013 in
addition to several other awards which include Mr. and Mrs. Fong Wu Cheng award, Women’s Council
Graduate Assistance Fund Award and the UMKC-MSA outstanding Leadership Award.
526
During the course of her Ph.D. Ikhlas has co-authored various papers and attended numerous
conferences and workshops, which are listed below:PUBLICATIONS:

I. D. Darkhalil, J. J. Klaassen, R. M. Gurusinghe, M. J. Tubergen, J. R. Durig “Microwave and infrared
spectra, adjusted r0 structural parameters, vibrational assignments and ab initio calculations of
cyclohexylamine,” Under preparation

J. R. Durig, I. D. Darkhalil, B. S. Deodhar, G. A. Guirgis, J. K. Wyatt, C. W. Reed, J. J. Klaassen
“Vibrational assignments, theoretical calculations, structural parameters and conformer stability
determinations of cyclobutyldichlorosilane,” Submitted to Asian Journal of Physics (January 2014) to
honor Professor Mohan who is an outstanding spectroscopists

I. D. Darkhalil, J. J. Klaassen, N. Nagels, W. A. Herrebout, B. J. van der Veken, R. M. Gurusinghe, M.
J. Tubergen, J. R. Durig “Raman and infrared, Microwave spectra, conformational stability, adjusted r 0
structural parameters and vibrational assignments of cyclopentylamine,” Submitted to Journal of Raman
Spectroscopy (December 2013)

I. D. Darkhalil, N. Nagels, W. A. Herrebout, B. J. van der Veken, R. M. Gurusinghe, M. J. Tubergenc, J.
R. Durig “Microwave spectra and conformational studies of ethylamine from temperature dependent
Raman spectra of xenon solutions and ab initio calculations,” Submitted to Journal of Molecular
Structure (December 2013)

I. D. Darkhalil, C. Paquet, M. Waqas, T. K. Gounev, J. R. Durig “R0 Structural Parameters,
Conformational Stability and Vibrational Assignment of ethyldichlorophosphine,” Accepted by
Spectrochimica Acta (December 2013)

J. R. Durig, J. J. Klaassen, D. K. Sawant, B. S. Deodhar, S. S. Panikara, R. M. Gurusinghe, I. D.
Darkhalil, M. J. Tubergen “Microwave, Structural, Conformational, Vibrational Studies and ab initio
Calculations of Isocyanocyclopentane,” Accepted by Spectrochimica Acta (December 2013)

J. J. Klaassen, I. D. Darkhalil, B. S. Deodhar, T. K. Gounev, R. M. Gurusinghe, M. J. Tubergen, P.
Groner, J. R. Durig “Microwave and Infrared Spectra, Adjusted r0 Structural Parameters,
Conformational Stabilities, Vibrational Assignments, and Theoretical Calculations of
Cyclobutylcarboxylic Acid Chloride,” J. Phys. Chem. A 2013, 117, 6508

G. A. Guirgis, J. J. Klaassen, B. H. Pate, N. A. Seifert, I. D. Darkhalil, B. S. Deodhar, J. R. Durig,
“Microwave, infrared, and Raman spectra, structural parameters, vibrational assignments and theoretical
calculations of 1,3-disilacyclopentane,” J. Mol. Struct. 2013, 1049, 400

B. H. Pate, N. A. Seifert, G. A. Guirgis, B. S. Deodhar, J. J. Klaassen, I. D. Darkhalil, J. A. Crow, J. K.
Wyatt, H. W. Duke, J. R. Durig,“Microwave, infrared, and Raman spectra, structural parameters,
vibrational assignments and theoretical calculations of 1,1,3,3-tetrafluoro-1,3-disilacyclopentane,”
Chem. Phys. 2013, 416, 33
527

J. R. Durig, J. J. Klaassen, B.S. Deodhar, I. D. Darkhalil, W. A. Herrebout, J. J.J. Dom B. J van der
Veken, S. S. Purohit “Conformational and Structural Studies of Ethynylcyclopentane from Temperature
Dependent Raman Spectra of Xenon Solutions and ab Initio Calculations,” J. Mol. Struct. 1044, 2013,
10

J. R. Durig, I. D. Darkhalil, J. J. Klaassen, N. Nagels, W. A. Herrebout, B. J. van der Veken
“Conformational and structural studies of 2,2,2 trifluoroethylamine from temperature dependent Raman
spectra of xenon solutions and ab initio calculations,” J. Mol. Struct. 1032, 2013, 229

J. R. Durig, I. D. Darkhalil, J. J. Klaassen, “Infrared and Raman Spectra r0 Structural Parameters,
Conformational Stability, and Vibrational Assignment of 2-cyanoethylamine,” J. Mol. Struct., 1023,
2012 ,154

J. R. Durig, I. D. Darkhalil, J. J. Klaassen, W. A. Herrebout , J. J.J. Dom, B. J. van der Veken,
“Conformational and structural studies of n-propylamine from temperature dependent Raman and farinfrared spectra of xenon solutions and ab initio calculations,” J. Raman Spectrosc. 43, 2012, 1329

J. R. Durig, J. J. Klaassen, I. D. Darkhalil, W. A. Herrebout , J. J.J. Dom, B. J. van der Veken,
“Conformational and structural studies of isopropylamine from temperature dependent Raman spectra of
xenon solutions and ab initio calculations,” J. Mol. Struct. 1009, 2012 ,30

G. A. Guirgis, I. D. Darkhalil, J. J. Klaassen, A. Ganguly, J.K. Wyatt, J.R. Durig, “Conformational,
vibrational, and structural studies of 2,2,3,3,3-pentafluoropropylamine from Raman and infrared spectra
of gas, liquid, xenon solutions and solid, supported by ab initio calculations,” J. Raman Spectrosc. 43,
2012 ,116

J. R. Durig, J. J. Klaassen, S. S. Panikar, I. D. Darkhalil, A. Ganguly, G. A. Guirgis, “Conformational
and structural studies of 2,2-difluoroethylamine from temperature dependent infrared spectra of xenon
solution and ab initio calculations,” J. Mol. Struct. 993, 2011,73

J. R. Durig, H. Deeb, I. D. Darkhalil, J. J. Klaassen, T. K. Gounev, A. Ganguly, “The r0 structural
parameters, conformational stability, barriers to internal rotation, and vibrational assignments for trans
and gauche ethanol,” J. Mol. Struct. 985, 2011, 202
CONFERENCES, WORKSHOPS, AND SYMPOSIUMS (*Presenter):

“Microwave, Raman and infrared spectra, conformational stability, r0 structural parameters, and
vibrational assignment of cyclohexylamine” Ikhlas Darkhalil*, Wouter Herreboutb, Benjamin van der
Veken, James Durig at the (Pittcon), March 3, 2014, Chicago, Illinois

“Microwave, Raman and infrared spectra, conformational stability, r0 structural parameters, and
vibrational assignment of cyclopentylamine” Ikhlas Darkhalil*, Wouter Herreboutb, Benjamin van der
Veken, James Durig at the (FACSS), October 2, 2013, Milwaukee, Wisconsin

FT-Microwave workshop at Kent State University, June 19-25, 2013

“Conformational and Structural Studies of n-propylamine from Temperature Dependent Raman and Far
Infrared Spectra of Xenon Solutions and ab initio Calculations.” Ikhlas Darkhalil*, Joshua Klaassen,
Wouter Herreboutb, Benjamin van der Veken, James Durig at the (FACSS), October 4 2012, Kansas
City, Missouri
528

“Identification of Multiple Conformers of Molecules which Associate by Utilizing Rare Gas Solutions
with Variable Temperature Vibrational Spectroscopy.” James R. Durig*, Joshua J. Klaassen, Ikhlas D.
Darkhalil at the (FACSS), October 2012, Kansas City, Missouri

“Conformational and structural studies of n-propylamine from temperature dependent Raman and far
infrared spectra of xenon solutions and ab initio calculations.” J. J. Klaassen*, I. D. Darkhalil, W. A.
Herrebout, J. J.J. Dom, B. J. van der Veken, J. R. Durig at European Congress of Molecular
Spectroscopy (EUCMOS), 2012, in Cluj-Napoca, Romania

“Conformational and structural studies of isopropylamine from temperature dependent Raman spectra of
xenon solutions and ab initio calculations.” Joshua J. Klaassen*, Ikhlas D. Darkhalil, James R. Durig
the International Symposium on Molecular Spectroscopy (67th Meeting Columbus Ohio; June 20-24,
2012)

“Infrared and Raman Spectra r0 Structural Parameters, Conformational Stability, and Vibrational
Assignment of 2-cyanoethylamine." Ikhlas D. Darkhalil*, James R. Durig at the Committee poster
constant which was held at the University of Missouri-Kansas City, Missouri, April 25, 2012

“Infrared and Raman Spectra r0 Structural Parameters, Conformational Stability, and Vibrational
Assignment of 2-cyanoethylamine." Ikhlas D. Darkhalil*, James R. Durig at the National Spectroscopy
meeting, Orlando, Florida, March 13, 2012

“Conformational and structural studies of isopropylamine from temperature dependent Raman spectra of
xenon solutions and ab initio calculations.” J. J. Klaassen*, I. D. Darkhalil, J. R. Durig at the National
Spectroscopy meeting, Orlando, Florida, March 13, 2012

“Raman Spectra and Conformational Stability of Gaseous and Liquid CF3CF2CH2NH2 and
CF3CF2CH2ND2” Joshua Klaassen*, G.A. Guirgis, I.D. Darkhalil, J.R. Durig at the National
Spectroscopy meeting (Pittcon), 2011, Pittsburgh, USA

“Conformational stability of some organoamines by utilizing variable temperature infrared spectra of
rare gas solutions” James R. Durig*, Savitha S. Panikar, Joshua J. Klaassen, Ikhlas D. Darkhalil,
Takuya Iwata at European Congress of Molecular Spectroscopy (EUCMOS), 2010, Florence, Italy

“Raman Spectra and Conformational Stability of Gaseous and Liquid CF3CF2CH2NH2 and
CF3CF2CH2ND2” Joshua Klaassen*, G.A. Guirgis, I.D. Darkhalil, J.R. Durig at the International
Conference on Raman Spectroscopy (ICORS), 2010, Boston, USA
529
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