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Microsolvation of reactive systems in the gas phase via Fourier transform microwave spectroscopy

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Microsolvation of Reactive Systems in the Gas Phase via Fourier Transform
Microwave Spectroscopy
A THESIS
SUBM ITTED TO THE FA C U LTY OF THE GRADUATE SCHOOL
OF THE U N IV ERSITY OF M INNESOTA
BY
Carolyn Sue Brauer
IN PARTIAL FULFILLM ENT OF THE REQUIREM ENTS
FO R THE D EG REE OF
DOCTOR OF PHILOSOPHY
Dr. K enneth R. Leopold, Advisor
October 2006
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UMI Number: 3234906
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© Carolyn S. Brauer 2006
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UNIVERSITY OF MINNESOTA
This is to certify that I have examined this copy o f a doctoral dissertation by
Carolyn S. Brauer
and have found that it is complete and satisfactory in all respects, and that any and all
revisions required by the final examining committed have been made.
Dr. Kenneth R. Leopold
Name of Faculty Advisor
Signature o f Faculty Advisor
Date
GRADUATE SCHOOL
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Acknowledgem ents
There are many people to whom I am grateful for accompanying me on this journey. First, I
want to thank my husband, William, who has been an ardent supporter and has always been
with me, even when we could not be together. Our shared vision has sustained us through long
periods of separation, and I am grateful for a life partner to whom I am so perfectly suited.
Truly, I could not have done this without him. I also am thankful to my son, Alex, who spent
nearly 1/3 of his life with his mother in graduate school. He never fails to amaze and inspire
me.
I have been blessed with a family who has always provided support and encouragement. My
love of puzzles, which led me to spectroscopy, came from my mother, and I always will be
grateful for that, as well as for all she taught me about life. My father, who has always cheered
me on, instilled in me a desire to do better, and I thank him for his constant faith in me. The
many, sometimes long, telephone conversations with my sisters helped me through the trials of
graduate school, and I appreciate the humor and advice from them both.
My group members have been more than coworkers, and have become good friends. Working
with Sherri was always a joy, and I treasure the friendship and sage advice of my wise little
sister. Kelly’s gentle soul and warm humor make him one of my favorite people. If not for
Matt, I would never know the dates of special Grateful Dead concerts, nor would I know what it
is like to have a little brother. Galen, who has always been “special”, and Erik, brought light
into a lab with no windows, and I shall miss our discussions and their companionship.
And finally, I am thankful to Ken, an advisor in the truest sense of the word. If I can be a
fraction as eloquent, patient, thoughtful and wise as he, I will count myself a success.
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To William and Alex
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Carolyn Sue Brauer
262 words
Abstract
Fourier transform microwave spectroscopy has been used to study a number of reactive
systems, with the primary goal of probing the effects of solvent on a molecule or a weakly
bound acid-base system at the small cluster level. Because these systems are particularly
sensitive to their first, nearest neighbors, the studies focus on examining structural changes and
electronic rearrangement that occurs with the addition of a single solvent molecule, or
microsolvent.
The structural effects of microsolvation were examined on two prototypical acid-base systems.
The first sought to ascertain the effect of microsolvent polarity by microsolvating HCN-S03
with Ar and CO, forming the complexes HCN-SOj-Ar and HCN-S03-"C0. Dipole moments
and ab initio calculations also are reported. The second examined the effect of microsolvation
on the primary hydrogen bond distance of (CH3)3N—FIF, by adding a single HF molecule,
forming the complex (CH3)3N- HF—HF.
The Stark effect was measured on a series of hydrogen halide complexes. These systems are
prototypical complexes with which to study proton transfer across a hydrogen bond.
The
resulting dipole moments are discussed in terms of the degree of proton transfer. The dipole
moment also was determined for the H2S04" H20 complex, which provides an important model
system for understanding rates of binary homogeneous nucleation, and a series of ab initio
calculations were performed in support of the results.
Finally, the microwave spectrum of the radical complex OFI-H2 O was observed and analyzed
using a two-state model which accounts for nuclear motion on the2A' and 2A ” potential
surfaces. The results provide insights into the effects of the partial quenching of orbital angular
momentum.
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Table o f Contents
_________ __________
_______ _____ Title________________
__
List o f Tables
v
List o f Figures
viii
Introduction
Chapter 1
Pages
x
M icrosolvation o f a Partially Bonded Complex by a
N on-Polar M icrosolvent: A M icrowave and Ab Initio
Study o f H CN -SO 3 - C O and H C N -S 0 3- A r
A ppendix to Chapter 1
30-40
Chapter 2
M icrowave Investigation o f (CH 3 ) 3 N --H F-"H F
A ppendix to Chapter 2
41-69
70-77
Chapter 3
Dipole M oments o f Am ine Hydrogen Flalide
Complexes
A ppendix to Chapter 3
Chapter 4
1-29
78-104
105-110
Stark Effect M easurem ents on the H 2 SO 4 -H 2 O
Complex
A ppendix to Chapter 4
111-143
Effects o f Partially Q uenched Orbital A ngular
M om entum on the M icrowave Spectrum and M agnetic
Hyperfine Splitting in the O H-W ater Complex
A ppendix to Chapter 5
156-181
182-187
Appendix A
D ischarge Source
188-197
A ppendix B
H eated N ozzle
198-201
Chapter 5
144 -15 5
iv
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List of Tables
Table
Table 1. 1
Table 1. 2
Table 1. 3
Table 1. 4
Table 1. 5
Table 1 .6
_ _________
Caption
Observed spectroscopic constants for H CN-SO 3 -X (X = Ar,
CO).
Theoretical binding energies (De) and relevant structural
param eters o f H CN/SO 3 /X (X=Ar, CO) molecular isomers
at M P2/aug-cc-pVDZ using M OLPRO 2000.1.
Theoretical binding energies (De), dipole mom ents and
relevant structural param eters o f HCN-SO 3 -X, H CN-SO 3
and SO 3 -X (X=Ar, CO) at M P2/aug-cc-pV TZ using
M OLPRO 2000.1.
Structural param eters o f H CN-SO 3 -X, H CN-SO 3 and SO 3 X (X=Ar, CO).
Dipole mom ents o f H CN-SO 3 -X, H CN-SO 3 and SO 3 -X
(X=Ar, CO).
Comparison o f experim ental geometries and dipole
m om ents w ith counterpoise corrected ab initio results.
Table
Table
Table
Table
Table
Table
Table
Table
Table
Table
A l.
1 Observed Rotational Transitions for HCN-S 0 3 -Ar.
A l . 2 Observed Rotational Transitions for HCN- 3 4 S 0 3 -Ar.
A l. 3 Observed Rotational Transitions for DCN-S 0 3 -Ar.
A l . 4 O bserved Rotational Transitions for H C 15N -S 0 3 -Ar.
A l. 5 Observed Rotational Transitions for H C 15N - 3 4 S 0 3 -Ar.
A l. 6
O bserved Rotational Transitions for H CN-SO 3 -CO.
A l . 7 Observed Rotational Transitions for H C 15 N -S 0 3 -C 0 .
A l. 8
O bserved Rotational Transitions for HCN-S 0 3 - 1 3 C 0 .
A l. 9 Observed Rotational Transitions for D CN-SO 3 -CO.
A l. 10 Observed Transitions o f H C 15N -S 0 3 -Ar at N on-Zero
Electric Field.
Table A l . 11 Observed Transitions o f H C 15N -S 0 3 -C 0 at N on-Zero
Electric Field.
Page
8
10
10
16
18
18
31
31
32
33
33
34
35
35
36
37
39
v
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List of Tables, continued
Table
Caption
Page
Spectroscopic constants of (CH 3 ) 3 N-HF-HF.
Theoretical changes imposed on experimental structure.
Inertial defect (A) for (CF^^N and (CH 3 )3N-HF-HF.
Results from series of structure fits.
Experimental values of Ppt for NH 3 -HF-HF and (CFE^NHF-HF.
51
53
54
55
59
Observed Rotational Transitions for (CH 3 )3 N-HF-HF.
Observed Rotational Transitions for (CH 3) 3 15N-HF-HF.
Observed Rotational Transitions for (CH 3) 3 15N-DF-DF.
Observed Rotational Transitions for (CH 3) 3 15N-DF-HF.
Observed Rotational Transitions for (CH 3) 3 15N-HF-DF.
Observed Rotational Transitions for l3 CH 3 (CH 3 ) 2 15N-HFHF.
Observed Rotational Transitions for 13 CH 3 (CH 3 ) 2 15N-HFHF.
Observed Rotational Transitions for (CD 3) 3N-HF-HF
Cartesian coordinates for final (average) (CIU^N-HF-HF
structure.
Unassigned transitions dependent on (CFE^N and HF.
71
72
73
73
73
74
Table 3. 1
Experimental and theoretical dipole moments of
15 NH 3 H 3 5 C1 and 15 N(CH 3 ) 3 H 3 5 C1.
87
Table A3. 1
Observed Transitions o f 15 NH 3 -H 3 5 C 1 at Non-Zero Electric
Field.
Observed Transitions o f (CH 3 ) 3 15NH 3 -H 3 5 C1 at Non-Zero
Electric Field.
Table
Table
Table
Table
Table
2.
2.
2.
2.
2.
1
2
3
4
5
Table A2.
Table A2.
Table A2.
Table A2.
Table A2.
Table A2.
1
2
3
4
5
6
Table A2. 7
Table A2. 8
Table A2. 9
Table A2. 10
Table A3. 2
Table 4. 1
Table 4. 2
Table 4. 3
Table 4. 4
Experimentally determined dipole moments for A- and 13states, from a series o f least squares fits.
Experimental and theoretical dipole moments (p.tot) and
binding energies (De) o f H 2 SO4 , H20 and H2 SO 4 -H 2 O at
various levels o f theory and basis sets.
Experimental and theoretical structural parameters of
H 2 SO 4 .
Comparison o f experimental and theoretical structures of
H 2 SO 4 -H 2 O.
74
75
76
77
106
108
121
126
127
129
v:
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List of Tables, continued
Table
Caption
Page
145
148
152
153
154
154
Table A4. 7
Stark Transitions o f H 2 SO 4 -H 2 O, A-State.
Stark Transitions o f H 2 SO 4 -H 2 O, B-State.
Summary o f Stark effect fits for A-State.
Summary o f Stark effect fits for B-State.
Experim ental Cartesian coordinates in principle axis system.
M P2/aug-cc-pV TZ Cartesian coordinates (including dipole
m om ent) in principle axis system.
Theoretical dipole m om ents and com ponents o f H 2 SO 4 -H 2 O.
Table 5. 1
Spectroscopic constants for OH-OH 2 .
169
O bserved Rotational Transitions for 1 6 O H - 1 6 OH2.
O bserved Rotational Transitions for 1 8 O H - 1 8 OH 2 .
O bserved Rotational Transitions for 18 O H - 1 6 OH 2 .
O bserved Rotational Transitions for 1 6 O H - 1 8 OH 2 .
Spectroscopic constants for OH-H 2 O from previous analysis.
183
184
185
186
187
Dim ensions o f the various discharge nozzle bodies.
K nown radical species observed and the best nozzle
configuration for each at the tim e o f observation.
192
193
Table
Table
Table
Table
Table
Table
Table
Table
Table
Table
Table
A4.
A4.
A4.
A4.
A4.
A4.
A5.
A5.
A5.
A5.
A5.
1
2
3
4
5
6
1
2
3
4
5
Table A. 1
Table A. 2
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155
List of Figures
Figure
_ _____
Figure 1. 1
Figure 1.2
Figure 1. 3
Figure 1.4
_
Caption
_____ _
Concentric needle assembly used to form HCN-SO 3 -CO
complex.
(a) Portion of the J=5<—4 o f HCN-S 0 3 -Ar, and (b) HCN-SO 3 CO.
Coordinates defining the structures o f (a) HCN-S 0 3 -Ar and
(b) HCN-SO 3 -CO.
(a) Stark transitions of HCN-SO 3 -AJ. (b) Stark transitions o f
HCN-SO 3 -CO.
Page
5
7
11
17
Figure 2. 1
Figure 2. 2
Figure 2. 3
J = 10 i ^ 2 02 transition of (CH 3 ) 3N-HF-HF.
Fitted structural parameters for (C F^N -H F-H F.
Theoretical values o f ppx for the series (CH 3 )nH 3 .nN-HX.
49
55
58
Figure 3.1
J = 1 ^ 0 , F = 3/2<—1/2 transition of 15NH 3 -H 3 5 C1 at 87.2
V/cm.
J = 2<—1, F = 7/2<—5/2 and F=5/2<—3/2 transitions o f
15 N(CH 3 ) 3 -H 3 5 C1 at 25.3 V/cm.
Stark effect for several components of (CHs^N-FlCl that do
not conform to second order behavior.
Contribution of distortion, polarization and charge transfer to
the induced dipole moments (A|j,ind) o f four amine-HX
complexes from BLW decomposition analysis.
Percent ionic character, f, for N H 3-H X and (C H 3)3-H X , X =
Cl, Br, I.
|j,ind vs. % Ionic Character.
p.ind vs. Normalized Proton Affinity Difference, A.
83
Figure 3 .2
Figure 3. 3
Figure 3. 4
Figure 3. 5
Figure 3. 6
Figure 3. 7
84
86
89
91
92
93
v iii
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List of Figures, continued
Figure_________________ _ _ _______
Figure 4. 1
Figure 4. 2
Figure 4. 3
Figure 4. 4
Figure 4. 5
Figure 4. 6
Figure 4. 7
Caption___
_ _ _ _ _ _______
A portion o f the J = 2 o2 <— loi transition o f H 2 SO 4 -H 2 O taken
at 25.8 V/cm.
The m easured Stark shifts for J = 2 o2 <—loi vs. S 2 o f the BState.
The three theoretical structures obtained by al N atsheh for
H 2 SO 4 -H 2 O, using PW 91/A V TZ level o f theory.
Theoretical D e (left axis) and p (right axis) for H 2 SO 4 -H 2 O
w ith respect to the rotation o f the torsional angle at
P W 9 1P W 9 1/aug-cc-pVTZ.
A tom num bering o f H 2 SO 4 .
A tom num bering o f H 2 SO 4 -H 2 O.
O rientation o f m onom er dipoles in the geom etry o f the
H 2 SO 4 -H 2 O complex, their sum ( |lsum), the total dipole
m om ent (p tot) and the enhancem ent due to complexation
Page
117
119
123
125
127
128
130
(Pind) from M P2/aug-cc-pV TZ calculations.
Figure 5. 1
Figure 5. 2
Figure 5. 3
Figure 5. 4
Figure A. 1
Figure A. 2
Figure A. 3
Figure A. 4
Figure B. 1
Figure B. 2
The O H-HO 2 complex.
Supersonic discharge source used to produce OFI-OH 2 .
The (J,Fi,F) = (1/2, 1, 2) <-(1/2, 0, 1) transition o f
1 6 O H - 1 6 OH2.
Vector diagrams for H und’s cases (a) and (b).
158
161
167
Initial discharge nozzle for generating radical species.
Discharge nozzle designed to contain discharge w ithin the
nozzle.
Discharge nozzle that attaches to the General Valve Series 9
pulsed nozzle assembly.
Experim ental set-up for observation o f OH radical.
188
189
(a) Pulsed nozzle assembly, (b) Enlarged view o f heating
sleeve.
Sample reservoir made from m odified Swagelock™ reducing
union.
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172
191
194
198
199
Introduction
Understanding the ways in which a molecular system interacts with its local
environment is a topic o f keen interest across a broad spectrum o f scientific disciplines.
In biology, molecular responses to surrounding molecules are important in protein
folding and enzyme function, as well as a host o f other mechanisms . 1
In the
atmosphere, condensed phase media, particularly water in the form o f clouds, rain and
fog, has prompted interest in aerosols and small clusters which fall in the intermediate
regime between the gas and condensed phases . 2 Nonetheless, the effect of solvent on a
molecular system is at the core of chemical understanding. Indeed, one of the earliest
lessens learned by organic chemistry students is that “like dissolves like”, and this is so
because o f the intermolecular forces at work between the solvent and solute. Yet a gulf
exists between the properties o f isolated molecules and those in the bulk, and one o f the
goals o f cluster science is to “bridge this gap ” . 3 In particular, gas phase studies of
weakly bound complexes4 ' 8 and molecular clusters9 ' 12 have laid the foundation for
understanding the fundamental principles o f intermolecular forces 8 ,1 1 ‘ 14 that govern the
effects o f solvation . 15' 20
While the study of weakly bound complexes and molecular clusters has provided
insights into the fundamental aspects o f solvation, there has been increasing emphasis
on complexes containing more reactive constituents . 2 1 ' 2 4 Because the latter systems are
especially sensitive to their first near neighbors, they can yield a great deal of
x
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information in the early stages o f solvation - at the microsolvent level. Among these
systems are a type o f Lewis acid-base adduct that has been termed “partially bound ” . 25
Much o f the early microwave work in the Ken Leopold lab concentrated on this type of
system , 26 and the effects of microsolvation on partially bonded molecules . 2 1 ' 2 4 While
work continues in this area, it has not been limited to Lewis acid-base complexes, and
reactive systems which have the potential for proton transfer across a hydrogen bond
also are an active area of study.
Microwave spectroscopy provides an excellent tool with which to study these
interactions.
This high-resolution technique allows us to determine very accurate
molecular structures, such that, even minute modifications to molecular structure can be
discerned when a complex if formed or when a small molecular cluster is “solvated”
with a single molecule. In addition, the use o f low temperature, supersonic jets enables
the study o f these systems, which may otherwise fully react in solution, in the pre­
reactive stage.
Traditional application o f microwave spectroscopy also enables one to elucidate the
changes in electronic structure that take place as a complex forms, through the analysis
of quadrupole coupling constants, and by observing the Stark effect, dipole moments
can be determined, giving a fundamental measure o f charge distribution.
In more
recent studies o f open-shell complexes, 2 7 this technique has even been used to provide
information about specific electronic states.
xi
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Overview
All of the studies presented here were performed using a Fourier transform microwave
spectrometer (FTMW), the details of which are provided elsewhere .2 8 ,2 9
However,
much of the work is supplemented with ab initio calculations, which prove invaluable
in predicting molecular structures, thereby decreasing the amount o f time required to
locate spectra.
We begin with a study that addresses a fundamental question in Lewis acid-base
chemistry, by examining the effect o f a non-polar and weakly polar microsolvent on a
partially bonded system.
The complex, HCN-SO 3 was microsolvated with argon,
forming the complex HCN-SCV-Ar and with carbon monoxide, forming HCNSO3 •••CO, in an attempt to draw conclusions regarding the effect o f the polarity of the
local environment on structural changes that occur on going from the small cluster stage
to the bulk. Ab initio results for both complexes as well as dipole moments also are
given.
The second chapter continues the theme o f microsolvation - this time, with a BronstadLowry acid-base complex. In a previous study by a former coworker, 21 the hydrogen
bonded complex, NH 3 -HF was microsolvated with an additional HF molecule. The
present work is somewhat o f a follow-up to that, and investigates the effect o f increased
basicity o f the amine on the primary hydrogen bond, by microsolvating the complex
(CH 3 ) 3 N-HF with an additional HF molecule.
xii
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The third and fourth chapters diverge from microsolvation and structural determination,
and concentrate on analyzing charge distribution through determining the dipole
moments of several complexes. Fundamental aspects o f hydrogen bonding and proton
transfer are addressed in chapter three, through Stark effect experiments o f two aminehydrogen halide complexes .30
In chapter four, the dipole moment of sulfuric acid monohydrate is presented.
While
the structure o f this complex was determined a few years ago,23b the dipole moment was
not. Although sulfuric acid is a well-known prototype for atmospheric nucleation, it
has recently been proposed that the dipole moment plays an important role in
determining nucleation rates . 31 An understanding o f these rates, in turn, is critical to
fully elucidating the role o f sulfuric acid aerosols in the atmosphere.
The final chapter o f this thesis represents the evolution o f work in this laboratory into
new and largely unexplored territory (at least from this group’s perspective).
Free
radical systems, with few exceptions, represent the ultimate in reactive systems. Thus,
it is only natural to delve into this type o f molecular system. This study focuses on the
partial quenching o f the orbital angular momentum o f the OH-HO 2 complex, through
the analysis of the rotational spectrum.
Two appendices also are included, which provide information on instrumental
adaptations made to the molecular source. Detailed information on the development of
xiii
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the discharge source, used in the production of radical species, is given in Appendix A.
A heated nozzle adaptor also was developed with which to generate gas phase
molecules from low vapor pressure solids. While it was not used in any o f the studies
in this document, it may be useful in future studies. Details o f the heated nozzle are
provided in Appendix B.
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References
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Stryer, L. (1995) Biochemistry, W.H. Freeman and Company: New York, 1995:
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Xu, S.; Nilles, J.M.; Bowen, Jr., K.H., “Zwitterion formation in hydrated amino acid,
dipole bound anions: How many water molecules are required?” J. Chem. Phys.,
119 , 10696-10701 (2003).
21
Hunt, S.W.; Higgins, K.J.; Craddock, M.B.; Brauer, C.S.; Leopold, K.R. /'Influence
o f a polar near-neighbor on incipient proton transfer in a strongly hydrogen
bonded complex", J. Am. Chem. Soc., 125 , 13850-13860 (2003).
22
Craddock, Matthew B., “Microwave Spectroscopic Studies o f Weakly-Bound and
Hydrogen-Bonded Molecular Complexes.”
Ph. D Thesis, University of
Minnesota, 2005.
xvii
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23
(a) Hunt, S. W.; Brauer, C. S.; Craddock, M. B.; Higgins, K. J.; Nienow, A. M.;
Leopold, K. R., "Microwave Observation of H 3 N-SO 3 -H 2 O Using a Concentric,
Dual-Injection Nozzle Source." Chem. Phys., 305, 155-164 (2004). (b) Fiacco,
D. L.; Hunt, S. W. and Leopold, K. R., “Microwave investigation of sulfuric
acid monohydrate.” J! Am. Chem. Soc., 124(16), 4504-4511 (2002).
24
(a) Fiacco, D.L.; Leopold, K.R., “Partially bound systems as sensitive probes of
microsolvation: A microwave and ab initio study of HCN-HCN-BF 3 .” J. Phys.
Chem. A, 107, 2808-2814 (2003). (b) Fiacco, D.L.; Hunt, S.W.; Leopold, K.R.,
“Structural change at the onset o f microsolvation: Rotational spectroscopy of
HCN-HCN-SO 3 .” J. Phys. Chem. A, 104, 8323-8327 (2000).
25 See, fo r example, (a) Leopold, K. R., “Partially bonded molecules and their transition
to the crystalline state.” in Advances in Molecular Structure Research, Vol
II.,edited by Hargittai, M. and Hargittai, I., JAI Press: Greenwich, CT, 1996, pp.
103-127. (b) Leopold, K. R.; Canagaratna, M.; Phillips, J. A., “Partially bonded
molecules from the solid state to the stratosphere.” Acct. Chem Res., 30, 57-64
(1997).
(c) Phillips, J.A.; Britton, D.; Leopold, K.R., “Gas - solid structure
differences in the donor - acceptor complex (CH 3 ) 2 HN-S 0 2 .” J. Chem.
Crystallogr., 26, 533-538 (1996).
(d) Phillips, J. A.; Canagaratna, M.;
Goodfriend, H.; Grushow, A.; Almlof, J.; Leopold, K. R., “Microwave and ab
initio investigation o f HF-BF 3 ” J. Am. Chem. Soc., 117, 12549-12556 (1995).
(e) Bums, W.A.; Phillips, J.A.; Canagaratna, M.; Goodfriend, H.; Leopold,
K.R., “Partially Formed Bonds in HCN-SO 3 and CH 3 CN-SO 3 : A comparison
xviii
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between donor-acceptor complexes of SO3 and BF3.” J. Phys. Chem. A, 103,
7445-7453 (1999).
26 See, fo r example, (a) Hunt, S.W.; Leopold, K.R., “Molecular and electronic structure
of C 5 H 5 N-SO 3 : Correlation o f ground state physical properties with orbital
energy gaps in partailly bound Lewis acid-base complexes.” J. Phys. Chem. A,
105, 5498-5506 (2001). (b) Fiacco, D.L.; Mo, Y.; Hunt, S.W.; Roberts, “Dipole
moments o f partially bound Lewis acid-base adducts.” J. Phys. Chem. A, 105,
484-493 (2001). (c) Hunt, S.W.; Fiacco, D.L.; Craddock, M.; Leopold, K.R.,
“Correlation o f dative bond length and donor proton affinity in adducts of SO3 :
A good predictor for HCCCN-SO 3 .” ./. Mol. Spec., 212, 213-218 (2002).
27
Brauer, C. S.; Sedo, G.; Grumstrup, E. K.; Leopold, K. R.; Marshall, M. D.; Leung,
H. O., “Effects of partially quenched orbital angular momentum on the
microwave spectrum and magnetic hyperfine splitting in the OH-water
complex.” Chem. Phys. Lett., 401, 420-425 (2005).
28
Balle, T. J.; Flygare, W. H., “Fabry-Perot cavity pulsed Fourier transform microwave
spectrometer with a pulsed nozzle particle source.” Rev. Sci. Inst., 52, 33-45
(1981).
29
Phillips, J. A., “Structure and dynamics o f partially-bound molecular complexes.”
University o f Minnesota, Minneapolis, MN, USA. Ph.D. Thesis, 1996.
30
Brauer, C. S.; Craddock, M. B.; Kilian, J.; Grumstrup, E. M.; Orilall, M. C.; Mo, Y;
Gao, J. and Leopold, K. R.’ “Amine - hydrogen halide complexes: Experimental
R eproduced with perm ission of the copyright owner. Further reproduction prohibited without permission.
electric dipole moments and theoretical decomposition o f dipole moment and
binding energies.” J. Phys. Chem. A, Web release: 07/29/2006.
31
Nadykto, A. B. and Yu, F., “Dipole moment o f condensing monomers:
A new
parameter controlling the ion-induced nucleation.” Atmos. Chem. Phys. 4,
016101 (2004).
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Chapter 1
Microsolvation of a Partially Bonded Complex by a NonPolar Microsolvent: A Microwave and Ab Initio Study of
HCN-SO3-CO and HCN-SO3 -Ar
C. S. Brauer, M. B. Craddock* K. J. Higgins/ and K. R. Leopold
Department o f Chemistry
University o f Minnesota
Minneapolis, MN
*Department o f Chemistry
Columbia University
N ew York, N Y
^Department o f Chemistry and Chemical Biology
H arvard University
Cambridge, MA
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Abstract
Rotational spectra of HCN-SCV-Ar and HCN-SO 3 —CO, as well as several o f their
isotopically substituted derivatives, have been observed by Fourier transform
microwave spectroscopy. Both complexes are symmetric tops with the Ar or CO
moieties weakly bound to the sulfur atom on the side opposite the HCN. These systems
represent the first step in the microsolvation of partially bound HCN-SO 3 by non- or
weakly polar species and, combined with previous results on HCN—HCN-SO 3 , test the
effect o f microsolvent polarity on a partially bonded system. Dipole moments and ab
initio results for both complexes also are reported. We find that HCN-SCV'CO displays
marked increases in the N-S and S-C distances relative to those observed in bare HCNSO3 and SO 3 -CO, respectively. Specifically, the N-S partial bond length (2.656(2)
A) is
A larger than in FICN-SO3 , and the C-S van der Waals bond length (3.011(5)
A) increases by 0.157(7) A with respect to SO 3 -CO. For the argon complex, the
0.079(8)
changes are less dramatic, with increases o f the N-S and S-Ar distances with respect to
HCN-SO 3 and AX-SO3 of about 0.014(8)
A and 0.096(6) A, respectively. Ab initio
calculations concur with the experimental structures. The structural results are in sharp
contrast to previous work on H C N -H C N -S 0 3 , in which the addition o f a polar HCN
microsolvent molecule caused a large contraction o f the partially formed N-S bond.
Stark effect measurements have been performed and the resulting dipole moments are
4.230(10) D for HCN-SO 3 -Ar and 3.678(11) D for HCN-SO 3 -C O .
Both of these
results are very near the differences between the dipole moments o f the constituent
dimers, suggesting that the dipole moments of the trimers are the vector sums o f the
dimer moments.
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Introduction
The effect o f solvent on partially bonded complexes has been a topic o f interest in this
laboratory for a number of years . 1' 9 Because these types o f complexes tend to be
especially sensitive to their immediate environment, they provide an opportunity to
examine the effects of solvation at the small cluster level. Indeed, microsolvation of a
number of partially bonded complexes has shown them to respond dramatically to the
addition o f the first nearest neighbors. Most notably, the dative bonds in HCN-SO 34
and HCN-BF 3 10 contracted by 0.107(21)5 A and 0.174(57),9 A respectively, when
microsolvated with a single HCN molecule.
In general, the formation o f these complexes is accompanied by a rapid increase in
dipole moment as the donor acceptor bond forms. This is entirely reasonable from a
chemical point of view and has been substantiated by a number o f ab initio
calculations .6 ,1 1 ,1 2 In the crystal, one way to minimize the interaction energy within
neighboring molecules is to increase the dipole moment. This can be accomplished by
contraction o f the bond. Thus, increasing polarity with shorter bond distance lowers the
intermolecular interaction energy and provides the energetic incentive to compress the
bonds.
Given that partially bonded molecules are subject to large medium effects which may
be due to the electric field imposed by the surrounding environment, and a significant
percentage o f the structural changes often result from near neighbor interactions with
3
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polar binding partners, the present study was motivated by considering the polarity of
the microsolvent “environment” and its contribution to the structural changes that take
place on crystallization of partially bonded complexes. In order to explore the role of
microsolvent polarity, an experiment was devised in which the microsolvent molecule
was: 1) non-polar (Ar) and 2) weakly polar (CO). Thus, if the medium effect is indeed
due to the polarity of the microsolvent, the non-polar microsolvent would have no effect
on the structure of the dimer, and the weakly polar microsolvent would have only a
minor effect.
H C N -SO 3 was chosen as a prototype system primarily because it has been shown to be
sensitive to near neighbor interactions . 5 However, for the complexes in question, there
are two possible isomers. The microsolvent could be oriented on the same side as the
H C N unit, analogous to H C N -H C N -S O 3 , or it could interact more strongly with the SO3
moiety, and form a complex in the H C N -S O 3 -X (X=Ar, C O ) conformation. Ultimately,
the latter isomer was the only one observed experimentally, therefore, in this work, ab
initio results are reported for both isomers, and experimental results are reported for the
H C N -S O 3 -X form.
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Experimental
Spectra were recorded on a pulsed nozzle Fourier transform microwave spectrometer,
the details o f which are described elsewhere . 13' 15 Briefly, argon was passed over a
sample o f polymerized S O 3 held at 0° C, and expanded through a pulsed nozzle (Series
9, General Valve) with a 0.8 mm orifice at a stagnation pressure o f 2 atm. For H C N S 0 3 -Ar, a 32% mixture o f FICN in Ar flowed through stainless steel hypodermic tubing
(0.008” I.D.) at a flow rate o f 1.5 standard cubic centimeters per minute (seem) and was
introduced into the early stages o f the supersonic expansion.
H C N -S O 3 -C O was
formed by using a concentric needle assembly 16 (shown in Figure 1.1), which allowed
for H C N and CO to be introduced independently. A 32% mixture o f H C N was flowed
CO
HCN
pulsed
solenoid
valve
Figure 1.1 Concentric needle 16 assembly used to form H C N -S O 3 -C O complex.
5
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through the outer needle (0.023” dia.) at 1.5 seem and neat CO was flowed through the
inner needle (0.008” dia.) at 4 seem. The flow rates were metered by a mass flow
controller (MKS Corporation). All isotopologs o f HCN were prepared according to
literature procedures . 17 Isotopically enriched 13CO (Cambridge Isotopes, 99 atom %
13
C) was used to observe HCN-S 0 3 - 13 CO. HCN-3 4 S0 3 -Ar was observed with 34S in
natural abundance.
Dipole moments were determined by measuring the Stark effect, as described
elsewhere . 6 ,1 9 Briefly, a pair o f rectangular aluminum Stark plates (30 x 40 cm) which
operate in a bipolar configuration are installed inside the FTMW such that, when equal
and opposite dc voltages are applied to each plate, a uniform electric field is produced
perpendicular to both the cavity axis and the molecular source. The J = 4 <— 3 (K=0)
transition o f HCN-S 0 3 -Ar and the J = 5
4 (K=0) transition o f HCN-SO 3 -CO were
observed at a series o f field strengths. The J = 4<—3, K = ± 3 transition o f Ar-SCb (p. =
0.2676(3) D ) 18 was calibrated to determine the effective spacing between the Stark
plates, as described elsewhere . 19 In order to mitigate effects due to diffusion pump oil
accumulating on the plate surfaces , 2 0 the distances between the plates were calibrated
both before and after experimental data were collected. Data were used in the analysis
only if the calibrations agreed within experimental error and the effective spacing
between the plates was taken to be the average o f these two measurements.
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Results
Zero-Field Spectra and Rotational Constants
Spectral searches for both molecular orientations, X -H C N -S O 3 and H C N -S O 3 -X , were
performed, however, after extensive searching, only transitions corresponding to the
H C N -S O 3 -X isomer were located. With the exception o f the 15N substituted species, all
spectra exhibited 14N nuclear quadrupolar hyperfine structure, which is evident in the
sample spectra shown in Figure 1. 2. Spectra for both complexes were characteristic o f
symmetric tops with equivalent off-axis oxygens and were fit to the standard
expression:
V
= 2(J” + 1)(B - D j k K 2) - 4Dj(J” + l ) 3 + AEquad
(1. 1)
where AEquad is the difference in the quadrupole hyperfine energies for the upper and
lower states and the other symbols have their usual meaning . 21 14N and 2H hyperfine
(a)
|
ill V
6386
6386.4
Frequency (MHz)
6386.8
F requency (MHz)
Figure 1. 2 (a) Portion o f the J=5<—4 o f FICN-SCb-Ar, and (b) FICN-SO3-CO. Both
exhibit characteristic 14N nuclear hyperfine structure.
7
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structure, when observed, were analyzed according to standard methods for one or two
coupling nuclei, where appropriate . 21
In both complexes, K = 0 and ± 3 transitions were observed, however, K = ± 1 and ± 2
spectra were absent. This was expected and follows the application o f Bose-Einstein
statistics to equivalent spinless atoms (oxygen), restricting the values o f K to integer
multiples of 3.
The observed transitions were analyzed using Herb Pickett’s spectral fitting program,
SPFIT , 22 and the results are given in Table 1. 1. Residuals were generally within
experimental error and the full list of measured transitions are tabulated in Tables A 1.1A 1.9 in the appendix to this chapter.
Table 1 .1 Observed spectroscopic constants3 for HCN-SO 3 -X (X = Ar, CO).
HCN-S03-Ar
HCN-34 S 03-Ar
DCN-S03-Ar
HC15N-S03-Ar
HC 15N-34 S 0 3-Ar
h c n - s o 3- c o
d c n -s o 3- c o
h c n - s o 3-13c o
h c 15n - s o 3- c o
B
(MHz)
638.65158(35)
638.4826(11)
617.89339(34)
631.75693(55)
631.61451(61)
Dj
(kHz)
0.4571(60)
0.452(17)
0.4072(40)
0.4411(66)
0.4398(76)
(kHz)
5.896(24)
5.922(44)
5.389(19)
5.720(41)
5.744(44)
694.49413(31)
671.68387(45)
686.65305(48)
687.31607(56)
0.2130(55)
0.2143(52)
0.2088(82)
0.2101(54)
3.268(23)
3.244(20)
3.161(28)
3.174(31)
D jk
eQq (14N)
(MHz)
-3.9672(58)
-3.976(76)
-3.98(10)
eQ q^H )
(MHz)
-
0.190(11)
-
-
-
-
-4.0130(45)
-4.017(30)
-4.0213(89)
-
.
0.192(12)
-
-
(a) Standard error is given in parentheses.
As noted in the Experimental section, the high sensitivity o f Fourier transform
microwave spectrometers (FTMW) allows the observation o f many isotopic species in
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natural abundance and in this case, the HCN- 3 4 S0 3 -Ar isotopolog was observed.
However, spectra of the HCN-3 4 SC>3 -CO isotopolog were predicted to be near the K=±
3 transitions o f HCN-SO 3 -CO. Careful searching o f this area o f the spectrum yielded
no candidates for the 34S substituted species and it is likely obscured by these observed
transitions.
Computational Methods and Results
Ab initio geometries, binding energies (De), and dipole moments o f the HCN/SO 3/X
(X=Ar, CO) system in both the HCN-SO 3 -X and X-HCN-SO 3 orientations were
calculated at the MP2/aug-cc-pVDZ level, 2 3 and at the MP2/aug-cc-pVTZ level for
HCN-SO 3 -X (X=Ar, CO). All calculations were performed using MOLPRO 2000.12 4
and were corrected for basis set superposition error (BSSE) using the method o f Boys
and Bemardi. 25
Counterpoise corrected binding energies were determined from the
difference between the energy o f the complex and the energies o f its constituent
monomers, in the complex geometry, and calculated with the full basis set o f the
complex. The distortion energy of the monomers, calculated with the monomer basis
set only, was then subtracted 2 6 Counterpoise corrected geometries were obtained by
minimizing o f the counterpoise corrected energy, using the method o f Simon and
Duran2 7 and now implemented in the MOLPRO package. Zero point energies were not
determined.
The counterpoise corrected binding energy (Decp) and the relevant
counterpoise corrected structural parameter results for the two conformers, HCN-SO 3 -X
and X-HCN-SO 3 (X=Ar, CO) at the MP2/aug-cc-pVDZ level are listed in Table 1. 2.
9
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Table 1. 2 Theoretical binding energies (De)a and relevant structural parameters® of
HCN/SO 3 /X (X=Ar, CO) molecular isomers at MP2/aug-cc-pVDZ using MOLPRO
2000.1 b
H C N -SO 3-X
(DeCP)C
rNSd
rArSd
rCSd
(Xe
X=Ar
-6.560
2.765
3.599
—
91.06
x= co
-8.191
2.804
—
X-HCN-SO3"
H CN-SO3
X=Ar
-6.278
2.757
7.979
x= co
-8.063
2.738
—
—
—
7.542
91.20
91.07
3.150
90.68
91.09
-5.902
2.760
—
(a) Counterpoise corrected, (b) Ref. 24 (c) Energies are in kcal/m ol. (d) B ond lengths are in
angstroms, (e) N SO angle, in degrees.
Counterpoise corrected binding energies (De), geometries and dipole moments o f HCNS O 3 -X , and their relevant dimers, calculated at MP2/aug-cc-pVTZ, are given in Table 1.
3. The structural features are in excellent agreement with experimentally determined
values.
Table 1. 3 Theoretical binding energies (De), dipole moments and relevant structural
parameters® o f H C N -S O 3 -X , H C N -S O 3 and S O 3 -X (X=Ar, C O ) at MP2/aug-cc-pVTZ
using MOLPRO 2000.l.b’c
De
rNS
rA rS
rCS
a
HCN-SO3-X
X=Ar
X=CO
-8.0964
-9.9035
2.597
3.449
—
91.546
4.3401
2.658
—
3.023
90.926
3.5708
HCN-SO3
-7.274
2.588
—
—
91.56s
4.5177
s o 3- x d
X=Ar
X=CO
-1.08
-3.92
__
3.348
—
90.04f
0.2792h
__
---
2.860
90.678
1.0478 h
(a) Counterpoise corrected binding energies and geometries, (b) Ref. 24 (c) Energies are in
kcal/mol, bond lengths are in angstroms, dipole m om ents are in Debye and angles are in degrees,
(d) R e f 33 (e) N SO angle, (f) ArSO angle, (g) CSO angle, (h) Ref. 33.
10
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Structure D eterm inations
The coordinates used to describe the structure o f H CN-SO 3 -X (X = Ar, CO) are shown
in Figure 1.3. In H CN -S 0 3 -Ar, Ri is the distance between the A r atom and the centerof-mass o f the com plex (COM), and R 3 is the distance between COM and the center-ofmass o f the H CN molecule. In H CN-SO 3 -CO, Ri is defined as the distance between
COM and the center-of-m ass o f the CO molecule and R 3 is the distance between COM
and the center-of-m ass o f the HCN molecule.
R 2 (not shown) is the distance between
COM and the center-of-m ass o f the SO 3 molecule.
o
O
o
o
COM
Figure 1. 3 Coordinates defining the structures o f (a) H CN-S 0 3 -Ar and (b) H CN -SO 3 CO.
The m onom ers are allowed to undergo large am plitude vibrations and the instantaneous
angular excursions from equilibrium geom etry are represented by yi, Ji and %. Because
the com plexes are symmetric tops, <%> = <yi> = < j 2 > = 0. The angle, a , which
11
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measures the distortion o f the SO 3 from its usual planar geometry, can, in principle, be
extracted from the
32
S/34S isotopic substitution . 2 1 *5
In term s o f these coordinates, the zero point vibrationally averaged m om ent o f inertia
about the b-inertial axis, Ibb=h 2 / 8 7 i2 B, is given by:
<Ibb> = m(X)Ri2 + m (S 0 3)R22 + m(HCN)R32
+ (l/2)Ibb(X )[l +
< c o s 2Y ! > ]
+ (l/2 )Ibb(S 0 3)[ l + <cos2x>]
(1 2)
+ (l/2)Icc(S 0 3)<sin2x > + (l/2 )Ibb(H CN)[l + <cos2y2>]
where m is the mass o f the m onom er, X is either A r or CO, Igg represents the m om ent
o f inertia o f the m onom er about its gth inertial axis and yi, y2 and % are the angles
shown in Figure 1.3. O f course, the Ibb(X) term is irrelevant in the case o f the A r atom
and was, thus, only included in the structural determ ination o f H CN-SO 3 -CO.
The excursion angles, y , y2 and %, could not be determined in the fit and so reasonable
estimates o f upper and low er limits were established from experim ental parameters.
Two m ethods were used to estim ate y2eff in each complex. Values o f y2eff from the
tensor projection formula:
eQqcomplex = eQqo<P 2 Cos(y2)> = eQ q 0[3<cos2y 2> - l]/2,
(1. 3)
ranged from 7.1° to 18.9° for H C N -S 0 3-Ar and 5.2° to 18.2° for HCN-SO 3 -C O . 2 8 2 9 In
this equation, eQqcompiex is the quadrupole coupling constant o f either 14N or 2H in the
12
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complex, and eQqo are the analogous param eters in the HCN monom er.
complexes, the low er value o f
In both
72
is derived from eQq( 2 H) in DCN and the larger
num ber is from eQ q( 14 N ) in HCN.
The disparity in y2 eff is not surprising, and likely
indicates electronic rearrangem ent on the HCN, since Equation (1.3) is only valid
insofar as the electric field gradient at the coupling nucleus rem ains unchanged w ith
respect to the HCN m onom er on com plexation . 4 ,3 0 Since the nitrogen atom is closest to
the interm olecular interaction, its value is m ore likely affected by the electronic
rearrangem ent and is thus less reliable. Therefore, in estimating
only the values
derived from the deuterium data were used.
A double substitution K raitchm an analysis 3 1 ’3 2 yielded a value o f 72= 6 . 5 ° for
Ar
and 6 .8 ° for
H C N -S O 3 -C O .
Given the relative agreement betw een the two methods,
the value o f y2 eff was taken to be the average o f the two, i.e., 6.8(3)° for
and 6.0(8)° for
H C N -S O 3-
H C N -S C V A r
H C N -S O 3 -C O .
For yieff, (the CO excursion angle), no quadrupole coupling constants are available and,
since only the carbon atom was isotopically substituted, a double substitution
Kraitchm an analysis cannot be perform ed. In light o f this, yieff was taken to be 6.5(15)°,
the same as applied to the van der W aals complex,
0
C -S 0 3 -A r . 33
In estimating the S O 3 excursion angle (%), the same reasoning applies as in the HCNSO 3 com plex , 4 thus the value o f Xeff m ust to be between 0° and the 15.6° observed in
13
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Ar-SC>3 . 18 Given the similarity between the trimers and the dimer and the relatively
weak dependence of Equation (1. 2) on %, this seems to be a reasonable estimation.
The angle, a , which measures the degree of distortion of the SO 3 monomer from
planarity, can, in principle, be determined in the fit from
32
S/34S substitution. However,
values close to 90° are difficult to fit because as a approaches 90°, the dependence of
Ibb on this parameter vanishes . 10 Since the presence o f Ar is unlikely to significantly
perturb the angular potential o f the HCN-S 0 3 -Ar complex, a was initially assumed to
be the same as in the HCN-SO 3 dimer, 91.8°. This assumption was confirmed by the ab
initio calculations at the MP2/aug-cc-pVTZ level o f theory, which indicated the change
in a to be -0.02° relative to the HCN-SO 3 dimer. Thus, a for H C N -S03-Ar was taken
to be 91.8°. As a check, identical structure fits were done with a held at 90° and the
values o f the fit parameters were virtually unchanged.
The theoretical change in a
relative to HCN-SO 3 in HCN-SO 3 -CO was -0.6°, considerably larger than in HCN-SO 3 Ar and much closer to planarity.
Therefore, a was held at 91.2°, which is the
theoretical change imposed on the experimental value of a in HCN-SO 3 . Again, as
with HCN-SC>3 -Ar, identical fits fixing a at 90° showed virtually no change in the fitted
parameters.
The structures were determined through a series of non-linear least squares fits of the
observed rotational constants to Equation (1. 2), fixing the values o f yi, J 2 and % at the
14
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maxim um and m inim um estimated values established above. In all fits, the m onom er
bond distances were constrained to their free m onom er values, and, for H CN-SCf-A r,
are inferred from their relationships to the center-of-m ass distances, viz,
rArS
= Ri + R2
Rj
(1 - 4 )
rNS
=
- R2-
r3
(1- 5)
rNS
= R2 + R3-
r3
^
^
rCS
= Ri - R2-
rj
(1 .
6)
Similarly, for H CN-SO 3 -CO,
The results, taken as the average o f the values obtained from the fits using the ranges
established for yi (5.0° and 8.0°), y2 (6.5° and 7.1° for H CN-SCf-Ar, 5.2° and
HCN-SO 3 -CO) and %(0° and 15.6°), are given in columns 1 and 2 o f Table 1. 4.
R eproduced with perm ission of the copyright owner. Further reproduction prohibited without permission.
6
. 8 ° for
Table 1. 4 Structural parameters of HCN-SO3 -X, HCN-SO3 and SO3 -X (X=Ar, CO).
HCN-so3-x a
X=Ar_ _
X=CO
rArS (A) ......3.446(5)
rNS (A)
2.591(2)
2.656(2)
rCS(A)
—
3.011(5)
h c n - so 7
“ — “
2.577(6)
—
SO,-X
X=Arc
X=COd
3.350(1)
— ~
—
—
—
2.854(2)
Changes in Bond Distances6
ArArS (A)
ArNS (A)
ArCS (A)
+0.096(6)
+0.014(8)
—
—
+0.079(8)
+0.157(7)
(a) This work, b) Ref. 4 (c) Ref. 18 (d) Ref. 33 (e) Relative to respective dimers.
Dipole Moments
Stark shifted frequencies can be found in Tables A 1.10 and A 1.11 at the end o f this
chapter. Both AMf=0 and AMF=+1 transitions were observed by orienting the direction
of the electric field vector o f the microwave radiation either parallel or perpendicular to
the static electric field. A total o f 67 transitions at electric fields ranging from 30 to 107
V/cm for HCN-S 0 3 -Ar and a total o f 46 transitions at electric fields ranging from 55 to
110 V/cm for HCN-SO 3 -CO were observed. The maximum Stark shift for the HCNSC>3 -Ar complex was approximately 500 kHz, and approximately 300 kHz for HCNSO 3 -CO. Plots of the frequency shifts (Av) vs. the field squared (S2), shown in Figure
1.
4, are linear, in keeping with the expected quadratic dependence o f the observed
transitions with the field.
16
R eproduced with perm ission of the copyright owner. Further reproduction prohibited without permission.
The Stark shifted frequencies were fit to within their experimental uncertainties using
the QSTARK program . 3 4 The results are listed in Table 1.5, along with the dipole
moments and differences between the dipole moments o f the constituent dimers.
0 .6
G M —0 -0
(a)
0 .5
■ M =l-1
0 .4
*M=l-2
0 .3
X
0 .2
N
x
• M=3-4
0 .1
S
<i
M=2-2
o M=3-3
1
0 .0
* *5
I *"
-0 .1
e
- 0 .2
-0 .3
-0 .4
-0 .5
4000
2000
6000
8000
10000
12000
14000
£V(V/cm)
0 .3 5
(b)
0 .3 0
9
0 .2 5
m
«
0 .2 0
§
0 .1 5
N
X
SS
0 .1 0
9
1
0 .0 5
•
t
9
»
■
n $
1
<3
* I
0 .0 0 1
-0 .0 5
t
%
%
&
1
B
¥.
p
#
-0 .1 0
I
%
s
*
i
* M=0-0
□ M= l-1
* M=l-2
* M=2-2
x M=2-3
« M=3-3
+ M=3-4
- M=4-4
a M«4-5
-a 1 5
_n
0
2000
4000
6000
8000
10000
12000
14000
5s/ (V /c m )
Figure 1. 4 (a) Stark transitions o f HCN-SC>3 -Ar. (b) Stark transitions o f HCN-SO 3 CO.
17
R eproduced with perm ission of the copyright owner. Further reproduction prohibited without permission.
Table 1. 5 Dipole m om ents o f H CN-SO 3 -X, H CN-SO 3 and SO 3 -X (X=Ar, CO ).ab
h c n - s o 3-x c
X=Ar
X=CO
p
4.2 3 0 (10)
3.678(11)
p(HCN-S03)p (S 0 3-X)
4 .1 4 9 6 (31)
3 .5684(34)
HCN-S03d
X=Are
4.4172(31)
s o 3-x
X=COf
0.2 6 7 6 (3 )
0 .8488(13)
(a) D ip o le m o m en ts in D ebye, (b) U n certain ties, in parentheses, include errors in the
calib rated p late spacing, (c) T his w ork, (d) R ef. 6 (e) R ef. 18 (f) R ef. 33
Values listed in Table 1.
6
com pare the experim ental geometries and dipole m om ents o f
H C N - S O 3 - X (X=Ar, C O ) w ith the ab initio results at both M P2/aug-cc-pV D Z and
M P2/aug-cc-pV TZ levels, and include the binding energies for both basis sets.
The
counterpoise corrected bond distances at the M P2/aug-cc-pV TZ level are in excellent
agreement w ith the experimental observations,
falling to
w ithin
0 .0 1 1
A
of
experimental values. Furtherm ore, w hile De decreases by 17% to 19% w ith respect to
basis set, the theoretical geom etries are relatively stable, with bond distances changing
by, at most, 6 %, suggesting reasonable convergence o f the m olecular geometries.
Table 1. 6 C om parison o f experimental geom etries 3 and dipole m om ents ’5 with
counterpoise corrected ab initio results . 0
rNS
rArS
rCS
De
Experim ental
X=Ar
x=co
2.591(2)
2.656(2)
3.446(5)
3.011(5)
4.230(10)
3.678(11)
—
—
—
—
MP2/aug-cc-pVDZ
X=Ar
x=co
2.804
2.765
3.599
3.150
4.0163
3.4141
-6.566
-8.186
—
—
MP2/aug-cc-pVTZ
X=Ar
x=co
2.658
2.597
3.449
3.023
3.5708
4.3401
-9.9040
-8.0968
—
—
(a) B o n d len g th s are in an g strom s, angles are in d egrees, (b) D ip o le m o m en ts are in D ebye,
(c) C o u n terp o ise co rre c ted b in d in g energies and g eom etries. E n ergies are in kcal/m ol.
18
R eproduced with perm ission of the copyright owner. Further reproduction prohibited without permission.
Discussion
The experim entally determ ined structures concur w ith theory that, 1) the structures are
symmetric tops o f the H CN-SO 3 -X form, and 2) the bond distances increase with
respect to the constituent dimers. A lthough the observed geom etric orientation o f these
com plexes is not unique, it is certainly unusual for strong acids such as SO 3 and B F 3 .
In a recent study o f a similar van der W aals system, OC-SCb-Ar , 3 3 it was reasoned that
the H CN-H CN -M X 3 com plexes likely form in the same-side orientation because the
hydrogen bond in (HCN ) 2 is very strong and, the opposite side configuration HCNM X 3 -NCH, would make it difficult for the M X 3 moiety to optimize its out-of-plane
distortion for both HCN units. Thus the w eak Ar-CO interaction does not provide the
impetus to form the same side configuration. This reasoning is applicable to the current
study, as well. W hile the OC-HCN and Ar-HCN interactions are stronger than Ar-CO,
they are still likely w eak com pared to the OC-SO 3 and A r-S 0 3 interactions. In addition,
since there is little out-of-plane distortion in O C-SO 3 and A r-S 0 3 , the problem o f
optimizing this param eter on both sides o f the SO 3 plane is alleviated.
The nitrogen quadrupole coupling constant o f H CN -S 0 3 -Ar (eQq(N)= -3.9672(58)
M Hz) is virtually unchanged from the H CN-SO 3 (eQq(N)= -3.9779(49) M Hz ) 3 dimer,
and there rem ains a substantial change in eQq(N) w ith respect to the HCN m onom er
(eQ q(N )=-4.70789(8) M Hz),29a likely indicating that the argon atom does little to
mitigate the degree o f electron transfer in the trimer.
This is not the case w ith the
H CN-SO 3 -CO com plex, w hich becom es m ore negative (eQq(N)= -4.0130(45) MHz)
19
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with the addition of the CO microsolvent.
This is not unexpected in light o f the
increased N-S bond distance.
The binding energies of the HCN-SO 3 -X (X=Ar, CO) indicate a slight favoring o f the
HCN-SO 3 -X (X=Ar, CO) isomer, likely explaining the lack o f evidence o f this
conformation in the spectral searches. The rotational temperature in the jet expansion
of the FTMW is nominally ~2 K. From Table 1. 2, the difference in De between the
two conformers o f the CO species is 0.128 kcal/mol, which translates to 64 K. It is thus
unlikely that the OC-HCN-SO 3 isomer would be observed. However, this difference is
relatively small compared to the difference in De between the two basis sets, which is
1.5 kcal/mol in the HCN-SO 3 -AJ complex, and 1.7 kcal/mol in HCN-SO 3 -CO. This
may imply that the MP2/aug-cc-avtz energies are not fully converged. Nonetheless, the
only observed isomer is in the HCN-SO 3 -X conformation, despite extensive spectral
searches.
As stated in the Introduction, one of the goals o f this study is to ascertain the effect of a
non-polar (or weakly polar) microsolvent on a partially bonded system in comparison
to a polar microsolvent.
Ideally, for a direct comparison to a polar microsolvated
species such as H C N -H C N -S O 3 , the microsolvent should be oriented in the same
relative position, i.e., X -H C N -S O 3 (X=Ar, C O ). However, with experimental results
for the opposite-side conformation only, it is useful to examine the theoretical results
for both conformers.
20
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The most striking difference between the two conformations in Table 1. 2 is that, while
there is very little change predicted in the N-S distances with respect to the HCN-SO 3
dimer, this bond length decreases in both species for the X-HCN-SO 3 isomer,
analogous to HCN-HCN-SO 3 , and increases in both species o f the HCN-SO 3 -X isomer.
Furthermore, the Ar atom has virtually no effect on the N-S distance in either case, with
a 0.003
A contraction for Ar-HCN-S0 3 and a 0.005 A elongation in HCN-SCA-Ar. The
effect is more pronounced in the CO species, with a 0.022
SO3 and a 0.044
A contraction in OC-HCN-
A elongation in HCN-SO 3 -CO. In addition, the NSO angle, a,
increases for the X-HCN-SO 3 isomers, and decreases for the HCN-SO 3 -X isomers.
Again, the change is negligible in the Ar complex, but more pronounced in the CO
complex.
There are two possible approaches in considering the dipole moments. The complexes
could be viewed as a dipolar unit (HNC-SO 3 ) and a polarizable sphere (Ar or CO), in
which case, there would be an induced dipole moment, making the dipole moment of
the trimers greater than the HCN-SO 3 dimer. However, in both complexes examined
here, the overall dipole moments are essentially the difference between the dipole
moments of the constituent dimers. This implies that, as in the case o f OC-S 0 3 -Ar, the
dipole moments o f the HCN-SO 3 -X complexes are more appropriately interpreted as
“bond moments”, which partially cancel as negative charge is moved toward the SO 3
units from both sides.
21
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Conclusion
Analysis o f the microwave spectra of the H C N /S O 3 X (X=Ar, C O ) system indicate a
favored structural conformation of the form H C N -S O 3 -X . While extensive searches
were performed for the same-side conformation, X -H C N -S O 3 , spectra were not located
for this molecular orientation.
The structures for both complexes demonstrate a
lengthening of both the donor-acceptor (N -S ) bonds and the van der Waals (S -X ) bonds.
This is in agreement with ab initio calculations, which indicate a preference for placing
the S O 3 unit between the H C N and the microsolvent and predict the bond distances to
within
0 .0 1 1
A o f experimental values. The dipole moment results support the notion
that these complexes are best described as having opposing bond moments, rather than
as a dipolar unit acting on a polarizable sphere.
The binding energies in OC-SCb-Ar indicated that the interactions were additive, with
the binding energy o f the trimer within about 2% of the sum o f the OC-SO 3 and Ar-SC>3
dimers. This is not entirely the case in the present study. While the binding energy of
HCN-SC>3 -Ar is within about 0.26 kcal/mol (-4% ) o f the sum o f the dimers, the binding
energy of HCN-SO 3 -CO is 1.3 kcal/mol (~13%) less than the sum o f the dimers. This
is perhaps owing to the stronger relative interactions in the trimers examined here.
The results of this study came as a rather pleasant surprise. In the process of trying to
address the effect of microsolvent polarity on a partially bonded system, we instead
observed a complex that was unexpected.
While not the anticipated effect, the
22
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lengthening of the N-S and C-S bonds comes as no surprise in light o f the orientation of
the microsolvent molecule on the opposite side of the S O 3 moiety.
Thus, while no
conclusion can be drawn from this study with regard to the effect of the polarity o f the
microsolvent in comparison to a polar medium, the results do provide insight into the
nature of structure and bonding of Lewis acid-base complexes. In addition, this serves
as a reminder that experimental research is often accompanied by surprises and
serendipitous discoveries.
Acknowledgements
This work was supported by the National Science Foundation, the donors o f the
Petroleum Research Fund, administered by the American Chemical Society, and the
Minnesota Supercomputing Institute.
23
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References
1
Leopold, K. R., “Partially bonded molecules and their transition to the crystalline
state.” in Advances in Molecular Structure Research, Vol II.,edited by Hargittai,
M. and Hargittai, I., JAI Press: Greenwich, CT, 1996, pp. 103-127.
2
Leopold, K. R.; Canagaratna, M.; Phillips, J. A., “Partially bonded molecules from the
solid state to the stratosphere.” Acct. Chem Res., 30, 57-64 (1997).
3
Phillips, J.A.; Britton, D.; Leopold, K.R., “Gas - solid structure differences in the
donor - acceptor complex (CH 3 )2 HN-SQ 2 .” J. Chem. Crystallogr., 26, 533-538
(1996).
4
Bums, W.A.; Phillips, J.A.; Canagaratna, M.; Goodfriend, H.; Leopold, K.R.,
“Partially Formed Bonds in H C N -S O 3 and C H 3 C N -S O 3 : A comparison between
donor-acceptor complexes of S O 3 and BF 3 .” J. Phys. Chem. A, 103, 7445-7453
(1999).
5
Fiacco, D.L.; Hunt, S.W.; Leopold, K.R., “Structural change at the onset of
microsolvation: Rotational spectroscopy o f H C N -H C N -S O 3 .” J. Phys. Chem. A,
104, 8323-8327 (2000).
6
Fiacco, D.L.; Mo, Y.; Hunt, Ott, M.E.; S.W.; Roberts, “Dipole moments o f partially
bound Lewis acid-base adducts.” J. Phys. Chem. A, 105, 484-493 (2001).
7
Hunt, S.W.; Leopold, K.R., “Molecular and electronic structure o f C 5 H 5 N -S O 3 :
Correlation o f ground state physical properties with orbital energy gaps in
partailly bound Lewis acid-base complexes.” J. Phys. Chem. A, 105, 5498-5506
(2001).
24
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8
Hunt, S.W.; Fiacco, D.L.; Craddock, M.; Leopold, K.R., “Correlation o f dative bond
length and donor proton affinity in adducts of SO3 : A good predictor for
HCCCN-SO 3 .” J. Mol. Spec., 212, 213-218 (2002).
9
Fiacco, D.L.; Leopold, K.R., “Partially bound systems as sensitive probes of
microsolvation: A microwave and ab initio study of H C N -H C N -B F 3 .” J. Phys.
Chem. A, 107, 2808-2814 (2003).
10
Reeve, S.W.; Burns, W.A.; Lovas, F.J.; Suenram, R.D.; Leopold, K.R., “Microwave
spectra and structure of hydrogen cyanide-boron trifluoride: an almost weakly
bound complex.” J. Phys. Chem., 97, 10630-10637 (1993).
11
Jiao, H.; Schleyer. P.v.R., “Large effects o f medium on geometries. An ab initio
study.” J. Am. Chem. Soc., 116, 7429-7430 (1994).
12
Choo, J.; Kim, S.; Kwon, Y., “Theoretical molecular structures for partially bonded
complexes of trimethylamine with SO2 and SO 3 :
functional theory calculations.” J. Mol. Struct.:
Ab initio and density
THEOCHEM, 594, 147-156
(2002 ).
13
Balle, T. J.; Flygare, W. H., “Fabry-Perot cavity pulsed Fourier transform microwave
spectrometer with a pulsed nozzle particle source.” Rev. Sci. Inst., 52, 33-45
(1981).
14
Phillips, J. A.; Canagaratna, M.; Goodfriend, H.; Grushow, A.; Almlof, J.; Leopold,
K. R., “Microwave and ab initio investigation o f H F -B F 3 ” J. Am. Chem. Soc.,
117, 12549-12556(1995).
25
R eproduced with perm ission of the copyright owner. Further reproduction prohibited without permission.
15
Phillips, J. A., “Structure and dynamics of partially-bound molecular complexes.”
University o f Minnesota, Minneapolis, MN, USA. Ph.D. Thesis, 1996.
16
Hunt, S. W.; Brauer, C. S.; Craddock, M. B.; Higgins, K. J.; Nienow, A. M.; Leopold,
K. R., “Microwave Observation of H 3 N-SO 3 -H 2 O Using a Concentric, DualInjection Nozzle Source.” Chem. Phys., 305, 155-164 (2004).
17
a.) Rego, C. A.; Batten, R. C.; Legon, A. C., “The properties o f the hydrogen-bonded
dimer
trimethylamine"'hydrogen
cyanide
((CH 3 ) 3 )N'"HCN)
from
an
investigation of its rotational spectrum.” J. Chem. Phys., 89, 696-702 (1988).
b.)
Fiacco, Denise Lynn, “Microwave investigation o f partial and hydrogen
bonded molecular complexes.” University o f Minnesota, Minneapolis, MN,
USA. Ph.D. Thesis, 2001.
18
Bowen, K. H.; Leopold, K. R.; Chance, K. V.; Klemperer, W., “Weakly Bound
Complexes o f Sulfur Trioxide: The Structure o f ArSC>3 and the Dipole Moment
of N 2 S O 3 ” , J. Chem. Phys., 73, 137-141 (1980).
19
Canagaratna, M.; Ott, M. E.; Leopold, K. R., “Determination o f the Dipole Moment
o f H 3 N -SO 3 in the Gas Phase”, Chem. Phys. Lett.. 281, 63-68 (1997).
20
Coudert, L. H.; Lovas, F. J.; Suenram, R. D.; Hougen, J. T., “New Measurements of
Microwave Transitions in the Water Dimer”, J. Chem. Phys., 87, 6290-6299
(1987).
26
R eproduced with perm ission of the copyright owner. Further reproduction prohibited without permission.
21
a) Townes, C. H.; Schawlow, A. L., Microwave Spectroscopy, Dover: New York,
1975. b) Gordy, W.; Cook, R. L., Microwave Molecular Spectra', Wiley: New
York, 1970.
22
Pickett, H. M., “The fitting and prediction of vibration-rotation spectra with spin
interactions” J. Mol. Spectrosc., 148, 371-377 (1991).
23
MP2/aug-cc-pVTZ calculations on the X -H C N -S O 3 conformers failed to converge.
24
H.-J. MOLPRO is a package o f ab initio programs written by H.-J. Werner and P. J.
Knowles, with contributions from J. Almlof, R. D. Amos, A. Beming, D. L.
Cooper, M. J. O. Deegan, A. J. Dobbyn, F. Eckert, S. T. Elbert, C. Hampel, R.
Lindh, A. W. Lloyd, W. Meyer, A. Nicklass, K. Peterson, R. Pitzer, A. J. Stone,
P. R. Taylor, M. E. Mura, P. Pulay, M. Schutz, H. Stoll, and T. Thorsteinsson.
C.
25
Boys, S.; Bernardi, F., “The calculation of small molecular interactions by the
difference o f separate total energies.
Mol. Phys., 19, 553-566 (1970).
Some procedures with reduced errors.”
[Note: this paper also can be located with the
following reference: Mol. Phys., 100(1), 65-73 (2002),]
26
Emsley, J. ; Hoyte, O.P.A.; Overill, R.E., “Ab initio calculations on the very strong
hydrogen bond o f the biformate anion and comparative esterification studies.”
J. Am. Chem. Soc., 100, 3303-3306 (1978).
27
Simon, S.; Duran, M, “How does basis set superposition error change the potential
surfaces for hydrogen-bonded dimers?” J. Chem. Phys., 105, 11024—11031
(1996).
27
R eproduced with perm ission of the copyright owner. Further reproduction prohibited without permission.
28
In calculating Y2 eff from the tensor projection formula, monomer values o f eQqo(14 N)
= -4.70789(8) MHz29a and eQq0 (D) = 0.1944(22) MHz29b for HC14N and
DC 14 N, respectively, were used.
29
a) Maki, A. G., “Microwave spectra of molecules of astrophysical interest, VI.
Carbonyl sulfide and hydrogen cyanide.” /. Phys. Chem. Ref. Data, 3, 221-244,
(1974). b) DeLucia, F.; Gordy, W., “Molecular-beam maser for the shortermillimeter-wave region: Spectral constants of HCN and DCN.” Phys. Rev., 187,
58-65 (1969).
30
Ruoff, R. S.; Emilsson, T.; Chuang, C.; Klots, T. D.; Gutowsky, H. S., “Experimental
separation of torsional and charge redistribution effects in rotational spectra of
HCN dimer.” Chem. Phys. Lett., 138, 553-558 (1987).
31
Kraitchman, J., “Determination of molecular structure from microwave spectroscopic
data.” Am. J. Phys., 21, 17-24 (1953).
32
Ruoff, R.S.; Emilsson, T.; Chuang, C.; Klots, T.D.; Gutowsky, H.S., “Experimental
separation o f torsional and charge redistribution effects in rotational spectra of
HCN dimers.” Chem. Phys. Lett. 138, 553-558 (1987).
33
Craddock, M.B.; Brauer, C.S.; Higgins, K.J.; Leopold, K.R., “A Microwave and ab
initio investigation of N2-SO3, OC-SO3, and OC-SCb-Ar.” J. Mol. Spec.,,222,
63-73 (2003).
34
See, fo r example, a) Kisiel, Z.; Kosarzewski, J.; Pietrewicz, B. A.; Pszczolkowski, L.,
“Electric Dipole Moments of the Cyclic Trimers (H 2 0 ) 2 HC1 and (H 2 0 ) 2HBr
from Stark Effects in their Rotational Spectra”, Chem. Phys. Lett., 325, 523-530
R eproduced with perm ission of the copyright owner. Further reproduction prohibited without permission.
(2000), b) Kisiel, Z.; Pietrewicz, B. A.; Fowler, P. W.; Legon, A. C.; Steiner, E.,
“ Rotational Spectra of the Less Common Isotopomers, Electric Dipole Moment
and the Double Minimum Inversion Potential of H 2 0-HC1”, J. Phys. Chem. A.,
104, 6970-6978 (2000).
R eproduced with perm ission of the copyright owner. Further reproduction prohibited without permission.
Appendix to Chapter 1
R eproduced with perm ission of the copyright owner. Further reproduction prohibited without permission.
Table A l. 1 Observed Rotational Transitions1 for HCN-S 0 3 -Ar.
J’
3
3
3
4
4
4
4
4
4
5
5
5
5
5
5
5
F'
J"
F"
K
2
2
1
0
2
2
6
6
3
4
4
5
3
4
5
3
5
5
6
4
4
5
6
6
7
5
5
6
6
5
6
4
7
5
6
6
6
6
6
2
3
3
3
3
3
3
4
4
4
4
4
4
4
5
5
5
5
5
4
5
5
5
3
4
4
6
6
6
6
7
5
6
6
6
7
6
1A ll
F’
4
5
J"
4
4
4
5
5
5
5
5
0
3
2
0
3
4
0
2
5
4
5
3
3
4
5
5
6
4
4
5
4
0
3
0
3
3
3
0
0
0
3
3
3
0
0
0
6
0
5
0
6
0
Table A l. 2 O b s e r v e d
J'
5
5
5
0
0
F"
3
4
5
5
6
4
5
6
~
Obs.a
3831.661
3831.860
3831.906
5107.783
5108.956
5109.011
5109.096
5109.125
5109.414
6384.937
6385.312
6385.921
6386.069
6386.240
6386.290
6386.307
7662.533
7662.890
7662.949
7663.396
7663.421
6387.931
7663.443
7665.057
7662.038
Obs.-Calc.
-0.00059
-0.00013
-0.00141
0.00956
-0.00215
0.00046
0.00036
-0.00073
-0.00129
0 .0 0 2 0 1
0.00163
-0.00004
-0.00165
0 .0 0 0 0 0
0.00272
-0 . 0 0 1 1 1
0.00073
-0.00086
0.00248
0 .0 0 2 0 2
-0.00307
-0.00946
0.00365
0.00984
-0.01295
R o t a t i o n a l T r a n s i t i o n s 1 f o r H C N - -34S 0 3- A r .
K
0
0
0
3
3
0
0
0
Obs.a
6384.551
6384.602
6384.621
7660.506
7660.865
7661.372
7661.398
7661.418
Obs.-Calc.
-0.00178
0.00183
-0.00005
0 .0 0 0 2 0
-0 . 0 0 0 2 0
0 .0 0 1 2 1
-0.00295
0.00174
f r e q u e n c ie s a r e in M H z .
R eproduced with perm ission of the copyright owner. Further reproduction prohibited without permission.
Table A l. 3 Observed Rotational Transitions' for DCN-S 0 3 -Ar.
J’
5
5
5
5
5
5
5
5
5
5
F i'
5
5
6
6
6
6
6
6
6
6
7
7
7
7
7
7
7
7
7
1
6
0
6
6
0
6
7
0
8
8
6
6
4
4
4
4
5
6
3
5
3
5
5
7
5
7
7
7
6
6
6
6
6
6
6
6
6
F 2"
5
4
4
5
6
6
6
F i"
4
4
5
5
3
3
3
3
4
5
5
5
5
7
7
5
5
5
6
6
J"
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
5
5
f 2’
7
7
7
7
8
7
6
6
6
7
7
7
9
7
2
4
3
4
4
6
5
5
6
6
7
6
6
4
5
7
5
6
4
5
6
K
3
3
3
3
3
3
3
0
0
0
3
3
3
3
3
3
0
6
0
0
6
7
7
6
6
6
6
6
7
6
3
3
3
3
3
3
6
6
6
6
7
6
7
5
5
5
7
8
6
6
7
0
8
9
6
7
8
0
8
6
6
6
8
6
5
6
0
O bs.a
6177.794
6177.816
6178.401
6178.429
6178.542
6178.555
6178.574
6178.681
6178.731
6178.751
7412.912
7412.933
7412.995
7413.524
7413.546
7413.885
7413.908
7413.941
7413.971
7414.339
7414.365
7414.385
8649.105
8649.122
8649.132
8649.333
8649.358
8649.377
8649.930
8649.943
8649.960
Obs.-Calc.
-0.00265
0 .0 0 0 2 2
0.00400
-0.00157
-0.00338
0.00109
0.00186
0 .0 0 0 1 1
-0.00183
0.00074
-0.00542
0.00514
0.00325
-0.00235
-0.00233
-0.00085
0.00529
-0.00106
0.00018
0.00160
-0.00276
0.00147
-0.00162
0.00460
-0.00738
-0.00355
0.00053
0.00867
0.00297
-0.00503
-0.00004
All frequencies are in MHz.
R eproduced with perm ission of the copyright owner. Further reproduction prohibited without permission.
Table A l. 4 Observed Rotational Transitions1 for HC15N-S 0 3 -Ar.
J'
4
4
5
5
7
7
J"
3
3
4
4
6
6
K
3
0
3
0
3
0
Obs.a
5053.531
5053.942
6316.834
6317.349
8843.271
8843.992
Obs.-Calc.
0.00030
-0.00053
0.00001
0.00024
-0.00018
0.00013
Table A l. 5 Observed Rotational Transitions1 for HC15N-34S0 3 -Ar.
J'
4
5
5
7
7
J"
3
4
4
6
6
K
0
3
0
3
0
Obs.a
5052.804
6315.407
6315.926
8841.277
8841.999
Obs.-Calc.
0.00048
-0.00129
0.00076
0.00092
-0.00082
1 All frequencies are in MHz.
R eproduced with perm ission of the copyright owner. Further reproduction prohibited without permission.
Table A l. 6 Observed Rotational Transitions1 for HCN-SO3-CO.
J'
3
3
3
3
3
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
6
6
6
6
6
6
6
6
6
F' .
2
3
4
2
4
3
4
5
5
3
3
5
5
4
6
4
5
6
4
6
6
6
7
5
5
6
7
5
J"
2
2
2
2
2
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
F"
3
1
2
3
2
4
2
3
4
4
2
3
5
4
4
5
3
4
5
4
6
6
5
6
4
4
5
6
5
K
0
0
0
0
0
0
0
0
0
3
3
0
0
3
3
3
0
0
0
0
0
3
3
3
3
0
0
0
0
O bs.a
4165.652
4166.742
4166.942
4166.991
4168.745
5554.557
5555.813
5555.901
5555.930
5555.951
5556.416
5557.618
6943.468
6944.089
6944.259
6944.709
6944.786
6944.838
6944.855
6946.506
8332.355
8332.996
8333.133
8333.497
8333.554
8333.717
8333.741
8333.763
8335.389
Obs.-Calc.
-0.00013
-0.00296
0.00103
0.00021
0.00139
-0.00404
0.00055
0.00247
0.00104
-0.00259
0.00001
-0.00061
0.00111
-0.00029
0.00222
0.00199
-0.00099
0.00319
-0.00088
-0.00108
-0.00154
0.00013
-0.00164
-0.00037
0.00032
-0.00453
0.00190
0.00202
0.00164
1 All frequencies are in MHz.
R eproduced with perm ission of the copyright owner. Further reproduction prohibited without permission.
Table A l. 7 Observed Rotational Transitions1 for HC 15N-S 0 3 -C 0 .
J"
4
5
5
6
6
7
7
J'
5
6
6
7
7
8
8
K
0
3
0
3
0
3
0
Obs.a
6873.055
8247.269
8247.611
9621.737
9622.137
10996.169
10996.627
Obs.-Calc.
-0.00060
0.00055
-0.00025
0.00029
0.00035
0.00026
-0.00067
Table A l. 8 Observed Rotational Transitions1 for HCN-S 0 3 - 1 3 C 0 .
J’
4
4
4
4
5
5
5
5
5
5
5
6
6
6
6
6
6
F'
3
4
5
5
5
5
6
4
5
6
4
6
7
5
5
6
7
.
J"
3
3
3
4
4
4
4
4
4
4
5
5
5
5
5
5
F"
2
3
4
4
5
4
5
3
4
5
4
5
6
4
4
5
6
K
0
0
0
3
0
3
3
0
0
0
0
3
3
3
0
0
0
Obs.a
5493.087
5493.174
5493.202
5493.230
6865.052
6865.691
6866.307
6866.379
6866.430
6866.445
6868.100
8239.055
8239.421
8239.479
8239.626
8239.652
8239.674
Obs.-Calc.
0.00231
0.00305
0.00055
-0.00432
-0.00337
0.00170
-0.00129
0.00081
0.00389
-0.00223
-0.00183
-0.00136
0.00116
0.00273
0.00027
-0.00423
0.00229
1 All frequencies are in MHz.
R eproduced with perm ission of the copyright owner. Further reproduction prohibited without permission.
Table A l. 9 Observed Rotational Transitions1 for DCN-SO3-CO.
J'
"5
5
5
5
6
6
6
6
6
6
6
7
7
7
7
7
7
7
7
7
7
F i'
6
6
6
6
6
6
5
6
7
7
7
6
8
7
7
8
7
8
7
8
7
f 2’
6 '
7
5
7
6
7
4
5
6
7
8
6
8
7
7
7
6
8
7
9
8
J"
4
4
4
4
5
5
5
5
5
5
5
6
6
6
6
6
6
6
6
6
6
F i"
~5
5
5
5
5
5
5
5
6
6
6
6
7
6
7
7
7
7
6
7
7
F2"
5 “
6
4
6
5
6
4
4
5
6
7
6
7
6
7
6
6
7
6
8
8
K
3
3
0
0
3
3
3
3
0
0
0
3
3
3
3
3
3
0
0
0
0
O bs.a
6715.985
6716.599
6716.679
6716.749
8059.422
8059.789
8059.814
8059.845
8059.992
8060.018
8060.038
9402.677
9402.699
9402.719
9402.743
9402.966
9402.991
9403.272
9403.282
9403.301
9403.356
Obs.-Calc.
0.00423
0.00095
-0.00259
-0.00260
-0.00217
0.00125
-0.00271
0.00102
0.00260
-0.00203
0.00205
-0.00256
-0.00759
0.00150
0.00327
0.00626
-0.00236
-0.00721
0.00103
0.00966
-0.00202
1 All frequencies are in MHz.
R eproduced with perm ission of the copyright owner. Further reproduction prohibited without permission.
Table A l. 10 Observed Transitions1 of HC 'N-S 0 3 -Ar at Non-Zero Electric Field.
Rotational Transition
-
Obs.a
5053.942
Obs.-Calc.
-0.00052
E (V/cm)
0.00
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
5053.907
5053.896
5053.867
5053.853
5053.832
5053.821
5053.792
5053.661
5053.557
-0.00422
-0.00132
-0.00511
0.00099
0.00241
0.00342
0.00062
-0.00098
-0.00438
30.74
36.94
46.11
52.29
58.42
61.45
67.60
92.20
107.5
0
0
0
0
0
0
0
0
0
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
5053.924
5053.917
5053.902
5053.889
5053.875
5053.868
5053.850
5053.771
5053.711
0.00055
0.00202
0.00238
0.00162
0.00124
0.00158
-0.00048
-0.00076
0.00039
30.74
36.94
46.11
52.29
58.41
61.45
67.60
92.20
107.6
0
0
0
0
0
0
0
0
0
0
0
2
2
2
2
2
2
2
2
2
2
2
1
1
1
1
1
1
5053.906
5053.885
5053.870
5053.857
5053.833
5053.806
5053.791
5053.757
5053.681
5053.594
5053.460
0.00235
-0.00152
0.00386
0.00196
0.00284
0.00381
0.00388
0.00259
0.00112
0.00089
-0.00684
30.73
36.88
43.08
46.10
52.25
58.41
61.45
67.62
79.90
92.16
107.6
K
0
M"
M'
-
0
0
0
0
0
0
0
0
0
1
1
1
1
1 All frequencies are in MHz.
37
R eproduced with perm ission of the copyright owner. Further reproduction prohibited without permission.
Table A l. 10 Observed Transitions o f HC15N-S03-Ar at Non-Zero Electric Field, Cont.1
Rotational Transition
K
Obs.a
Obs.-Calc.
E (V/cm)
M"
M'
4 < -3
0
2
2
5053.961
0.00086
30.74
2
2
5053.967
0
-0.00097
36.88
5053.982
0
2
2
-0.00019
46.11
2
2
5053.994
0
0.00046
52.29
2
2
5054.007
58.41
0
0.00082
0
2
2
5054.012
-0.00099
61.45
0
2
2
5054.028
0.00018
67.60
2
0
2
5054.100
-0.00153
92.25
0
2
2
5054.160
0.00130
107.5
0
0
0
0
0
0
0
0
0
0
0
3
3
3
3
3
3
3
3
3
3
3
2
2
2
2
2
2
2
2
2
2
2
5053.932
5053.923
5053.914
5053.906
5053.899
5053.898
5053.879
5053.868
5053.840
5053.802
5053.757
0.00488
0.00268
0.00175
-0.00186
0.00000
0.01210
-0.00195
0.00001
0.00163
-0.00209
0.00293
30.73
36.88
43.08
46.10
58.41
52.25
61.45
67.62
79.91
92.19
107.6
0
0
0
0
0
0
0
3
3
3
3
3
3
3
3
3
3
3
3
3
3
5054.024
5054.055
5054.120
5054.173
5054.227
5054.258
5054.325
0.00270
-0.00131
0.00016
0.00249
-0.00012
0.00042
0.00059
30.74
36.94
46.11
52.29
58.42
61.47
67.67
0
0
0
0
0
0
0
0
0
0
0
4
4
4
4
4
4
4
4
4
4
4
3
3
3
3
3
3
3
3
3
3
3
5053.974
5053.988
5054.005
5054.016
5054.035
5054.057
5054.072
5054.101
5054.162
5054.234
5054.340
-0.00104
-0.00138
-0.00143
0.00028
-0.00155
-0.00302
-0.00095
0.00099
-0.00044
-0.00128
-0.00066
30.73
36.87
43.08
46.10
52.25
58.41
61.54
67.62
79.91
92.19
107.5
1 All frequencies are in MHz.
R eproduced with perm ission of the copyright owner. Further reproduction prohibited without permission.
Table A l. 11 Observed Transitions1 o f HC 15N-S 0 3 -C 0 at Non-Zero Electric Field.
R otational Transition
5 <—4
K
0
M"
M’
Obs-Calc
-0.00095
E (V/cm)
-
Obs
6873.055
-
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
6873.021
6873.011
6872.990
6872.976
6872.957
6872.922
-0.00103
-0.00311
-0.00066
-0.00104
-0.00503
0.00112
55.37
61.50
76.85
84.50
92.21
110.64
0
0
0
0
0
0
0
1
1
1
1
1
1
1
1
1
1
1
1
1
1
6873.031
6873.026
6873.009
6872.999
6872.988
6872.975
6872.955
0.00061
0.00158
0.00225
0.00251
0.00282
-0.00048
0.00080
55.37
61.50
76.85
84.50
92.21
98.34
110.64
0
0
0
2
2
2
1
1
1
6873.008
6872.999
6872.909
-0.00268
-0.00122
-0.00463
55.42
61.49
98.34
0
0
0
0
0
0
0
2
2
2
2
2
2
2
2
2
2
2
2
2
2
6873.055
6873.055
6873.054
6873.055
6873.054
6873.055
6873.054
-0.00046
-0.00035
-0.00103
0.00015
-0.00066
0.00049
-0.00018
55.37
61.50
76.85
84.50
92.21
98.34
110.64
0
0
0
3
3
3
2
2
2
6873.023
6872.995
6872.955
0.00029
0.00294
0.00359
55.42
76.84
98.34
0
1
2
6873.117
0.00063
98.34
1 All frequencies are in MHz.
R eproduced with perm ission of the copyright owner. Further reproduction prohibited without permission.
0.00
Table A l. 11 Observed Transitions o f HC15N-S03-CO at Non-Zero Electric Field,
Cont.1
K
M"
M'
R otational T ransition
Obs
Obs-Calc
E (V/cm)
0
-0.00025
5 < -4
3
3
6873.097
55.37
0
3
3
6873.107
0.00010
61.50
0
3
3
6873.134
-0.00151
76.85
0
3
3
6873.152
-0.00015
84.50
6873.171
0
3
3
0.00049
92.21
0
3
3
6873.185
-0.00127
98.34
0
3
3
6873.219
-0.00189
110.64
0
0
0
0
4
4
4
4
3
3
3
3
6873.052
6873.048
6873.050
6873.041
0.00051
-0.00245
0.00262
-0.00093
55.42
61.48
76.84
98.34
0
0
0
0
0
4
4
4
4
4
4
4
4
4
4
6873.155
6873.176
6873.253
6873.283
6873.338
-0.00076
-0.00308
0.00478
-0.00543
0.00523
55.37
61.50
76.85
84.50
92.21
0
0
0
5
5
5
4
4
4
6873.095
6873.104
6873.149
-0.00202
-0.00250
-0.00248
•
55.42
61.49
84.53
1 All frequencies are in MHz.
40
R eproduced with perm ission of the copyright owner. Further reproduction prohibited without permission.
Chapter 2
Microwave Investigation of (CH 3)3N***HF***HF
Carolyn Brauer, Matthew Craddock, Sherri Hunt and Kenneth R.
Leopold
Department o f Chemistry
University o f Minnesota
Minneapolis, MN
Jens-Uwe Grabow
Lehrgebiet Physikalische Chemie A,
Institut fu r Physikalische Chemie und Elektrochemie
Universtat Hannover
Hannover, Germany
R eproduced with perm ission of the copyright owner. Further reproduction prohibited without permission.
Abstract
Rotational spectroscopy has been used to examine the effect o f a single HF “solvent”
molecule on gas phase proton transfer in the hydrogen bonded complex (CHh^N-'-HF.
The rotational spectra o f (C T H ^N -H F -H F and seven of its isotopically substituted
derivatives have been observed by Fourier transform microwave spectroscopy. This
follows a previous study by our group on H 3 N -H F -H F , in which the addition o f the
second HF decreased the N - H hydrogen bond distance in H3N--HF by 0.21(6)
A.
The present study investigates the effect o f increased basicity o f the amine on the
N —H hydrogen bond. We observe a simple asymmetric rotor spectrum with strong aand b-type transitions, consistent with a ring structure for the CN-- HF-- HF frame. No
evidence o f internal rotation is observed.
hydrogen bond distance of 1.24(4)
Structural analysis indicates a primary
A, approximately halfway between the hydrogen
bond distance in H3N "FLF of 1.7 A and the N-H covalent length o f 1.1
A.
Previously reported ab initio calculations [Hunt, S. W., Ph.D. Thesis, University of
Minnesota, 2002] concur with experiment, indicating that, as with H 3N --H F—HF, the
trimethylamine complex forms a ring in which both the N--H hydrogen bond and the
HFH angle are significantly perturbed. This complex provides the first step in
microsolvation of (CTH^N-HF and is useful in understanding the role o f local
environment in promoting proton transfer.
42
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Introduction
In the near century since it was first characterized by Latimer and Rodebush , 1
hydrogen bonding has been an active area o f interest. 2 Given the key roles it plays in
areas as diverse as simple gas phase attractions , 3 to intricate biochemical
interactions , 4 ' 7 it is no wonder that a plethora of research has been devoted to simply
identifying the nature of the hydrogen bond . 8 ' 11 Much of this attention has been
focused on elucidating the degree o f proton transfer, and the factors that influence
proton transfer reactions in the highly prototypical hydrogen halide complexes . 1 2 ' 2 0
While early experimental work on these systems sought simply to identify ammonium
chloride by mass spectrometry , 21 later studies emphasized the bonding interaction in
an attempt to classify these systems as hydrogen bonded complexes or ion pairs . 1 3 ,1 4 ,2 2
In a series o f matrix isolation studies, Pimentel, et al . 2 2 established that the bonding in
hydrogen halide complexes span a range o f values that they classified as Type I, II and
III hydrogen bonds. Type I is described as a “traditional” hydrogen bond, where the
proton is considered attached to the acid. Type III refers to an ion pair, where the
proton is fully transferred to the base. Type II lies in between the two extremes, and
the proton is shared between the acid and base in a quasi-symmetric hydrogen bond.
In the analogous gas phase systems, Tony Legon 13,23 contributed to this field through a
series of microwave studies, aimed at analyzing the rotational spectra and structures of
(CH 3 )nN-HX (n=0-3, X=F, Cl, Br, I) in terms o f these bonding types.
By using
43
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nuclear quadrupole coupling constants and force constants, he was able to
approximate the degree of proton transfer.
While most of these types of bonds generally fall into one o f two categories hydrogen bond or ionic bond - there is nothing to preclude the existence o f Pimentel’s
Type II bonds.
Indeed, recent attention has been focused on the “short-strong”
hydrogen bond as playing a critical role in enzyme catalysis. 7
Present interest in (C H ^-H F-H F was prompted by previous work done in our lab.
There is a great deal of evidence, primarily from matrix isolation work , 231,24 that amine
hydrogen halide complexes are sensitive to the local environment and a number of
recent theoretical studies have focused on understanding these matrix effects .2 5 ,2 6 In
order to examine these effects at the microsolvent level, Hunt, 12 et al. demonstrated
that adding a single HF molecule can be an effective method o f driving a proton in the
NH 3 -HF complex toward the base and hence, toward proton transfer. In addition to
perturbing the structures of the constituent dimers, NH 3 and (HF)2 , from their free
geometries, formation o f the trimer, NH 3 -HF-HF, contracted the primary hydrogen
bond by 0.205(12)
A. Clearly, this complex responded to the “medium,” which
stabilizes the charge separation associated with the proton transfer.
In order to determine the effect of a stronger base on this process, Hunt2 7 performed
an ab initio calculation on the (CH 3 ) 3 -HF-HF system. The binding energy of the
complex is 32.85 kcal/mol, which is over 27% larger than the sum o f the (CH 3 )3 N and
44
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(H F ) 2 dimers. These energies are not additive, which is consistent with the predicted
structure, where the primary hydrogen bond is 1.354
A. The most striking feature,
from our standpoint, is that this bond distance is predicted to be exactly halfway
between the hydrogen bond in the N H 3 -H F dimer and a covalent N -H bond, such as
one finds in ammonium ion. Thus, this was a particularly interesting system to look at
experimentally.
Experimental
Spectra for (CTH^N-HF-HF were recorded on the Minnesota Fourier transform
microwave spectrometer, 28 details o f which are described elsewhere . 2 9 The complex
was produced in situ, with the method similar to those used for other reactive
systems .2 7 ,3 0 Briefly; a mixture of trimethylamine in argon was pulsed through a
pinhole nozzle (0.8mm) at a rate o f 6 Hz. Neat HF was flowed through stainless steel
hypodermic tubing (0.028” O.D.), and introduced into the supersonic expansion
approximately 0.29” below the nozzle aperture.
HF flow rate, stagnation pressure and percentage o f trimethylamine in argon were
varied throughout the spectral search, and were optimized to improve signal-to-noise.
Stagnation pressure ranged from 4 to 15 psig, however signal intensities were
relatively insensitive to this parameter.
While generally optimizing at the higher
pressure, the improvement was minor. HF flow rates varied from 0.5 to 10 seem,
45
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again with the higher rate producing the best signal-to-noise.
In contrast to the
stagnation pressure, signal intensities were significantly improved with higher flow
rates. However, the lower rate was used to conserve reactant, especially the costly DF
isotopolog, whenever possible. In general, unlike most trimers, the signal intensities
were comparable to those of the dimer, (Q H ^N -H F. This unusually high signal-tonoise ratio allowed for the aforementioned flexibility in experimental conditions,
without sacrificing experimental results.
For the parent and
used, and the
15
15
N(CH 3 ) 3 isotopologs, a 2.5% mixture o f 14 N(CH 3 ) 3 in argon was
N(CH 3 ) 3 -HF-HF was observed in natural abundance.
A 0.5 %
mixture o f (CDs^N (98 atom %, Cambridge) in argon was employed to observe
(CD 3 )3 N-HF-HF.
The remaining species were observed with 15N labeled
trimethylamine, synthesized in house according to established procedures . 31 Briefly,
15
NH 4 C1 (99.5 atom %, Icon) and paraformaldehyde (Aldrich) were heated to form
(CH 3 ) 3 15NH+CF. A concentrated solution o f NaOH was then added to release the
(CH 3 ) 3 15N. The product was collected in an evacuated ballast and a 0.5% mixture in
Ar was used to observe the (CH 3 ) 3 15 N-HF/DF, (CH 3 )3 15N-DF-DF, as well as the
l3 CH 3 ( 12 CH 3 )2 15N-HF-HF isotopomers with 13C in natural abundance.
A 50/50
mixture of HF/DF (99 atom %, Icon) was made to observe the mixed isotopomers.
46
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Results
Spectra and Rotational Constants
The observed transition frequencies o f (C t^ N -H F -H F and seven isotopically
substituted species are listed in Tables A2.1 through A2.8 in the Appendix o f this
chapter. Both a- and b-type transitions were observed for all but the symmetric form
of the
13
C-substituted species,
13
CsH 3 (CaH 3) 2 15N-FIF-HF .3 2
The component o f the
dipole moment along the c-axis was predicted to be zero, thus c-type transitions were
not expected and nor were they found in this study . 33
Most o f the observed transitions were broad, with full width at half maximum
intensity (FWHM) typically 15 kHz, but as large as 80 kHz. This broadening is likely
due to a number of factors inherent in complexes containing several atoms with non­
zero nuclear spin. The primary cause o f spectral broadening in this complex likely
arises from the interaction between the magnetic moments o f the hydrogen and
fluorine nuclei in the HF moieties. While magnetic hyperfine structure is not typically
observed in microwave spectroscopy o f closed shell molecules, the large magnetic
moments of both hydrogen and fluorine, coupled with the short molecular bond, give
rise to nuclear spin-nuclear spin interactions large enough to be detected.
The
splitting, however, is complicated by the presence o f two HF molecules and the
deuterated species are further complicated by quadrupolar hyperfine structure from the
47
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2H nucleus.
Thus, the spectral lines are broadened considerably, but individual
hyperfine components generally cannot be resolved.
A number o f transitions were found that depended on HF and N(CH 3 ) 3 , but did not
correspond to any known lines, nor were they consistent with a- b- or c-type
transitions from any o f the measured isotopologs. Furthermore, no distinct pattern
could be discerned among these anomalous transitions that belonged to any o f the
likely, or even possible isotopologs. While one or two isolated transitions may have
been consistent with the predicted spectrum o f an unlikely isotopically substituted
candidate, no pattern emerged to confirm assignments.
These transitions may be
attributed to a variety o f factors, and were thoroughly investigated. Many were small
and within 100 kHz of an assigned transition. These may be the few resolvable spinspin hyperfine components, or, in the deuterated species, quadrupolar hyperfine
components.
Approximately ten o f the unassigned lines were near predicted
transitions involving very high rotational levels o f the observed species. While these
generally fit well, their inclusion did not change the results and, as they were not
expected to have sufficient intensity to be observed, they were not included in the final
analysis.
O f the remaining unassigned transitions, a large number were found during the search
for the
13
C/15N substituted species, which depended both on HF and
15
N(CH 3 ) 3 . 3 4
Many o f the lines were as intense, or more so, than the assigned transitions for these
isotopomers. Furthermore, three o f the lines could be fit to a rigid rotor spectrum.
48
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However, subsequent searches for additional transitions to confirm this assignment
did not yield additional lines.
It should be noted that, although the spectra o f the 13C substituted species was located
amidst a dense collection o f unknown lines, the assignments lead to rotational
constants that are very close (within 1 MHz) to those predicted by the structure
without 13C substitution. All unassigned transitions are listed in Table A2.9 o f the
Appendix o f this chapter.
A portion of the parent spectrum is shown in Figure 2. 1. While transitions seen on our
instrument are typically
~ 8
kHz wide (FWHM), the transition shown is 16 kHz.
Despite the broadness o f the lines, signal-to-noise for this complex was unusually
high, indicating a strong interaction. The strong intensity o f the main peak in this
figure nearly obscures the nuclear quadrupolar hyperfine structure, which is seen as
the small bump to the left o f the large peak.
J
6416.2
6416.4
6416.6
6416.8
6417
6417.2
Frequency (MHz)
Figure 2 .1 J = lox—>2 o2 transition o f (C T ^N -H F-H F taken over 93 seconds.
Signal to noise in this spectrum is 900.
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49
A- and b-type spectra from eight isotopologs were fit to a Watson A reduced
Hamiltonian using Herb Pickett’s spectra fitting program, SPFIT , 3 5 and the resulting
rotational constants are listed in Table 2. 1. The spectrum did not display evidence of
internal rotation, which is not unusual for trimethylamine complexes. 3 6 Steric
hindrance precludes rotation of the methyl groups, as evidenced by the high barrier
(1530 cm '1), determined by Lide, et al . 3 7 Furthermore, while rotation o f the
trimethylamine about its pseudo-C 3 axis is possible, this motion also is likely to have a
high reduced barrier . 36
The close agreement o f the observed rotational constants to those predicted from the
ab initio structure , 27 help to confirm that the structure determined here is, indeed,
close to predicted. However, the structure determination was by no means trivial, and
incorporates both experimental and theoretical results in the final structure.
50
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Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Table 2 .1 Spectroscopic constants o f (CH 3 ) 3N-HF-HF.a
u N(CH3)r HF-HF
(MHz)
3715.6026(68)
B
(MHz)
1689.2727(17)
(MHz)
1523.9249(26)
(kHz)
1.98(11)
(kHz)
-2.53(32)
1SN(CH3)3-HF-HF
3715.7958(50)
1685.0955(40)
1520.5271(42)
1.76(14)
-2.53”
15N(CH3) 3-D F-D F
3697.168(18)
1654.2038(68)
1492.0680(71)
1.75(34)
-2.53b
1s N(CH3)3-H F-D F
3714.415 (18)
1659.8927(64)
1499.7462(74)
1.82(34)
-2.5 3b
1sN(CH 3) 3-D F-H F
3698.373 (29)
1678.9714(43)
1512.3992(55)
1.72(13)
-2.53b
14N (C D 3) 3-H F-HF
3064.1919(76)
1492.8196(56)
1376.6634(65)
1.33(33)
-2.53b
15N13CsH3(CH 3)2-H F-HF
3702.87 (31)
1659.4433(22)
1497.6207(29)
1.36(51)
-2.53b
15N13C aH3(CH 3)2-H F-HF
3656.24(26)
1674.1768(20)
1514.4437(22)
1.71(66)
-2.53“
A
C
A3
A jk
XbirXcc
Xaa
(MHz)c
-1.2521(85)
(MHz)'
-2.618(19)
-1.173(27)
-2.676(82)
(a) V alues in parentheses are one standard error, (b) H eld fixed in fit. (c.) Quadrupole coupling constant for the 14N nucleus.
Structure Analysis
The structure analysis posed a number o f challenges, not the least o f which is the
shear number o f structural parameters necessary for full determination. In weakly
interacting species, constraining the monomer geometries to free monomer values
typically alleviates this problem.
However, given the strong interactions in this
complex, the inner HF is likely to be significantly altered from its monomer geometry,
thus, constraining the HF bond distance would not be appropriate in this case.
Furthermore, the calculations predict small, but very real changes in the
trimethylamine geometry in the complex, and constraining its geometry to free
monomer values would be equally inappropriate.
In order to accommodate the structural changes in the monomer units, the theoretical
changes from the ab initio calculation 2 7 were imposed on a number o f parameters in
the experimental structure. These values (tabulated in Table 2. 2) were held fixed
throughout the fits. Specifically, the HF bond distances in the inner and outer HF
were predicted to elongate by 0.1657
A and 0.0278 A, respectively. These values
were added to the experimental HF bond length3 8 for each o f the HF molecules in the
complex. In the trimethylamine, the positions o f the hydrogen atoms in the methyl
groups were treated in a similar manner, imposing the small, calculated changes in
bond distances and angles, on free trimethylamine.
The carbon-nitrogen distances
also were fixed at the theoretical differences, however, the bond angles were allowed
to float. As a check, the carbon-nitrogen distances were varied by 20% in the fits, to
52
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ascertain the effect o f these parameters on the structure. While this had little effect on
most o f the parameters, the values o f % and \\i did change somewhat outside the
experimental uncertainties. The value of % ranged from 30.4° to 33.5°, and \|/ ranged
from 129.0° to 132.7°.
Table 2. 2 Theoretical changes imposed on experimental structure.
Parameter3
rCaN
rCsN
rCaH22d
rCsHn
rCaH21e
rCsH12f
Ti
ZNCaH22g
ZN CsHn
ZN CaH2jh
z n c sh 121
rHFj
rHF2
Calculated
Changeb
+0.0219
+0.0338
+0.0001
-0.0053
-0.006
-0.0052
-1.0
-0.607
-1.740
-2.586
-0.995
+0.1657
+0.0278
Experimental
Monomer Value'
1.451(3)
1.451(3)
1.109(8)
1.109(8)
1.088(8)
1.088(8)
110.9(6)
111.7(4)
111.7(4)
110.1(5)
110.1(5)
0.925595
0.925595
Value Held Fixed
in Fit
1.473
1.483
1.109
1.104
1.082
1.083
109.9
111.1
110.0
107.5
109.1
1.091295
0.953395
(a) Bond lengths are in A, angles in degrees. See Figure 2. 2 for structural definitions, (b) Relative to
free monomers calculated at MP2/aug-c-avdz in Ref. 27. (c) N(CH3)3 parameters are from Ref. 39
and HF bond distance is from Ref. 38. (d) By symmetry, rCaH22=rCaH32. (e) By symmetry, rCaH2i =
rCaH2 3 = rCaH31 = rCaH33. (f) By symmetry, rCsHi2=rCsH13. (g) By symmetry, NCaH22=NCaH32. (h)
By symmetry, NCaH2i = NCaH23 = NCaH3, = NCaH33. (i) By symmetry, NCsH]2=NCsH i3.
Calculations indicate that the two HF molecules lie in a plane bisecting the CNC
angle. In order to test the validity o f constraining the geometry to contain this plane
of symmetry, the inertial defect, A, of the complex was compared to that of free
trimethylamine. The values are close, differing only by ~2 amu A2 (see Table 2. 3)
from the monomer. While it thus seems reasonable to assume this symmetry plane,
the HF molecules are likely to undergo some angular excursion. To account for this, a
53
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series of fits was performed, allowing the HF hydrogens to move out o f the plane.
Unfortunately, the experimental data do not allow for experimental determination of
these angles. Therefore, upper limits were established based on the 14(1)° excursion
angle, estimated by Legon and Rego , 4 0 o f the HF about its center o f mass in (CH3)3HF and the 10° angle in (HF)2 , given by Howard, et al . 41
Table 2. 3 Inertial defect (A) for (CH3)3N and (CH 3 ) 3N-HF-HF.
A (am u A2)
(CH 3 ) 3 N-HF-HF (calc)a
-104.114
(CH 3 ) 3 N-HF-HF (obs)b
-103.555
(CH 3 ) 3 Nc
-101.392
(a) Ref 27. (b) This work, (c) Ref. 37.
The structure, shown in Figure 2. 2, was determined through a series o f non-linear
least squares fits o f the rotational constants.
A total o f nine structure fits was
performed, allowing for out-of-plane angular excursion of the HF units, as given in
Table 2. 4. The final structure was based on an average o f the fitted values, and the
error in the average structure represents the spread in the fitted values from the nine
structural fits. Residuals in the rotational constants were less than 0.4%, and most
were less than 0.1% (~2 MHz). The largest errors were observed in the A rotational
constant o f the DF/HF and DF/DF isotopomers, which ranged from about 9 MHz to
about 15 MHz. Although the residuals in B and C of these species were somewhat
higher than most, they were generally less than 5 MHz ~(0.3%).
54
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Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Table 2. 4 Results from series of structure fits. Bond lengths are in angstroms, angles are in degrees.
rNH,
rFF
X
Tfi
72
r2
¥
O O P,8
O O P,8
Structure 1
1.22723(9)
2.6663(1)
31.907(4)
Structure 2
1.22258(9)
2.6654(1)
31.794(4)
Structure 3
1.22808(9)
2.6578(1)
31.685(4)
Structure 4
1.26697(9)
2.66889(1)
31.529(4)
Structure 5
1.26019(9)
2.6731(1)
31.511(4)
Structure 6
1.26697(9)
2.6689(1)
31.529(4)
25.11(2)
23.65(2)
23.52(2)
32.16(1)
18.719(5)
31.44(1)
32.16(1)
18.719(5)
17.95(6)
110.50812(8)
18.190(5)
131.022(4)
110.59851(7)
131.099(4)
46.086(9)
15
11
46.838(9)
0
11
Structure 7
1.22723(9)
2.6663(1)
31.907(4)
25.11(2)
Structure 8
1.22258(9)
2.6654(1)
31.794(4)
23.65(2)
Structure 9
1.22808(9)
2.6578(1)
31.685(4)
23.52(2)
Average
1.24(4)
2.67(2)
31.7(4)
26(9)
17.946(6)
18.804(5)
18.273(5)
110.52776(7) 111.09514(7) 111.19195(7) 111.09514(7) 110.50812(8)
131.022(4)
131.242(4)
131.388(4)
131.242(4)
130.968(4)
18.190(5)
18.273(5)
18.3(9)
110.59851(7)
131.099(4)
110.52776(7)
46.086(9)
-15
-11
46.838(9)
0
-11
110.7(7)
131.1(4)
45(4)
46.994(9)
-15
11
43.215(8)
15
0
43.531(8)
0
0
43.215(8)
-15
0
130.968(4)
46.994(9)
15
-11
(a) Values o f out-of-plane (OOP) angles o f HF moieties held fixed in fit. (b) Standard error in parentheses, (c) Error of
average structure given by spread o f averaged quantities (see text).
H
H 23
x-axis
K-y,
CO
CO
Figure 2. 2 Fitted structural parameters. Cs is the
carbon atom in the symmetry plane o f the complex.
Only one o f the Ca methyl groups is visible in the
perspective o f 2.a.
(b)
n /a
n /a
Discussion
While the structural changes in the geometry o f the N(CH 3 ) 3 moiety were relatively
small, the changes in v/ and Ti (which can be inferred from the relation o f the fitted
parameters V|/ = cos'1[cos(Ti)/cos(T2/2)]) were greater than predicted. Ti, which was
predicted to increase by only 0.2° actually increased by 1.0°. These differences are,
however, within the combined experimental errors o f the monomer and the trimer.
Similarly, the actual increase in V|/ is within experimental error o f the theoretical
increase.
The most significant structural feature of this complex is the short primary hydrogen
bond distance, which implies that the addition o f a single HF molecule has a
significant impact in promoting proton transfer in the (CH 3 ) 3 N-HF dimer.
Nonetheless, while the short primary hydrogen bond in (CH 3 ) 3 N-HF-HF and the large
binding energy o f this complex certainly suggests a strong interaction, it is not, in
itself, an indication o f proton transfer in a hydrogen halide system.
Because real
proton transfer is accompanied by the elongation o f the HF bond as it breaks, it is
necessary to use a method to evaluate the degree o f proton transfer that incorporates
both the decreasing N-H distance, and the elongation o f the HF bond as the proton is
transferred to the base.
56
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A number of methods have been proposed to this end,13’22b however, because our work
is primarily structural, the proton transfer parameter, pPT, is particularly useful since it,
too, is intrinsically structural in nature. Introduced by Kumig and Scheiner,.42 pPT is
defined as:
pPT= [ArHX-ArNH]
(2.1)
where ArHX is the change in the hydrogen halide bond distance relative to the free
monomer and ArNH is the change in NH distance relative to the covalent bond in a
fully protonated amine. When the “stretch” in the NH distance exceeds that in the HX
distance, pPx < 0, and the system is hydrogen bonded. In the limit o f proton transfer,
ArNH vanishes, and pPx > 0. If pPx = 0, the proton shared equally between the acid
and the base.
The plot in Figure 2. 3 illustrates the trend in pPT across a series o f amine - hydrogen
halide complexes. Experimental values are not available for all o f the complexes in
the plot, therefore, the values are from recent theoretical calculations.43 The figure
clearly demonstrates the increasing value o f pPT with increasing hydrogen halide
acidity, and with increased methylation on the base.
57
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Figure 2 . 3 Theoretical values of pPT for the series (CH3)nH3.nN-H X.a
0 .4
^ f! * " m
m
f. "
^
0.2
m ’
#«*
*
00
/
/
- O - HBr
Xc
- *
- HC1
£
a
'°'2
x
b /
»
-0 .4
*
;
M
-°-6
-
0.8
_<
/
*
<HF)>
M
~
-
*■
i * * * ’ ”
m !•
m- 9
*
"
-o - hf
~
*
....................................................
(a) Counterpoise corrected values at M P2/6-311++G(d,p) level o f theory from
Ref. 43. (b) Experimentally based value o f Ref. 12. (c) This work.
The experimental values o f pPT for NH 3 -HF-HF and (CH 3 )N-HF-HF, inserted in
Figure 2. 3 as stars, also are listed in Table 2. 5 for comparison. A few things are
immediately apparent from the plot. First, although methylation increases pPT for the
HF complexes (shown as open circles), these are still well within the hydrogenbonded regime.
For HBr, however, adding even one methyl group to the base
dramatically increases the degree o f proton transfer, and the methylamine-HBr
complex is closer to an ion pair than a hydrogen-bonded complex.
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Table 2. 5 Experimental values o f ppx for NH3-HF-HFa and (CH3) 3N-HF-HF.b
ArHF (A)
ArNH (A)
H3N-HF-HF
0.06
0.40
-0.34
(CH3)3N-HF-HF
0.17
0.21
-0.04
p PT
(A)
(a) Ref. 12. (b) This work
More significant, however, is the effect of adding a single HF molecule to the amineHF complexes.
While all o f the theoretical values o f ppx for the HF complexes fall
squarely in the hydrogen-bonded arena, concurring with known experimental
findings, 13 the additional HF molecule in NH 3 -HF-HF and (CH 3 ) 3 N-HF-HF make a
significant contribution to driving the hydrogen bond toward completion.
In
particular, ppx = -0.04 for (CH 3 )3 N-HF-HF indicates that this complex is near the
proton shared regime.
It is important to bear in mind that ppx in (CH 3 ) 3 N-HF-HF is not a wholly
experimental value. Instead, it is a combination o f experiment (ArNH) and theory
(ArHF). However, the proton transfer parameter serves to illustrate a trend, rather
than assign a rigid value. As noted above, the close agreement o f experimental results
with ab initio values helps to confirm the essential character o f this complex as having
a shared proton in the primary hydrogen bond position.
59
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Summary
The rotational spectra of eight isotopic forms o f (CH 3 )3 N-HF-HF have been observed
by Fourier transform microwave spectroscopy in order to investigate the effect of
microsolvation on the (CH3)3-HF dimer. The addition of a single HF molecule has
been shown to have a significant effect on the structure o f the dimer. In particular, the
primary N---H hydrogen bond is found to be 1.24(4) A, which, through analysis using
Kumig and Schemer’s proton transfer parameter, Ppt, 4 2 indicates the complex is in the
proton shared regime. Previous ab initio calculations are in close agreement with the
present experimental results.
Acknowledgements
This work was supported by the National Science Foundation, the donors o f the
Petroleum Research Fund, administered by the American Chemical Society, and the
Minnesota Supercomputing Institute.
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References
1
Latimer, W. M. and Rodebush, W. H., “Polarity and ionization from the standpoint
o f the Lewis theory of valence.” J. Am. Chem. Soc., 42, 359-362 (1920).
2
See, for example, a.) Jeffrey, G. A., An introduction to hydrogen bonding', Oxford
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3
a.) Alongi, K. S.; Dibble, T. S.; Shields, G. C.; Kirschner, K. N., “Exploration o f the
potential energy surfaces, prediction o f atmospheric concentrations and
prediction of vibrational spectra for the H 0 2 '"(H 2 0 )„ (n= l- 2 ) hydrogen bonded
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directional character o f the hydrogen bond in gas-phase dimers.” J. Mol.
Struct., 237, 1-18 (1990).
4
Borg, J.; Jensen, M. H.; Sneppen, K.; Tiana, G., “Hydrogen bonds in polymer
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5
Alberts, B., Molecular Biology o f the C e ll; Garland Publishing, New York 1994.
6
Vianello, R.; Kovacevic, B.; Ambrozic, G.; Mavri, J.; Maksic, Z., “Hydrogen boding
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61
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8
Issacs, E. D.; Shukla, A.; Platzman, P. M.; ITamann, D. R.; Barbiellini, B.; Tulk, C.
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6 6
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Li, J.-C.; Ross, D. K.; Hayes, M. I. B., “The existence of two H-bonds o f different
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12
Hunt, S.W.; Higgins, K.J.; Craddock, M.B.; Brauer, C.S.; Leopold, K.R. /'Influence
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Legon, A.C., “The nature o f ammonium and methylammonium halides in the vapor
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14
Schriver, L.; Schriver, A.; Perchard, J.P., “Spectroscopic evidence for proton
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15
(a) Biczysko, M.; Latajka, Z., “Accuracy o f theoretical potential energy profiles
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(b) Biczysko, M.;
Latajka, Z., “The influence o f water molecules on the proton position in H3 N HX (X=F, Cl, Br) complexes.” Chem. Phys. Lett., 313, 366-373 (1999).
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(a) Latajka, Z.; Scheiner, S.; Ratajczak, H., “The proton position in hydrogen
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Lett., 110, 464-468 (1984), and references
therein.
63
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17
Alkorta, I.; Rozas, I.; Mo, O.; Yanez, M.; Elguero, J., “Hydrogen bond vs. proton
transfer between neutral molecules in the gas phase.” J. Phys. Chem. A, 105,
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18
Brciz, A.; Karpfen, A.; Lischka, H.; Schuster, P., “A candidate for an ion pair in the
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19
Chaban, G.M.; Gerber, R.B.; Janda, K.C., “Transition from hydrogen bonding to
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anharmonic vibrational spectroscopy.” J! Phys. Chem. A, 105, 8323- (2001).
20
Heidrich, D., “Ion pair formation modelled by NH 3 (HF)„ (n = 3-5).” J. Mol. Struct.
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21
Goldfinger, P. and Verhaegen, G., “Stability o f the gaseous ammonium chloride
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22
(a) Ault, B. S; and Pimentel, G. C., “Infrared spectra o f the ammonia - hydrochloric
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(b) Ault, B.S.; Steinback, E.; Pimentel, G.C., “Matrix isolation studies of
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615-620(1975).
23
(a) Legon, A.C.; Wallwork, A.L.; Rego, C.A., “The rotational spectrum and nature
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6397-6407 (1990). (b) Howard, N.W.; Legon, A.C, “Nature, geometry, and
binding strength o f the ammonia-hydrogen chloride dimer determined from the
64
R eproduced with perm ission of the copyright owner. Further reproduction prohibited without permission.
rotational spectrum of ammonium chloride vapor.” J. Chem. Phys.,
4701 (1988).
8 8
, 4694-
(c) Legon, A.C.; Rego, C.A., “An investigation of the
trimethylammonium chloride molecule in the vapor phase by pulsed-nozzle,
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(1989). (d) Howard, N. W.; Legon, A. C., “An investigation o f the hydrogenbonded dimer H 3N-HBr by pulsed-nozzle, Fourier-transform microwave
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8 6
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(1987). (e) Legon, A. C.; Rego, C. A., “ 14N- and D-nuclear quadrupole
coupling in the rotational spectrum of (CH 3 ) 3 14 N-H(D)F: Modification o f the
electric field gradients at the N ad D nuclei.” Chem. Phys. Lett., 157, 243-250
(1989). (f) Barnes, A. J. and Legon, A. C., “Proton transfer in amine-hydrogen
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Comparison o f low temperature matrices with the gas
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24
(a) Andrews, L.; Wang, X., “Infrared spectra of the HsN-HBr complex in solid Ne,
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(d) Andrews, L.; Wang, X., “Infrared spectra o f the H 3 N-HI
complex in solid Ne, Ne/Ar Ar, Kr and N 2 .
Strong matrix effects on a
65
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hydrogen-bonded complex.” J. Phys. Chem. A, 105, 7541-7550 (2001). (e)
Barnes, A.J.; Latajka, Z.; Biczysko, M,, “Proton transfer in strongly hydrogenbonded molecular complexes: matrix effects.” J. M ol Struct., 614, 11-21
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25
(a) Bevitt, J.; Chapman, K; Crittenden, D.; Jordan, M.J.T.; Del Bene, J.E., “An ab
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“Unraveling Environmental Effects on Hydrogen-Bonded Complexes: Matrix
Effects on the Structures and Proton-Stretching Frequencies o f HydrogenHalide Complexes with Ammonia and Trimethylamine.” J. Am. Chem. Soc.,
122, 2101-2115 (2000). (d) Del Bene, J.E.; Jordan, M.J.T., “A comparative
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Jordan, M.J.T.; Gill, P.M.W.; Buckingham, A.D., “An ab initio study of
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3.” Mol. Phys., 92, 429-440 (1997).
26
(a) Chemg, B.; Tao, F.-M., “Formation of ammonium halide particles from pure
ammonia and hydrogen halide gases: A theoretical study on small molecular
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particles from HC1 and NH 3 vapors.” J. Chem. Phys., 110, 11121-11124
66
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(1999). (c) Li, R.-J.; Li, Z.-R.; Wu, D.; Hao, X.-Y.; Li, Y.; Wang, B.-Q.; Tao,
F.-M.; Sun, C.-C., “Density functional study o f structures and interaction
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27
Hunt, S. W., “Structural Studies o f Partially Bonded and Hydrogen Bonded
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Balle, T. J.; Flygare, W. H., “Fabry-Perot Cavity Pulsed Fourier Transform
Microwave Spectrometer with a Pulsed Nozzle Particle Source”, Rev. Sci.
Instrum., 52(1), 33-45 (1981).
29
(a) Phillips, J. A.; Canagaratna, M.; Goodfriend, H.; Grushow, A.; Almlof, J.;
Leopold, K. R., “Microwave and ab initio Investigation o f HF-BF 3 ”, J. Am.
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Dynamics o f Partially-Bound Molecular Complexes”, Ph.D. Thesis, University
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30
Hunt, S. W.; Brauer, C. S.; Craddock, M. B.; Higgins, K. J.; Nienow, A. M.;
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Concentric, Dual-Injection Nozzle Source." Chem. Phys., 305, 155-164
(2004).
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31
(a) Organic Synthesis, 2nd. Ed:, Wiley: New York 1964; Collect. Vol. 1. (b) Clippard,
P.H., Ph.D. Thesis, University of Michigan, 1969.
32
The carbon atoms in the methyl groups out o f the NFF plane are labeled Ca, and
termed the asymmetric carbons. The carbon in the NFF plane is labeled Cs,
and termed the symmetric carbon.
Refer to Figure 2.2 for a definition of
atomic coordinates.
33
One of the unassigned transitions found during the search for the 13C substituted
isotopomers yielded a line that corresponded to a c-type transition of the
asymmetric form o f
13
CH 3 (CH 3 )2 -HF-HF, however, given that other c-type
transitions could not be found to confirm its assignment, it was not included in
the fit.
34
Note that
13
CaH 3 ( 12 CH 3 )2 N-HF-HF was observed using
observed in natural abundance.
15
N ( 1 2 CH 3 )3 , with the 13C
Therefore, it was impossible to tell, under
these experimental conditions, on which isotopic form o f carbon the lines
depended.
35
Pickett, H. M., “The fitting and prediction o f vibration-rotation spectra with spin
interactions” /. Mol. Spectrosc., 148(2), 371-377 (1991).
36 See, fo r example, (a) Forest, S. E.; Kuczkowski, R. L., “The structures of
cyclopropane-amine van der Waals complexes.” J. Am. Chem. Soc., 118, 217224 (1996). (b) Odom, J. D.; Barnes, J. A.; Hudgens, B. A.; Durig, J. R.,
“Spectra and structure o f boron-nitrogen compounds. II. Infrared and Raman
spectra o f trimethylamine-borane.” J. Phys. Chem., 78, 1503-1509 (1974).
R eproduced with perm ission of the copyright owner. Further reproduction prohibited without permission.
37
Lide, D. R. and Mann, D. E., “Microwave spectra o f molecules exhibiting internal
rotation. III. Trimethylamine” J. Chem. Phys., 28(4), 572-576 (1958).
38
Lovas, F.J.; Tiemann, E.; Coursey, J.S.; Kotochigova, S.A.; Chang, J.; Olsen, K.;
Dragoset,
R.A., NIST Diatomic Spectral DataBase.
Available
<http://physics.nist.gov/PhysRefData/MolSpec/Diatomic/index.html>,
from:
and
references therein.
39
Wollrab, J.E.; Laurie, V. W., “Structure and conformation o f trimethylamine.” J.
Chem. Phys., 51, 1580-1583 (1969).
40
Legon, A. C. and Rego, C. A., “H 19F nuclear spin-nuclear spin coupling in the
rotational spectrum o f (CH 3 )3 15 N ' HF and the lengthening o f the HF bond.”
Chem Phys. Lett., 154(5), 468-472 (1989).
41
Howard, B. J.; Dyke, T. R. and Klemperer, W., “The molecular beam spectrum and
the structure of the hydrogen fluoride dimer.” J. Chem. Phys., 81(12), 54175425.
42
Kumig, I. J. and Scheiner, S., “Ab initio investigation of the structure o f hydrogen
halide-amine complexes in the gas phase and in a polarizable medium.” Int. J.
Quantum Chem., Quantum Biol. Symp., 14, 47 (1987).
43
Brauer, C. S.; Craddock, M. B.; Kilian, J.; Grumstrup, E. M.; Orilall, M. C.; Mo, Y;
Gao, J. and Leopold, K. R.’ “Amine -
hydrogen halide complexes:
Experimental electric dipole moments and theoretical decomposition o f dipole
moment and binding energies.” J. Phys. Chem. A, Web release: 07/29/2006.
69
R eproduced with perm ission of the copyright owner. Further reproduction prohibited without permission.
Appendix to Chapter 2
R eproduced with perm ission of the copyright owner. Further reproduction prohibited without permission.
Table A 2 .1 Observed Rotational Transitions1 for (CH 3) 3N-HF-HF.
J'
1
1
1
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
3
3
3
3
2
2
2
2
2
2
3
3
3
3
3
3
3
3
1
Kp'
0
0
0
1
1
1
1
1
0
0
0
0
0
1
1
1
1
1
0
0
0
0
1
1
1
1
1
1
1
1
1
1
0
0
0
0
K0’
1
1
1
2
2
2
2
2
2
2
2
2
2
1
1
1
1
1
3
3
3
3
2
2
2
2
2
2
3
3
3
3
3
3
3
3
F'
1
2
0
2
2
3
1
1
2
1
2
3
1
1
2
3
2
1
2
4
3
2
2
2
1
3
1
1
3
2
4
2
3
3
4
2
J"
0
0
0
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
2
2
2
2
1
1
1
1
1
1
2
2
2
2
2
2
2
2
Kp"
0
0
0
1
1
1
1
1
0
0
0
0
0
1
1
1
1
1
1
1
1
1
0
0
0
0
0
0
1
1
1
1
0
0
0
0
Ko"
0
0
0
1
1
1
1
1
1
1
1
1
1
0
0
0
0
0
2
2
2
2
1
1
1
1
1
1
2
2
2
2
2
2
2
2
F"
1
1
1
2
1
2
0
1
2
0
1
2
1
1
1
2
2
0
1
3
2
2
2
1
0
2
2
1
2
1
3
2
3
2
3
2
Obs.
3212.872
3213.247
3213.812
6260.465
6260.680
6261.093
6261.150
6261.646
6416.209
6416.329
6416.576
6416.650
6417.270
6591.027
6591.363
6591.734
6591.970
6592.485
7729.858
7730.033
7730.154
7730.836
8286.777
8287.159
8287.172
8287.408
8287.756
8288.135
9385.393
9385.449
9385.531
9386.424
9600.285
9600.720
9600.791
9601.398
FWHM
0.012
0.034
0.005
0.034
0.032
0.026
0.016
0.023
0.019
0.011
0.025
0.016
0.019
0.031
0.045
0.034
0.025
0.026
0.024
0.045
0.032
0.019
0.015
0.005
0.025
0.033
0.005
0.032
0.020
0.010
0.014
0.028
0.014
0.019
0.017
0.022
Obs.-Calc.
-0.00463
-0.00527
-0.00374
-0.01109
-0.00104
-0.00509
0.01370
-0.00262
-0.01085
-0.00323
-0.01949
. -0.00431
-0.00134
-0.00821
-0.01361
-0.00360
0.01285
-0.00151
-0.00399
0.00034
-0.00111
0.00643
-0.00560
0.00076
-0.01470
0.00340
0.00583
0.00919
0.00771
0.00619
0.00839
0.01361
0.00161
0.00215
0.00805
0.00569
All frequencies are in MHz.
71
R eproduced with perm ission of the copyright owner. Further reproduction prohibited without permission.
J'
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
Table A 2 .1 Observed Rotational Transitions for (CH3)3N-HF-HF, Continued.1
Obs.Obs.
FWHM
Kp’ Ko' F' J" Kp" Ko" F"
Calc.
2
2
3
2
2
1
2
9639.107
0.035
-0.01927
2
2
4
2
2
1
3
9639.529
0.030
0.00030
2
2
2
2
2
1
1
9639.739
0.028
-0.01329
2
1
2
2
2
2
0
9677.676
0 .0 1 1
0.00005
2
1
3
2
2
0
2
0.041
9677.770
-0.01687
2
1
2
2
2
0
3
9678.090
0.041
-0.02041
2
1
4
2
2
0
3
9678.132
0.025
-0.00712
2
1
3
2
2
0
3
9678.224
0.005
0.00266
2
1
2
2
2
0
1
0.025
9678.343
-0.00872
1
2
2
2
1
1
2
9880.912
0.013
-0.00700
1
1
2
3
2
1
2
9881.064
0.019
-0.00678
4
1
2
2
1
1
3
9881.172
0 .0 2 0
-0.00586
1
1
2
2
2
1
1
9881.276
0.026
0.01560
1
2
0
3
3
2
3
11255.532
0.015
0.01843
1
3
3
2
0
2
2
11255.952
0.005
0.00396
1
4
2
0
2
3
3
11256.260
0.005
-0.01290
1
2
0
2
1
11256.314
3
2
0.017
0.01671
Table A2. 2 Observed Rotational Transitions1 for (CH 3) 3 15N-HF-HF.
J'
Kp’
Ko'
J"
Kp"
K„"
1
0
1
0
0
0
1
1
1
0
0
0
2
1
2
1
1
1
2
1
0
1
1
1
1
1
0
3
2
1
2
2
1
2
1
0
1
3
3
3
3
1
3
3
2
1
2
2
0
2
1
2
2
1
1
1
3
2
0
2
2
2
3
2
2
1
1
1
0
4
0
4
3
0
3
Obs.a
3205.622
5236.335
6246.620
6401.586
6575.777
7702.730
8277.342
9364.003
9578.483
9857.352
11239.747
12667.91100
12727.93690
FWHM Obs.-Calc.
0.025
0.00677
0.040
0.01423
0.025
-0.01094
0.016
-0 . 0 0 1 2 2
0.016
0.00986
0.028
-0.00643
0.017
0.00603
0.015
-0 . 0 0 2 1 2
0.026
-0.00309
0 .0 2 0
-0.00736
0.055
-0.00647
0.015
-0.00271
0 .0 2 0
0.00583
1 All frequencies are in MHz.
72
R eproduced with perm ission of the copyright owner. Further reproduction prohibited without permission.
Table A2. 3 Observed Rotational Transitions1 for (CH 3) 315N-DF-DF.
J'
1
2
2
2
3
2
3
3
3
3
K P’
Ko'
1
1
0
1
0
1
1
0
2
1
1
2
2
1
3
2
3
3
2
2
J"
0
1
1
1
2
1
2
2
2
2
Kp"
Ko"
0
1
0
1
1
0
1
0
2
1
0
1
1
0
2
1
2
2
1
1
Obs.
5189.243
6130.350
6283.213
6454.651
7511.624
8173.309
9189.788
9401.746
9438.661
9675.866
FW H M
0.030
0.040
0.015
0.030
0.019
0.049
0.024
Obs.-Calc.
0.00940
-0.01185
-0.00253
0.01753
0.00079
-0.02205
-0.01177
0.00747
-0.02588
-0.00870
0.011
0.048
0.017
Table A2. 4 Observed Rotational Transitions1 for (CH 3 ) 3 15N-DF-HF.
J'
2
2
2
3
2
3
3
3
3
4
4
4
K p'
Ko'
1
0
1
0
1
1
0
2
1
0
1
0
2
2
1
3
2
3
3
2
2
4
4
4
J"
1
1
1
2
1
2
2
2
2
3
3
3
K p"
Ko"
1
0
1
1
0
1
0
2
1
1
1
0
1
1
0
2
1
2
2
1
1
3
3
3
Obs.
6216.128
6372.805
6549.291
7671.884
8235.527
9318.069
9534.614
9573.960
9817.430
11022.030
12413.213
12668.219
FW H M
0.050
0.015
0.030
0.041
0.045
Obs.-Calc.
0.00407
0.00395
0.02251
-0.00184
-0.00392
-0.00880
-0.00172
-0.02660
0.00278
-0.00255
0.00888
-0.00551
0.020
0.011
0.041
0.024
0.035
0.014
0.014
Table A2. 5 Observed Rotational Transitions 1 for (CH 3 ) 3 15N-HF-DF.
J'
1
2
2
2
3
2
3
3
3
3
1
Kp’
Ko’
1
1
0
1
0
1
1
0
2
1
1
2
2
1
3
2
3
3
2
2
J"
0
1
1
1
2
1
2
2
2
2
Kp"
Ko"
0
1
0
1
1
0
1
0
2
1
0
1
1
0
2
1
2
2
1
1
Obs.a
5214.160
6159.070
6310.216
6479.381
7539.516
8213.617
9233.037
9442.904
9478.770
9713.158
FW H M
Obs.-Calc.
0.020
0.050
0.015
0.025
0.034
0.035
0.017
0.010
0.055
0.020
0.00037
-0.01337
-0.00164
0.00445
0.00695
0.00584
-0.00231
0.00173
-0.01145
-0.00454
All frequencies are in MHz.
73
R eproduced with perm ission of the copyright owner. Further reproduction prohibited without permission.
Table A2. 6 Observed Rotational Transitions1 for 13CaH3(CH3) 215N-HF-HF.a
J'
K P’
Ko'
J"
KP"
Ko"
2
1
2
1
1
1
2
0
2
1
0
1
2
1
3
3
3
4
4
4
5
1
1
1
0
2
1
2
0
3
3
2
0
2
1
2
2
1
1
4
4
3
5
3
3
3
4
1
3
3
1
1
0
1
0
0
1
2
0
4
Obs.
FW H M
6217.462 -0.00115
6367.921 0.00498
6536.909 -0.02046
9320.440 -0.01739
9528.801 -0.00141
9799.341 0.02107
12417.032 0.00437
12663.167 -0.00173
13053.935 -0.00817
15765.611
0.01298
Obs.-Calc.
0.00472
0.00821
-0.01582
-0.01289
0.00116
0.02379
0.00228
-0.00006
-0.00266
-0.01226
(a) Substituted carbon is on the asymmetric (out-of-plane) methyl group (bisecting NFF plane).
Table A2. 7 Observed Rotational Transitions 1 for 1 3 CSH 3 (CH 3 ) 2 1SN-HF-HF a
J"
KP''
2
1
0
1
2
1
2
0
3
3
2
0
2
1
2
2
1
1
4
4
5
3
3
4
1
0
3
3
4
J'
Kp'
Ko'
2
0
3
3
3
4
4
5
1
1
0
1
1
Ko"
Obs.
6304.863
9222.705
9434.309
9707.845
12286.757
12537.314
15343.026
FW H M
0.013
0 .0 1 1
0.008
0.009
0 .0 1 1
0.009
0.009
Obs.-Calc.
0.01356
-0.00538
0.00546
-0.00314
-0.00609
-0.00372
0.00354
(a) Substituted carbon is on the symmetric (in-plane) methyl group (in NFF plane).
1
All frequencies are in MHz.
R eproduced with perm ission of the copyright owner. Further reproduction prohibited without permission.
Table A2. 8 Observed Rotational Transitions1 for (CD 3) 3N-HF-HF.
J'
Kp’
Ko’
F'
J"
KP"
Ko"
F"
_ _
_
2
1
1
1
1
2
1
2
3
1
1
1
2
2
0
2
2
1
0
1
2
2
0
2
2
1
0
1
1
2
0
2
3
1
0
1
2
2
0
2
1
1
0
1
1
2
1
1
2
1
1
0
1
3
1
1
0
2
0
_
1
1
2
1
1
1
1
1
0
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
1
3
3
2
1
2
3
2
1
2
2
0
3
3
3
3
3
3
3
3
1
2
1
2
1
2
1
1
0
0
0
2
2
1
2
1
4
3
3
4
2
1
2
2
0
2
3
3
2
0
2
2
2
0
2
3
2
2
2
0
2
2
2
1
1
2
2
1
1
2
2
3
4
2
1
1
3
1
2
2
2
1
1
1
1
2
3
3
4
2
1
1
3
2
0
2
2
2
0
2
3
2
2
0
2
1
3
1
1
0
2
1
3
3
3
2
1
1
1
FW H M
_
2
1
Obs.
5622.491
5622.864
5732.335
5732.712
5732.772
5733.327
5854.805
5855.140
5855.854
8429.615
8430.204
8430.277
8430.369
8583.177
8583.583
8583.648
8584.213
8778.299
8778.458
8778.576
8778.644
8778.708
9891.474
9891.836
9891.914
10569.267
0.048
0.023
0.058
0.049
0.035
0.037
0.027
0.065
0.026
0 .0 2 1
0.024
0.025
0.029
0.034
0.075
0.040
0.017
0.030
0.053
0.034
0.060
0.043
0.025
0.060
0.040
0 .0 2 2
1 All frequencies are in MHz.
R eproduced with perm ission of the copyright owner. Further reproduction prohibited without permission.
Obs.Calc.
0.00678
-0.01334
-0.00910
0.01597
0.02094
-0.00209
0.00843
0.00763
-0.01036
0.00671
-0.02296
0.00173
0.01205
0.01246
0.01150
0.01552
0.00974
0.00411
-0.02947
-0.01039
-0.02661
-0 . 0 2 1 1 0
-0.02381
-0.00351
0.03855
0.00228
Table A2. 9 Cartesian coordinates for final (average) (CH 3) 3N-HF-HF structure.
Atom
N
C
H
H
H
C
H
H
H
C
H
H
H
H
F
H
F
X
-0.074971
0.762464
0.989403
1.682709
0.212484
0.762464
0.989403
0.212484
1.682709
-1.055564
-2.088048
-0.895679
-0.895679
-0.944712
-1.281941
0.061090
0.987360
Y
0.000000
1.211667
1.540021
0.971464
1.987600
-1.211667
-1.540021
-1.987600
-0.971464
0.000000
0.000000
-0.889998
0.889998
0.000000
0.000000
0.000000
0.000000
z
0.865821
0.865821
1.900609
0.349900
0.349900
0.865821
1.900609
0.349900
0.349900
1.978086
1.588046
2.573733
2.573733
-0.003920
-1.041803
-2.228519
-2.454319
R eproduced with perm ission of the copyright owner. Further reproduction prohibited without permission.
T able A 2 . 10 Unassigned transitions dependent on (C IU ^N and H F.1
va
3212.839
3212.884
3212.895
6261.615
6261.678
6263.262
6272.780
6272.804
6272.821
6272.901
6272.942
6273.135
6273.164
6453.679
6590.971
9169.396
9169.415
9341.229
9341.364
9392.330
9400.706
9400.800
9406.740
9406.760
9406.870
9411.355
9411.383
9411.510
9639.237
9639.389
9639.473
9639.645
9639.772
9665.138
9665.205
In ten sity
2.7
5.4
4.1
28.6
26.9
18.0
1 0 .0
29.0
19.8
14.7
8 .0
2.9
9.0
17.9
17.7
23.8
52.0
14.3
51.4
38.0
49.3
34.6
57.0
54.1
80.3
26.7
38.0
71.3
vb
6356.695
6394.849
6394.853
9306.837
9353.880
9354.024
9424.157
9424.222
9448.842
9461.224
9461.660
9461.884
12641.867
In ten sity
477.0
53.0
50.0
938.0
1750.0
711.0
1009.0
1127.0
54.0
84.0
62.0
32.0
1 0 .0
Vc
6215.648
6306.826
6306.881
6362.437
6362.463
6399.610
9222.110
9222.392
9315.934
9316.162
9430.386
9431.075
9436.097
9511.871
9522.394
9522.428
9796.152
9796.418
12534.121
In ten sity
56.0
82.0
32.0
130.0
56.0
87.0
38.0
25.0
8 .0
1 2 .0
501.0
1046.0
26.0
70.0
24.0
17.0
23.0
25.0
24.0
6 .0
26.0
2 2 .0
9.0
1 0 .0
13.0
22.9
(a) Observed w ith parent isotopic form s o f N (C H 3 ) 3 and HF. (b) Observed w ith parent
form o fN (C H 3)3 and DF/HF mixture, (c) O bserved w ith 15N (C H 3 ) 3 and HF.
1 All frequencies are in M Hz and intensities are in arbitrary units.
77
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Chapter 3
Dipole Moments of Amine Hydrogen Halide Complexes
C. S. Brauer, M. B. Craddock,* E. M. Grumstrup, M. C. Orilall/
and K. R. Leopold
Department o f Chemistry
University o f Minnesota
Minneapolis, MN
*Department o f Chemistry
Columbia University
New York, N Y
fDepartment o f Chemistry
Cornell University
Ithica, N Y
Reproduced in part with permission from The Journal o f Physical Chemistry A, Web release
date: 07/29/2006. © Copyright 2006 American Chemical Society.
R eproduced with perm ission of the copyright owner. Further reproduction prohibited without permission.
Abstract
Amine hydrogen halide complexes are prototypical systems in which to study the
transfer o f a proton across a hydrogen bond.
(4.05865 ± 0.00095 D), and
15
The dipole moments of:
15
NH 3 3 5 C 1
N(CH 3 ) 3 3 5 C1 (7.128 ± 0.012 D) were determined via the
Stark effect using a pulsed nozzle Fourier transform microwave spectrometer.
The
results are discussed in term of the degree of proton transfer, as previously elucidated
by Barnes and Legon [Barnes, A.J. and Legon, A.C., J. Mol. Spec. 448, 101 (1998).]
and should be o f interest in view of a large body o f matrix isolation work in which the
interaction between the complex and the host matrix has been investigated.
79
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Introduction
Among the widely studied hydrogen bonded systems are the highly prototypical amine
hydrogen halides . 1' 2 0 Early experimental work on these systems involved mass
spectrometric detection by Goldfinger and Verhaegen . 1 This work was followed by the
matrix isolation work of Pimentel and coworkers . 2 Later contributions were made by
Andrews, 3’4 Barnes 5 ,6 and others . 7 ' 9 In the analogous gas phase systems, Legon
contributed to this field through a series o f microwave studies. 8
This body o f work gave us insights into the nature of hydrogen bonding in these
systems, and the early matrix infrared work o f Pimentel, et al2b helped to establish that
proton transfer in hydrogen bonded complexes such as these depends on the difference
between the proton affinities o f the base and the acid anion. This concept suggested a
continuum o f hydrogen bond types. Amine hydrogen halides indeed display a range of
bonding, from the gas phase NH 3 HF and NH 3 HCI, which can best be described as
having a traditional hydrogen bond where the HX bond is only slightly elongated, to
(CH 3 ) 3N+HT, which, as Barnes and Legon8a showed, even in the gas phase, exists as an
ion pair.
There is a great deal of evidence from matrix isolation work that these types of
complexes are sensitive to the local environment and a number o f recent experimental4 ,6
and theoretical12,13 studies have focused on understanding these matrix effects. Much
of the recent work done in our laboratory has focused on this type o f bonding,7,21, and
80
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for some time, we have been interested in acid base reactions . 2 2 One o f the fundamental
properties that one can measure in these types o f systems is the dipole moment and,
when speaking o f proton transfer, it is an essential piece of information to have, as it is
a fundamental measure of charge distribution. Given this, the goal o f this study is to
determine the dipole moments o f NH 3 -HCI and (CH 3 )3 -HC 1, in order to extract
information as to how the dipole moments of these complexes change as the degree of
proton transfer increases.
81
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Experimental
Spectra were recorded using a pulsed nozzle Fourier transform microwave
spectrometer, 23 the details o f which have been described in detail elsewhere 2 4 A pair of
rectangular aluminum Stark plates (30 x 40 cm), which operate in a bipolar
configuration, are installed inside the instrument cavity, such that, when equal and
opposite dc voltages are applied to each plate, a uniform electric field is produced
perpendicular to both the cavity axis and the molecular source. The J = 1 <— 0, K=0
transition o f OCS (|X = 0.71521(20) D ) 2 5 was used to calibrate the effective spacing
between the Stark plates, as described elsewhere . 2 6 In order to account for effects due
to diffusion pump oil accumulating on the plate surfaces,
97
the distance between the
plates was calibrated both before and after experimental data were collected. Data were
used in the analysis only if the calibrations agreed.
The nitrogen atom in each complex was substituted with
15
N, thus reducing each to one
quadrupolar nucleus and simplifying the spectra and analysis considerably.
15
NH 328 and
15
N(CH 3 ) 32 9 were synthesized as described previously.’ Briefly,
produced by reacting solid KOH with solid
15
N(CH 3) 3 was synthesized by heating
15
producing (CH 3 ) 3 15N+C1\ from which free
15
15
Both
NH 3 was
NH 4 C 1 (Icon Isotopes, 99.5 atom %).
NH 4 C 1 and paraformaldehyde (Aldrich),
15
N(CH 3 ) 3 was released on addition o f a
concentrated NaOH solution.
82
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The complexes were formed in a manner similar to previously reported,21b,3° by pulsing
a 0.5% mixture o f 15NH 3 or 15N(CH 3 ) 3 in argon, at a stagnation pressure o f 15 psig,
through a pulsed nozzle (General Valve, Series 9) containing an 0.8 mm pinhole, at a
repetition rate o f 5 Hz.
Neat HC1 was flowed through 0.016” inner diameter (ID)
hypodermic tubing at 5 standard cubic centimeters per minute (seem) and entered the
expansion approximately 0.244” below the nozzle oriface.
Results
Four previously assigned hyperfine components o f 15NH3-H35C1 (three in the J = 1 <— 0,
K=0 transition and one in the J = 2 <— 1, K±1 transition),80 and three o f (CH 3 )3 15NH35C1 (J = 2 <— 1, K=0)8d were observed at a series o f field strengths. The maximum
shift obtained was about 6 MHz, with a voltage range o f 20 to 5000 V. A total o f 68
f..........
8209
8209.2
8209.4
8209.6
8209.8
8210
8210.2
8210.4
F requency (MHz)
Figure 3. 1 J = 1<—0, F = 3/2<—1/2 transition o f 15NH3-H35C1 at 87.2 V/cm. The
unlabeled peaks to the left o f the labeled transitions are instrumental artifacts.
83
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Stark-shifted transitions for 15NH3-H35C1 and 74 Stark-shifted transitions for (CH 3 ) 3 15NH35C1 were measured.
The J = 14—0, F = 3/2<—1/2 transition o f 15NH3-H35C1 with a field of 80.1 V/cm applied
is shown in Figure 3. 1 and the J = 2<—1, F = 7/2<—5/2 and F = 5/24-3/2 transitions of
15N(CH3)3-H35C1 at 25.3 V/cm is shown in Figure 3. 2. The 15N(CH3)3-H35C1 spectra
are much denser, owing, in part, to the higher rotational level, which has more
hyperfine components that are much more closely spaced.
F = 7/24—5/2
M f = 5/24-3/2
F = 7/24-5/2
M f = 7/24-5/2
F
= 1/24-1/2 ^
7184.0
7184.5
7185.0
7185.5
F r e q u e n c y (M H z)
Figure 3. 2 J = 24-1, F = 7/24-5/2 and F=5/2.4—3/2 transitions o f 15N(CH3)3-H35C1 at
25.3 V/cm. The Stark effect o f the F=1/24-1/2 transition was not measured, but is
visible in this spectrum. The small peaks between the F=l/24—1/2 and F=5/24—3/2
transitions are instrumental artifacts.
Most o f the observed Stark shifts in the K = 0 components of both complexes displayed
the expected linear relationship with the field squared (S 2), however, four o f the
84
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components in 15N(CH3)3-H35C1 deviated considerably from second order behavior. A
plot o f these shifts (Av) vs. 5, shown in Figure 3. 3, illustrate the deviation from second
order behavior. Similarly, the two Stark components measured in the J = 2<—1, K = ± 1
of N H 3 -H C I deviate from the expected first order behavior. This behavior is likely due
to the similarity between the Stark energies and the quadrupolar energies in these
complexes.
Given this, a treatment beyond second order perturbation theory is
necessary to accurately analyze the 15N(CH3)3-H35C1 complex. Therefore, spectra were
analyzed using the program QSTARK,31 developed by Zbigniew Kisiel, which is
available on the PROSPE Database.32 This program performs a least squares fit by
direct diagonalization o f the full energy matrix:
H = H, + H q + H e
j)
which is constructed in the |I,J,F,K,M[> basis, with matrix elements described
elsewhere.33,34,35
The agreement between the observed and calculated shifts is excellent and the data
were fit to within their experimental uncertainties. This is evident in Figure 3. 3 and,
though not shown, also follows with all o f the Stark components.
A table o f all
measured transitions is provided in the Appendix to this chapter.
85
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0 .4 5
0
20
40
60
80
£ (V/cm)
Figure 3. 3 Stark effect for several components o f (CHs^N-HCl that do not conform to
second order behavior. Observed are represented as points and calculated as smooth
curves.
The dipole moments o f these and two related complexes21b,3° measured concurrently
with this study are presented in Table 3. 1, along with theoretical dipole moments
obtained from a study done in collaboration with Gao, Mo and Kilian. Theory differs
anywhere from roughly 15 to 22%, which, although done at the HF level o f theory, is
comparable to other studies at higher levels o f theory.12
86
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Table 3. 1 Experimental and theoretical dipole moments o f 15NH3H35C1 and
15N(CH3)3H35C1.
11(D)
H in d ( D ) f
Exp.a
Theoryd,e
% Diff
Exp.
4.05865(95)
4.78
17.8
1.4787(16)
15N(CH3)3H3SC lb
7.128(12)
8.72
22.3
5.408(16)
15NH3H79Brc
4.2577(22)
4.92
15.6
1.9591(27)
8.397(14)
9.77
16.4
6.958(17)
Complex
°N H3H3SCIb
15N(CH3)3H79Brc
(a) V alues in parentheses represent the standard error in the least squares fit, but w hen errors
in the calibration are included, the absolute uncertainties are betw een 0.20% and 0.26% . (b)
This w ork (c) Ref. 21b (d) Ref. 30 (e) Calculated at the H F/aug-cc-pVDZ level/basis set. (f)
Ref. 36.
Also included in Table 3. 1 are the induced dipole moments, defined as:
A (X ind = |X complex — f t amine
” HhX
(3. 2)
This quantity is especially useful in comparing trends across a series o f complexes that
are composed of monomers with diverse dipole moments.
Discussion
While the substantial induced dipole moments o f all four complexes listed in Table 3. 1
suggest strong interactions, those of the trimethylamine complexes, (CH3)3N-HC1 and
(CH3)3N-HBr, are much larger than the ammonia complexes, NH3-HC1 and NH3-HBr,
even though the dipole moment of trimethylamine is much smaller than that of
87
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ammonia.
This suggests that the induced m om ents are due to m ore than simple
polarization. As these com plexes are in varying stages o f proton transfer, it is useful to
find a succinct w ay to describe, in simple term s, proton transfer in a way that can be
related to the m olecular dipole moment. A lthough a more rigorous treatm ent is offered
elsewhere,30 in very general term s, if we can think o f proton transfer across a hydrogen
bond in an am ine - hydrogen halide com plex in term s o f the distance the proton m ust
travel from the hydrogen bonded or proton shared position to the bond distance in an
ion, then the change in the dipole m om ent, A pmd, can be given as:
Ajijnd = qAd
(3. 3)
where q is simply the charge on the proton and Ad is the change in the proton position.
The degree o f proton transfer, then, should be given by Ap/qAd. O f course, this is a
gross oversim plification, as this change in distance is not the only contribution to the
dipole moment.
Indeed, the induced m om ent is com prised o f several components,
including proton transfer, but polarization and m onom er distortion also contribute.
Figure 3. 4 graphically illustrates a portion o f the results o f a theoretical analysis
recently perform ed by Kilian, Mo and Gao on a series o f amine - hydrogen halide
com plexes,30 w hich used the Block-Localized W avefunction (BLW )41 m ethod to break
the induced dipole m om ents (Apmd) dow n into their polarization, distortion and charge
transfer com ponents, viz.,
88
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Ajiind —A|x,iist + AjXpoi + A|1 ct
(3.4)
where Ajidist is the contribution from each monomer’s distortion, A|ipoi is the
contribution from polarization o f each monomer, and A|ici arises from charge transfer
in the complex. In the figure, each column represents the total Apmd o f an amine hydrogen halide complex and the contribution from the individual components are
shown as different shaded blocks within the columns.
ApcT
Appoi(amine)
■ Ap-dist(amine)
m A p pol(HX)
I i A p dis,(HX)
HC1 Complexes
HBr Complexes
Figure 3. 4 Contribution o f distortion, polarization and charge transfer to the
induced dipole moments (Apmd) o f four amine-HX complexes from BLW
decomposition analysis.30 Units are in Debye. Complexes shown are, from left
to right, NHs-HCl, N(CH3)3-HC1, NH3-HBr, and N(CH3)3-HBr.
It is readily seen from the figure that monomer polarization (shown in dark grey and
light grey patterned in the figure) makes up the bulk o f the induced dipole moments of
all four complexes. However, charge transfer contributes as well, though to a greater
89
R eproduced with perm ission of the copyright owner. Further reproduction prohibited without permission.
extent for the trimethylamine complexes. This comes as no surprise, as trimethylamine
is a stronger base. Monomer distortion is the smallest component and, distortion o f the
amine (shown in black in the figure) is actually slightly negative.
The small
contribution o f hydrogen halide distortion, which is nearly zero for the ammonia
complexes, concurs with experimental and theoretical consensus8,30 that there is little
change to the HX geometry on complexation with N H 3 .
The BLW decomposition helps to confirm that the dipole moments o f these systems are
sufficiently complicated to require a more in-depth treatment than the simple A|ijnd =
qAd treatment used earlier. In order to analyze the trends in these amine hydrogen
halide complexes, we can find in the literature a number o f existing methods that
researchers have devised to assess the degree o f proton transfer.
In their analysis of amine-hydrogen halide complexes, Barnes and Legon used the
quadrupole coupling constant o f the halide (%h x ) to establish the fraction of ionic
character,/ in the complex. This is defined as:
(3.5)
where
% (X )hb
is the quadrupole coupling constant o f the limiting case o f hydrogen
bonding, established as that o f HCN-HX, and x(X)iP is in the limiting case o f an ion
90
R eproduced with perm ission of the copyright owner. Further reproduction prohibited without permission.
pair, defined as the quadrupole coupling constant o f N a+X \ The quadrupole coupling
constant o f the amine hydrogen halide is %(X)c.
Barnes and Legon calculated / for
N H 3 -HX and (CH 3 ) 3 -HX, X = Cl, Br, I and the result is shown in Figure 3. 5. The
hydrogen bonding lim it o f H CN-H X is shown to the far left o f the plot, and to the far
right, is the proton transfer limit o f N a+X \
HCN
HC1
HBr
HI
Ion
Figure 3. 5 Percent ionic character, / for N H 3-FfX and (CH3) 3-HX, X = Cl, Br, I from
Ref. 8.
While the degree o f proton transfer increases across the series, HC1, HBr, HI, for both
amines shown in the figure, the am m onia complexes all lie squarely in the hydrogen
bonded arena.
The degree o f proton transfer for the trim ethylam ine com plexes,
however, is significant across the series. A plot o f the induced dipole m om ent vs. the
percent ionic character (J) is shown in Figure 3. 5. It displays a rapid, nearly linear,
increase in Ajlind w ith in c re a s in g /
91
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SJ -
N(CH3)3 -HBrb
10 SJ ^
5.0
&
4J
J
3.0
2.0
f* *
11
NH 3-H B rb
N H j - H C l"
OJ <
0
11
21
30
50
40
Sfl
TO
30
90
% Io n ic C h a r a c t e r (/)
Figure 3. 6 [iind vs. % Ionic Character, (a) This work, (b) Ref. 21b.
A similar trend can be seen in com paring o f A pmci to Pim entel’s N orm alized Proton
Affinity Difference, A,3 w hich is defined as:
A _ PA(B) - PA(A')
~~ PA(B) + PA(A‘)
(3 .6)
where PA(B) is the proton affinity o f the base and PA(A ') is that o f the anion in the
acid.
W hen PA(A ') is significantly larger than PA(B), A approaches -1, w hich
corresponds to the hydrogen bonded limit.
Full proton transfer is associated w ith a
value o f A = +1. Like the correlation w i t h / a plot o f | l md vs. A, shown in Figure 3. 7,
displays a rapid increase in |ijnd w ith increasing A.
92
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SJ7.0
6J
5.0
s 4J
"■4P
■1.0
2.0
1. 0 -
m
415
N(CH3)3 -H B i*
N(CH3) 3-HCF
• ..........*"
NHr H tla
4.24
41S
NH,-HBrb
4.22
4.21
41
4.11
4.18
4.1?
A
Figure 3. 7 jjiind vs. N orm alized Proton A ffinity Difference, A. (a) This work, (b) Ref.
21b.
Sum mary
Dipole m om ents o f 15NH3-H35C1 and 15N(CH3)3-H35C1 have been determ ined by using
the Stark effect. B oth com plexes show a significant dipole m om ent inducement, w hich
is especially noticeable in 15N(CH3)3-H35C1, where A |imci is over 5 Debye. Analysis o f
these, along w ith 15N H 3 -H79Br and 15N (C H 3 ) 3 -H79Br,21 indicate that these induced
dipole m om ents parallel established measures o f proton transfer in amine hydrogen
halide systems., in that, as the degree o f proton transfer increases, Ajimd increases as
well.
93
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Acknowledgements
This work was supported by the National Science Foundation, the donors o f the
Petroleum Research Fund, administered by the American Chemical Society, and the
Minnesota Supercomputing Institute. Jacob Kilian, Yirong Mo and J. Gao also are
gratefully acknowledged for their theoretical work.
94
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Reference
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and
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Legon, A. C., “An investigation o f the hydrogen-bonded dimer HsN-HBr by
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Clementi, E., “Study of the electronic structure of molecules. II. Wavefunctions for
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11
(a) Biczysko, M.; Latajka, Z., “Accuracy o f theoretical potential energy profiles
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(b) Biczysko, M.;
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Latajka, Z., “The influence of water molecules on the proton position in H 3 N HX (X=F, Cl, Br) complexes.” Chem. Phys. Lett., 313, 366-373 (1999).
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(a) Bevitt, J.; Chapman, K; Crittenden, D.; Jordan, M.J.T.; Del Bene, J.E., “An ab
initio study o f anharmonicity and field effects in hydrogen-bonded complexes of
the deuterated analogues o f HC1 and HBr with NH 3 and N(CH 3 ) 3 .” J. Phys.
Chem. A, 105, 3371-3378 (2001). (b) Jordan, M.J.T.; Del Bene, J.E.,
“Unraveling Environmental Effects on Hydrogen-Bonded Complexes: Matrix
Effects on the Structures and Proton-Stretching Frequencies o f Hydrogen-Halide
Complexes with Ammonia and Trimethylamine.” J. Am. Chem. Soc., 122, 21012115 (2000). (d) Del Bene, J.E.; Jordan, M.J.T., “A comparative study of
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Gill, P.M.W.; Buckingham, A.D., “An ab initio study o f anharmonicity and
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(a) Chemg, B.; Tao, F.-M., “Formation o f ammonium halide particles from pure
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(2001). (b) Tao, F.-M., “Direct formation of solid ammonium chloride particles
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R.-J.; Li, Z.-R.; Wu, D.; Hao, X.-Y.; Li, Y.; Wang, B.-Q.; Tao, F.-M.; Sun, C.C., “Density functional study o f structures and interaction hyperpolarizabilities
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o f NH 3 -HC1-(H2 0)„ («=0-4) clusters.” Chem. Phys. Lett., 372, 893-898 (2003).
(d) Snyder, J.A.; Cazar, R.A.; Jamka, A.J.; Tao, F.-M., “Ab Initio Study o f GasPhase Proton Transfer in Ammonia-Hydrogen Halides and the Influence of
Water Molecules.” J. Phys. Chem. A, 103, 7719-7724 (1999).
14
(a) Latajka, Z.; Scheiner, S.; Ratajczak, H., “The proton position in hydrogen halideamine complexes. BrH-NH 3 and BrH-NH 2 CH3.” Chem. Phys. Lett., 135, 367372 (1987). (b) Latajka, Z.; Scheiner, S., “Ab initio comparison o f H bonds and
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15
Alkorta, I.; Rozas, I.; Mo, O.; Yanez, M.; Elguero, J., “Hydrogen bond vs. proton
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16
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17
Corongiu, G.; Estrin, D.; Murgia, G.; Paglieri, L.; Pisani, L.; Valli, G.S.; Watts, J.D.;
Clementi, E., “Revisiting the potential energy surface for [H3N
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119-134(1996).
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18
Chaban, G.M.; Gerber, R.B.; Janda, K.C., “Transition from hydrogen bonding to
ionization in (HC 1)„(NH3 )„ and (HC 1)„(H2 0 )„ clusters: Consequences for
anharmonic vibrational spectroscopy.” ./ Phys. Chem. A, 105, 8323- (2001).
19
Kumig, I.J.; Scheiner, S., “Ab initio investigation o f the structure o f hydrogen halideamine complexes in the gas phase and in a polarizable medium.” Int. J.
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20
Heidrich, D., “Ion pair formation modelled by NH 3 (HF)„ (n = 3-5).” J. Mol. Struct.
(Theochem.), 429, 87-94 (1998). (b) Heidrich, D.; van Eikema Hommes, J.R.;
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models
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21
See, fo r example, (a) Chapter 2 of this document, “Microwave and ab initio
investigation of (CH 3 ) 3 N —HF” HF.” (b) Craddock, Matthew B., “Microwave
Spectroscopic Studies o f Weakly-Bound and Hydrogen-Bonded Molecular
Complexes.” Ph. D Thesis, University o f Minnesota, 2005.
22
(a) Hunt, S. W.; Brauer, C. S.; Craddock, M. B.; Higgins, K. J.; Nienow, A. M.;
Leopold, K. R., "Microwave Observation o f H 3N-SO 3 -H 2 O Using a Concentric,
Dual-Injection Nozzle Source." Chem. Phys., 305, 155-164 (2004). (b) Fiacco,
D.L.; Leopold, K.R., "Partially Bound Systems as Sensitive Probes of
Microsolvation: A Microwave and Ab Initio Study o f HCN-HCN-BF 3 ." J. Phys.
Chem. A, 107, 2808-2814, (2003). (c) Hunt, S.W.; Fiacco, D.L.; Craddock, M.;
Leopold, K.R., "Correlation o f Dative Bond Length and Donor Proton Affinity
R eproduced with perm ission of the copyright owner. Further reproduction prohibited without permission.
in Adducts of S03: A Good Predictor for HCCCN-SO 3 ." J. Mol. Spec., 212,
213-218 (2002). (d) Fiacco, D.L.; Mo, Y.; Hunt, S.W.; Roberts, A., “Dipole
Moments o f Partially Bound Lewis Acid-Base Adducts." J. Phys. Chem. A, 105,
484-493 (2001).
23
Balle, T. J.; Flygare, W. H., “Fabry-Perot Cavity Pulsed Fourier Transform
Microwave Spectrometer with a Pulsed Nozzle Particle Source”, Rev. Sci.
Instrum., 52(1), 33-45 (1981).
24
(a) Phillips, J. A.; Canagaratna, M.; Goodfriend, H.; Grushow, A.; Almlof, J.;
Leopold, K. R., “Microwave and ah initio Investigation o f HF-BF 3 ”, J. Am.
Chem. Soc., 117(50), 12549 (1995). (b) Phillips, J. A., “Structure and Dynamics
o f Partially-Bound Molecular Complexes”, Ph.D.
Thesis, University of
Minnesota, 1996.
25
Muenter, J. S., “Electric dipole moment of carbonyl sulfide”, J. Chem. Phys., 48,
4544-4547 (1968).
26
Canagaratna, M.; Ott, M. E.; Leopold, K. R., “Determination o f the Dipole Moment
o f H 3 N -SO 3 in the Gas Phase”, Chem. Phys. Lett.. 281(1), 63-68 (1997).
27
Coudert, L. H.; Lovas, F. J.; Suenram, R. D,; Hougen, J. T., “New Measurements of
Microwave Transitions in the Water Dimer”, J. Chem. Phys., 87(11), 6290-6299
(1987).
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28
Fiacco, D.L., “Microwave Investigation o f Partial and Hydrogen Bonded Molecular
Complexes.” Ph.D. Thesis, University of Minnesota, Minneapolis, 2001.
29
(a) Organic Synthesis, 2nd. Ed.', Wiley: New York 1964; Collect. Vol. 1. (b) Clippard,
P.H., Ph.D. Thesis, University of Michigan, 1969.
30
Brauer, C. S.; Craddock, M. B.; Kilian, J.; Grumstrup, E. M.; Orilall, M. C.; Mo, Y;
Gao, J. and Leopold, K. R., “Amine -
hydrogen halide complexes:
Experimental electric dipole moments and theoretical decomposition o f dipole
moment and binding energies.” J. Phys. Chem. A, Web release: 07/29/2006.
31 See, fo r example, a) Kisiel, Z.; Kosarzewski, J.; Pietrewicz, B. A.; Pszczolkowski, L.,
“ Electric Dipole Moments of the Cyclic Trimers (H 2 0 ) 2 HC1 and (H 2 0 ) 2HBr
from Stark Effects in their Rotational Spectra”, Chem. Phys. Lett., 325, 523-530
(2000), b) Kisiel, Z.; Pietrewicz, B. A.; Fowler, P. W.; Legon, A. C.; Steiner, E.,
“ Rotational Spectra of the Less Common Isotopomers, Electric Dipole Moment
and the Double Minimum Inversion Potential of H 2 0-HC1”, J. Phys. Chem. A.,
104, 6970-6978 (2000).
32
Z.
Kisiel,
PROSPE -
Programs
for
Rotational
Spectroscopy,
http://info.ifpan.edu.pl/~kisiel/prospe.htm.
33
Keenan, M. R.; Wozniak, D. B.; Flygare, W. H., “Rotational Spectrum, Structure,
and Intramolecular Force Field of the ArClCN van der Waals Complex”, J.
Chem. Phys., 75(2), 631-640 (1981).
34
Read, W. G.; Flygare, W. H., “The Microwave Spectrum and Molecular Structure of
the Acetylene-HF Complex”, J. Chem. Phys., 76(5), 2238-2246 (1982).
R eproduced with perm ission of the copyright owner. Further reproduction prohibited without permission.
35
Benz, H. P.; Bauder, A.; Gunthard, Hs. H., “Exact Quadrupole Interaction Energies in
Rotational Spectra”, J. Mol. Spectrosc., 21(1-4), 156-164 (1966).
36 Rind
=
^complex
-
( |! hx
+ Ramine).
Values used to calculate these quantities are:
(i( 14NH3) = 1.47149(15) D , 37 |i(!5 N(CH3)3) = 0.612(3) D , 3 8 (t(HCl) = 1.1086
D , 39 and |i (H 79 Br) = 0.8271(3) D .4 0
37
Marshall, M.D.; Muenter, J.S. J. Mol. Spectrosc. 1981, 85, 322. Note that although
the value for
14
NH 3 is used here, little error is expected to be incurred in
calculating |0 ,jnd for the
38
15
NH 3 complexes.
Lide, D.R.; Mann, D.E., “Microwave Spectra o f Molecules Exhibiting Internal
Rotation. III. Trimethylamine.” J. Chem. Phys., 28, 572-576 (1958).
39
Kaiser, E.W., “Dipole Moment and Hyperfine Parameters o f H 3 5 C1 and D 3 5 C1.” J.
Chem. Phys., 53, 1686-1703 (1970).
40 Van Duk, F.A.; Dymanus, A., “Hyperfine and Stark spectrum o f DBr in the
millimeter-wave region.” Chem. Phys., 6 , 474-478 (1974).
41
(a) Mo, Y; Peyerimhoff, S.D., “Theoretical analysis o f electronic delocalization.” J.
Chem. Phys., 109, 1687-1697 (1998). (b) Mo, Y.; Gao, J.; Peyerimhoff, S.D.,
“Energy decomposition analysis o f intermolecular interactions using a blocklocalized wave function approach.” J. Chem. Phys, 112, 5530-5538 (2000). (c)
Mo, Y.; Zhang, Y.; Gao, J., “A simple electrostatic model for trisilylamine:
Theoretical examinations o f the n—>G* negative hyperconjugation, p„—>(1*
bonding, and stereoelectronic interaction.” J. Am. Chem. Soc., 121, 5737-5742
103
R eproduced with perm ission of the copyright owner. Further reproduction prohibited without permission.
(1999). (d) Mo., Y.; Gao, J., “Polarization and charge-transfer effects in Lewis
acid-base complexes.” J. Phys. Chem. A, 105, 6530-6536 (2001).
104
R eproduced with perm ission of the copyright owner. Further reproduction prohibited without permission.
Appendix to Chapter 3
R eproduced with perm ission of the copyright owner. Further reproduction prohibited without permission.
Table A 3.1 Observed Transitions1 o f 15NH 3-H 5C1 at Non-Zero Electric Field.
R otational
T ransition
Mf "
F'
Mf '
Obs.
Obs.-Calc.
E (V/cm)
0
0
0
0
0
0
0
1.5
1.5
1.5
1.5
1.5
1.5
1.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
8208.483
8209.143
8209.795
8210.003
8210.216
8210.696
8210.964
-0.003
-0.007
-0.008
-0.001
-0.003
-0.002
0.001
0.00
62.25
87.16
93.41
99.61
112.1
118.3
0
0
0
0
0
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
8209.147
8209.803
8210.005
8210.226
8210.693
-0.004
-0.002
-0.002
0.004
-0.009
62.25
87.15
93.39
99.61
112.1
0
0
0
0
1.5
1.5
1.5
1.5
_
0.5
0.5
0.5
1.5
1.5
1.5
1.5
0.5
0.5
0.5
8187.060
8187.553
8187.776
8188.329
0.002
-0.003
0.001
0.002
0.00
62.25
74.74
99.61
0
0
0
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
0.5
0.5
0.5
8187.558
8187.776
8188.333
0.001
0.000
0.002
62.25
74.71
99.61
0
0
1.5
1.5
0.5
0.5
1.5
1.5
1.5
1.5
8188.225
8189.102
0.007
0.002
74.71
99.61
0
0
0
0
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
8187.865
8188.222
8189.102
8192.254
-0.003
0.001
-0.001
0.011
62.25
74.74
99.61
161.8
0
0
0
0
0
0
0
0
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
_
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
_
8198.969
8199.167
8199.348
8199.762
8200.316
8200.756
8201.266
8201.819
0.009
0.005
-0.010
-0.006
-0.002
-0.004
0.007
0.006
0.00
31.12
43.61
62.25
80.92
93.41
105.8
118.3
-
0.5
0.5
0.5
0.5
0.5
0.5
0.5
-
0.5
0.5
0.5
0.5
0.5
0.5
0.5
1 All frequencies are in MHz.
106
R eproduced with perm ission of the copyright owner. Further reproduction prohibited without permission.
Table A 3 .1 Observed Transitions1 of 15NH3-H35C1 at Non-Zero Electric Field, Cont.
Rotational
K F"
F' Mf'
Obs.a
Obs.-Calc. E (V/cm)
Transition
1.5 2.5 0.5
0 1.5
8199.168
0.005
31.13
0-*- 1
0 1.5
1.5 2.5 0.5
8199.771
0.002
62.25
0 1.5
1.5 2.5 0.5
8200.313
-0.007
80.94
1 —> 2
0
0
0
0
0
0
1.5
1.5
1.5
1.5
1.5
1.5
0.5
0.5
0.5
0.5
0.5
0.5
2.5
2.5
2.5
2.5
2.5
2.5
1.5
1.5
1.5
1.5
1.5
1.5
8199.130
8199.306
8199.666
8200.160
8200.557
8201.029
-0.004
0.003
0.003
0.002
-0.006
-0.005
31.13
43.60
62.26
80.94
93.40
105.9
0
0
0
0
0
0
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
2.5
2.5
2.5
2.5
2.5
2.5
1.5
1.5
1.5
1.5
1.5
1.5
8199.130
8199.306
8199.656
8200.557
8201.035
8201.577
-0.004
0.002
-0.008
-0.010
-0.001
0.004
31.12
43.61
62.25
93.41
105.8
118.3
0
0
0
0
0
0
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
8199.076
8199.185
8199.420
8199.740
8199.993
8200.291
0.002
0.000
0.000
0.001
-0.004
-0.001
31.13
43.60
62.26
80.94
93.40
105.8
1
1
1
1
1
1
1
1
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
_
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
16386.735
16387.034
16387.095
16387.122
16387.159
16387.193
16387.217
16386.242
-0.009
0.004
0.008
-0.009
-0.008
-0.007
-0.009
-0.005
0.00
1.127
1.269
1.381
1.481
1.585
1.679
1.774
1
1
1
1
1
1
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
16386.730
16386.717
16386.713
16386.684
16386.665
16386.646
-0.004
-0.005
0.007
0.000
0.004
0.009
0.800
0.975
1.126
1.269
1.381
1.478
1 All frequencies are in MHz.
107
R eproduced with perm ission of the copyright owner. Further reproduction prohibited without permission.
Table A3. 2 Observed Transitions1 of (CH3)315NH3-H35C1 at Non-Zero Electric Field.
Rotational
Transition
1 -> 2
K
F"
Mf "
F'
Mf'
Obs
Obs-Calc
0
0
0
0
0
1.5
1.5
1.5
1.5
1.5
0.5
0.5
0.5
0.5
1.5
1.5
1.5
1.5
1.5
0.5
0.5
0.5
0.5
7188.668
7188.707
7188.768
7188.845
7188.950
0.001
-0.005
-0.001
-0.001
0.000
E
(V/cm)
0.0
12.47
18.78
24.83
31.09
0
0
0
0
0
0
0
0
0
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
7188.621
7188.569
7188.506
7188.429
7188.359
7188.297
7188.258
7188.254
7188.286
-0.002
-0.003
-0.002
-0.006
-0.005
-0.006
-0.005
-0.001
-0.002
12.47
18.70
24.79
31.03
37.18
43.43
49.61
55.93
62.09
0
0
0
0
0
1.5
1.5
1.5
1.5
1.5
0.5
0.5
0.5
0.5
0.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
7188.709
7188.766
7188.843
7188.947
7189.079
-0.003
-0.003
-0.005
-0.008
-0.009
12.47
18.70
24.79
31.03
37.18
0
0
0
0
0
0
0
0
0
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
7188.620
7188.569
7188.508
7188.438
7188.373
7188.326
7188.304
7188.324
7188.394
-0.003
-0.003
-0.002
-0.003
-0.003
-0.002
-0.003
-0.001
0.002
12.47
18.78
24.83
31.10
37.37
43.36
49.76
55.81
61.97
0
0
0
0
1.5
1.5
1.5
1.5
0.5
0.5
0.5
0.5
2.5
2.5
2.5
2.5
1.5
1.5
1.5
1.5
7184.807
7184.895
7184.987
7185.118
0.000
-0.017
-0.006
0.019
0.00
18.70
24.87
31.13
1 All frequencies are in MHz.
R eproduced with perm ission of the copyright owner. Further reproduction prohibited without permission.
Table A3. 2 Observed Transitions1 of (CH3)315NH3-H35C1 at Non-Zero Electric
Cont.
R otational
ObsF"
K
Mf "
F'
Obs
Transition
Calc
0
1.5
1.5
2.5
1 —»■2
1.5
7184.758
-0.006
0
1.5
2.5
1.5
1.5
7184.708
-0.008
0
1.5
1.5
2.5
1.5
7184.650
-0.003
0
1.5
1.5
2.5
1.5
7184.512
0.000
Field
E
(V/cm)
12.58
18.75
24.95
37.40
0
0
0
0
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
7184.749
7184.689
7184.609
7184.517
-0.005
-0.002
-0.001
0.001
12.43
18.70
24.87
31.13
0
0
0
0
0
0
0
0
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
3.5
3.5
3.5
3.5
3.5
3.5
3.5
3.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
7184.807
7184.783
7184.762
7184.724
7184.696
7184.680
7184.690
7184.739
0.000
0.000
0.006
-0.001
-0.001
-0.002
-0.003
-0.004
0.00
12.58
18.75
24.95
31.21
37.40
43.56
49.70
0
0
2.5
2.5
1.5
1.5
3.5
3.5
0.5
0.5
7184.820
7184.804
0.001
-0.005
24.87
31.13
0
0
0
0
0
0
0
2.5
2.5
2.5
2.5
2.5
2.5
2.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
3.5
3.5
3.5
3.5
3.5
3.5
3.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
7184.771
7184.728
7184.695
7184.639
7184.615
7184.602
7184.624
-0.005
-0.011
0.000
-0.011
-0.001
-0.001
-0.004
12.43
18.70
24.87
31.13
37.28
43.54
49.87
0
0
0
0
0
2.5
2.5
2.5
2.5
2.5
1.5
1.5
1.5
1.5
1.5
3.5
3.5
3.5
3.5
3.5
1.5
1.5
1.5
1.5
1.5
7184.809
7184.799
7184.787
7184.756
7184.715
0.002
-0.003
-0.002
-0.005
0.004
12.58
18.75
24.95
31.21
37.40
1 All frequencies are in MHz.
109
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Table A 3.1 Observed Transitions1 of (CH3)315NH3-H35C1 at Non-Zero Electric Field
Cont.
R otational
ObsE
K
F"
F*
Obs
Mf "
Mf '
T ransition
Calc
(V/cm)
0
3.5
1 —> 2
2.5
1.5
2.5
7184.792
0.000
12.43
0
2.5
3.5
2.5
7184.767
-0.001
1.5
18.70
7184.726
0
2.5
1.5
3.5
2.5
-0.003
24.87
0
2.5
1.5
3.5
7184.672
2.5
0.004
31.13
0
0
0
2.5
2.5
2.5
2.5
2.5
2.5
3.5
3.5
3.5
2.5
2.5
2.5
7184.857
7184.922
7185.135
-0.003
-0.003
-0.001
12.58
18.75
31.22
0
0
0
0
0
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
3.5
3.5
3.5
3.5
3.5
3.5
3.5
3.5
3.5
3.5
7184.833
7184.872
7184.923
7184.989
7185.067
-0.004
-0.001
-0.001
-0.001
-0.002
12.43
18.70
24.87
31.13
37.28
1 All frequencies are in MHz.
110
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Chapter 4
Stark Effect Measurements on the H 2 SO4 -H 2 O Complex
C. S. Brauer, Galen Sedo, and K. R. Leopold
Department o f Chemistry
University o f Minnesota
Minneapolis, M N
A portion of the following has been submitted for publication in Geophysical Research
Letters, September, 2006.
R eproduced with perm ission of the copyright owner. Further reproduction prohibited without permission.
Abstract
Sulfuric acid water mixtures provide important model systems for understanding
nucleation rates. An understanding o f these rates, in turn, is critical to fully elucidating
the role o f sulfuric acid aerosols in the atmosphere. Though a variety o f mechanisms for
particle growth have been proposed, none have been shown to be universal under
atmospheric conditions.
The problem is made difficult by the strong tendency of
sulfuric acid to form gas phase hydrates, and thus accurate information about
H2SO4/H2O clusters is of considerable interest.
In this work, Stark effect measurements have been performed on three rotational
transitions o f the H2SO4-H2O complex. Results for the two tunneling states, labeled A
and B, are: pa = 2.630(20), |itot = 3.024(94), and (ia = 2.6402(40), p tot = 3.052(15),
respectively. Due to high correlation between |Ub and |ic, these two components were
difficult to determine independently, however, a series o f least square fits shows the
values o f |ia, as well as the total value of the dipole moment, jitot to be well determined
from the data.
A variety o f theoretical calculations also have been performed on H2SO4-H2O and its
constituent monomers to ascertain the relationship between the torsional angle o f the
unbound proton o f the H2SO4 unit to the dipole moment, as well as to determine the
most accurate method o f predicting the dipole moment and to examine the relationship
between theoretical structure and dipole moments.
112
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Introduction
Sulfuric acid is one of the principle sources of atmospheric aerosols.1 The impact of
these aerosols is varied, affecting the health o f the Earth as well as its inhabitants. By
scattering radiation, sulfate aerosols may have a mitigating affect on global warming.1
Yet at the same time, they may indirectly affect global climate by acting as cloud
condensation nuclei (CCN),2'5 altering the number and size o f cloud droplets and
ultimately increasing the cloud albedo.1,6 Furthermore, atmospheric aerosols are
implicated in the destruction of ozone by providing surfaces on which heterogeneous
reactions can occur, as well as in their role in the formation o f polar stratospheric clouds
(PSC).1
Sulfate aerosols are ubiquitous in the atmosphere, forming in both the troposphere7'9
and stratosphere,10 in the marine boundary layer,11' 13 and in remote Arctic regions14' 16
as well as in polluted urban air.
17
Despite intensive study, the mechanisms for aerosol
formation are still uncertain. A number o f theories for gas-to-particle conversion have
been proposed, including:
Binary Homogeneous Nucleation (BHN),18'23 Ternary
Homogeneous Nucleation (THN),5,24-29 Ion Induced Nucleation30'34 and Ion Mediated
Nucleation.35'37 While none have been shown to be universal under atmospheric
conditions, models based on each theory have been applied to successfully predict
nucleation rates under varying conditions.
113
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Historically, the mechanism for sulfate particle formation was thought to be binary
homogeneous nucleation (BHN) via H 2 SO4 and water vapor . 19,3 8 However, binary
nucleation theory alone has been insufficient in predicting nucleation rates with a
reasonable degree of uncertainty, and models often fall far short o f the experimentally
observed values . 5,39
Sulfuric acid’s affinity for water, and its propensity to form hydrates precludes the use
of classical binary nucleation theory without correcting for hydration to estimate
nucleation rates . 19,20,40,41 Developed by Heist and Reiss in 1974,40 the classical hydrates
interaction model for sulfuric acid/water nucleation accounted for the reduced
nucleation rates attributed to the stabilization o f the vapor by the hydrates. This model
was refined in 1987 by Jaecker-Voirol, et al . 4 2 and further developed by Noppel, 4 3 ,4 4 to
resolve thermodynamic inconsistencies in the original model.
BHN theory tends to break down in areas where the concentration o f H2SO4 vapor is
low, such as in the remote troposphere or the marine boundary layer . 2 6 In this case, one
viable alternative to BHN is Ternary Homogeneous Nucleation theory (THN), which
incorporates a third molecule, such as ammonia, to the H2SO4/H2O system . 2 4 Ammonia
is thought to stabilize the critical embryos, increasing the nucleation rate and bringing
the model more in line with observed nucleation rates . 6
In addition to homogeneous nucleation, aerosols also may form by heterogeneous
nucleation - that is via condensation on an existing particle or prenucleation embryo.
114
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In that respect, ion-induced nucleation (UN) may be considered a form of
heterogeneous nucleation, where ionic clusters (or neutral clusters formed from the
recombination of charged clusters) serve as embryos around which sulfuric acid/water
vapor condenses. The presence o f ions has been shown to enhance nucleation rates by
several orders o f magnitude, 4 5 ’33 likely due to the thermodynamic advantage provided
by the electrostatic interaction between the electrically charged embryo and the
condensing vapor. 32 Though UN models have been effective in explaining nucleation
events in the middle troposphere,
problematic
global application o f the theory remains
46
One limitation o f classical UN is that it does not allow for the interaction between
colliding particles and assumes the vapor concentration in the vicinity o f the growing
particle or cluster is unaffected by the surrounding field 4 7 Since the presence o f polar
monomers is thought to enhance growth rates4 8 ’4 9 UN tends to underestimate nucleation
rates of small cluster ions that are composed o f polar molecules.
Ion Mediated
Nucleation (IMN ) , 37 which was developed as an enhancement to classical UN, accounts
for this interaction through inclusion of the dipole-charge interaction.
Although the importance o f sulfuric acid hydrates has been evident for quite some time,
the gas-phase structure o f H2SO4-H2O wasn’t determined until 2002 . 5 0
In this
microwave spectroscopic study, Fiacco and coworkers observed two rigid rotor states
(labeled A- and B-states) in the rotational spectrum, likely due to internal motion on the
water unit.
Although appreciable values were predicted for both the a- and c115
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components o f the dipole moment (pa and pc), only a-type spectra were assigned. A
thorough analysis o f eighteen isotopically substituted species allowed for an accurate
determination of the structure. However, the dipole moment was not experimentally
determined at that time.
Given the potential importance o f the dipole charge interaction in predicting nucleation
rates using IMN theory, it is perhaps surprising that only one theoretical study could be
found in the literature that specifically address the dipole moment o f sulfuric acid
hydrates . 51 In it, DFT methods were used to predict the dipole moments o f gas phase
H 2 S0 4 -(H 2 0 )n (n=0-3). These dipole moments were found to be strongly dependent on
the structure o f the sulfuric acid monomer unit and, for the monohydrate, three minima
corresponded to different structures with dipole moments varying by nearly a Debye.
The impetus for the present study arose from the lack o f experimental dipole moments
of sulfuric acid hydrates, which are necessary in implementing IMN theory to
determine nucleation rates.
116
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Experimental
Spectra were recorded using
a pulsed nozzle Fourier transform microwave
spectrometer, 52 which has been described in detail elsewhere . 5 3 The instrument is
adapted with a pair of rectangular aluminum Stark plates (30 x 40 cm), which operate in
a bipolar configuration. The plates are installed inside the cavity such that, when equal
and opposite dc voltages are applied to each plate, a uniform electric field is produced
perpendicular to both the cavity axis and the molecular source.
assigned transitions (2 o2 <—loi,
strengths.
2n<—
lio and
2 | 2 <—
Three previously
In ) were observed at a series o f field
The J = 4<—3, K = ± 3 transition of Ar-SC>3 (p = 0.2676(3) D ) 5 4 was
calibrated to determine the effective spacing between the Stark plates, as described
elsewhere. 55 In order to mitigate effects due to diffusion pump oil accumulating on the
plate surfaces, 56 the distances between the plates were calibrated both before and after
experimental data were collected.
Data were used in the analysis only if the
calibrations agreed.
Due to its low vapor pressure, H2SO4 was formed in situ via the reaction between H 20
and SO3 using a co-injection source in a manner similar to previous studies . 5 0 Argon,
seeded with the vapor above solid, polymerized SO3, was introduced through the pulsed
valve at a stagnation pressure o f ~2.5 atm. Water vapor was then introduced through a
needle (0.023 in. ID) situated immediately below the pulsed nozzle, by flowing argon
117
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over liquid water at 5.0 standard cubic centimeters per second (seem). This process
formed the H 2 SO4 -H 2 O complex in the early stages o f the supersonic expansion.
Results
A sample o f the spectrum, showing the A and B tunneling states o f the J = 2 o2 <—loi
transition, with a field o f 25.8 V/cm applied is shown in Fig. 4.1. Little to no spectral
broadening was observed as the electric field was increased, however, the transition
B-State
A-State
M=0-0
7554
7554.3
M=l-1
7554.6
7554.9
Frequency (MHz)
Figure 4. 1 A portion o f the J = 2 o2
loi transition o f H 2 S 0 4 -H20 taken at 25.8 V/cm. The
two com ponents on the left are the M j = 0 <— 0 com ponents o f the B and A states,
respectively. The tw o components on the right are the M j = 1 <— 1 com ponents o f the B and A
states. The strong, sharp feature dividing the two sets o f transitions is an instrum ental artifact
and not a m olecular transition. The splitting in the A-state lines is not due to the electric field,
and also is present in the field free spectrum.
intensity diminished with increasing field strength, and this factor limited the degree to
which the lines could be shifted.
While both states were observed to shift with
increasing field strength, the A-state was much smaller in intensity than the B-state. As
118
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a consequence, fewer transitions were observed. The maximum Stark shift for the Bstate was - 6 0 0 kHz, with a maximum field o f - 8 0 V/cm for the 2 o2 <—loi transition,
- 7 5 0 kHz at - 6 . 2 V/cm for the 2 n < —1 io and - 7 6 0 kHz at - 6 . 2 V/cm for the
For the A-state, the maximum shift in the
2
2
i 2 <—I n .
o2 <—loi transition was - 4 0 0 kHz, at - 6 0
V/cm, - 6 0 0 kHz at - 5 .5 V/cm for the 2 n < —lio and - 5 0 0 kHz at - 5 .5 V/cm for the
2
i 2 <—I n -
Preliminary analysis o f the Stark shifted spectra was done via the method o f Golden and
Wilson 57 in which the component o f the Stark energy due to p g is given by the non­
degenerate perturbation expression:
r F (2>i
J
-
u,2r 2Y
^
JVr'
Fo _ Fo
Jt
(.
>1
n
JV
Where p g is the gth component o f the dipole moment, Oz g, the direction cosine between
the gth axis of the complex and the space-fixed z-axis, S is the field and J, Mj and x
have their usual meaning. The K=0 transitions (J = 2 o2 <—loi) were found to depend on
all three dipole moment components. However, the K=±l transitions (J = 2n<—li 0 and
J =
2
i2 <—I n ) were dominated by p a and were virtually independent o f pb and p c.
Furthermore, the Stark coefficients of pb and p c in the J =
2
o2 <—loi transitions were
nearly identical. This posed a challenge in trying to fit all three dipole moment
119
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components simultaneously and gave rise to high correlation coefficients between pb
and (J-c-
0.8
«M = 0-0
0.6
■ M = 0 -1
• Mss 1-1
» M=l-2
0.4
0.2
N
a
0.0 ^5
a
>
<
-
0.2
-
0.4
-
0 .6
■
2000
4000
6000
8000
10000
12000
S 1 (V/cm ) 2
Figure 4. 2 The measured Stark shifts for J = 2 02 <—loi vs. £ 2 o f the B-State.
Figure 4. 2 shows a plot o f the shift vs. S 2 for various components o f the J =
2 02
<—loi
transition. The Stark shifted lines display a linear dependence with the field squared,
which is consistent with the expected second order behavior o f an asymmetric top with
no accidental near degeneracies. 57 Similar behavior is observed in the J = 2n<—lio and J
= 2 i2 <—In transitions.
The final analysis o f the data was performed by fitting the observed Stark shifts using
the program QSTARK , 58 developed by Zbigniew Kisiel, which is available on the
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PROSPE Database.59 This program performs
a least squares
fit by
direct
diagonalization o f the full energy matrix:
H = Hr + Hq + He
(4. 2)
which is constructed in the |I,J,F,K,M f> basis, with matrix elements described
elsewhere.60'62 For H2S 0 4-H20 , H Q = 0.
A total o f 116 B-state and 91 A-state lines, including three field free transitions for each
state, were included in the fit and the results are shown in Table 4. 1. Tables o f the
measured Stark transitions are located in Table A4. 1 and Table A4. 2, in the appendix
to Chapter 4.
Table 4 .1 Experimentally determined dipole moments for A- and B-states.a
Ha (D)
A-State
2.630(20)
B-State
2.6402(40)
(hot (D)
3.024(94)
3.052(15)
(a) Values in parentheses are one h alf the total spread from the series o f fits listed in Table A4. 3
and Table A4. 4, and account for uncertainties in the calibrated plate spacing.
Because the dependence on jib and |lc was nearly identical in the available transitions,
the results o f these two components were highly correlated and were thus difficult to
determine independently.
Therefore, as a check, in addition to fitting all of the
components in the each state, a series of fits was performed, fixing either |Xb or |ic at a
variety o f values,63 and freeing the other two parameters. In each case, the value o f jra
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remained essentially the same and, although the value of the other floated parameter
changed, the overall dipole moment did not change significantly.
The relatively large uncertainties in the fitted values o f dipole moments are a reflection
of the lack o f available transitions that display a strong, uncorrelated dependence on pb
and |ic. A consequence of only having a-type spectra is that there is a large uncertainty
in the A rotational constant. Using Herb Pickett’s spectral fitting program, SPFIT , 6 4 the
published rotational transitions were fit and a value o f A=5053±157 MHz was obtained
for both the A- and B-states. As an additional check, Stark effect fits were performed,
using the range implied by the standard error in the A rotational constant. While this
resulted in variations for the individual values o f pb and p c, the values o f p a and ptot
remained relatively stable.
While the results reported in Figure 4. 1 are from least squares fits using the QSTARK
program, in which all of the dipole moment components are floated parameters, the
uncertainties are assigned based on the range of values obtained from the series of fits
detailed above, and account for uncertainties in the calibrated plate spacing. Results of
the various fits are summarized in Table A4. 3 and Table A4. 4 in the appendix to
Chapter 4.
Theoretical Methods and Results
The experimental dipole moment of H 2 SO 4 -H 2 O differs considerably from the only
theoretical value (ptof=2.147 D) in the literature . 51
While one cannot expect perfect
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agreement between theory and experiment, theoretical dipole moments are generally in
much better agreement than in this case . 3 1 ,6 5 This nearly 30% discrepancy was initially
a cause for concern, and was investigated using a number of theoretical methods.
While each o f the three local minima in the DFT study by A1 Natsheh, et al . 51 differed
somewhat with respect to the position o f the water relative to the sulfuric acid, the
SW-a
(0 , = 2.147 D
E tb = -1202.49 kcal/mol
SW-b
(X= 2.393 D
E tb= -1202.46 kcal/mol
SW-c
jx = 3.411 D
E tb= -1200.59 kcal/mol
Figure 4. 3 The three theoretical structures obtained by al Natsheh 51 for H2SO4-H2O,
using PW91/AVTZ level o f theory.
primary structural difference was in the torsional angle of the unbound proton on the
sulfuric acid (Figure 4. 3). The differences in the calculated dipole moments among the
three structures, however, were considerable, ranging from 2.147 D in the global
minimum structure to 3.411 D. To further investigate the possible dependence o f the
dipole moment on the structure o f the sulfuric acid unit, a theoretical study was
conducted in the present work to examine the change in the dipole moment with respect
123
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to this torsional angle.
A series o f geometry optimizations was performed using
Gaussian 0366 at the PW91/PW91/aug-cc-pVTZ level, fixing the O1-S-O2-H2 torsional
angle (see Figure 4. 6 ) at a variety o f positions to rotate the proton in increments o f 15°.
Both the sulfuric acid monomer and the monohydrate complex display a broad range of
dipole moments throughout the full 360° rotation of the unbound proton. The binding
energy, De, is determined by taking the difference between the energy o f the H2SO4H 2 O dimer in the rotated geometry and the sum o f the monomers in their normal
configuration (unrotated H2SO4). Figure 4. 4 shows a plot o f the De (open squares, left
axis) and dipole moment, jn, (closed circles, right axis) o f H2SO4-H2O as the torsional
angle is rotated through 360° from its equilibrium position. The dipole moment ranges
from 1.8 to 4.1 D, indicating that the structure of the sulfuric acid unit does, indeed,
strongly influence the dipole moment. However, given the substantial barrier to this
motion, it is unlikely that the discrepancy between the experimental and published
theoretical dipole moments is due entirely to the floppiness o f the complex.
Nonetheless, although these arguments do not account for possible large amplitude
zero-point torsional motion, the
1000
cm ' 1 barrier seemed large, and another
explanation for this discrepancy was sought.
Given that the published theoretical results used DFT, we decided to explore the effect
of different theoretical methods and basis sets on the dipole moments o f the complex
and its constituent monomers. Ab initio calculations were performed, again using the
Gaussian 03 package o f computer codes, 6 6 at the HF/aug-cc-pVTZ level, the MP3/augcc-pVDZ and at the MP2 level of theory, using a variety of basis sets listed in Table 4.
124
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2. The geometries were allowed to fully optimize in each case, except for the MP2/augcc-pVQZ level, in which it was held at the geometries optimized at the MP2/aug-ccpVTZ level.
4.1
- 10.00
3.9
3.7
- 10.50
3.5
11.00
3.3
- 11.50
2.9
-
«
2.7
2.5
- 12.00
2.3
-
12.50
-
13.00
Rotation Angle (°)
Figure 4. 4 Theoretical De (left axis) and p. (right axis) for H 2 SO4 -H 2 O with respect to
the rotation of the torsional angle at PW91PW91/aug-cc-pVTZ.
Table 4. 2 lists the theoretical values o f the total dipole moments (|Xtot) and the binding
energies, (De), defined as the difference between the energy o f the dimer and the sum of
the monomers, viz,
De = E(H2S 0 4-H20 ) - [E(H2S 0 4) + E(H20)]
(4. 3)
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for the complex and its constituent monomers at a variety o f levels o f theory and basis
sets, and the experimentally determined values. It is evident from these results that,
while the DFT method determines the values of the dipole moment o f the monomers
which is in good agreement with experiment, it does not agree with the experimental
results of the dimer.
Conversely, while the ab initio methods all approach the
experimental value of the dimer, their determinations o f the dipole moments o f the
monomers do not.
Table 4. 2 Experimental and theoretical dipole moments (|itot) and binding energies
(De) of H 2 S O 4 , H 2 O and H 2 S O 4 -H 2 O at various levels o f theory and basis sets.
PW 91/TZPa
HF/AVTZ
MP2/AVDZ
MP3/AVDZ
M P2/AVTZ
M P2/AVQZ
Experimental
H (D )
H 2 SO 4
11(D)
H20
2.817
3.475
3.311
3.340
3.422
3.417
2.725(15)°
1.813
1.940
2.017
2.005
1.994
1.957
1.8546(6)d
11(D)
H 2 SO 4 h 2o
2.147
3.428
3.003
2.954
2.970
2.965
3.052(15)°
lla(D)
H 2 SO 4 h 2o
1.971
2.966
2.890
2.615
2.614
2.611
2.6402(27)°
De
(kcal/mol)
H 2 SO 4 -H 2 O
" -_E..... "
-9.6471
-12.6864
-12.6079
-13.0152
-13.5955
-
(a) Ref. 51 (b) The total bonding energy ( E tb ) is reported as - 1 2 0 2 .4 9 kcal/mol. Binding
energy (De) is not given, (c) Ref. 67 (d) Ref. 68 (e) This work, B-state.
Table 4. 3 lists the structural parameters o f H2S 0 4, obtained from the various
calculations and compares them to the experimental structure. The atom numbering can
be found in Figure 4. 5. The mean unsigned errors in the bond distances (M UEr), the
bond angles (MUEZ) and the torsional angles (M UEd) also are given in Table 4.3. These
are found by taking the average o f the absolute values o f the differences between the
observed experimental and theoretical parameters, and they provide a means of
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comparing the
agreement between
the various
^ h1
theoretical methods with the experimental structure.
While all levels of theory reproduce the experimental
structures fairly well, the MP2/aug-cc-pVTZ results
°4
h 2H
are in closer agreement with respect to bond
distances and angles. Although the dihedral
angles ??
^ ^ ^ to m numbering of
X 120O 4.
are reproduced best by the HF/aug-cc-pVTZ level, it is possible that this agreement is
fortuitous, as the torsional angles are likely most susceptible to large amplitude motion.
However, from Table 4. 2, the “best” dipole moment for sulfuric acid is predicted by
the PW91/TZP level.
Table 4. 3 Experimental and theoretical structural parameters o f H 2 S O 4 .
PW91/
TZPa
HF/
AVTZ
MP2/
AVDZ
MP3/
AVDZ
MP2/
AVTZ
Exp.b
rChH,0
0.980
0.947
0.976
0.969
0.970
0.97(1)
rSOic,d
1.626
1.553
1.647
1.627
1.600
1.574(1)
rS 0 3c’e
1.441
1.400
1.467
1.455
1.434
1.422(1)
ZH A S^
106.5
111.0
106.3
107.3
107.6
108.5(15)
Z C hS O /
102.3
102.2
101.4
101.6
101.9
101.3(10)
Z 0 3S 0 4f
125.0
122.9
125.4
124.8
124.4
123.3(10)
c /H A S O /
-25.4
-23.6
-27.5
-26.6
-27.0
-20.8(10)
rftijO jS O /
85.1
88.6
82.8
84.3
84.0
90.9(10)
MUErc
0.027
0.022
0.041
0.029
0.013
M UE/
1.57
1.27
1.47
1.00
0.87
MUEdf
5.20
2.55
7.40
6.20
6.55
(a) Refs. 51 and 69 (b) Ref. 67 (c) Values in angstroms, (d) By sym m etry, rS 0 i= r S 0 2 (e)
By sym m etry, r S 0 3 = rS 0 4 (f) Values in degrees (g) By symmetry, Z H i 0 iS = Z H 2 0 2 S.
R eproduced with perm ission of the copyright owner. Further reproduction prohibited without permission.
The results are somewhat different in
the H 2 SO4 -H 2 O dimer, whose atom
numbering is shown in Figure 4.
6
Hi
.
Experimental and theoretical structural
parameters o f H 2 S 0 4 -H 2 0 , and the
mean unsigned errors are listed in Table
S
02
4. 4. Here, the theoretical method for
determining
the
“best”
structure
Figure 4. 6 Atom numbering o f H 2 SO 4 -H 2O.
(MP2/aug-cc-pVTZ) also reproduces the dipole moment closest to the experimental
value. In particular, while the MUE in the dihedral angles for this ab initio method is
larger than the PW91/TZP level, it performs the best at reproducing the torsional angle
of the unbound proton (-163.3°), which may be a key contributor to the dipole moment.
Nonetheless, the structural differences across the theoretical methods and basis sets are
small, particularly in light of the large gap between the largest and smallest calculated
values o f the dipole moments.
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Table 4. 4 Comparison o f experimental and theoretical structures o f H2S 0 4-H20 . a
PW 91/
T Z Pa
H F/
AVTZ
M P2/
AVDZ
M P3/
M P2/
AVDZ
AVTZ
Exp.b
rChfV
1.029
0.964
1.004
0.992
1.000
1.04(1)
r 0 2H2c
0.980
0.947
0.976
0.968
0.970
0.95e
rSOic
1.589
1.535
1.618
1.603
1.572
1.567(1)
rS 0 2c
1.629
1.555
1.649
1.629
1.602
1.578(3)
rSOsc
1.457
1.407
1.477
1.464
1.444
1.464(1)
rS 0 4c
1.443
1.403
1.468
1.456
1.435
1.410(4)
rf^C V
1.608
1.786
1.684
1.709
1.657
1.645(5)
Z H 10 1Sd
107.0
111.6
106.4
107.5
107.8
108.6e
Z H 20 2Sd
106.2
110.9
106.3
107.2
107.5
108.5e
Z O iS O /
103.2
102.9
102.4
102.5
102.8
101.8(2)
Z 0 3S 0 4d
122.7
121.8
123.6
123.2
122.7
123.3e
Z O jS O /
109.0
109.0
109.5
109.3
109.2
106.71(6)
Z 0 2S 0 3d
104.2
105.9
103.9
104.6
104.4
104.71(9)
Z O ^O s"
165.6
165.7
165.6
164.7
163.3
162.2(5)
c/H j O jS O /
-17.7
-23.0
-25.0
-24.6
-22.7
-13.7(5)
r/H20 2S 0 3d
-157.4
-159.2
-165.0
-162.8
-163.3
-163.4(5)
cM30 50 !S d
115.4
137.8
127.0
127.7
125.8
121.0e
MUErc
0.027
0.048
0.042
0.038
0.021
MUEz d
1.729
2.140
1.757
1.200
1.043
MUEdd
5.200
10.100
6.300
6.067
4.633
(a) Refs. 51 and 69 (b) Ref. 50 (c) Units are in A. (d) Units are in degrees (e) Inferred from
experimental structure.
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Discussion
It often is useful to examine the induced dipole moment, p md, in order to ascertain the
degree to which the dipole moment is enhanced when a complex forms. In this case,
however, since |ib and |ic are not well determined, it is impossible to know the direction
of the dipole moment, and (iind, therefore, cannot be determined experimentally.
However, given the degree o f convergence o f the ab initio calculations, we can use the
MP2/aug-cc-pVTZ results to approximate the induced dipole moment.
Figure 4. 7
c -a x is
Ms a
a -a x is
Figure 4. 7 O rientation o f m onom er dipoles in the geom etry o f the H2S 0 4-H 20 complex,
their sum (p sum), the total dipole m om ent (ptot) and the enhancem ent due to com plexation (g md)
from M P2/aug-cc-pV TZ calculations.
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shows the MP2/aug-cc-pVTZ dipole moments o f the sulfuric acid and water monomers
in the complex geometry, their sum (|ASum) and the total dipole moment obtained in this
work (|Atot). The magnitude o f the induced moment ( |lmd), determined by finding the
vector difference between |Xsum and | l tot, is 0.99 Debye, indicating significant
enhancement to the dipole moment as the complex forms.
The diversity o f results and their dependence on the theoretical method underscores the
need for accurate experimental (or theoretical) dipole moments o f sulfuric acid hydrates
if one is to apply IMN theory to nucleation models. Nadykto and coworkers70 recently
investigated the effect o f the structure o f sulfuric acid and its hydrates on the uptake of
sulfuric acid by charged clusters. They found the uptake coefficients to vary by as
much as 230% due to the large variations in the dipole moments. For the monohydrate,
the uptake coefficient using the average of the theoretical dipole moments (2.65 D) was
1.5 times higher than that determined from the ground state structure.70
The
experimental dipole moment determined in the present work is significantly larger than
the average of the three theoretical minima, indicating an even larger adjustment to the
uptake coefficient.
The ability o f theory to reproduce relatively accurate molecular structures without
correspondingly accurate dipole moments is somewhat disconcerting.
A cautionary
note is thus in order: The “best” theoretical structure does not necessarily provide the
most accurate dipole moment. More to the point, the best method for calculating the
131
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structure may not be the best for predicting dipole moments. We have found that the
theoretical method that predicts the dipole moment o f H 2 S 0 4 monomer closest to the
experimentally determined value does not reproduce the dipole moment o f the
monohydrate very well.
Assuming the experimental dipole moment for H 2 SO4 is
correct, it is an important lesson to apply the appropriate theoretical method for the
particular parameters desired. On the other hand, it may be worthwhile to re-examine
the experimental dipole moment o f H 2 S 0 4,
Conclusion
The experimental results o f this work will be useful in developing models for
atmospheric nucleation using the IMN mechanism. While higher hydrates o f sulfuric
acid play an important role in the atmosphere, the current results represent a first step in
providing accurate experimental data to help solve an important problem. However, the
significance o f the results goes beyond experimental value o f the dipole moment of
H2 S 0 4 -H20 and serves as a reminder that simple geometry optimizations may not
always be enough to accurately predict dipole moments.
Acknowledgements
This work was supported by the National Science Foundation, and the Minnesota
Supercomputing Institute.
The authors also with to gratefully acknowledge A. A1
Natsheh for providing his Cartesian coordinates and dipole moment components, and
Prof. S. Kass for helpful insights.
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55 Canagaratna, ML; Ott, M. E.; Leopold, K. R., “Determination o f the dipole moment of
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Z.
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141
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61 Read, W. G.; Flygare, W. H., “The microwave spectrum and molecular structure of
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62 Benz, H. P.; Bauder, A.; Gunthard, Hs. H., “Exact quadrupole interaction energies in
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142
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67 Kuczkowski, R. L.; Suenram, R. D. and Lovas, F. J., “Microwave spectrum,
structure, and dipole moment o f sulfuric acid.” J. Am. Chem. Soc., 103, 2561
(1981).
68 Clough, S. A.; Beers, Y.; Klein, G. P.; Rothman, L.S., “Dipole moment o f water from
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69 A1 Natsheh, A, private communication.
70 Nadykto, A. B. ; A1 Natsheh, A.; Yu, F.; Mikkelsen, K. V.; Ruuskanen, J., ’’Effect of
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by Charged Clusters/Ultrafine Particles,” Aero. Sci. & Tech., 38, 349-353
(2004).
R eproduced with perm ission of the copyright owner. Further reproduction prohibited without permission.
Appendix to Chapter 4
R eproduced with perm ission of the copyright owner. Further reproduction prohibited without permission.
Table A4. 1 Stark Transitions1 o f H2SO4-H2O, A-State.
Rotational
Transition
2 o2 <— loi
M"
M'
Vobs
Av 0|,s
7554.600
Obs.-Calc.2
E (V/cm)
-0.0040
0.00
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
7554.586
7554.585
7554.558
7554.545
7554.523
7554.501
7554.461
7554.382
-0.014
-0.015
-0.042
-0.055
-0.077
-0.099
-0.139
-0.218
-0.0053
0.0052
-0.0067
-0.0007
0.0024
0.0083
0.0014
0.0023
12.30
18.48
24.59
30.73
36.95
43.03
49.15
61.49
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
7554.615
7554.632
7554.654
7554.689
7554.734
7554.778
7554.825
7554.890
7554.967
0.015
0.032
0.054
0.089
0.134
0.178
0.225
0.290
0.367
0.0003
-0.0005
-0.0039
-0.0015
0.0034
0.0001
-0.0068
-0.0035
0.0041
12.30
18.48
24.59
30.73
36.95
43.02
49.16
55.35
61.49
1
1
1
0
0
0
7554.438
7554.386
7554.343
-0.162
-0.214
-0.257
-0.0036
-0.0064
0.0054
43.05
49.19
55.31
2
2
2
2
2
2
2
2
2
2
1
1
1
1
1
1
1
1
1
1
7554.610
7554.610
7554.617
7554.626
7554.630
7554.643
7554.650
7554.660
7554.711
7554.724
0.010
0.010
0.017
0.026
0.030
0.043
0.050
0.060
0.111
0.124
0.0021
-0.0029
-0.0010
0.0021
-0.0015
0.0032
0.0006
0.0007
0.0006
-0.0024
24.60
30.72
36.86
43.04
49.19
55.32
61.47
67.61
92.20
98.35
1 All frequencies are in MHz.
2 Obtained from initial fit, freeing all parameters.
R eproduced with perm ission of the copyright owner. Further reproduction prohibited without permission.
Table A 4 . 1 Stark Transitions o f H2SO4-H2O, A-State, C ontinued.1
Rotational
Obs. v
Obs.-Calc.2 E (V/cm)
M"
M'
Obs. Shift
Transition
0.000
-0.0013
7533.772
2l2 <— 111
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
7533.770
7533.766
7533.767
7533.777
7533.770
7533.772
7533.773
7533.771
7533.768
7533.770
-0.002
-0.006
-0.005
0.008
-0.002
0.000
0.001
-0.001
-0.004
-0.002
-0.0029
-0.0067
-0.0053
0.0045
-0.0024
-0.0008
0.0003
-0.0017
-0.0053
-0.0035
0.636
0.945
1.262
1.553
2.185
2.470
2.759
3.086
3.401
3.733
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
7533.782
7533.790
7533.802
7533.820
7533.844
7533.865
7533.886
7533.927
7533.959
7534.000
7534.051
7534.133
0.010
0.017
0.030
0.048
0.072
0.093
0.114
0.155
0.187
0.228
0.279
0.361
0.0009
-0.0009
-0.0029
-0.0010
0.0012
-0.0030
-0.0084
0.0015
-0.0036
-0.0030
0.0034
-0.0104
0.636
0.945
1.262
1.553
1.867
2.184
2.463
2.758
3.082
3.400
3.712
4.322
1
1
1
1
1
1
1
1
0
0
0
0
0
0
0
0
7533.768
7533.767
7533.781
7533.773
7533.771
7533.768
7533.765
7533.756
-0.004
-0.005
0.009
0.001
-0.001
-0.004
-0.007
-0.016
-0.0042
-0.0045
0.0104
0.0054
0.0040
0.0021
0.0005
-0.0013
0.933
1.261
1.554
2.497
2.805
3.086
3.094
4.622
1 All frequencies are in MHz.
2 Obtained from initial fit, freeing all parameters.
R eproduced with perm ission of the copyright owner. Further reproduction prohibited without permission.
Table A 4 . 1 Stark Transitions of H2SO4-H2O, A-State, Continued.1
Rotational
M"
M'
Obs. Shift
Obs.-Calc.2 E (V/cm)
Obs. v
Transition
2l2 <— 111
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
2n <— 110
7533.788
7533.801
7533.814
7533.833
7533.856
7533.879
7533.909
7533.942
7533.980
7534.019
7534.059
7534.103
7534.137
7534.201
7534.301
0.016
0.029
0.042
0.061
0.084
0.107
0.136
0.170
0.208
0.247
0.287
0.331
0.365
0.429
0.529
7575.537
-0.0003
0.0002
-0.0014
-0.0027
0.0007
-0.0042
-0.0034
0.0010
0.0031
0.0028
0.0027
0.0009
-0.0091
0.0045
-0.0041
0.933
1.260
1.554
1.879
2.156
2.497
2.804
3.083
3.398
3.716
4.015
4.337
4.626
4.935
5.550
0.0000
0.000
0
0
0
0
0
0
0
0
7575.541
7575.538
7575.536
7575.534
0.004
0.001
-0.001
-0.003
0.0039
0.0006
-0.0008
-0.0028
0.639
0.933
1.575
2.166
1
1
1
1
1
1
1
1
1
1
1
1
7575.525
7575.516
7575.433
7575.383
7575.336
7574.920
-0.012
-0.021
-0.104
-0.155
-0.201
-0.617
-0.0039
-0.0035
-0.0095
-0.0004
-0.0097
-0.0106
0.639
0.933
2.172
2.776
3.092
5.571
1
1
0
0
7575.536
7575.543
-0.001
0.006
-0.0028
-0.0001
1.578
2.778
2
1
7575.395
-0.142
-0.0051
2.780
1 All frequencies are in MHz.
2 Obtained from initial fit, freeing all parameters.
R eproduced with perm ission of the copyright owner. Further reproduction prohibited without permission.
Table A4. 2 Stark Transitions1 o f H2SO4-H2O, B-State.
Rotational
Transition
2 o2
M"
M'
Obs. v
Obs. Shift
7554.539
<— loi
Obs-Calc.2
E (V/cm)
-0.0000
0.000
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
7554.527
7554.523
7554.505
7554.482
7554.462
7554.431
7554.397
7554.362
7554.318
7554.224
7554.168
-0.012
-0.016
-0.034
-0.057
-0.077
-0.108
-0.142
-0.177
-0.221
-0.315
-0.371
-0.0030
0.0038
0.0013
-0.0021
0.0027
0.0001
-0.0015
0.0012
-0.0015
0.0003
-0.0009
12.30
18.48
24.59
30.73
36.95
43.03
49.15
55.36
61.49
73.69
79.82
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
7554.554
7554.570
7554.597
7554.630
7554.672
7554.720
7554.774
7554.835
7554.910
7555.069
7555.176
0.015
0.031
0.058
0.091
0.133
0.181
0.235
0.296
0.371
0.530
0.637
0.0006
-0.0021
-0.0015
-0.0016
-0.0016
-0.0013
-0.0026
-0.0056
-0.0017
-0.0049
0.0091
12.30
18.48
24.59
30.73
36.95
43.02
49.16
55.35
61.49
73.69
79.82
1
1
1
1
1
1
1
0
0
0
0
0
0
0
7554.488
7554.459
7554.426
7554.380
7554.332
7554.274
7554.215
-0.051
-0.080
-0.113
-0.159
-0.207
-0.265
-0.324
0.0017
0.0017
0.0050
0.0016
0.0023
-0.0001
0.0027
24.60
30.72
36.86
43.05
49.19
55.31
61.47
1 All frequencies are in MHz.
2 Obtained from initial fit, freeing all parameters.
R eproduced with perm ission of the copyright owner. Further reproduction prohibited without permission.
Table A 4 . 2 Stark Transitions o f H2SO4-H2O, B-State , Continued.1
Rotational
Obs. v
Obs-Calc.2 E (V/cm)
M'
Obs. Shift
M"
Transition
2(32<— loi
2
2
2
2
2
2
2
2
2
2
1
1
1
1
1
1
1
1
1
1
2n <— 110
7554.546
7554.550
7554.556
7554.563
7554.570
7554.578
7554.586
7554.597
7554.647
7554.663
0.007
0.011
0.017
0.024
0.031
0.039
0.047
0.058
0.108
0.124
7575.484
-0.0007
-0.0006
0.0000
0.0005
-0.0001
0.0001
-0.0019
-0.0005
-0.0016
-0.0015
24.60
30.72
36.86
43.04
49.19
55.32
61.47
67.61
92.20
98.35
0.0000
0.000
0
0
0
0
0
0
0
0
7575.484
7575.488
7575.484
7575.486
0.000
0.004
0.000
0.002
-0.0005
0.0039
0.0002
0.0015
0.639
0.933
1.575
2.166
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
7575.473
7575.466
7575.434
7575.388
7575.328
7575.291
7575.111
7574.869
7574.736
-0.011
-0.018
-0.050
-0.096
-0.156
-0.193
-0.373
-0.615
-0.748
-0.0025
-0.0004
0.0005
-0.0003
-0.0007
-0.0009
-0.0013
-0.0039
-0.0059
0.639
0.933
1.575
2.172
2.776
3.092
4.315
5.571
6.156
1
1
0
0
0
7575.490
7575.483
7575.492
0.006
-0.001
0.008
0.0051
-0.0032
0.0017
1.235
1.578
2.778
2
2
2
2
2
2
1
1
1
1
1
1
7575.456
7575.441
7575.345
7575.163
7574.944
7574.839
-0.028
-0.043
-0.139
-0.321
-0.540
-0.645
-0.0001
0.0014
-0.0013
0.0046
-0.0029
0.0071
1.238
1.578
2.780
4.298
5.562
6.152
1 All frequencies are in MHz.
2 Obtained from initial fit, freeing all parameters.
149
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Table A4. 2 Stark Transitions o f H2SO4-H2O, B-State, C ontinued.1
Rotational
Obs. v
Obs.-Calc.2 E (V/cm)
M'
Obs. Shift
M"
Transition
-0.0007
0.000
7533.701
2l2 <— 111
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
7533.705
7533.698
7533.700
7533.703
7533.700
7533.700
7533.700
7533.702
7533.702
7533.703
7533.701
0.004
-0.003
-0.001
0.002
-0.001
-0.001
-0.001
0.001
0.001
0.002
0.000
0.0042
-0.0031
-0.0007
0.0017
-0.0011
-0.0008
-0.0012
0.0002
0.0001
0.0014
-0.0012
0.636
0.945
1.262
1.555
1.870
2.185
2.470
2.764
3.086
3.401
3.733
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
7533.723
7533.736
7533.746
7533.773
7533.798
7533.819
7533.855
7533.892
7533.933
7533.979
7534.071
7534.182
7534.312
7534.464
0.022
0.035
0.045
0.072
0.097
0.118
0.154
0.191
0.232
0.278
0.370
0.481
0.611
0.763
0.0041
0.0023
-0.0043
0.0008
0.0001
-0.0044
0.0000
-0.0002
-0.0008
0.0011
-0.0031
-0.0019
0.0010
0.0090
0.945
1.262
1.553
1.867
2.184
2.463
2.758
3.082
3.400
3.712
4.322
4.929
5.557
6.200
1
1
1
1
1
1
1
1
1
1
0
0
0
0
0
0
0
0
0
0
7533.697
7533.693
7533.701
7533.700
7533.697
7533.696
7533.695
7533.696
7533.691
7533.685
-0.004
-0.008
0.000
-0.001
-0.004
-0.005
-0.006
-0.005
-0.010
-0.016
-0.0038
-0.0069
0.0015
0.0016
-0.0006
-0.0005
0.0001
0.0020
-0.0011
-0.0005
0.934
1.261
1.559
1.879
2.151
2.500
2.805
3.086
3.401
4.623
1 All frequencies are in MHz.
2 Obtained from initial fit, freeing all parameters.
R eproduced with perm ission of the copyright owner. Further reproduction prohibited without permission.
Table A4. 2 Stark Transitions o f H2SO4-H2O, B-State, Continued.1
Rotational
Transition
212 <— 111
M"
M’
Obs. v
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
7533.717
7533.729
7533.743
7533.762
7533.781
7533.811
7533.840
7533.869
7533.905
7533.944
7533.985
7534.031
7534.074
7534.126
7534.235
0
0
1
1
7533.840
7533.871
Obs.-Calc.2
E (V/cm)
0.016
0.028
0.042
0.061
0.080
0.110
0.139
0.168
0.204
0.243
0.284
0.330
0.373
0.425
0.534
0.0002
-0.0001
-0.0014
-0.0021
-0.0030
-0.0020
-0.0016
-0.0015
-0.0015
-0.0022
-0.0016
-0.0021
-0.0034
-0.0020
-0.0025
0.934
1.261
1.559
1.879
2.151
2.497
2.804
3.084
3.398
3.716
4.015
4.337
4.626
4.936
5.550
0.139
0.170
0.0077
0.0052
2.497
2.804
Obs. Shift
1 All frequencies are in MHz.
2 Obtained from initial fit, freeing all parameters.
R eproduced with perm ission of the copyright owner. Further reproduction prohibited without permission.
Table A4. 3 Summary o f Stark effect fits for A-State.a
Fit Parameters
Transitions
# of
Value of
used in fit
Lines A(MHz)
All
All
All
2o2<—101
2n<—1io
2l2<—111
All
All
All
All
All
All
All
All
All
All
All
All
Mean
Max
Min
Spread
91
91
91
31
14
46
91
91
91
91
91
91
91
91
91
91
91
91
5053
5210
4896
5053
5053
5053
5053
5210
4896
5053
5210
4896
5053
5210
4896
5053
5210
4896
Pa
pb
2.6300(38)
2.6319(51)
2.6283(38)
2.6224(78)
2.6597(69)
2.6221(46)
2.6303(36)
2.6307(36)
2.6300(37)
2.6298(36)
2.6301(36)
2.6297(37)
2.6303(36)
2.6306(36)
2.6300(37)
2.6274(37)
2.6278(39)
2.6270(37)
0.43(67)
0.15(44)
0.92(29)
0.65(44)
0.43b
0.43b
0.04°
0.04°
0.04°
0.509(32)
0.592(28)
0.401(39)
0.14d
0.14d
0.14d
1.258(13)
1.293(13)
1.220(13)
2.6305
2.6597
2.6221
0.0376
Pc
1.43(19)
1.81(34)
1.14(22)
1.34(20)
1.43b
1.43b
1.495(10)
1.526(10)
1.462(10)
1.41°
1.41°
1.41°
1.489(10)
1.520(10)
1.456(10)
0.84d
0.84d
0.84d
Pt
2, 2,1/2
(P ib + P c )
3.02(37)
3.20(43)
3.01(35)
3.02(38)
3.05e
3.02e
3.03(2)
3.04(2)
3.01(2)
3.03(2)
3.04(2)
3.01(2)
3.03(2)
3.04(2)
3.01(2)
3.03(2)
3.05(2)
3.02(2)
1.49(70)
1.82(73)
1.46(71)
1.49(73)
1.49®
1.49®
1.50(2)
1.53(2)
1.46(2)
1.50(2)
1.53(2)
1.47(2)
1.50(2)
1.53(2)
1.46(2)
1.51(2)
1.54(2)
1.48(2)
3.04
3.20
3.01
0.19
1.51
1.82
1.46
0.35
(a) Values are in Debye, unless noted otherwise. Numbers in parentheses are one standard
error, (b) Values held fixed in fit. (c) Held fixed in fit at MP2/aug-cc-pVTZ value, (d)
Held fixed at PW91/TZP value from Ref. 51. (e) Values excluded from statistics of pT and
( P b 2 + P c 2) 1/2.
152
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Table A4. 4 Summary o f Stark effect fits for B-State.a
Fit Parameters
Transitions
used in fit
# of
Lines
Value
of A
(MHz)
All
All
All
2 o2 <—loi
2n<—1io
2l2<—111
All
All
All
All
All
All
All
All
All
All
All
All
116
115
115
40
23
53
115
115
115
115
115
115
115
115
115
115
115
115
5053
5210
4896
5053
5053
5053
5053
5210
4896
5053
5200
4896
5053
5210
4896
5053
5210
4896
Mean
Max
Min
Spread
M>a
2.6402(16)
2.6426(16)
2.6380(16)
2.6373(32)
2.6424(28)
2.6403(23)
2.6412(16)
2.6426(15)
2.6399(17)
2.6408(15)
2.6421(15)
2.6396(17)
2.6412(16)
2.6426(15)
2.6399(17)
2.6394(15)
2.6408(17)
2.6380(16)
2.6405
2.6406
2.6373
0.0053
Jib
0.95(17)
0.20(90)
1.27(12)
0.87(18)
0.95b
0.95b
0.04°
0.04°
0.04c
0.617(11)
0.676(10)
0.545(13)
0.14d
0.14d
0.14d
1.2956(55)
1.3241(60)
1.2637(55)
lie
1.20(13)
1.54(12)
0.82(18)
1.26(12)
1.20b
1.20b
1.5368(46)
1.5618(46)
1.5090(50)
1.41°
1.41°
1.41°
1.5310(46)
1.5561(46)
1.5031(50)
0.84d
0.84d
0.84d
(It
3.05(21)
3.07(23)
3.04(20)
3.05(21)
3.05e
3.05e
3.06(1)
3.07(1)
3.04(1)
3.06(1)
3.07(1)
3.04(1)
3.06(1)
3.07(1)
3.04(1)
3.06(1)
3.07(1)
3.04(1)
1.53(41)
1.55(41)
1.51(40)
1.53(40)
1.53e
1.53e
1.54(1)
1.56(1)
1.51(1)
1.54(1)
1.56(1)
1.52(1)
1.54(1)
1.57(1)
1.51(1)
1.55(1)
1.57(1)
1.52(1)
3.06
3.07
3.04
0.30
1.54
1.57
1.51
0.06
(a) Values are in Debye, unless noted otherwise. Numbers in parentheses are one standard
error, (b) Values held fixed in fit. (c) Held fixed in fit at MP2/aug-cc-pVTZ value, (d)
Held fixed at PW91/TZP value from Ref. 51. (e) Values excluded from statistics of |i[ and
(|Xb2 + He2)172-
153
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Table A4. 5 Experimental Cartesian coordinates3 in principle axis system.
a
H
H
H
H
O
O
O
O
O
S
3.480100
1.348960
2.296470
-1.673620
0.309211
0.355640
2.752200
-0.921102
-1.720650
-0.559721
b
-0.510697
-0.681945
0.464794
0.903599
0.566544
-0.922034
0.020776
1.157650
-0.769241
-0.032402
c
0.298045
-0.598025
0.658375
-1.374970
1.132030
-0.791527
-0.086911
-0.853779
0.429616
0.117379
(a) Ref. 50 of chapter 4. (b) Units are in A.
Table A4. 6 MP2/aug-cc-pVTZ Cartesian coordinates3 (including dipole moment) in
principle axis system.
a
H
H
H
H
O
O
O
O
O
S
1.323360
-1.571030
2.308440
3.439570
0.375132
-0.838661
0.244553
-1.769040
2.723230
-0.541192
-2.613720
|Xb
b
-0.715568
0.830434
0.487231
-0.493321
-1.017250
1.130900
0.621208
-0.713092
0.024219
-0.026438
c
-0.574968
-1.506990
0.658737
0.295579
-0.686040
-0.946400
1.144100
0.403843
-0.085361
0.120522
0.039655
1.410790
(a) Ref. 50 of chapter 4. (b) Units are in A.
154
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Table A4. 7 Theoretical dipole moments and components of HiSCVtfeO.8
Level/Basis Set
Ill/AVI Z
MP2/AVDZ
MP3/AVDZ
MP2/AVTZ
MP2/AVQZb
|Xtot
3.4284
3.0031
2.9544
2.9704
2.9654
|4a
2.9656
-2.6889
-2.6146
-2.6137
-2.6103
|4b
-0.2250
0.1061
0.0334
0.0397
0.0397
Jic
." T 7 0 5 I
1.3331
1.3752
1.4108
1.4067
(a) Values are in Debye, (b) Geometry held fixed at MP2/AVTZ optimized geometry.
R eproduced with perm ission of the copyright owner. Further reproduction prohibited without permission.
Chapter 5
Effects of Partially Quenched Orbital Angular Momentum on
the Microwave Spectrum and Magnetic Hyperfine Splitting in
the OH-Water Complex
Carolyn S. Brauer, Galen Sedo, Erik. M. Grumstrup, Kenneth R.
Leopold
Department o f Chemistry
University o f Minnesota
Minneapolis, MN
M ark D. Marshall*and Helen O. Leung
D epartment o f Chemistry
Amherst College
Amherst, MA
Reproduced in part with permission from Chemical Physics Letters, 401(4-6), 420-425
(2005). © Copyright 2005 Elsevier..
156
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Abstract
The radical complex OH-H 2 O has been observed by rotational spectroscopy. Spectra
for 16OH-16OH2, 18OH-18OH2, 16OH-18OH2 and 18OH-16OH2 have been analyzed using a
two-state model which accounts for nuclear motion on both the 2A ' and 2A " potential
surfaces. Partial quenching o f the OH orbital angular momentum dramatically affects
the rotational spectra, and the 2A ' - 2A " energy separation, p, is determined to be
-146.17085(31) cm '1. The ground state o f the complex has ~86% ^4' character and the
vibrationally averaged OH-OH2 hydrogen bond distance is 1.945 A.
Magnetic
hyperfine structure reveals large changes in the OH electron distribution upon
complexation.
157
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Introduction
Understanding the physical and chemical properties o f open shell species is important
to areas as diverse as combustion, atmospheric science, and biology.1 Spectroscopic
studies o f molecular complexes offer a unique means o f investigating intermolecular
interactions and, for this reason, weakly bound complexes containing an open shell
moiety have been the subject of much active research.2"4 In recent years, moreover,
there has been a growing recognition that complexation can alter the chemistry and
photophysics o f key atmospheric species,5’6 and in this context, the study o f open shell
molecular complexes has acquired a new and perhaps unexpected application.
Among the most well studied open
shell species is the OH radical, and
correspondingly its complexes with
rare gas atoms have long provided
a basis for understanding open shell
intermolecular interactions in the
gas phase.2'4 Because of its central
role
in
atmospheric
chemistry,
however, its complex with water also
has been of considerable interest.7' 12
Although early ab initio calculations
Figure 5. 1 The OH-H O 2 com plex. The top and
bottom structures represent equivalent potential
m inim a that are connected by vibrational averaging
over a small barrier in the planar (center)
configuration.
suggested that the OH functions as a hydrogen bond acceptor in the H 2 O —OH
R eproduced with perm ission of the copyright owner. Further reproduction prohibited without permission.
complex,9 subsequent work revealed that this configuration is, in fact, only a local
minimum on the OH + H 2 O potential surface. The global minimum is a C, structure in
which the OH acts as a hydrogen bond donor, as shown in Figure 5. 1. With the
0
calculated energy o f the planar structure only slightly higher, the two equivalent nonplanar structures shown are connected by large amplitude, zero-point motion through
the small barrier, as for the analogous closed shell species, H 2O-HF . 13 The hydrogen
bond distance in OH-OH 2 is predicted to be -1.95 A, which is similar to that in the
(H20 ) 2,14 although the computed well depth o f -5 .6 kcal m ol'1 is somewhat greater than
that for the water dimer (4.9 kcal mol'1).15
An important effect that arises in nonlinear systems containing a 2II moiety arises as
complexation breaks the cylindrical symmetry o f the free monomer and lifts the
electronic degeneracy of the n state.16' 19 The result is a pair o f states which, in the Cs
symmetry o f OH-H 2 O, are labeled 2A ' and 2A ", and one spectral manifestation is a
splitting o f rotational energy levels with the same angular momentum properties but
with opposite parities (P-type doubling).
The rotational energies o f the system are
strongly affected by coupling between the spin and orbital angular momenta of the OH
with the overall rotation of the complex,16'21 as well as by the partial quenching o f these
angular momenta associated with the P-type doubling. Formally, this latter effect is
induced when the difference potential p = E(2/f) - E(2A") is comparable in magnitude
to the spin-orbit coupling constant for the OH monomer (Aso - -139 cm '1), and its
effect is to produce a very complex rotational energy level structure for the system.
159
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Various ab initio calculations predict values o f the difference potential ranging from
approximately -110 cm '1 to -190 cm '1 for OH-OH 2 at its equilibrium geometry,8’10’11
and thus the effects o f the quenching are expected to be prominent in both microwave
spectra and infrared band contours for this complex.20 Such effects have recently been
1 q *y 1 9 1
demonstrated in the infrared spectrum o f the OH-acetylene. ’ ’ In the pure rotation
spectrum, magnetic hyperfine structure will also be strongly affected by angular
momentum quenching, as it depends sensitively on the electron distribution of the
molecule.
Although the first gas phase observations o f the OH-H 2 O complex were not reported
until recently,23’24 there are several reports on its detection in cryogenic environments.
Indeed, three recent studies report infrared transitions attributed to the OH-water
complex.11,25,26
A recent report o f the microwave spectrum of 16OH-16OH 2 by Ohshima, et al.24
coincided with the initial publication o f this work.
These two studies differ, however,
in both the number of rotational transitions and isotopic species measured, as well as in
the analytical approach taken. In this work, microwave spectra for 16OH-16OH 2 , 18OH18OH2, 16OH-18OH 2 , and 18OH-16OH 2 are presented, that demonstrate the effects of the
difference potential and of the two interacting electronic states. The results provide a
detailed characterization o f this species in the gas phase, determining for the first time
the 2A ' - 2'A " energy difference, the hydrogen bond length, and aspects o f the molecular
charge distribution, as revealed by magnetic hyperfine structure.
160
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Experimental
Spectra
were
recorded
using
a
pulsed-nozzle
Fourier
transform
microwave
spectrometer27 which has been described elsew here28 The OH-H 2 O complex was
generated in a supersonic jet using a DC discharge source29 similar to those used by
many groups and based on the original work o f Engleking.30 The design used in this
Gas In
•*
----------------
Pulsed Solenoid Valve
Anode
Teflon Block
=A
Titanium Cathode
Teflon Cap
Figure 5. 2 Supersonic discharge source used to produce OH-OH 2 .31
particular study is shown in Figure 5. 2. A 1" long Teflon block with a 5/64” bore,
flared discharge channel is appended to a General Valve nozzle for creation o f the gas
pulses. Several Teflon blocks with varying bore diameters and flare angles were tried,
as preliminary tests indicated that the production o f Ar-OH, originally observed by
161
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Endo and coworkers,32 was sensitive to these parameters.
Titanium electrodes are
located above and below the Teflon block, with the lower one serving as the high
voltage cathode. The use of titanium appears to be important in reducing noise that
develops with other metals after short periods of running the discharge. The titanium
shows little sign of sputtering or corrosion relative to other metals we have tried, and
we speculate that its apparent superiority may be related to its chemical and/or physical
robustness. Optimum signals for the OH-H 2 O complex were obtained by bubbling Ar
though H 2 O or H2180 at a backing pressure o f 2.7 atm and expanding through the
nozzle/discharge assembly at a repetition rate of 5 Hz. Following the initial study of the
0H -H 20 complex,23 it was discovered that a non-uniform magnetic field existed inside
the spectrometer cavity, which perturbed the rotational energy o f the complex with a
Zeeman effect. The origin of the field was determined to be four large, 440 stainless
steel rods. These rods were replaced by four identical rods, made o f 316 stainless steel.
This minimized, but did not completely eliminate the inhomogeniety o f the internal
field. However, the field was homogeneous in a region sufficient for spectroscopic
studies. Three perpendicular sets o f Helmholtz coils surrounded the apparatus and were
used to cancel the Earth’s magnetic field inside the microwave cavity.
162
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Theoretical Methods
The model o f Marshall and Lester for the rotational energy level structure o f OHcontaining complexes with arbitrary quenching o f the electronic orbital angular
momentum19 has been extended to include the magnetic hyperfine interactions o f the
OH monomer. In addition to terms representing the rotational motion of the complex,
the spin-orbit interaction of OH, and the newly added magnetic hyperfine interactions,
the model includes the operator,
which causes the splitting o f the OH monomer orbital degeneracy into electronic states
that is responsible for the quenching. In Eq. 1, p is the difference potential, defined as
E(2A') - E(2A"), and the bars over the electronic orbital raising and lowering operators
indicate that they are "normalized" by the effective value o f /( / + l) for the molecule.
Hyperfine-split rotational energy levels are calculated via diagonalization of the
complete Hamiltonian matrix that results within the framework o f the two interacting
electronic states, 2A ' and 2A". Although the model was originally developed for the Tshaped OH-acetylene complex, it may be applied directly to OH-water, for which the
large amplitude motion o f the water subunit leads to rovibrational states o f Civ
symmetry. As a further consequence o f this large amplitude motion, OH-water will
163
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have ortho and para spin modifications that will not interconvert under most
experimental conditions.
In the model o f Marshall and Lester, rotational wavefunctions are constructed using a
primitive set of basis functions
I JpMXa) = |JpM)|tiX>|sa)
where J is the total angular momentum excluding nuclear spin, p is its projection onto
the a-inertial axis of the complex, and M is its projection onto a space-fixed axis. The
projections o f orbital and spin angular momenta, I = 1 and s = 14, o f the unpaired
electron onto the OH axis are given by A,= ±1 o = ± 14., respectively, and their sum is
designated co = X + o. The quantum number T] is included to allow for the general case
of different electronic states with the same value o f X. Linear combinations o f these
primitive functions produce wavefunctions of definite parity, viz,
IjP to e ) = (1/2)1/2{ IJPA.o) + 8(-l)J',/2 |j,-P,-^,-o)}
(5.3)
where P = | p |, £ = ±1, and M is suppressed in the absence o f an external field.
Rotational states observed in this work lie in the co = +3/2 manifold, and may be further
described by J, P, and £.
When hyperfine structure due to the OH proton spin, I, is included, the angular
momentum is Fi = I + J. Superhyperfine structure arises from further coupling of Fi
with Iw, the total proton spin o f the water unit, giving the total angular momentum, F =
Fi + Iw. The states observed in this work correlate to K* = 0 in the fully quenched limit
164
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and thus, accounting for the symmetry o f the electronic wavefunction, Fermi - Dirac
statistics require Iw = 1. Hyperfme matrix elements were taken from Mills, Western and
Howard33 and re-expressed in terms o f the parity-conserving basis o f Equation (5.3) to
complete the Hamiltonian matrix prior to diagonalization.
Results
As noted above, quenching o f the OH electronic angular momentum renders the
microwave spectrum strongly dependent on the potential difference, p, and spectral
searches were therefore initiated by assuming that p lies within a range bounded by
published theoretical values of -110 cm '1 and -190 cm '1,8’10,11 and calculations20 thus
placed two transitions o f the co = 3/2, P = V%manifold within the spectral range of the
apparatus: A stronger R branch line, |J,P,e) = |3/2,l/2,-l) <—|l/2 ,l/2 ,+ l), and a weaker
Q-branch line corresponding to a direct transition between + and - parity levels o f J = P
= Vi, viz., |l/2 ,l/2 ,-l) <—11/2,l/2,+l). Approximately 500 MHz o f searching eventually
yielded a set o f lines at ~14950 MHz, which were spread over about 23 MHz, required
the discharge, and disappeared in the absence o f H2 O.
Most o f these lines also
broadened and/or moved in frequency when the Helmholtz coils were turned off.
Preliminary calculations indicated that an R-branch transition at this frequency
corresponded to a potential difference o f approximately -147 cm '1, and calculations of
the magnetic hyperfine structure indicated that the observed 23 MHz spread was also
consistent with this value. The lines were roughly grouped into three components, as
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expected in the absence o f superhyperfine structure, but showed additional structure
suggesting further coupling to the water protons. Next, the isotope shift corresponding
to double - 180 substitution in the complex (-1500 MHz) was calculated using the
theoretical structure, and a set of hyperfine components similar to those o f the parent
species was readily located after just 30 MHz of scanning. Using the estimated -147
cm'1 value for p, the Q-branch transitions for 16OH-16OH 2 were then quickly located,
and isotopic scaling immediately allowed for detection of the corresponding I8OH18OH2 spectra.
Spectra for each isotope were analyzed using a weighted least squares fit to the model
Hamiltonian previously applied to OH-C2H2,19,21’22 with the rotational constants,
(B+C)/2 and p as adjustable parameters.
Values of A and (B-C)/2, to which the
observed transitions are insensitive, were fixed at the values corresponding to the
theoretical geometry.8 Although the observed hyperfine structure depends on each of
the magnetic hyperfine constants, a, bF, c, and d, the data were insufficient to determine
all of them independently. Thus, the following approach was taken: Dennis et al.34
have argued that for NO-HF, changes in the dipolar constants a, c, and, d, upon
complexation should all be similar, since all three depend on an average involving 1/r3
(where r is the electron-nucleus distance). Thus, using the known hyperfine constants
for OH (a = 86.1248 MHz, c = 130.22 MHz, and d = 56.6664 MHz),35 the hyperfine
constants o f the complex were written as the fractional change in the monomer
parameters, viz.,
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a complex — a 0H ( 1 + ■*
)
4a)
C om plex
= cOH(l + xOH)
(5.4b)
C
=
(5.4c)
mp,ex
d 0H (l
+ ^ 0H)
where x0H represents the fractional change upon complexation. With this approach, the
hyperfine structure was readily analyzed using only two parameters, xOH and bFOH. The
superhyperfine Fermi contact parameter, bFH2°, also was included in the fits, but the
dipolar superhyperfine parameters showed strong correlation and could not be
independently determined. Attempts to scale these parameters form those o f free OH
using the theoretical geometry o f the complex led to a variety o f sets o f values, none of
which significantly improved the results. Thus, aH2°, cH2° and dH2° were set to zero.
Once the inhomogeneous
magnetic field inside the
instrument was discovered
and
corrected
Experimental
follow-up
conducted
transitions
Section),
study
to
(see
refine
a
was
the
previously
9564
9564.2
9564.4
9564.6
9564.8
9565
9565.2
Frequency (MHz)
Figure 5. 3 The (J,Fi,F) = (1/2, 1, 2) <—(1/2, 0, 1) transition
reported,23 and to analyze
o f 16O H -16OH2, collected over a period o f 103 seconds.
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two additional isotopically substituted species, 16OH-18OH 2 and 18OH-16OH 2 . A sample
spectrum o f the newly observed parent isotopolog is shown in Figure 5. 3.
The new data allowed for inclusion o f the dipolar superhyperfine parameters in the fit,
and these were defined analogously to the hyperfine parameters, but instead, as the
fraction o f the OH monomer values:
h 2o
complex
(5. 5a)
, h 2o
(5.5b)
complex
,h 2o
complex
(5.5c)
All but two transitions, observed while scanning for the 16OH-18OH 2 isotopomer, were
assigned.
While both disappeared in the absence of the discharge and their signal
intensity decreased when the Helmholtz coils were detuned, neither had an analog in
any o f the other isotopic species and thus were omitted from the fit. These transitions
are listed in Table A5.4, in the appendix to Chapter 5.
It also should be noted that the results reported here are not in conflict with the initial
results23 obtained before the inhomogeneous magnetic field was discovered in the
instrument.
Differences in the measured rotational transitions were confined to the
superhyperfine structure, and were typically on the order of a few hundred kHz. Thus,
the only significant change in parameters from the initial study was in the
superhyperfine constant, bFH2°.
The dipolar superhyperfine parameters were not
168
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analyzed in the initial study (they were set to zero in the fit), thus, no change can be
ascertained in these parameters. The resulting spectroscopic constants from the most
recent measurements are listed in Table 5. 1, and those from the initial study can be
found in Tables A5.1-A5.4, in the appendix to Chapter 5.
a,b,c
Table 5 .1 Spectroscopic constants for OH-OH 2 .
_
OH- OH2
o h
- 18o
h
2
16
oh
-
oh
2
1SO H -1(iOH2
—146.17085(31)
-146.19034(32)'
-146.27589(32)'
-146.08139(36)'
0.32957(35)e
0.33124(32)e
0.33034(34)e
0.33045(38)e
bF
-154.936(52)
-155.213(48)
-155.080(51)
-155.084(57)
x h 2o
0.00775(20)e
0.00767(18)e
0.00767(20)e
0.00766(21)e
bFH>°
3.827(38)
3.828(34)
3.831(34)
3.837(41)
(B + Q /2
6580.7498(39)
5922.5257(35)
6294.4622(38)
6210.0162(41)
OH
OH
(a) All values are in M Hz, except where noted, (b) V alues in parentheses are one standard error,
(c) The rotational constants A and (B-C)/2 were set at 288044.1 M Hz and 50 M H z, respectively,
and assum ed isotopically invariant, (d) Values are in c m '1, (e) Values are unitless.
Discussion
As a consequence of the extremely high resolution and lack o f predissociation
broadening, the rotational spectrum of OH-OH2 provides an exquisitely detailed picture
of this molecule.
The hydrogen bond distance, determined from the measured
rotational constants, is 1.945 A, in good agreement with other experimental results24
and theoretical predictions.8 The potential difference, p, for 16OH-16OH2 is
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-146.17085(31) cm '1, and agrees with the value (— 150 cm '1) estimated by Ohshima, et
al.24 The independently determined value o f p for the 180 derivative agrees with the
parent species to within -0.02 cm '1, however, those determined for the mixed
isotopologs range from -146.08139(36) cm '1 to -146.27589(32) cm '1.
While the
differences may arise from subtle changes in vibrational averaging o f the water subunit,
it is impossible to rule out that it is due to an artifact o f the analysis. Nonetheless, the
differences are small, amounting to -0.1% over the full range o f values. Although the
rotational energies depend on |p|, the magnetic hyperfine structure depends strongly on
its sign, and it is through the measurement o f the parameters c and d that the sign o f p is
unambiguously determined in this work.
The potential difference lies squarely in the middle o f the -110 to -190 cm '1 range of
theoretical predictions. The fitted value o f p is only slightly larger in magnitude than
the spin-orbit constant o f OH, indicating partial, but not complete, quenching o f the OH
orbital angular momentum. For the levels observed here, the ground state is -86% A '
character and 14% A" character.
At this level o f mixing, approximately 2/3 o f the
orbital angular momentum remains unquenched as measured by the expectation value
of the effective spin-orbit operator, —Izsz . The lowest excited state o f the system lies at
approximately [p2 + Aso2]1/2, or -201 cm '1 above the ground state.
The negative value o f p indicates that the 2A ' state, with the unpaired electron in the
symmetry plane o f the complex, is of lower energy. This situation is similar to that for
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HF-NO,36 for which the unpaired electron is in the plane of the complex, but opposite
that of Ar-NO, in which it lies out o f the heavy atom plane. The -146 cm'1 value o f p
determined for OH-H 2 O is remarkably similar to the 148.1(7) cm '1 difference potential
previously obtained for OH-acetylene.21
The spin dipolar constants of 0H -H 20 increase by about 33% upon complexation while
the Fermi contact terms changes from -73.25 MHz in free OH to -154.9 MHz in the
complex. The negative values o f bF in both OH and 0H -H 20 indicate important
contributions from spin polarization that apparently increase upon complexation. While
significant, this is inherently a multi-electron effect and as such is difficult to interpret
in terms o f redistribution of the odd electron alone.37,38 In a simple one-electron
picture, on the other hand, the large increase in the spin dipolar parameters naively
suggests a drift in unpaired electron density toward the OH proton. However, small
changes in both the OH bond length and the angular distribution o f the odd electron
may also contribute and are difficult to disentangle from the radial contributions. In
addition, like the Fermi contact term, these parameters may be sensitive to spin
polarization.
Thus, while the large change in the OH magnetic hyperfine constants
relative to those in free OH suggests a substantial effect on the electron distribution of
the radical upon complexation, these parameters should not be over-interpreted.
The hyperfine structure depends on a linear combination of the hyperfine parameters, bF
and, in the terminology o f Frosch and Foley39, a, c and d.
Yet the values o f the
individual parameters appear to depend on the analytical approach taken. In particular,
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they depend on whether one chooses to ignore the partial quenching o f the orbital
angular momentum, and instead treat the complex as a non-linear, open-shell species, in
which the orbital angular momentum is typically fully quenched. The analysis o f OHOH 2 by this group, and by Ohshima and coworkers24 take two different approaches,
which, while they are in general agreement with respect to the major conclusions, differ
significantly in the value o f the Fermi contact term.
In order to address the two different approaches and the different values each yields, it
is helpful, at this point, to diverge briefly into a discussion o f ways in which angular
momentum coupling is represented in diatomic molecules. While a number o f coupling
schemes have been identified, most diatomic molecules typically fall into one o f two
types o f coupling, shown in Figure 5. 4.
R
V
A
a
Figure 5. 4 Vector diagrams for Hund’s cases (a) and (b).
172
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These schemes, established by Hund , 4 0 and designated Hund’s cases (a), (b), etc., are
ideal cases for which most molecules can be approximated.
The various angular
momenta are:
L - electronic orbital angular momentum,
S - electronic spin angular momentum,
J - total angular momentum,
N - total angular momentum excluding electron spin ( N = J - S ) ,
R - rotational angular momentum o f the nuclei (R = N - L ) .
For a diatomic molecule in a good Hund’s case (a), the electronic orbital angular
momentum is strongly coupled to the intemuclear axis and the electronic spin angular
momentum is coupled to the electronic orbital angular momentum. J is still a “good”
quantum number, and the projections o f L (A) and S (E) on the molecular axis are well
defined. Their sum is designated A +E = £X If A=0 (or if the spin-orbit interaction is
weak), S is no longer coupled to the inter-nuclear axis and E is not well defined. This is
best described by Hund’s case (b).
Here, the rotational angular momentum R is
coupled to L, forming a resultant N, The magnetic hyperfine interaction in Hund’s
cases (a) and (b) is most generally given by:
= aAI»k + bI»S +c(I»k)(S»k)
(5. 6 )
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where k is a unit vector along the molecular axis, a, b and c are the Frosch and Foley
hyperfine parameters, and the Fermi contact term, bF, has the relation, bp = b - c/3. The
nuclear spin, I, can couple in a variety o f ways, and the energy that results from the
interaction in Equation (5. 1) is evaluated according to the specific basis chosen. For a
Hund’s case (a), the magnetic hyperfine energy is typically given by:
W = [aA + (b + c)Z]£2/[(J(J+l)] I*J
( 5 . 7)
where I*J = (1/2)F(F+1) -1(1+1) - J(J+1).
The approach taken by Ohshima et al . 2 4 assumes the OH-OH 2 complex is an open-shell
non-linear molecule, in which the orbital angular momentum, L, is not quantized along
the molecular axis. Thus, the term dependent on L, which is due to the nuclear spinelectron orbital hyperfine coupling, can be ignored, and the basis is approximated by
Hund’s case (b). This analysis yields hyperfine parameters bF°H = -8.226(6) and Taa =
2/3c = 126.159(9). The analysis done in this work, however, assumes that the orbital
angular momentum is only partially quenched, and indeed, evaluation o f the
expectation value o f the spin-orbit operator, lzsz, shows that only about 1/3 o f the orbital
angular momentum is quenched. Therefore, we feel the appropriate choice of basis is
closer to Hund’s case (a) and the nuclear spin-electron orbital coupling is included,
leading to bF°H = -154.936(52) and c = 173.14(9). Nonetheless, while it is important to
note that Hund’s cases represent idealized angular momentum coupling, the difference
in the Fermi contact terms is significant.
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Yet it is clear from Equation (5. 7) that, in the absence o f the orbital part of the
hyperfine Hamiltonian, the only available parameters to “absorb” the hyperfine
interaction are c and bF. Therefore, it is altogether reasonable to expect these two
analytical approaches to the same spectroscopic data to give very different values o f the
hyperfine parameters. What is most surprising, perhaps, is the relative agreement in the
dipolar term (Taa = 2/3c) between the two analyses.
Acknowledgements
This work was supported by the National Science Foundation. MDM thanks the H.
Axel Schupf 57 Fund for Intellectual Life for partial support o f this visit. The authors
are grateful to Prof. Don Truhlar for valuable insights, and to Prof. Y. Endo and Dr. Y.
Sumiyoshi for information regarding measured transitions.
175
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40
8 8
, 1337-1349 (1952).
Herzberg, G., Spectra o f Diatomic Molecules (D. Van Nostrand Co., New York, NY,
1950).
R eproduced with perm ission of the copyright owner. Further reproduction prohibited without permission.
Appendix to Chapter 5
R eproduced with perm ission of the copyright owner. Further reproduction prohibited without permission.
Table A 5 .1 Observed Rotational Transitions1,2 for 16OH-16OH2.
1
2
J'
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
F i'
F'
0
1
0
1
0
1
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
2
3
2
2
2
1
1
0
1
1
1
2
1
1
1
0
1
1
1
2
1
0
1
1
1
1
1
1
1
2
1
2
1
2
1
0
1
1
1
2
1
1
J"
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
F i"
F "
1
2
1
1
1
0
0
1
0
1
0
1
1
1
1
2
1
1
1
0
1
2
1
1
1
2
1
1
1
0
0
1
0
1
0
1
1
2
1
2
1
1
1
1
1
1
1
0
Obs.
9484.334
9485.314
9485.736
9560.672
9562.038
9564.651
9569.527
9569.909
9570.893
9571.316
9572.523
9573.506
Obs.-Calc.
-0.0054
0.0182
-0.0381
-0.0298
0.0131
-0.0197
0.0037
0.0191
0.0466
-0.0086
-0.0128
0.0138
14951.418
14952.390
14952.779
14965.926
14965.946
14965.981
14973.802
14973.830
14974.757
14974.792
14974.826
14975.214
0.0175
0.0823
0.0258
0 .0 0 2 1
-0.0078
-0.0330
-0.0169
-0.0491
0.0116
0.0167
-0.0095
-0.0396
All transitions correspond to co = 3/2, P = 14, (£",£') = +1 —> -1.
All frequencies are in MHz. Experimental uncertainty is 20 kHz.
183
R eproduced with perm ission of the copyright owner. Further reproduction prohibited without permission.
Table A5. 2 Observed Rotational Transitions1,2 for 18OH-18OH 2.
J'
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1
2
F i'
F'
0
1
0
1
0
1
1
0
1
1
1
2
1
0
1
1
1
1
1
1
1
2
1
2
2
3
2
2
2
1
1
0
1
1
1
2
1
1
1
2
1
0
1
1
1
2
1
1
J"
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
F i"
F"
1
2
1
1
1
0
0
1
0
1
0
1
1
1
1
2
1
1
1
0
1
2
1
1
1
2
1
1
1
0
0
1
0
1
0
1
1
2
1
2
1
1
1
1
1
1
1
0
Obs.
8530.399
8531.375
8531.794
8606.788
8608.157
8610.766
8615.690
8616.074
8617.048
8617.472
8618.682
8619.669
Obs.-Calc.
-0.0066
0.0196
-0.0363
-0.0277
0.0206
-0.0115
0.0013
0.0144
0.0386
-0.0123
-0.0187
0.0185
13455.614
13456.584
13456.961
13470.109
13470.135
13470.170
13478.064
13478.095
13478.995
13479.025
13479.068
13479.459
0.0169
0.0831
0.0159
-0.0079
-0.0103
-0.0325
-0.0045
-0.0307
0.0051
0.0067
-0.0075
-0.0343
All transitions correspond to to = 3/2, P = V2 , (£",£') = +1 —>-1.
All frequencies are in MHz. Experimental uncertainty is 20 kHz.
184
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Table A5. 3 Observed Rotational Transitions1’2 for I8OH-l6OH 2.
J’
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1
2
F i'
F'
0
1
0
1
0
1
1
0
1
1
1
2
1
0
1
1
1
1
1
1
1
2
1
2
2
3
2
1
2
2
2
1
1
0
1
1
1
2
1
1
1
2
1
0
1
1
1
2
1
1
J"
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
F i"
F"
1
2
1
1
1
0
0
1
0
1
0
1
1
1
1
2
1
1
1
0
1
2
1
1
1
2
1
1
1
1
1
0
0
1
0
1
0
1
1
2
1
2
1
1
1
1
1
1
1
0
Ob s.
8944.393
8945.371
8945.793
9020.805
9022.172
9024.785
9029.645
9030.030
9031.014
9031.430
9032.639
9033.626
Obs.-Calc.
-0.0113
0.0164
-0.0368
-0.0230
0.0209
-0.0119
14110.253
14111.202
14111.220
14111.602
14124.767
14124.793
14124.831
14132.663
14132.688
14133.619
14133.649
14133.683
14134.054
0.0055
0.0796
0.0674
0.0043
-0.0194
-0 . 0 2 1 2
-0.0392
-0.0157
-0.0467
0.0178
0 .0 0 2 2
0.0144
0.0481
-0 . 0 1 1 1
-0.0224
0.0143
0 .0 2 0 0
-0 . 0 0 2 0
-0.0503
All transitions correspond to 0 ) = 3/2, P = Vi, (£",£') = +1 —> -1.
All frequencies are in MHz. Experimental uncertainty is 20 kHz.
185
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Table A5. 4 Observed Rotational Transitions1,2 for 16OH-18OH 2.
J'
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1
2
F i'
F'
0
1
0
1
0
1
1
0
1
1
1
2
1
0
1
1
1
1
1
1
1
2
1
2
2
3
2
2
2
1
1
0
1
1
1
2
1
1
1
2
1
0
1
1
1
1
J"
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
F t"
F"
1
2
1
1
1
0
0
1
0
1
0
1
1
1
1
2
1
1
1
0
1
2
1
1
1
2
1
1
1
0
0
1
0
1
0
1
1
2
1
2
1
1
1
1
1
0
Obs.
9072.065
9073.044
9073.466
9148.376
9149.747
9152.358
9157.288
9157.678
9158.659
9159.075
9160.285
9161.265
Obs.-Calc.
-0.0081
14299.553
14300.520
14300.902
14314.048
14314.072
14314.102
14322.001
14322.025
14322.951
14322.985
14323.393
0.0099
0.0726
-0.0004
-0.0044
-0.0305
-0 . 0 1 1 0
-0.0432
0.0172
0.0233
-0.0436
14299.489
14300.504
unassigned
unassigned
0 .0 2 1 2
-0.0318
-0.0290
0.0204
-0.0115
-0.0023
0.0158
0.0471
-0.0119
-0 . 0 2 0 1
0 .0 1 0 1
0 .0 1 0 0
All transitions correspond to co = 3/2, P = Vi, (e",e') = +1 —» -1.
All frequencies are in MHz. Experimental uncertainty is 20 kHz.
186
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Table A5. 5 Spectroscopic constants for OH-H 2 O from previous analysis . 23,31
16O H - 16O H 2
P [cm' ]
(B + C)/2 [MHz]
(B-C)/2 [MHz]
A [MHz]
x
bF (OH) [MHz]
bF (H20 ) [MHz]
-146.50780(45)
6580.0668(49)
46.2a
393600a
0.33040(44)
-155.336(70)
4.615(48)
8OH-18OH2
-146.54811(75)
5922.4606(88)
37.2a
393000a
0.33244(63)
-155.55(11)
4.638(70)
(a) Constrained in fit.
187
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Appendix A . Discharge Source.
In order to study radical species, such as OH-H 2 O , 1 a source
was developed to generate free radicals in the M innesota
<
I
V
©
R
I L
Fourier transform microwave spectrom eter (M N-FTM W ).
Given the available resources, the m ost straightforward
m ethod for this purpose w as a DC discharge, similar to that
|_ J
used by other researchers and based on the w ork o f
Discharge N ozzle
Engleking.3 In this work, a simple RL circuit, consisting o f a
Figure A. 1 R L C ircu it
15 Henry inductor and a 100 kQ resistor, was constructed in
order to apply a stable current from a high voltage DC pow er supply (Sorenson, 1003200) to a gas m ixture as it entered the supersonic expansion.
A
num ber
o f nozzle
configurations
were
designed to adapt the pulsed nozzle currently
in use for the production o f radical complexes.
These configurations, shown in Figures A .lA.3, were used w ith varying
degrees o f
e
efficacy to produce the known radical species
listed in Table A. 2. The discharge nozzles
fall into four general types. The first, shown
in Figure A .l, is the m ost basic type and is
Figure A. 2 Initial d ischarge nozzle for
generating radical species, (a) pu lsed valve
assem bly (G eneral V alve, Series 9) (b) ss
body/anode, 1/2” th ick (c) T eflo n insulator,
1/8” th ick (d) A1 o r ss cathode, 1/8” thick
(e) T eflo n insulator, 1/8” thick.
188
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composed o f a 1/8” thick A1 or stainless steel cathode that is sandwiched between two
1/8” thick Teflon insulators and attaches to the pulsed valve assembly/nozzle body
already in use in our lab. The stainless steel nozzle body acts as the anode. While this
configuration enabled the observation of OH radical, no other radical species were
observed with it. Also, it often produced electrical noise. A slight modification to this
configuration was made by replacing the stainless steel anode with titanium, and
inserting a 10 mm thick insulator between the anode and pulsed valve assembly. The
distance between the electrodes also was decreased from 1/8” to 1/16”. This decreased
the occurrence o f noise, and enabled the observation o f FeCO radical. However, signal
intensities were low, and improvements were necessary.
■Pulsed solenoid valve (General V alve, Series 9)
_J
f= V pBBi
Stainless steel body w ith 5/64” oriface at entrance
and ~ 1 2 ° flare
Ti anode, 0.002” thick
-----------
Teflon body, 1” .
3/16” bore with cylindrical glass insert (5/64” bore).
Cylindrical Ti cathode, 0.2" thick, 5/64” bore.
Opening for metal contact to electrode.
Figure A. 2 Discharge nozzle designed to contain discharge within the nozzle. See
text for discussion.
189
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The second, shown in Figure A.2, was designed to contain the discharge to the nozzle
body. It has a nozzle body that is constructed o f a solid, cylindrical block o f Teflon,
which attaches to the pulsed valve assembly/nozzle body used in our lab. The Teflon
block has a 3/16” bore drilled approximately 3/4” through the center, and a 5/64” bore
through the remainder o f the block (approximately 1/4” long). A titanium cylinder with
a 5/64” bore, fits into the nozzle body and acts as the cathode. A bore is drilled into the
side o f the block for a metal contact to the electrode. A titanium wafer, 0.2” thick, is
used as the anode. The initial design had a considerable amount o f dead space between
the inlet and the cathode, which likely disrupted the gas flow. To remedy this, a glass
insert with a 5/64” bore was cut to fit into this space. This configuration worked well
for OH radical, and the use o f the titanium cathode appears to be a factor in reducing
the occurrence of noise. However, neither Ar-OH nor Ar-SH could be observed with
this configuration, and while FeCO was observed, the signal intensity was low.
Therefore, further improvements were needed, and the next nozzle incorporated the
lessons learned about the use o f titanium to reduce noise, but enabled a smoother flow
of gas through the discharge body.
The third, shown in Figure A.3., consists o f a 1” long Teflon cylinder that attaches to
the pulsed valve assembly (the stainless steel nozzle body has been eliminated in this
design). A 3/16” thick Teflon “cap” fits onto the outlet end o f the cylinder. This cap
contains a recess into which the rectangular titanium cathode (0.065” thick, 0.6” by
. ”) sits, such that, when the discharge nozzle is assembled, the cathode is fully
0 8
encased in the insulator. The same titanium wafer used in the discharge nozzle shown
190
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in Figure A. 2 is used as the anode. A total o f five Teflon nozzle bodies were made,
with different channel dimensions and these are shown in Table A.2. While only the
first three in the table proved to be effective in observing radical species, this proved to
be the most versatile design and was used successfully in the detection o f the OH-H 2 O
radical complex . 1
Figure A. 3 Discharge nozzle that attaches to the General Valve Series 9 pulsed
nozzle assembly (a). It consists o f a 0.002” thick titanium wafer anode (b), a
Teflon discharge body with either a 5/64” dia. channel with a flair at the end (c) or
a continuous flair (d), a 5/16” thick, rectangular titanium cathode (e) that fits into a
recess o f the Teflon cap (f). See text and Table A. 1 for details o f nozzle body (c,
d) dimensions.
191
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Table A. 1 Dimensions of the various discharge nozzle bodies corresponding to
configuration shown in Figure A. 3.
Nozzle Body8
_
2
3
4
5
e f o r e ' F l a i r angle
_
3/4”
1/2”
0C
0C
_
-12°
-12°
-19°
-22°
Outlet opening
Zo'.22”
-0 .1 9 ”
-0 .2 8 ”
-0 .3 5 ”
-0 .4 1 ”
(a) N ozzle bodies arbitrarily labeled 1-5. (b) Inlet opening and channel diam eter = 5/64” for all
discharge nozzle bodies, (c) Flair is continuous throughout the discharge nozzle body.
Finally, a discharge nozzle was made which is identical to the nozzle used by Endo, et
al. for the production o f Ar-OH .4 Although this nozzle configuration improved the
signal intensities o f closed-shell complexes such as Ar-F^S , 5 it produced a great deal of
noise and none o f the radical species attempted were observed.
Table A. 2 lists the best discharge nozzle for each radical species, at the time the
rotational transitions were observed. Two points should be noted regarding the “best”
nozzles, however. First, these studies were done in the early stages o f development of
the DC discharge for the MN-FTMW and Helmholtz coils had not yet been employed
to null the effect o f the Earth’s magnetic field. Therefore, the radical species observed
may have exhibited Zeeman splitting. Second, at the time of these studies, the rods
onto which the mirrors were mounted were constructed o f 440 stainless steel, which had
been magnetized in the milling process.
These rods created an inhomogeneous
magnetic field inside the instrument, which again, may have caused Zeeman splitting of
the observed transitions. Thus, while the observed transitions were, in general, clean
192
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and in each case, found at the reported literature values, the signal intensities may have
been diluted in some cases. Having established the limitations of the original
evaluation, the experimental conditions used to observe each o f the known radical
species will be detailed individually in the following sections.
Table A. 2 Known radical species observed and the best nozzle configuration for each
at the time o f observation.
Species
Best Nozzle
OHa
Figure A. 2
Transition(s)
Observed
J=T/2<—1/2, F=1 <—l h
J=5/2<—5/2, F=2<—2‘
Ar-OHb Figure A. 3e J=5/2<—3/2, F=2<—2k
Figure A. 3f
5 6 FeCOc
J=l<-0
J=2<—1
Ar-SHd Figure A. 3e J=7/2<—5/2, F=3<-2j
Frequency
Intensii
(MHz)
This
Literature
Work
4750.657
4750.656 1.0539
6030.753
6030.749 2.7
15351.581
8585.505
17171.066
15351.584
8585.503
17171.068
1.125
0.0467
0.052
10985.736
10985.736
0.0047
(a) Ref. 6 (b) Ref. 4 (c) Ref. 7 (d) Ref. 8 (e) Corresponds to nozzle body #1, in Table A .l (f)
Corresponds to nozzle body #2, in Table A .l (g) A rbitrary units, (h) £2 = 1/2 (i) Q = 3/2 (j)
Transition corresponding to the low er parity doublet, (k) Transition corresponding to upper
parity doublet.
193
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OH and Ar-OH
OH radical w as produced by flowing A r over a reservoir o f liquid w ater at a stagnation
pressure o f 30 psig and expanding through the discharge nozzle at a repetition rate o f 5
HZ. The best discharge nozzle for OH radical, listed in Table A. 2, is shown in Figure
A. 3.
For the observation o f OH radical, the reservoir was placed outside the
instrum ent, immediately upstream o f the pulsed nozzle connection, as shown in Figure
A. 5.
This configuration, however, made it difficult to control the concentration o f
w ater vapor entering the cham ber while maintaining the optimal stagnation pressure.
For Ar-OH, a lower concentration o f w ater vapor was desirable , 4 so to achieve this, the
reservoir was placed farther upstream , approxim ately
8
feet from the nozzle connection.
Optimal signal intensities for Ar-OH were obtained with a stagnation pressure o f 25
HV connection
Figure A. 5 Experim ental set-up for observation o f OH radical.
194
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psig and a repetition rate of 5 Hz. The best nozzle configuration, listed in Table A. 2, is
shown in Figure A. 3, using nozzle body #1, from Table A. 1. The outlet opening is
nominally 0.19” diameter.
Ar-SH
Ar-SH was generated by pulsing a 0.15% mixture o f H 2 S in Ar at a stagnation pressure
of 20 psig through the discharge nozzle at a repetition rate of 5 Hz. As with Ar-OH, the
best nozzle configuration, listed in Table A. 2, is shown in Figure A. 3, using nozzle
body #1, from Table A. 1.
FeCO
FeCO was generated by pulsing a 0.4% mixture o f Fe(CO)s in Ar at a stagnation
pressure o f 6 psig through the discharge nozzle at a repetition rate o f 5 Hz. The mixture
was prepared by injecting a sample o f liquid Fe(CO)s into an evacuated, stainless steel
ballast through a septum. The ballast was then filled with argon to the desired pressure.
The best nozzle configuration, listed in Table A. 2, is shown in Figure A. 3, and uses
nozzle body #2, from Table A. 1.
195
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References
1
(a) Chapter 5, this document, (b) Brauer, C. S.; Sedo, G.; Grumstrup, E. M.; Leopold,
K. R.; Marshall, M. D.; Leung, H. O., “Effects of partially quenched orbital
angular momentum on the microwave spectrum and magnetic hyperfine
splitting in the OH-water complex.” Chem. Phys. Lett., 401, 420-425 (2005).
2
(a) Phillips, J. A., “Structure and dynamics o f partially-bound molecular complexes.”
University o f Minnesota, Minneapolis, MN, USA.
Ph.D. Thesis, 1996. (b)
Phillips, J. A.; Canagaratna, M.; Goodfriend, H.; Grushow, A.; Almlof, J.;
Leopold, K. R., “Microwave and ab initio investigation o f HF-BF 3 ” J. Am.
Chem. Soc., 117, 12549-12556 (1995).
3
Engleking, P. C., “Corona excited supersonic expansion.” Rev. Sci. Inst., 57, 22742277 (1986).
4
Ohshima, Y.; Minora, I.; Endo, Y., “Observation o f the pure rotational spectra o f the
ArOH and ArOD complexes by a Fourier-transform microwave spectrometer.”
J. Chem. Phys., 95, 7001-7003 (1991).
5
Gutowsky, H. S.; Emilsson, T.; Arunan, E., “Rotational spectra, structure, and internal
dynamics o f Ar-HhS isotopomers.” J. Chem. Phys., 106, 5309-5315 (1997).
6
(a) Radford, H. E., “Scanning microwave echo box spectrometer.” Rev. Sci. Instrum.
39, 1687-1691 (1968). (b) Meulen, J.J.; Meerts, W.L.; van Mierlo, G.W.M.;
Dymanus, A., “Observations o f population inversion between the A-doubled
states o f O Yir Phys. Rev. Lett., 36, 1031-1034 (1976).
196
R eproduced with perm ission of the copyright owner. Further reproduction prohibited without permission.
7
Kasai, Y.; Obi, K.; Ohshima, Y.; Endo, Y.; Kawaguchi, K., “Pure rotational spectrum
o f FeCO.” J. Chem. Phys., 103, 90-95 (1995).
8
Sumiyoshi, Y.; Endo, Y.; Ohshima, Y., “Intermolecular potential-energy surface for
the
Ar-SH(2Pi)
complex
studied
by
Fourier-transform
spectroscopy.” J. Chem. Phys., 113, 10121-10129(2000).
R eproduced with perm ission of the copyright owner. Further reproduction prohibited without permission.
microwave
Appendix B. Heated Nozzle.
Many interesting m olecules exist whose vapor pressure is so low at room tem perature
that gas phase studies are precluded. W hile creative m ethods have been developed to
overcome this barrier, such as in the production o f H 2 SO 4 in situ via the reaction o f SO 3
and w ater vapor , 1 it is m ost often the case that a solid or liquid m ust simply be heated to
produce vapor. To this end, a heating apparatus was designed to fit onto the pulsed
nozzle currently in use in the M innesota Fourier transform m icrowave spectrom eter
(M N-FTM W ) . 2
The heating apparatus is com posed o f a flat Kapton™ heater (W atlow, 0404c-04, 120
V, 125 W) that is rolled inside a cylindrical, Teflon insulating sleeve (Figure B .la).
This heating sleeve slides over the pulsed nozzle shaft (Figure B .lb ), onto w hich a
Kapton™
Fleater
Glass
wool
Connections
for heater and
therm ocouple
H eating —
sleeve
Figure B. 1 (a) Pulsed nozzle assembly, (b) Enlarged view o f heating sleeve.
198
R eproduced with perm ission of the copyright owner. Further reproduction prohibited without permission.
sample reservoir has been attached (Figure B.2).
The solid sample to be heated is
placed into the sample reservoir, and the heating sleeve is packed with glass wool for
thermal contact. A thermocouple wire is threaded through existing hypodermic tubing
(0.365” O.D.) and attached to the outside of the reservoir to monitor the temperature.
Fine screen cut to fit
over 1/4” tubing.
4 - 0 .0 5 ”
dia. holes.
3/4” to 1/4”
Swagelock™
reducing union
• •
Solid sample.
0.3” to top of
sealed tubing.
1/4” tubing, turned
down to press fit.
Sealed at top.
To solenoid valve
Figure B. 2 Sample reservoir made from modified Swagelock™ reducing union.
The sample reservoir was made by modifying a 3/4” to 1/4” Swagelock™ reducing
union by inserting a section o f stainless steel tubing that is press-fit into the 1/4”
opening of the reducing union. This section o f tubing is sealed at the top, and has four
holes (0.05” dia.) drilled into the side, immediately below the top of the tubing. The
R eproduced with perm ission of the copyright owner. Further reproduction prohibited without permission.
sample is placed into the bottom o f the reservoir, such that it occupies the space below
the 0.05” holes. To minimize the loss o f solid sample, a fine screen is cut to fit over the
1/4” tubing and rest on the top of the sample, just below the 0.05” holes. The reservoir
is attached to the nozzle assembly immediately before the General Valve Series 9
pulsed solenoid valve, and the heating sleeve is placed over the entire assembly.
This heating assembly was tested on a solid sample o f cp-V(CO)4 , and a portion o f the
microwave spectrum o f this molecule, originally measured by McKay and coworkers , 3
was readily observed.
200
R eproduced with perm ission of the copyright owner. Further reproduction prohibited without permission.
References
1 Fiacco, D. L.; Hunt, S. W. and Leopold, K. R., “Microwave investigation o f sulfuric
acid monohydrate.” J. Am. Chem. Soc., 124(16), 4504-4511 (2002).
2 (a) Phillips, J. A., “Structure and dynamics o f partially-bound molecular complexes.”
University o f Minnesota, Minneapolis, MN, USA.
Ph.D. Thesis, 1996. (b)
Phillips, J. A.; Canagaratna, M.; Goodfriend, H.; Grushow, A.; Almlof, J.;
Leopold, K. R., “Microwave and ab initio investigation o f H F -B F 3 ” J. Am.
Chem. Soc., 117, 12549-12556(1995).
3 McKay, R.T.; Hubbard, J. L.; Kukolich, S. G., “The microwave spectrum of
cyclopentadienyl vanadium tetracarbonyl, a fluxional molecule.”
Spectrosc., 172, 378-383 (1995).
R eproduced with perm ission of the copyright owner. Further reproduction prohibited without permission.
J. Mol.
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