close

Вход

Забыли?

вход по аккаунту

?

Fourier transform microwave spectroscopy of multiconformational molecules andvan der Waals complexes

код для вставкиСкачать
INFORMATION TO USERS
This manuscript has been reproduced from the microfilm master. UMI
films the text directly from the original or copy submitted. Thus, some
thesis and dissertation copies are in typewriter face, while others may
be from any type of computer printer.
The quality of this reproduction is dependent upon the quality o f the
copy submitted. Broken or indistinct print, colored or poor quality
illustrations and photographs, print bleedthrough, substandard margins,
and improper alignment can adversely afreet reproduction.
In the unlikely event that the author did not send UMI a complete
manuscript and there are missing pages, these will be noted. Also, if
unauthorized copyright material had to be removed, a note will indicate
the deletion.
Oversize materials (e.g., maps, drawings, charts) are reproduced by
sectioning the original, beginning at the upper left-hand comer and
continuing from left to right in equal sections with small overlaps. Each
original is also photographed in one exposure and is included in
reduced form at the back of die book.
Photographs included in the original manuscript have been reproduced
xerographically in this copy. Higher quality 6" x 9" black and white
photographic prints are available for any photographs or illustrations
appearing in this copy for an additional charge. Contact UMI directly
to order.
A Bell & Howell Information Com pany
300 North Z eeb Road. Ann Arbor. Ml 48106-1346 USA
313/761-4700 800/521-0600
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
WESLEYAN UNIVERSITY
Fourier Transform M icrowave Spectroscopy
o f M ulticonform ational M olecules
and van der W aals Com plexes
A thesis submitted to the Faculty o f Wesleyan University in
partial fulfillment o f the requirements for the degree o f
Doctor o f Philosophy
by
Angela R. Hight Walker
Middletown, Connecticut
September, 1994
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
UMI
Number: 9532486
Copyright 1995 by
Hight Walker, Angela Renee
All rights reserved.
OMI Microform 9532486
Copyright 1995, by UMI Company. All rights reserved.
This microform edition is protected against unauthorized
copying under Title 17, United States Code.
UMI
300 North Zeeb Road
Ann Arbor, MI 48103
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
To George E. Carver,
whose wonderous ability to share his knowledge and love
of physics has opened my mind,
and touched my heart.
ii
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Acknowledgements
Listed below are many o f the people who have influenced my life and career
during my time at Wesleyan University. Each person below has touched my life in a
special and unique way and I wish to thank them all.
Coworkers
Professor Kimberely Grant
Brian Bean
Wei Chen
Aaron Schoeffler
Visiting Professors
Professor Robert Bohn
Professor Karen Peterson
Committee Members
Professor Joe Knee
Professor Brian Stewart
Wesleyan Faculty
Professor Pete Pringle
Professor A1 Fry
Professor David Beveridge
Dr. Susan Sobolov
Wesleyan Staff
Lucile Blanchard
Jean Hazen
Don Albert
Holly Castelli
Bret Roth
Wesleyan M achine and Electronics Shop
Tom Castelli
David Boule
Dick Widlansky
Bruce Strickland
Matt Tidgewell
Harry Allen
Allen Alonzo
David Carlson
NIST
Dr. Frank Lovas
Dr. Richard Suenram
Dr. Gerald Fraser
m
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Family
Benjamin and Tana Hight
Alexis Hight Walker
Trevor J. Hight Walker
Ester B. Cottrill
Special Friends
Ron Lewis
Kieran Curley
M ohammad and Kim Al-Laham
Patrick and Ann DiSarro
Todd and Mamie Baggett
Matt and Tammie Erick
Everyone in CO-OP
Chris Retardies
Vonnie Christiansen
Tom and Holly Castelli
Tom and Nicole Marron
Mike and Michele Roberts
Susan and Burton Jaynes
Everyone on the softball teams
Most importantly, I would like to thank my research advisor, Professor
Stewart E. Novick. Not only is Dr. Novick my ‘Yount o f all knowlege,” he is also a
man of great character. Working for him has enabled me to develop as a scientist and
as a person. I will always respect him and his work, and I could not have had a more
perfect advisor.
IV
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Thesis A bstract
With the use of a Fourier transform microwave (FTM) spectrometer, structural
determinations of two types of species; multiconformational molecules and van der
Waals complexes, have been performed. Presented in this thesis are three sections
summarizing this research effort. The first section contains a detailed explanation o f
the FTM instrument. In Section n, the study o f three multiconformational molecules is
presented as two chapters . Finally, three chapters in Section
in outline the work still
in progress on many van der Waals complexes.
Section I was written to be a "manual" for the FTM spectrometer and to aid
new additions to the group in their understanding o f the instrument. An instruction
guide is necessary for home-built instruments such as this one due to their unique
design and application.
Vital techniques and theories are discussed and machine
operation is outlined. A brief explanation o f general microwave spectroscopy as
performed on an FTM spectrometer is also given.
Section II is composed o f two chapters pertaining to multiconformational
molecules. In Chapter 2, a complete structural analysis o f dipropyl ether is reported.
The only conforsaer assigned had Cs symmetry. Many transitions are yet unassigned.
Chapter 3 summarizes an investigation o f two nitrosamines; methyl ethyl and methyl
propyl nitrosamine. Only one conformer was observed for methyl ethyl nitrosamine,
v
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
but two were assigned to methyl propyl nitrosamine. Nuclear hyperfine structure and
internal methyl rotation complicated the spectra.
The final section, Section m , contains the ongoing progress on weakly bound
van der Waals complexes.
The analysis o f the OCS--HBr complex identified the
structure as quasi-linear with large amplitude bending motions. Five separate
isotopomers were assigned. Transitions originating from the HBr--DBr complex were
measured and presented in Chapter 5. Although early in the analysis, the structure was
determined to be bent and deuterium bonded.
The final chapter o f this section is
meant to be a permanent record o f transition frequencies whose molecular carrier is
still in question. Two different groups o f transitions from two different samples are
listed. Further work is needed to unambiguously assign the frequencies with a carrier
and quantum numbers, however the complexes (H 2 O)--(HC 1 ) 2 and NO--H 2 O are
considered possible suspects.
vi
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Table of Contents
Dedication.........................................................................................................................“
Acknowledgments...........................................................................................................i“
Thesis Abstract..M.................».........«»...«»m»....»..................................................... v
Table of Contents..........................................................................................................*vii
List of Figures....................................................................................................................*
List of Tables................................................................................................................... xi
Section I. The Technique of Fourier Transform Microwave Spectroscopy
Introduction......................................................................................................................2
C hapter 1.
Overview............................................................................................................................ 6
Pulsed Beam Sources.........................................................................................................8
Fabry-Perot Cavity........................................................................................................... 12
Electronic Components................................................................................................... 13
Data Collection and Results............................................................................................. 14
vii
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Section n . Multiconformational Molecules
Introduction...................................................................................................................38
C hapter 2.
Microwave Determination of the Structure of the Cs
Conformation of Dipropyl Ether.
Abstract........................................................................................................................... 40
Introduction.....................................................................................................................41
Experimental....................................................................................................................43
Results............................................................................................................................. 45
C hapter 3.
Rotational Spectra of Methyl Ethyl and Methyl Propyl
Nitrosamines. Conformational Assignment, Internal Rotation and
Quadrupole Coupling.
Abstract.......................................................................................................................... 61
Introduction.....................................................................................................................62
Experimental....................................................................................................................64
Results and Discussion....................................................................................................65
Structural Conclusions.....................................................................................................72
Supplementary Material.................................................................................................. 85
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Section HI. van der Waals Complexes
Introduction.................................................................................................................105
C hapter 4.
A Structural Determination of the OCS—H B r van der Waals
Complex
Abstract.......................................................................................................
107
Introduction................................................................................................................... 107
Experimental................................................................................................................. 109
Results...........................................................................................................................I l l
Conclusion.................................................................................................................... 117
C hapter 5.
Early Analysis of the HBr—DBr van der W aals Complex
Abstract.........................................................................................................................131
Introduction.................................................................................................................. 131
Experimental.................................................................................................................136
Results and Discussion.................................................................................................137
C hapter 6 .
Unassigned Transition Frequencies with Unknown Molecular
Carriers
Introduction..................................................................................................................148
H20-(HC1)2................................................................................................................149
N 0 - H 20 ......................................................................................................................155
ix
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
List of Figures
Section I.
Chapter 1.
Figure 1. Oversimplified circuit diagram o f the FTM spectrometer............... 16
Figure 2. Solenoid pulsed valve........................................................................17
Figure 3. Generalized circuit diagram............................................................... 18
Figure 4. Actual circuit diagram........................................................................19
Figures 5-16. Typical spectra o f OCS isotopomers, coaxial........................ 20
Figures 17-21. Typical spectra o f OCS isotopomers,perpendicular................32
Section II.
Chapter 2.
Figure 1. Cs conformation o f dipropyl ether.....................................................59
Chapter 3.
Figure 1. Two conformations o f MetEtNO.......................................................83
Figure 2. Two conformations of MetProNO.....................................................84
Section PI.
Chapter 4.
Figure 1. Determined structure o f QCS—HBr complex..................................119
Figure 2. Typical rotational transition o f OCS—HBr...................................... 120
Chapter 5.
Figure 1. Tunneling motion in hydrogen halide symmetric dimers................ 140
Figure 2. One dimensional potential energy surface o f (HX ) 2 .......................141
Figure 3. Correlation diagram for energy levels o f (HX ) 2 ............................. 142
Figure 4. Perturbation to unsymmetric well depths energy levels................. 143
Chapter 6 .
Figure 1. Structure ofH 2 0 ~(HCl) 2 complex.................................................161
Figure 2. Structure o f NQ—HrjQ complex...................................................... 162
x
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
List of Tables
Section II.
Chapter 2.
Table 1. Assigned transition frequencies for dipropyl ether (Cs conformer)...54
Table 2. Unassigned transition frequencies for dipropyl ether..........................55
Table 3. Spectroscopic constants for the Cs structure.................................... 56
Table 4. Structural parameters relating to the Cs conformation..................... 57
Table 5. Structural fits of the Cs conformation............................................... 58
Chapter 3.
Table 1. Calculation verses observed conformers for MetEtNO.................... 75
Table 2. Transition frequencies o f MetEtNO.................................................. 76
Table 3. Spectroscopic constants for MetEtNO............................................. 77
Table 4. Methyl group internal rotation analysis.............................................78
Table 5. Calculation verses observed conformers o f MetProNO................... 79
Table 6 . Transition frequencies for MetProNO.............................................. 80
Table 7. Spectroscopic constants for MetProNO............................................. 81
Table 8 . Methyl group internal rotation barriers................................................ 82
Section III.
Chapter 4.
Table 1. Transition frequencies o f OCS—H ^ B r and Q C S -H ^ B r............. 121
Table 2. More transition frequencies of OCS—H ^ B r and G C S -H ^ B r.... 122
Table 3. Transition frequencies o f OCS—D ^ B r and OCS—D ^ B r ............. 123
Table 4. More transition frequencies of OCS—H ^ B r and OCS—H ^ B r .... 124
xi
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Table 5. Transition frequencies of O C ^ S —H ^ B r .......................................125
Table 6. More transition frequencies o f O C -^S --H ^ B r.............................. 126
Table 7. Spectroscopic constants for five isotopomers o f OCS—HBr.......... 127
Table 8. Structural information for OCS—HBr.............................................. 128
Table 9. Dynamical information for OCS—HBr............................................. 129
Chapter 5.
Table 1. Transition frequencies o f H ^ B r—D ^ B r.........................................144
Table 2. Transition frequencies of H^ ^Br—
^Br.........................................145
Table 3. Transition frequencies o fH ^ B r—D ^ B r and/or H ^ B r-D ^ B r... 2 4 6
Chapter 6.
Table 1. Unassigned transitions possibly from H 2 O—(HC1)2........................163
Table 2. Unassigned transitions possibly from NO—H 2 0 ............................. 164
xii
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Section I:
The Technique of Fourier Transform Microwave Spectroscopy
1
IReproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Introduction
This section o f the thesis contains only one chapter titled The Technique of
Fourier Transform Microwave Spectroscopy. It was written as a manual for new
researchers joining the group such that they might gain insight into the particular
workings o f our pulsed beam Fabry-Perot Fourier transform microwave (FTM)
spectrometer.
Thus no new science is presented here.
Rather a review o f the
pertinent technical design and spectroscopic theory is given. Most importantly, this
section of the thesis provides future researchers with benchmark spectra of a test
molecule, carbonyl sulfide (OCS), which will allow for instrumental diagnostics to be
performed.
The building o f the FTM spectrometer spanned a significant portion o f my
graduate career. The funding for the construction o f the instrument came from an
NSF grant (CHE-9215098) that contained a special twist. The proposal claimed that,
if funded, this instrument would be shared by four co-principal investigators from four
different research universities.
Dr. Stewart Novick o f Wesleyan University, Dr.
Robert Bohn o f the University o f Connecticut, Dr. Karen Peterson o f the University o f
Rhode Island, and Dr. Mark Marshall o f Amherst College, collectively call themselves
the Southern New England Microwave Consortium.
This collaboration among the four co-principal investigators has evolved into a
steady stream o f laboratory visitors and projects from all areas o f microwave
spectroscopy 1. Furthermore, interaction with these scientific visitors has broadened
my research experience and provided me with an unique opportunity to pilot three
different spectrometers; the FTM spectrometer, the Hewlett Packard Stark cell and the
molecular beam electric resonance (MBER) spectrometer, all o f which belong to the
2
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
consortium.
Both the FTM and the MBER spectrometers are kept at Wesleyan
whereas the conventional Stark cell spectrometer is housed at UCONN.
As the design o f the FTM spectrometer was completed in the late seventies,
only a dozen o f these instruments are in existence today. However, with each new
spectrometer, subtle changes and innovations are made as the new technology moves
forward. Neither a text nor a manual is available to summarize the theory behind and
operation o f the spectrometer.
Thus, presented in this section is a single chapter
broken into several parts. After a short summary o f the experimental procedure, the
various componentry o f the instrument is detailed. Finally, results and typical spectra
are given in the conclusion.
Hopefully, this chapter will provide insight into any
questions an operator may have in the future.
1. Laboratory Visitors and Projects fas of 8/941
Professor Karen Peterson, University o f Rhode Island, 7/4/93, (CO)2
Professor Jim LoBue, Ursinus College (now Georgia Southern University), 7/5 7/9/93, further conformations o f dipropyl ether
Professor Robert Bohn and Lou Qi, University o f Connecticut, 7/19 - 7/27/93, methyl
ethyl nitrosamine
Dr. Dan Kohn, Department o f Chemistry, Harvard University, 8/2 - 8/6/93, various
radicals including CF3 and H 2 CCCH, using Peter Chen's flash pyrolysis source
Professor Kimberley Grant, College o f St Elizabeth, 8/9 - 8/13/93, PF 3 HC1 and NF 3
HC1
Dr. Mike McCarthy, Center for Astrophysics, Harvard Smithsonian, 8/17 - 8/20/93,
various radicals including H 2 CCCH, HNCN, SO, and HCCN
Dr. Mark Lindsay, Center for Astrophysics, Harvard Smithsonian, 8/17/93, to observe
pulse discharge radical source
Brian Bean, Amherst College, (Brian is an undergraduate student o f Professor Mark
Marshall, he spent the summer in the lab as a NECUSE student and
participated in all the projects, his own personal project was:) HBr OCS
3
Reproduced with permission o f the copyright owner. Further reproduction prohibited without permission.
Professor Robert Bohn, UCONN, 12/13 -12/15/93, 3,6 dichloropyridazine
Professor Jim LoBue, Georgia Southern University, 12/20/93 - 12/24/93, CO2 CgH^
Brian Bean, Amherst, 1/3 - 1/7/94, to continue work on HBr OCS
Professor Kimberley Grant, College o f St Elizabeth, 1/10 - 1/14/94, (PF3 ) 2
Professor Karen Peterson, University of Rhode Island, 5/13, 17, 19, 25/94, NO H 2 O
Professor Robert Bohn, University o f Connecticut, 5/31 - 6/3/94, methyl ethyl
formamide, CH3 (NCHO)CH 2 CH3
Professor Karen Peterson, University of Rhode Island, 6/7, 8 , 22,24/94, NO H 2 O
Professor Kimberley Grant, College of St Elizabeth, 8/1 - 8/5/94, A rN F 3
Professor Jim LoBue, Georgia Southern University,
8 /8
- 8/12/94, CO2 CgHg
Professor Robert Bohn, University o f Connecticut, 8/18 - 8/19/94, 3-hexyn-2-one,
CH3 CH2 C=C(CO)CH3
Professor Wallace Pringle and David McCamant, Wesleyan University, 8/29 - 9/2/94,
Ar cyclopropane and Ar cyclobutane
4
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Chapter 1; The Instrument: The Fourier Transform Microwave Spectrometer
5
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Overview
Microwave spectroscopy is a mature and well understood subject in the field o f
chemical
p h y s ic s ^ .
Therefore, the author will not review the theory o f spectral
analysis, but rather provide information as to how one obtains the rotational spectra.
Before discussing the main elements in the technique o f Fourier transform microwave
spectroscopy, a brief history o f the instrument is useful. In the late seventies, T. J.
Balle and W. H. Flygare designed and constructed a new type o f microwave
spectrometer which allowed for the first time the collection o f microwave spectra data
in the time rather than the frequency domain^. The resulting instrument provided high
sensitivity and high resolution for extremely accurate structural determinations o f both
stable and unstable species.
Since the original FTM spectrometer, built at the
University o f Illinois, over a dozen machines have been built in laboratories around the
world where various improvements and innovations have been made. Several review
articles, including the original by Flygare^, have been written summarizing the changes
and technological advances in the instrum entation^.
The National Institute o f Standards and Technology (NIST) has lead the way
in advancing rotational spectroscopy with the use o f the FTM spectrometer. It was
with the help of two o f their senior research scientists, Dr. Richard Suenram and Dr.
Frank Lovas, that the Wesleyan spectrometer came on-line in such a short amount o f
time. As a matter o f fact, the instrument at Wesleyan was constructed using the very
blueprints NIST used to build their second FTM spectrometer. The Novick group is
very grateful for the help and guidance provided by NIST.
The central feature o f the Fourier transform microwave spectrometer is the
Fabry-Perot cavity, which is formed by two concave spherical mirrors housed inside an
6
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
evacuated chamber. Within the center o f the cavity molecular absorption and
reemission takes place. An oversimplified circuit diagram o f the instrument, sketched
in Figure 1, is useful for this survey o f the experimental procedure. There are four
main elements o f the FTM spectrometer; the pulsed-beam source, the Fabry-Perot
cavity, the electronic components including the pulsed microwave source, and the data
collection system
The experimental procedure occurs in the following sequence. A gas pulse o f
in the simplest case, about 1 % of the species o f interest seeded in an inert carrier (such
as Ar or Ne) is supersonically expanded into the Fabiy-Perot cavity.
Once the
rotationally cooled beam travels to the center o f the cavity, a microwave pulse at
frequency v, to which the cavity is critically tuned, is coupled through a mirror via an
L-shaped antenna and into the cavity. The duration o f this microwave pulse is about 1
ps, thus the Heisenberg uncertainty principle deems a bandwidth, Av, centered around
v, to be on the order o f 1 MHz. The molecules and the radiation interact within the
cavity, and if the species under investigation has a rotational transition in the range of
Av about v, absorption occurs, inducing a macroscopic polarization o f the gas. After
the polarizing pulse of radiation dies away, the molecules that absorbed the energy will
reemit it at the resonant frequency, vr. These signals, called free induction decays
(FDD), are collected and averaged in order to give a suitable signal to noise ratio. A
Fourier transform is performed to obtain the power spectrum typically desired. The
frequency o f the pumping microwave radiation is stepped to the next frequency and
the process is performed again.
From the above summary o f the events involved in Fourier transform
microwave spectroscopy, the crucial nature o f the correct timing sequence is obvious.
7
Reproduced with permission o f the copyright owner. Further reproduction prohibited without permission.
Although not examined as its own topic below, references are made as to the order
and length o f the timings throughout the chapter.
Pulsed-Beam Sources
In this section,
the method o f introducing a gas pulse into the FTM
spectrometer is discussed.
In the opinion o f the author, the nozzle is the most
important feature of the spectrometer, so a more detailed explanation o f the gas
expansion and of the mechanical details is necessary.
There are real advantages to using a pulsed molecular beam instead o f a
continuous flow source. With a pulsed nozzle (0.5 mm diameter opened for 800 ps)
less gas is passed through the instrument and therefore being used than for the case o f
a nozzle o f the same size open continuously. Thus, there is a substantial reduction in
the rate of sample consumption. It has been determined that almost 15% more sample
is used for a continuous nozzle (diameter o f 25 microns opened continuously)
compared to the pulsed nozzle^. For a given pumping speed system, a much larger
diameter can be used in the pulsed nozzle than possible in the continuous nozzle.
Furthermore, dramatically lower rotational temperatures are achieved with the
pulsed nozzle.
The continuous nozzle produces a molecular beam rotational
temperatures on the order o f 10 K
However, as a direct result o f the larger nozzle
diameter, the pulsed molecular beam in the FTM spectrometer achieves a rotational
temperature o f less than or equal to 1 K, ensuring the sample to be in the lowest
possible energy state. Therefore, one can expect a simplified spectra resulting from
fewer populated energy levels.
The nozzles used in the Novick laboratory are commercially available from
General Valve Corporation. We have had nothing but a positive relationship with this
8
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
company. In addition to excellent service, General Valve has been open to considering
new design changes and ideas concerning the various nozzle improvements necessary
for advancement of the FTM technique. The author recommends using their products.
A diagram of the commercially available solenoid valve is drawn in Figure 2. It
is housed inside the vacuum chamber, but can be easily removed for repairs. The
nozzle is shown as a small circular orifice in the bottom o f the face plate ranging fiom
0.5 mm to 1.0 mm in diameter.
The Novick group typically uses a pulsed pinhole
nozzle o f 0.5 mm diameter with a stagnation pressure o f 1-2 atm.
Also shown in this figure is the poppet which plugs the hole between pulses to
ensure no gas throughput into the chamber. As mentioned in later chapters o f this
thesis, the poppet is in constant need o f attention. The teflon poppets normally used
for our experiments deform easily and must be replaced. The condition o f the poppet
will determine how well the cooling process via the supersonic expansion occurs.
Therefore, it is critical to monitor the pressure o f the chamber, and observe as each
pulse takes place, a rise and fall in the pressure. With a 0.5 mm nozzle, 1 atm of
backing pressure, and nozzle opening time of 800 (as, this should be done with an
approximate pressure o f 5 x 10*5 t 0 rr.
The solenoid valve is actually two separable sections, as shown by the two
different directions of the cross hatching in Figure 2. By unscrewing the face plate
from the body o f the valve, one has access to the poppet, two springs, the plunger, and
the o-ring. The poppet is encased in the open-ended cylindrical plunger which rests
upon a short spring (not shown in the diagram). On top o f the poppet, also enclosed
in the plunger is a longer spring which pushes and pulls the poppet in and out o f the
nozzle hole.
9
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
A second style o f face plate has been designed in cooperation with General
Valve, building on an idea found in Reference 9, which holds about five milliliters o f a
liquid sample. By drilling out the area o f this face plate and enlarging it, a valve was
constructed for use with samples whose vapor pressure forbade a gas mixture to be
made. With a small amount o f liquid in the reservoir, argon is blown over the sample
which grabs a small percentage for beam production. Furthermore, a heating band
exists which makes heating the liquid possible for temperatures under 150 degrees
Celsius.
Afier repairs have been made, reattaching the face plate to the body is another
critical detail in the performance o f the nozzle. The tension (number o f screw turns)
must be at a specific point for the pulsed nozzle to work correctly. Fine adjustments
are made by feeling the gas pulse beneath the nozzle hole and listening carefidly. The
sound of the pulses should be a clear snapping noise and it should feel like a quick
spirt o f gas symmetrically leaving the nozzle. Experience with this process is required
to fully understand the parameters. Most importantly, however, is to check for leaks.
This must be done before the valve is lowered back into the cavity. Simply stop the
pulsing, leave the gas flowing, and check for pressure leaving the nozzle hole.
Obviously, do not return a leaking nozzle into the chamber.
In most instances of unexplained decreases in the signal to noise ratio o f
transitions, the nozzle is to blame. It may be that the tension is not correct for beam
temperatures o f 1 K, or the poppet is deformed and leaking, or perhaps the O ring has
corroded and is also causing slight leaks o f gas into the chamber.
The author
suggests, at the first sign o f a problem, that the nozzle be checked.
10
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Now that the mechanical details o f the pulsed beam source have been
discussed, the supersonic expansion itself can be addressed.
A high pressure gas,
usually composed o f 1% species of interest and 99% inert rare gas, is pushed through
a pin hole nozzle into an evacuated Fabry-Perot cavity. The nozzle is held open only
for a short amount o f time, typically 800 ps. Thus, equilibrium expansion properties
are reached quite rapidly.
The resulting burst o f gas is neither skimmed nor
collimated. The repetition rate for the nozzle is between 10 and 30 Hz. It begins the
timing sequence in the FTM spectrometer.
The temperature o f the molecule beam plummets to almost zero as the
adiabatic expansion converts all random translational kinetic energy and internal
energy into directed mass flow as a result o f binary collisions. The rare gas, in huge
excess, dominates the expansion.
The formation o f dimers occurs readily in the
expansion through three body collisions, where the third body carries away the excess
energy thereby stabilizing the complex formed between the other two bodies. The
resulting molecular beam has rotational and transitional temperature o f < 1 K, a
narrow velocity distribution, and is rich in van der Waals complexes.
The pulsed molecular beam may be produced either perpendicular or parallel to
the Fabiy-Perot cavity. For liquid samples which require the liquid reservoir nozzle,
perpendicular is the only option at present, however It has been noticed by those at
NIST and elsewhere, that the sensitivity and the resolution o f the spectrometer
increase when the nozzle is parallel to the cavityl®.
To achieve this mode of
operation, a 1 mm diameter hole is drilled through a mirror in the Fabiy-Perot cavity.
The face plate is then attached directed to the back o f the mirror. With the nozzle in
this position, the molecules in the beam remain in the cavity longer, and the signal
11
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
emanating from the molecules lasts longer. These factors produce an enhanced signal
and extremely narrow rotational transition. Further discussion o f the two possible
orientations o f the pulsed nozzle is given in the Data Collection section o f this chapter.
Fabrv-Perot Cavity
Before discussing the Fabry-Perot cavity itself^ the vacuum chamber that
houses the cavity is a necessary topic to mention. The large stainless steel cylinder is
over a meter long and 56 cm in diameter. Each end is sealed with an O ring to a
stainless steel flange, one o f which is especially designed with hinges for a more
streamlined opening o f the chamber. Smaller ports and flanges are positioned on the
chamber to make access to the cavity quick and simple. A large flange on top o f the
instrument provides access for the perpendicular nozzle arrangement. A large port on
the bottom is for the Varian VHS-400 diffusion pump (ID=15"). This pump operating
at 80001/s combined with a 90 cfin mechanical pump create a pressure o f 2x10-7 torr
within the chamber. The only gate valve in the system is on the top flange used for
manipulation.
The Fabry-Perot cavity is defined by two parallel spherical concave mirrors
enclosed within an evacuated chamber. These two aluminum mirrors are 35.6 cm in
diameter with a radius o f curvature o f 84 cm. Their adjustable distance o f separation
is nominally 70 cm
The mirrors are suspended on four 3/4-inch diameter stainless
steel rods which bolt to aluminum end plates. For adjusting the mirror separation,
each mirror is fastened to four linear motion bearings which glide on the rods.
Furthermore, the aluminum end plates ride on bearings such that the entire mirror
assembly can be rolled out of the vacuum chamber for efficient servicing.
12
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
For adjusting the separation between the mirrors, one mirror is stationary and
the other is allowed to slide along the rails. A motorized micrometer, secured in the
aluminum end plate, pushes on the back o f one mirror, providing a total range of
motion o f 5 cm. Four springs are attached between the end plates and the back o f the
mirrors which provide the necessary tension and allow for backward motion.
With this mobility of the mirrors, it is possible to critically tune the cavity for a
specific frequency. The cavity was designed to operate between 6-24 GHz. However,
lower frequencies have been achieved without modification. In fact, the limiting factor
in the frequency range o f the instrument is largely the microwave componentry. The
spectral window covered by the FTM spectrometer is perfect for the rotational
transitions we are interested in probing. The pulse molecular beam populates only
low-J transitions which are typically in the 5-24 GHz region o f the microwave
spectrum.
The pulse o f microwave radiation is introduced into the cavity once the pulsed
molecular beam has reached the center o f the mirrors. To couple the radiation into the
cavity a 0.141" semirigid coaxial cable is used which extends through the center of the
mirror body and terminates into an L shaped antenna. Microwave powers on the
order o f 1 mW are typically used. However, the capability to go to 20 mW exists by
adding a second amplifier to the input circuit.
Electronic Components
A generalized circuit diagram o f the FTM spectrometer is given in Figure 3.
The preliminary features to note are the previously discussed pulsed nozzle and the
13
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Fabry-Perot mirrors.
Also, note the microwave synthesizer which produces the
microwave power used to stimulate the molecules in the chamber.
After being mixed with a 30 MHz frequency at the single side band modulator,
the microwave radiation is switched on and off. Once through the circulator, the
power is now spread over 1 MHz and thud may interact with the sample.
If a molecular transition occurs within the 1 MHz bandwidth, a bulk
polarization o f the sample results. Lasting longer than the polarizing pulse, the free
induction decay will be collected and averaged.
The exact circuit diagram of the FTM spectrometer as o f 9/6/94 is shown in
figure 4. The microwave components not mentioned previously will be discussed.
Data Collection
Once several time domain, free induction decays have been collected, a fast
Fourier transform manipulation is performed and the desired microwave spectra is
obtained. In order for the electronic equipment to with deal the frequency o f the
molecular transition, the signal must be amplified and "mixed down" twice, by a
process called super heterodyning..
As two types o f nozzle configurations exist, two different line shapes are
observed on the FTM spectrometer. In Figures 5-16 one can see that when the nozzle
is coaxial to the microwave pulse, a doublet appears.
In figures 17-21, test spectra were taken for isotopomers o f OCS, a single line
appears which has a dip in the center.
14
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
References
1. C.H. Townes and A.L. Schawlow, Microwave Spectroscopy. Dover, New York,
(1975)
2. W. Gordy and R.L. Cook, Microwave Molecular Structure. Wiley-Interscience,
New York, (1984)
3. T.J. Balle, W.H. Flygare, Rev. Sci. Instrum. 52, 33 (1981)
4. F J. Lovas, R.D. Suenram, J. Chem. Phys. 87, 2010 (1987)
5. A.C. Legon, Ann. Rev. Phys. Chem. 34, 275 (1983)
6
. H. Dreizler, Molec. Phys. 59, 1 (1986)
7. Y. Xu, W. Jager, M.C. Gerry, J. Mol. Spect. 151, 206 (1992)
8
. E.J. Campbell, L.W. Burton, T.J. Balle, M.R. Keenan, W.H. Flygare, J. Chem.
Phys. 74, 829 (1981)
9. Richard Suenram, private communication
10.
Jens Uwe-Grabow, private communication
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Pulsed Beam
Source
Microwave
Source and
Electronic
Components
It
------ 1 >-------
Data
Collection
Fabry-Perot Cavity
Figure 1: An Oversimplified Circuit Diagram o f the FTM Spectrometer
16
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Figure 2: Solenoid Pulsed Valve
17
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Id
Id
Ld
La
-1
u.
U.
Q.
Ld
Id
00
Ld
U.
CO
Ld
Ld
00
Figure 3: Generalized Circuit Diagram o f the FTM Spectrometer
18
Reproduced with
permission of the copyright owner. Further reproduction prohibited without permission.
A c ry s ta l d iode d e te c to r
p u ls e d n o z z le
circu lato r
a
I—
*o
■CU
o
a
fsl
Figure 4: The Exact Circuit Diagram o f the FTM Spectrometer as o f 9/6/94
19
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
CM "
.
lS-Feb-SM
16.88:55
HERSURE
1-
OFF RITSffla
Parameters
i0:Kfl(FFT(8))
28 kHz
48.8nV
Absolute
31.9nV
Wtf Vftt fvfe
— Reference—
c urso r
Track U S On
OiFFerence
. 1 ns
1
2
V
i v
1
Ext
OC 6.12 V
□
AUTO
18 Ms/s
0
*£*
4 0
2t>
\j^vu
U n i
-)(pi
\j/jd
^W b ft
*•
j j /?
.
\ y tai
($ V '0 &
O & fy s o J
1
Figure 5: Spectrum c f OCS (93.6% na) from the Coaxial Nozzle Configuration
20
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
15-feb-91
16:66:12
. 1 as
8.S6 V
MEASURE
off Bnasm
2x
Paraneters
Amplitude
Q: HOCFFTt ( | ) )
28 kite
28.6nV
-9 .1 M
£
-
fib
Absolute
-
j
-p u s
j
s
r-fle fe re n c
cursor
Track B23 On
ulFference
cu rso r
n.e utz^j S "
2 1 v 9)
a mo
E x t. DC 8.22 V
18 Ms/s
Q
Q C ^ S ,
L S t t O
V * jr ^
1
% ( j? 6 - 4 6 j Z
fa o O p y
fad
Figure 6 : Spectrum o f OC34S (4.22% na) from the Coaxial Nozzle Configuration
21
Reproduced with permission o f the copyright owner. Further reproduction prohibited without permission.
1
■
lS -F eb -9 4
16:11:51
MEASURE
2x
nraa
A w plltude
A bso lu te
Ext
Q
OC 8 .2 2 V
AUTO
18 Hs/s
O l3 d
s
Figure 7: Spectrum of 0 13CS (1.11% na) from the Coaxial Nozzle Configuration
22
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
.1 ns
0.58 V
2x
*0 <
—Reference—
c u rs o r
TracK E33 On
v so
Cxt
DC 8.22 V
Q C
o 6cS
Figure 8 : Spectrum of OC3JS (0.25% u » ftom the Coaxial N o rfe Configuration
23
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission
lS -F e b -9 1
1 6 :2 6 :5 5
MEASURE
2x
— R eF ere n ce—
cut' s o r
T r a c t Wi3 An
SBQ
Ext
□
DC 0.22 V
AUTO
10 Hs/s
a
F
■
f
a
'
3 I'l
oc35s
(2
0
II-
Figure 9: Spectrum o f OC33S (0.38% na) from the Coaxial Nozzle Configuration
24
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
1 5 -F e b -iH
MEASURE
16:29:39
.1 us
8.58 V
Zx
Absolute
|— Reference— >
I cu rso r
I
|Tract B?3 On [
■an
2
V
1
tx t
□
OC 8.22 V
RUTO
18 Ms/s
f
J -
F
-
I'
J -
Figure 10: Spectrum of OC33S (0.13% na) from the Coaxial Nozzle Configuration
25
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
HEfiSURE
OFF iUUdlilifcl
.1 ns
2x
Parameters
e.se v
iiffra
ftupIItude
10:11ft(FFTtfi)>
28 kHz
2 8 .8 kU
A b s o lu te
-ll.enU
:eFerence—
cursor
Track R53 On
DiFFerence
cu rso r
2
1
V
Ext
DCB.22 V
\1
O cs
Y p-
A D -O J Z
j. cf0^ -^ >
(fiU d / &
' u *
0
Figure 11: Spectrum e f "OCS (0.20% na) from the Coaxial Nozzle Configuration
26
Reproduced w ilt, permission o f the copyright owner. Further reproduction prohibited without permisston
15-Feb-94
16 :3 8 :3 4
HEfiSURE
f1---------. 1 ns
6.59 V
OFF BIB.-BHH
Paraneters
2x
-nodefln p litu d e
MWKFFTC#))
28 k b
28.6nV
28.8nV
type—
R e la tive
w ir ir ? " P W 7 [ ' F I W W W
■JliliMHIilnife LjjtkillilliMl
millllHH
cursor
P o s itio n
.1 ns
1 .5
2
V SO
1 V SO
Freq
E xt
186.8 kHz
0
DC 6.22 V
6UT0
16 Hs/s
D'3 C 311S
Y f//
Figure 12
% 2
3
- 2 Q c /
Spectrum o f 0 13C34S (0.05% na) from the Coaxial Nozzle Configuration
27
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
t5 -F e b -9 1
1 6 :5 8 :6 0
MCftSURE
off
.1 n s
2*
g v itU .ro
Paraneters
s .s e v
-n o d e ftn p lltu d e
(QitmCFFTtfi))
26 urn
2B.BnV
lS .ln V
— type----R e la tiv e
ilL
nmmg
cursor
P o s itio n
.1 ns
SQ
2
l
Freq
V
Ext
□
DC 6.22 V
BUTO
18 Ks/s
//
Figure 13: Spectrum of 17OCS (0.02% na) fiom the Coaxial Nozzle Configuration
28
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
lS-Feb-91
17:83:51
. 1 us
288 rV
hersure
OFF fillJj.lnU
Paraneters
2x
ROde-
R nplitude
^MfllFFTCan,
28 kHz
type—
R e la tive
IIJ
F^Wfl'iTii*' ip riV B 'n rjpqrqi.
B.B rV
A.irti
w
cursor
P o s itio n
Ml
m
.1 RS
1 .2
2
V aa
iv s
Freq
Ext
223.8 kHz
□
DC 8.22 V
ftUTO
18 Ms/s
9
Figure 14: Spectrum of OC36S (0.02% na) from the Coaxial Nozzle Configuration
29
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
1 5 -F e b -9 1
1 7 :1 2 :8 2
MEASURE
E33SE
Parameters
off
IffiH
fln p lltu d a
*MPe
itjEfara
Absolute
— ReFereno
cu rsor
T ra ct Egg On I
n
OlFFerence
cu rso r
1 .2
2
V 5SQ
1 V SO
E xt
OC 0.22 V
□
AUTO
10 Ms/s
\%
Figure 15: Spectrum of 18OC34S (0.01% na) from the Coaxial Nozzle Configuration
30
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
h ^ -w x
erf/-?
15-Feb-94
15:44:25
MEASURE
1-
OFF
Paraw eters
- jo d e -
m
ftw plltude
10: MB(FFTCJ))
28 kHz
40.8nV
14.6nV
Absolute
S£h
— ReFerence—
cu rso r
Track H33 On
OiFFerence
cu rs o r
.1 ns
1
1 V 900
2
l v so
48.0 kHz
Ext
DC 0.12 V
f l r - DCS
V p =
\oi'2|z.
\ D m p L t* \O i
^ ( 9 0 /W ^
50 sat*
^ ) 6 4
^ ^
04
<P Oo%%Ofy/o
-
Figure 16: Spectnxm of Ar--OCS Complex from the Coaxial Nozzle Configuration
31
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
O
2 0 -J u n -9 4
1 6 :4 9 :2 7
c ^
£
TIMEB0SE
T /d iv 20 ps
fer
<J 0 J J S .
2 « 18000
sanples a t
50 Hs/s
C 20 n s /p t)
For .2 ns
2x
0 V
|0:Nfl(FFT(9))
- S a n p l in n " 1
.1 MHz
IB.BnV
2 .0 n V
r-Sanole Clock
ECU 8 v TTL
—Sequence----2 segments
OFF Bil Hrap
-Max. segment
20 p s
1B
K
T
sanples
I n s
2 IB nV
Freq
SO
Ext
720.B kHz
□ BUIO
OC 0.21 V
50 Ms/s
C o
S f^ '"
^
) | g s > Q .^ z
*20
^ c / r £ (b
l . l &
Figure 17: Spectrum o f OC34S (4.22% na) from the Perpendicular Nozzle
32
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
28-Jun-94
16:53:55
TIHEBflSE
T /d iv 28 ps
2 « 18888
sanples a t
58 Ms/s
( 26 n s /p t)
For .2 r s
1 .6 8 V
l@:MflCFFT(fl))
.1 HRz
48.B*U
3 .1nV
fio tz tc
r5 a n p lin q
s in g le s S h o t
r - 5 ai n p l e C lock
lo c
KlUJiiihll
ECl 8 v TTL
— Sequence----2 segnents
OFF B3 Hrap
rM ax. segnent'
16K
sanples
28 ps
1
1
V
SO
2 18 nV
500
Freq
Ext
728.8 klfe
DC 8.21 U
□
fiUTO
58 Ms/s
^ /d ^
( (p fjt& f)
%
S < y o jM >
Figure 18: Spectrum of 0 13CS (1.11% na) from the Perpendicular Nozzle
33
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
28-Jun-94
16:57:28
TIHEBflSE
T / d i v 28 p s
2 > leeaa
sanples a t
58 Ms/s
< 28 n s /p t)
For .2 ns
L.80 V
i0 :H A (F F T C |»
an plinq------
.1 KHz
48.8nV
2.8nV
a in g le i'S h o t
pSanple Clock
ECU 8 v TTL
—Sequence----2 segnents
OFF Bil Wrap
r-Max. segnent'
18K
sanples
28 ps
1
1 V 500
Freq
2 18 nV SB
Ext
728.8 kHz
0C 8.24 V
□
AUTO
58 Ms/s
jyoii-
o i n
V f
Ci*
D '^ c L g
I
Af=-+i
s_
Figure 19: Spectrum o f OC33S (0.38% na) from the Perpendicular Nozzle
34
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
/*
D
2 8 - J u n -94
1 7 : 8 8 :T8
t-
28 p s
c S
1IME8ftSC
T /d iv 28 ps
2 « tease
2x
sanples a t
58 Ms/s
( 28 n s /p t)
For .2 ns
i.ee v
|0 :MflCFFTCft))
-S an plinq
.1 Mte
4 8 .8nV
8.6nV
f-Sanple Clocfc
ECl 8 v ITL
— Sequence----2 segnents
OFf ffil Wrap
r-Max. seqnent
18K
sanples
28 ps
1
1
V 5BQ
2 18 nV
SB
Freq
Ext
728.8 kHz
□
OC 8.24 V)
AUTO
58 Ms/s
^
11 q - < f t S o X
Figure 20! Spectrum of 18OCS (4.22% ua) ftom tie Petpeudicular Nozzle
35
Reproduced w ith permission ot the copyright owner. Further reproduction p ro h M e d without permission.
;4 i/- 0 C S
20-Jun-SM
17:05:10
TIMEBflSE
T /d iv 20 ps
1-------20 ps
1.00 V
2 « 10008
sanples a t
50 Ms/s
( 28 n s /p t)
For .2 b s
2x
|Q:MB(FFT(ft))
.1 MHz
40.0nV
2.4«V
J
r S;anplinq-----
s
.
S in g le ! S h o t
it
1 nF
\
/
ki LnM
. . . .1
W V
lULfu tyftfcp.!
t-lr
1
ECL 0 v TIL
— Sequence----- <
2 segmentsI
OFF 03 Wrap |
r-Max. seqnent
10K
sanples
20 ps
1
f-Sawple Clock
IfilcJiM ill
V
Freq
SO
2 10 M/ SBO
Ext
720.0 kHz
0
OC 0.24 V
fiUTO
50 Ms/s
5-0
\0 4 -
G?-
I
Figure 21: Spectrum of A r-O C S Complex from the Perpendicular Nozzle
36
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Section II:
Multiconformational Molecules
37
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Introduction
This section o f the thesis is composed o f two chapters;
Structural
Determination o f Dipropvl Ether, and Rotational Spectra o f Methyl Ethyl and Methvl
Propyl Nitrosamine. Conformational Assignment. Internal Rotation and Ouadrupole
Coupling. The first chapter is a published paper which is included in this thesis for
completeness. Although a draft of this work is a large part o f another thesis* from this
research group, many changes were made before it was submitted to the journal. The
document presented here is unchanged from the published paper.
The second chapter o f this section was recently submitted to the Journal o f
Molecular Structure for publication. The work on the nitrosamine compounds was a
large part of my research career and constitutes a significant portion o f the thesis. This
was the first project performed on the Fourier transform microwave spectrometer
described thoroughly in Section I.
The study o f these nitrosamines presented
considerable experimental challenges during which I became extremely proficient with
the spectrometer.
It was through a productive collaboration with Professor Robert Bohn's
research group at The University o f Connecticut that our group became interested in
multiconformational organic molecules. A State o f Connecticut, Department o f Higher
Education, Apollos Kinsley Collaborative Grant began our joint venture with the
dipropyl ether investigation. Success led to grant renewals and was crucial in getting
the NSF grant that funded the building o f the FTM spectrometer. The collaboration
with Professor Bohn's group was a particularly positive influence on my graduate
career and on our laboratory.
1. Kim Grant, Molecular Beam Studies o f van der Waals Complexes and the
Structure o f the Dipropvl Ether Cs Conformer. 1993
38
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Chapter 2: Microwave Determination of the Structure of the Cs Conformation
of Dipropyl Ether
39
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Microwave Determination of the Structure of the Cs Conformation of
Dipropyl Ether
Kimberley J. Grantt, A.R. Hight Walker, Stewart E. Novick
Department o f Chemistry, Wesleyan University, Middletown, CT 06459
Robert K. Bohn, Lou Qi, Timothy Wheeler
Department of Chemistry, University o f Connecticut,
Storrs, CT 06269-3060
James M. LoBue
Department o f Chemistry, Ursinus College, Collegeville, PA 19426
Mohammad A. Al-Laham
AT&T Bell Laboratories, Holmdel, NJ 07733
Microwave spectroscopy of dipropyl ether has been performed by supersonic jet
molecular beam electric resonance spectroscopy and by molecular beam pulsed-jet
Fourier Transform spectroscopy. Calculations suggest that there are four distinct
conformations o f dipropyl ether whose energies are within half a kcal/mol o f each
other and have geometries o f C2 V, C2 , Cs, and C j symmetry. The geometry o f the Cs
conformation has been determined. The rotational constants o f this conformation are
A=4793.6393(5) MHz, B=1242.5098(3) MHz, C=1053.3520(2) MHz, 5j=0.117(6)
kHz, Sk = 1.25(5) kHz, Ajk =-2.807(7) kHz, Aj=0.448(2) kHz, and AK=11.57(3) kHz.
The outer torsion angles (the O-C-C-C angles) are approximately 65°. Unassigned
transitions in the microwave spectrum suggest that more than one additional
conformation is present in both cw and pulsed molecular jets.
This paper is dedicated to the memory o f E. Bright Wilson, who inspired us all
t Present address: Department of Chemistry, College of Saint Elizabeth, Morristown, NJ 0796
40
Reproduced with permission o f the copyright owner. Further reproduction prohibited without permission.
Introduction
Dipropyl ether is a multi-conformational molecule. A 1977 gas-phase electron
diffraction study suggests that perhaps three conformations are present at room
temperature. 1 These are the all trans conformer aaaa, where a stands for a dihedral
angle in the anti or trans configuration, a conformation o f C2 V symmetry; a g^aag^
conformer, where g stands for a dihedral angle in the gauche configuration, a
conformation o f C2 symmetry; and an aaag conformer o f symmetry C j. In addition,
molecular mechanics calculations predict that the conformer o f Cs symmetry (g^aag)
is also low in energy. Indeed, all four o f these configurations are predicted to be within
half a kcal/mol of each other. Other conformations are also possible. There has been
no microwave structural determination o f dipropyl ether attempted before now,
probably due to the complication o f the simultaneous population o f so many
conformations. A microwave determination o f diethyl ether has been carried out and
the structure of the aa all trans conformer was solved. 2 The authors state that they
find many weak transitions which may belong to the other conformers o f diethyl ether,
but that the spectra were so complicated that the analysis was abandoned.
We began this study in the hope that the extreme cooling o f the supersonic jet
molecular beam source would cool the molecule into the lowest energy conformation.
This, of course, would greatly simplify the spectrum The other possibility is that the
energy barrier for transformation between the conformations is high enough that the
conformational population of the jet is frozen at the room temperature distribution.
Each conformation is then separately cooled by the expansion into a low rotational
41
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
state distribution. Each conformation present at room temperature is then present in
the jet, each essentially a separate molecule non-convertable into the other and each at
a rotational temperature near 0 K This, o f course, would also be a simplification over
a straight room temperature spectrum This, in feet, is what we observed. This paper
presents the determination of the structure o f the Cs conformation. Fifty four
transitions ranging in rotational quantum number from
2
to
11
have been assigned to
this conformation. In addition we have observed, to date, 35 transitions which we have
not yet been able to assign,and which we believe are not due to van der Waals
complexes. These are most likely due to one or more o f the other conformations.
Dipropyl ether is, for us, the first in a series o f multi-conformational molecules whose
structures we will determine by microwave spectroscopy. It was deemed to be simpler
spectroscopically than a structurally related molecule which we hope to study in the
future: N-nitroso-di-n-propylamine an environmental pollutant and carcinogen.
42
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Experimental
The microwave spectrum of dipropyl ether was obtained on two separate instruments.
The data was measured using both a molecular beam electric resonance (MBER)
spectrometer and a Fourier transform (FT) spectrometer. The A and B state selecting
and state analyzing fields and the mass spectrometer detector o f the MBER apparatus
have been described
e ls e w h e r e ^ A
The molecular beam pulsed-jet Fourier Transform
(FT) spectrometer o f the Flygare-Balle design^ in the laboratory o f Professor Robert
Kuczkowski of the University o f Michigan is also described elsewhere^. The two
instruments employ very different nozzles and thus have different resulting jet
conditions.
In the MBER apparatus, argon is bubbled through liquid dipropyl ether,
CH3 CH2 CH 2 OCH2 CH2 CH3 , and expanded through a 25 micron diameter nozzle
with a backing pressure between 0.7 and 1 atm The vapor pressure o f dipropyl ether
at room temperature is approximately 60 torr; thus the saturated argon vapor contains
between
8
and 11% dipropyl ether. The nozzle is just a pin-hole and is continuously
open. It is usually assumed that this arrangement produces rotational distributions
characterized by a temperature of between 5 and 10 K. Spectroscopy is performed
while monitoring a mass peak (CH 3 CH2 CH2 +) o f the dipropyl ether.
In the FT apparatus, a 1 mm diameter nozzle is pulsed. The pulse duration is between
0.25 and 0.75 ms with a repetition rate o f 6 Hz. Dipropyl ether was placed in a sample
bulb, cooled to 0 °C (vapor pressure 20 torr), and Ar or a neon rich mixture o f Ne/He
43
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
was added to obtain a total pressure o f 1 - 1.7 atm. These conditions should produce
rotational temperatures o f approximately 1 K. Thus, not only should we expect
different rotational temperatures in the two experiments; it is also possible that there
will be different conformational distributions observed on the two instruments. This
will be discussed more fully later.
The microwave frequency range available on the FT apparatus is
6
-18 GHz; on the
MBER it is 3.16 - 26.5 GHz. In addition to the zero external electric field microwave
transitions on both the MBER and FT machines, Stark effect measurements were
performed on the MBER apparatus. A constant electric field (C field) is produced
between two parallel, gold coated, pyrex plates. Microwave radiation is introduced
into this C field region through either unterminated X band or tapered G band
waveguides oriented so that the microwave electric field vector is parallel to the
constant electric field maintained in the C field. Some twisting and spreading of the
microwave radiation occurs, resulting in the observation o f some AM=1 along with the
expected AM=0 transitions and in some broadening o f the transition linewidths.
44
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Results
A. Spectroscopic Assignment and Constants
Table I gives the frequencies and spectral assignments o f the transitions o f the Cs
conformation o f dipropyl ether. Transitions labeled with a pound sign (#) were
measured on the FT apparatus, those with an asterisk (*) were measured on both
machines, the remainder were measured on the MBER apparatus. Table n presents
other transitions that we have found that are not yet assigned. Some o f these
transitions will eventually be assigned to one or more additional conformations of
dipropyl ether. The transitions in Table I were least-squares fit with a Watson ,4reduction asymmetric top Hamiltonian^
8
using a program written by Maki^. Table III
gives the spectroscopic constants o f the Cs conformer.
The rotational constants A, B, and C, in Table III are consistent with a conformation
of Cs symmetry. Indeed, the assignments were found by predicting the general values
o f these rotational constants based on assuming this structure (and others) as
generated by molecular mechanics calculations. It was found that the three rotation
constants predicted by molecular mechanics for the Cs conformation o f dipropyl ether
were all within 40 MHz o f the experimental values. This was an aid in spectral
assignment, but due to the presence of transitions from multiple conformations (among
which, o f course, the mass spectrometer of the MBER was unable to distinguish) this
level o f prediction was only sufficient for a preliminary assignment.
45
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
The dipole moment components pt, and pc were determined by measuring the
frequency splittings and shifts o f the
5 2 4 -5 1 4
,
6 2 5 -6 1 5
,
4 3 2 -4 2 3
, and the
transitions in the presence o f a constant external electric field o f up to
1000
4 3 1 -4 2 3
,
v/cm pa
was set equal to zero and not allowed to vary in the fit. A pure 2nc* order Stark effect
was assumed***. As shown in Table m , pjj and pc were determined to be 0.79(4) and
0.58(4) D respectively, which results in a total dipole moment o f 0.98(4) D.
B. Ab Initio Calculations
Geometry optimization o f the Cs conformer o f dipropyl ether was performed at the
Hatree-Fock double zeta plus polarization (HF/6-31G**) level o f theory**. This level
o f theory is known to produce reliable results for the geometry optimization**. In
addition, geometry optimizations at higher levels o f theory are not practical at this time
due to the size o f the molecular system The HF/6-31G
calculations predicted A, B,
and C rotational constants that are within 11, 2, and 2 MHz, respectively, o f the
experimental results.
Ab initio methods were also used to calculate the dipole moment o f the molecule. The
calculated dipole moment o f 1.14 D at the HF/6-31G** level is about 16% higher than
the experimental value o f 0.98(4) D, as given in Table HI. However, the calculated
dipole moment at the second-order Moller-Plesset perturbation theory, using the 6 31G** basis set and the HF/6-31G** optimized geometry, is 0.95 D. This is in
excellent agreement with the experimental value. The total dipole moment o f the
molecule was used to obtain the appropriate atomic charges and bond moments at the
46
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
optimized HF/6-31G** geometry. These bond moments were then scaled down by a
factor of 0.85 to account for the above mentioned overestimation o f the dipole
moment at the HF/6-31G** level o f theory.
In this study, these ab-initio results were used to augment our experimental results.
The following section explains how we combined these results to arrive at the
structure o f the Cs conformation o f dipropyl ether.
C. Structure
Considering only the heavy atoms, and imposing the plane o f symmetry which bisects
the C-O-C angle, there are eight structural parameters to be determined in the Cs
conformer of dipropyl ether. They are: three bond angles ( C-O-C, O-C-C, and C-C-C
); three bond lengths ( O-C, H 2 C-CH2 , and H 2 C-CH 3 ); and two dihedral angles
C-O-C-C (rotation about the O-C bond, the "inner" dihedral angle) and O-C-C-C
(rotation about the H 2 C-CH2 bond, the "outer" dihedral angle). Since we have
measured only the major isotopomer we have only three perameters A, B, and C, that
we can use to constrain the geometric fit. There is also structural information implicit
in the two measured dipole moment components,
and p c. Table IV presents
structural parameters of diethyl ether from a microwave study 1 ; and o f dipropyl ether
from a gas-phase electron diffraction study^, from an MM2 molecular mechanics
calculation, and from the ab-initio calculation. As mentioned above, this calculation
resulted in relatively accurate rotational constants. Thus we felt justified in using some
o f the structural parameters from this calculation to augment the experimental
47
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
rotational constants in aid o f finding a structural fit. We proceeded to find a
representative structural fit to the rotation constants by fixing all but three o f the
structural constants at the ab-initio values and fitting the remaining three bond
lengths/angles in a least-squares sense to A, B, and C. The inner dihedral angle,
rotation about the O-C bond, always came to within one degree of 180°. We chose,
therefore to set this angle to 180° for all fits. (180° for a dihedral angle is the trans or
anti configuration, where the two outer atoms o f the four atoms that define the
dihedral angle are at their maximum distance from each other). Also set was the H 3 CCH2 bond distance, which did not vary significantly in the fits. All C-H distances were
set to their ab-initio values. Thus, with the imposition o f Cs symmetry, there are six
molecular parameters to be fit. Since we are concerned with the conformations o f
dipropyl ether, we will always choose to fit the outer dihedral angle (rotation about the
H 2 C-CH 2 bond). We can now fit any two o f the remaining five geometrical
parameters. There are ten ways to choose which two to fit and which three to set (at
their ab-initio values). All ten independent fits are presented in Table V, with each row
in the table being a separate fit. The values in parenthesis are the ab-initio values
which are not varied in that particular fit. The eleventh row in Table V is a leastsquares fit to the three rotational constants varying only the outer dihedral angle. The
twelfth row in the table is the ab-initio structure. Also presented in Table V are the
two dipole moment components that are estimated by taking the bond moments
deduced from the ab-initio calculation. The dipole components o f each o f the eleven
fits give gc within experimental error; the pfo's are a bit on the high side but all are
within two standard deviations of the experimental pjj value. Thus our hope o f being
able to use the dipole components to distinguish between the various fits has not been
48
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
realized. This is basically because all the geometric fits presented in Table V are similar
to each other. The microwave data is actually being used to confirm and refine the
results from the HF/6-31G** calculation o f the Cs conformation o f dipropyl ether.
The range o f values generated by these various fits are presented in the last row in
Table IV. Figure 1 presents a perspective drawing o f the Cs conformation o f dipropyl
ether. Some o f the bonds in this drawing are foreshortened due to the perspective; the
major purpose of the drawing is to clearly define the O-C-C-C dihedral angle, labeled
X in the diagram.
D. The Unassigned Transitions and the Barrier to Conformational Conversions
There are enough transitions that are unassigned to account for the expected three
additional conformations, C2 V> C2 , and C j. (The reason, of course, that there are so
many more assigned (Cs) transitions than unassigned (C 2 V> C 2 , and C j) is that once
high resolution spectroscopic constants are known additional weaker transitions are
easy to find. The relative intensities of the transitions (both assigned and unassigned)
differ on the FT and the MBER apparatus. This is in part due to the relative
populations within a conformation, as we recognize when we recall that the beam
temperature in the pulsed valve o f the FT spectrometer is of the order o f 1 K or less
and that o f the cw jet in the MBER apparatus is between 5 and
10
K. However, there
could also be a contribution due in part to the relative populations o f the
conformations in the two instruments. This is not a temperature effect, per se, since at
even 10 K the relative population o f a conformation o f .5 kcal/mol above the lowest
energy conformation would only be 10'H and absolutely undetectable. However, at
49
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
room temperature, the relative population o f this conformation would be 0.4 and quite
easy to detect. So the issue is one o f "what is the effective conformational temperature
in the two experiments?". The two extreme cases are (1) the conformational
temperature cools in the jet to the final rotational temperature and ( 2 ) the
conformational populations are frozen into the room temperature distribution that they
had before the expansion. It seems likely that the expansion in the pulsed nozzle o f the
FT apparatus has a better chance o f affecting conformational cooling than does the cw
nozzle o f the MBER instrument. This is because o f the relative diameters o f the two
nozzles; 25 pm for the cw nozzle and 1000 pm for the pulsed nozzle. Both jets use
approximately the same backing pressure on the order o f 1 atm o f Ar (and sometimes
Ne). The molecules cool with increasing Mach number until the Mach shock disk is
reach ed ^ .
All things being equal, the distance from the nozzle to the Mach disk is
proportional to the nozzle diameter. Thus the molecules in the pulsed jet undergo
cooling collisions over a distance that is a factor o f forty greater than the distance
travelled by the molecules in the cw jet. In a case like ours where the conformational
energy differences are on the order o f kT for room temperature, the initial collisions
(the first 5 nozzle diameters downstream?) can affect the conformational populations.
Thus we expect that the highest energy conformation that is populated in the cw jet
might have a negligible population in the pulsed jet.
Chemical intuition and
molecular mechanics calculations suggest that the lowest energy conformation will be
the all-trans C2 V structure. This structure will have a sparse microwave spectrum and
it is not unreasonable that transitions from this conformation (if found at all) remain in
the "unassigned" category. The Cs conformation shows up well in both experiments
with the MBER (cw jet) instrument tending to find higher J transitions and the FT
50
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
(pulsed jet) apparatus tending to find lower J transitions. We can speculate that when
all the conformations are assigned, the highest energy conformation appearing in the
MBER apparatus will be very weak or absent in the FT spectrometer.
51
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Acknowledgements
This work was sponsored by the Yankee Ingenuity Initiative, Apollos Kinsley
Collaborative Grants, Connecticut Innovations, Inc., Department o f Economic
Development, grants 9IK006 and 92K004. We wish to thank Professor Robert
Kuczkowski o f the University o f Michigan, who graciously allowed one o f us (RKB)
the use o f his Fourier transform microwave spectrometer.
52
Reproduced with permission o f the copyright owner. Further reproduction prohibited without permission.
References
1.
Astrup, E.E. Acta Chem. Scand. A 1977,3 1 ,4.
2.
Hayashi, M.; Kuwada, K Bull. Chem. Soc. Jpn. 1974, 47, 3006.
3.
LoBue, J.M.; Rice, J.K.; Novick, S.E. Chem. Phys. Lett. 1984,112, 376.
4.
LoBue, J.M.; Rice, J.K ; Blake, T.A.; Novick, S.E. J. Chem. Phys. 1986, 85,
4261.
5.
6
.
Balle, T.J.; Flygare, W.H. Rev. Set. Instr. 1981, 5 2 ,33.
Hillig, K.W. II; Matos, J.; Scioly, A.; Kuczkowski, R.L. Cem. Phys. Lett.
1987,133, 359.
7.
Watson, J.K.G. J. Chem. Phys. 1967, 46, 1935.
8.
Watson, J.KG. Vibrational Spectra and Structure 1977, 6, 1.
9.
Maki, A.G. private communication.
10.
Beaudet, R.A. computer program, private communication.
11.
For a general introduction to the Hartree-Fock based methods, see W. J. Hehre,
L. Radom, P.v.R. Schleyer, and J.A. Pople, Ab Initio Molecular Orbital
Theory (Wiley, New York, 1986).
12.
Miller, D.R.; in Atomic and Molecular Beam Methods ed. Scoles, G. 19 8 8 ,1,
15, (Oxford University Press).
13.
Ruoff R.S.; Klots, T.D.; Emilsson, T.; Gutowsky, H.S. J. Chem Phys. 1990,
93,3142.
53
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Table I Dipropyl Ether Frequencies and Assignments
for the Cs (g+aag") Structure
Freq/MHz
3821.883
5263.727
7953.695 #
8265.114*
8521.146#
9329.685 #
9329.844 #
9550.648
9804.059
9968.669 #
10005.631#
10115.230
10374.308
10411.024
10548.895 #
10653.278 #
10660.626 #
11097.197#
11103.375#
11220.724#
11228.080#
11509.003#
11545.710#
11895.814#
11901.374#
12005.406 #
12111.307
Dev/kHz
10
5
-3
0
2
1
-2
-1
0
-3
0
5
7
17
-2
1
-8
0
-1
•O
-3
-2
-1
1
-3
-2
1
Transition
Freq/MHz
818 ‘ 726
5 l4 “ 505
2 1 2 - !0 1
60 6 ' 514
2 11 - * 0 1
643 ' 734
642 - 734
52 4 - 514
52 3 “ 5 14
3 1 3 - 202
423 - 4 13
422 ' 4 13
12382.734 #
12971.126*
13470.371
13530.504#
13634.443 #
13766.877 #
13791.558#
14454.910
15585.108
16537.860
16598.975
17206.317
17287.568
17379.093
17627.524
18044.010
18142.896
18144.137
18252.492
18253.740
18506.024
18587.252
23292.515
23322.746
23695.521
23791.742
24951.492
3 2 2 " 3 12
" 3 12
817 ■ 726
2 2 1 ‘ 2 11
2 2 0 - 2 11
60 6 - 5 i5
3 12 ' 2 02
2 21 - 2 12
2 2 0 - 2 12
322 “ 3 13
3 2 1 _ 3 13
423 - 4 14
4 14 - 303
422 - 4 14
9 18 - 826
3 21
Transition
Dev/kHz
52 4 - 5 15
625 - 6 16
624 ' 6 16
9 1 8 ' 827
70 7 “ 6 16
515 “ 404
4 13 ■ 303
827 - 818
6 i 6 ‘ 505
1 ° 1 9 '928
514 " 404
836 ■ 826
835 “ 826
7 17 ‘ 606
734 - 725
532 - 523
432 - 422
43 1" 422
432 - 423
431 ” 423
734 - 726
909 " 818
62 5 - 514
2
-1 1
5
-3
0
-2
-2
9
2
2
12
-5
3
2
3
6
2
-9
3
0
0
1
-5
-3
0
-7
0
iio,n-10i,io
144
, l H 4 3 ,ll
6 2 4 -5 14
1147 "H 38
* This line was seen on both the FT and the MBER spectrometer
# This line was only seen on the FT spectrometer
Due to the lower temperature o f the jet in the FT apparatus compared to the MBER
machine, the lower energy states show up well on the FT and the higher energy states
are observed on the M BER There are also "beam selection rules" in the MBER
experiment that tend to rule out observing some o f the low quantum transitions on this
spectrometer.
54
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Table n Unassigned Transitions
3522.442
15519.169
3790.583
15839.042
4110.500
16778.654
7308.638 # (m)
18495.934
7345.155 # (s)
18723.387
7772.596 * (m)
18824.638
8517.336 *(w )
18893.693
8828.050
19117.325
8920.660 * (w)
20377.492
9203.111 #(m )
21091.309
10429.182
21100.042
10522.044 * (m)
21405.588
10656.576 # (s)
21842.790
10678.917 # (m)
22004.124
10808.155 #(m )
22653.637
11064.834 * (w)
24047.442
12245.347 * (s)
24148.893
12540.145 # (s)
* This line was seen on both the FT machine and the MBER
# This line was only seen on the FT machine
(s) intense, (m) medium strength, and (w) very weak on the FT apparatus
MBER observed intensities are dependant upon A and B field focusing voltages and
not simply on population; thus they are not very useful in intuiting assignments and
thus are left out o f this table
55
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Table 111 Spectroscopic Constants for the Cs (g+aag") structure
A = 4793.6393(5) MHz
B = 1242.5098(3) MHz
C = 1053.3520(2) MHz
8j
=
0.117(6) kHz
5k =
1.25(5) kHz
Aj k = -2.807(7) kHz
Aj =
0.448(2) kHz
AK = 11.57(3) kHz
Ha =
0
Pb = 0.79(4) D
pc = 0.58(4) D
ptot = 0.98(4) D
All numbers in parenthesis are one standard deviation from least-squares fitting.
56
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission
Table IV Structural Parameters relating to the Cs Conformation of CH3CH2CH2OCH2CH2CB3
C-H
diethyl ether
O-C
H2C-CH2
Dihedrals3
Bond Angies
Bond Lengths / A
Method
h 2 c-c h 3
C-O-C
o-c-c
c-c-c
C-O-C-C
o-c-c-c
1.516
112.25°
108.37°
1.524
1.524
116.1°
109.2°
112.1°
1.41-1.42
1.53-1.54
1.53-1.54
112-113°
108.5-110.0°
111.5-112.5° 179.8-180.2° 62.5-63.6°
1.09e
1.397
1.519
1.527
114.74°
1 0 9 .2 9 °
113.40°
1.09e
1.36-1.40
1.51-1.54
1.527
115.0-116.7° 108.7-108.8°
1.09
1.408
1.116
1.404
1.09-1.12
(uwave)b
electron
180°
71.8°
diffraction0
Molecular
mechanics'*
HF/6-31G**
179.75°
111.8-113.6° 180°
63.03°
62.4-64.6°
This Study ^
Zero degrees for the dihedral angles is an "eclipsed" orientation where the two end atoms are on the same side of the bond formed between
the two central atoms.
b. Ref. 2. The microwave study is of all trans diethyl ether. Thus some of the bond lengths, bond angles, and both o f the dihedral angles are not
applicable to this row o f the table.
c. Ref. 1.
d. Both standard MM2 and the commercial program Hyperchem, using various force fields, were employed. The two stated values give the
range of values obtained.
e. Average value for all C-H bonds.
f. These numbers represent the range of values generated in Table V.
a.
Table V Structural Fits of the Cs Conformation of CH 3 CH 2 CH 2 OCH 2 CH 2 CH 3
Each row o f this table represents a seperate fit to the three rotational constants. The values in parenthesis in each row are
not varied in the fit but are set at the calculated ab-initio value. The next to the last row in the table is a least-squares fit to
the three rotational constants, varying only the dihedral angle. The last row in the table is the ab-initio fit.
r(0-C)/A
r(Me-CH2)/A
r(H2C-CH2)/A
ZC-O-C
ZO-C-C
ZC-C-C
zc-o-c-c
ZO-C-C-C
Mb' 0 3
He'D3
(1.397)
(1.527)
(1.519)
115.0°
108.8°
(113.4°)
(180°)
64.2°
0.801
0.544
(1.397)
(1.527)
(1.519)
116.0°
(109.3°)
1 1 2 .8 °
(180°)
64.0°
0.791
0.538
(1.397)
(1.527)
(1.519)
(114.74°).
108.7°
113.6°
(180°)
64.3°
0.804
0.545
1.387
(1.527)
(1.519)
116.1°
(109.3°)
(113.4°)
(180°)
64.6°
0.787
0.537
1.399
(1.527)
(1.519)
(114.74°)
108.7°
(113.4°)
(180°)
64.1°
0.805
0.545
b
(1.527)
(1.519)
(114.8°)
(109.3°)
b
(180°)
b
b
b
(1.397)
(1.527)
1.508
116.7°
(109.3°)
(113.4°)
(180°)
64.6°
0.779
0.531
(1.397)
(1.527)
1.521
(114.74°)
108.7°
(113.4°)
(180°)
64.1°
0.805
0.546
(1.397)
(1.527)
1.537
(114.74°)
(109.3°)
1 1 1 .8 °
(180°)
63.1°
0.811
0.549
1.365
(1.527)
1.542
(114.74°)
(109.3°)
(113.4°)
(180°)
64.5°
0.805
0.550
(1.397)
(1.527)
(1.519)
(114.74°)
(109.3°)
(113.4°)
(180°)
62.4°
0.820
0.538
(1.397)
(1.527)
(1.519)
(114.74°)
(109.3°)
(113.4°)
(180°)
(63.0°)
0.786
0.539
These dipole moments are estimated by taking the bond moments from the ab-initio calculation, scaled down 0.85 (see text). These bond
moments add to give the pjj and pc for the various geometry choices in the table.
b.
This combination of parameters to be set and fit produced multiple solutions o f dubious numerical significance
CH^
2m f
^CH"
2-c h
Figure I A perspective drawing o f the Cs conformation o f dipropyl ether. The
purpose of the drawing is to clearly define the O-C-C-C dihedral angle, labeled % in
the diagram.
59
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
C hapter 3: Rotational Spectra of Methyl Ethyl and Methyl Propyl Nitrosamines
Conformational Assignment, Internal Rotation and Quadrupole Coupling
60
Reproduced with permission o f the copyright owner. Further reproduction prohibited without permission.
Rotational Spectra of Methyl Ethyl and Methyl Propyl
Nitrosarnines. Conformational Assignment, Internal
Rotation and Quadrupole Coupling.
A. R. Hight Walker a>b, Qi Louc, Robert K Bohnc, and Stewart E. Novicka
aDept. of Chemistry, Wesleyan University, Middletown, CT 06459-0180
^present address: Molecular Physics Division, National Institute o f Standards and
Technology, Gaithersburg, Maryland 20899
cDept. of Chemistry, University of Connecticut, Storrs, CT 06269-3060
A bstract
A structural determination o f two carcinogenic nitrosarnines, methyl ethyl and
methyl propyl nitrosamine, was performed. Microwave spectra were gathered from
both a Stark cell spectrometer and a pulsed jet Fabry-Perot Fourier transform
microwave spectrometer. Each rotational transition is split into quadrupole hyperfine
components by two nitrogen nuclei. This quadrupole pattern is doubled by a low
barrier methyl rotor which produces resolvable A and E states. Rotational spectra
were assigned for one conformer o f methyl ethyl nitrosamine and two conformers of
methyl propyl nitrosamine.
The lowest energy conformers o f each compound,
according to empirical force field calculations, were assigned. The structure found for
methyl ethyl nitrosamine has the nitrosyl oxygen on the methyl side with the terminal
methyl group o f the ethyl chain in the gauche position (OMG).
Both o f the
conformers o f methyl propyl nitrosamine have the same skeletal structure as the
methyl ethyl compound; one conformer has the terminal methyl o f the propyl group in
the anti position (OMGA) while the other conformer has this methyl in the gauche
61
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
position (OMGG").
Rotational constants and quadrupole coupling constants are
reported for each assigned species. A barrier to internal rotation o f the N-methyl
group in each compound is also reported.
Introduction
Many nitrosarnines are carcinogens.
In a large and comprehensive 1984
review, Preussmann and Stewart (1) summarize carcinogenic activity o f 235 different
nitrosarnines and another
different studies.
100
nitrosamides and nitrosoureas, citing more than
About 90% o f those 335 compounds are carcinogenic.
1000
This
immense research effort on N-nitroso compounds is stimulated by at least three
factors. First, a few N-nitroso compounds are noncarcinogenic. One subgroup o f
nitrosarnines which appear to all be noncarcinogenic are those with an N-t-butyl group
(1).
Secondly, many nitrosarnines are found in the environment and some are
synthesized in vivo. Beer, dried fish and tobacco all contain traces o f nitrosarnines.
Finally, there are unusual organ-specific as well as species-specific effects that
accompany the carcinogenicity o f the N-nitroso compounds. For example, in Fischer
rats N-methyl-N-ethylnitrosamine (MetEtNO) is a carcinogen producing tumors in the
liver and nasal cavity (1).
N-Methyl-N-n-propylnitrosamine (MetProNO) is also
carcinogenic and produces tumors specifically in the esophagus and forestomach ( 1 ).
A peculiar example of the organ specificity for tumors is demonstrated by a series o f
unsymmetrically substituted N-methyl-N-n-alkyl nitrosarnines. If the n-alkyl group has
an even number ( 8 , 10, 12, or 14) o f carbon atoms, tumors appear in the urinary
bladder of rats. However, if the n-alkyl group has an odd number (7, 9, or 11) of
carbon atoms, the tumors appear in organs other than the urinary bladder ( 1 ).
62
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Our perspective oa this wealth o f information is that there is a structure
specific binding step for each o f these compounds along the metabolic pathway leading
to tumor genesis. We suggest that this structural specificity is a significant factor
affecting the pattern o f carcinogenicity among related compounds and the organ
specificity for tumors sites among groups o f related carcinogens. We propose that a
carcinogenic nitrosamine binds to a geometry-specific site analogous to the lock and
key model applicable to enzyme-substrate, antibody-antigen, or drug-receptor binding.
A given binding site only binds substrates whose geometry or structure is consistent
with that site. Therefore, nitrosarnines in certain conformations do not fit the binding
site and are excluded. Due to the fact that such binding sites have not yet been
identified, we are currently restricted to studies and characterization o f the substrates.
A literature survey o f experimental structure determinations o f nitrosarnines
reveals that little work has been accomplished. X-ray crystallographic studies o f four
compounds have been reported: nitrosopyrrolidine (2),
nitrosodiazepine (3),
dinitrosopiperazine (4), and dimethylnitrosamine (5). Only one microwave structural
determination has been reported: dimethylnitrosamine ( 6 ). We have embarked on a
program o f characterizing a series of N-nitrosamines and report here the structural
analysis
of
N-methyl-N-ethylnitrosamine
(MetEtNO)
and
N-methyl-N-n-
propylnitrosamine (MetProNQ).
Our goal is to identify and characterize the conformational structures present in
these carcinogenic compounds.
However, it became necessary to analyze and
characterize other spectroscopic details o f the compounds due to the presence o f
quadrupole hyperfine and methyl internal rotation splittings in the microwave spectra.
63
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Experimental
Both samples were obtained from Sigma Chemical Co.
*H NMR spectra
revealed no significant impurity in either sample but did reveal that each compound is a
mixture o f at least two conformational isomers.
The samples were used without
further purification.
Preliminary structural predictions were made using empirical force field
calculations (7). The molecular geometries and a set o f assumed bond dipole moments
were used to calculate rotational constants and dipole moment components along the
principal axes. These spectroscopic constants were used to predict the rotational
spectra which guided initial searches.
The Hewlett-Packard 8460A spectrometer was used to gather low resolution
spectra from 8-40 GHz for both compounds.
Experimental conditions included a
Stark field o f 2000 V/cm, sample pressures o f 20-50 mtorr and sample temperatures of
about -150 c . A fast scan rate (5 MHz/sec) and a long detector time constant (1 sec)
were used to gather the spectra.
The pulsed jet Fabry-Perot Fourier transform spectrometer newly constructed
at Wesleyan University provided the high resolution necessary to resolve hyperfine
components not observed with the Stark cell spectrometer. The liquid sample was
placed in a modified General Valve series 9 nozzle and heated to approximately 100° C
(vapor pressure ~ 50 torr). One atmosphere o f argon was used as the carrier gas for
these experiments.
64
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Results and Discussion
N-Methyl-N-ethylnitrosamine (MetEtNO)
Empirical force field calculations (7) predict four stable conformers of
MetEtNO, two of which are much more stable than the others. The results o f these
calculations are presented in Table 1 and the two most stable conformers are
diagrammed in Figure 1. In both low energy conformers, all the heavy atoms are
coplanar except for the carbon atom o f the terminal methyl o f the ethyl group which is
rotated out o f the plane in a gauche configuration.
In one conformer the nitrosyl
oxygen lies on the methyl side (OMG, Oxygen on Methyl side-Gauche) and in the
other it lies on the ethyl side (OEG, Oxygen on Ethyl side-Gauche). The OMG form
is predicted to be a nearly symmetric prolate top
(k
= -0.83 ) with a large dipole
moment component along the a-axis based on vector addition o f assumed bond
dipoles. This structure is expected to display a-type R-branch pileups in its rotational
spectrum at intervals o f B+C = 4540 MHz. The OEG form is a very asymmetric top
(k
= +0.30) with b-type selection rules. It is not expected to have any pileups or easily
distinguishable features in its rotational spectrum.
The lH NMR spectrum displays spectra o f two distinguishable conformers
with a population ratio 4:1 at room temperature corresponding to AG =
0 .8
kcal/mol.
The calculated AE value of 0.5 kcal/mol between the OMG and OEG conformers
(Table 1) agrees reasonably well with the observed AG value. The more stable
conformer seen in the NMR spectrum has the nitrosyl oxygen on the methyl side
(OMG) of the molecule. Transitions from the OEG conformer should also be present
in the rotational spectrum obtained from the Stark spectrometer and the FT
spectrometer.
Although many unassigned transitions exist in the Stark cell data,
65
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
attempts to assign some of these lines to an OEG structure with b-type selection rules
were unsuccessful.
Ruoff et al . have demonstrated that conformational isomers
which are separated by a barrier o f400 cm’ * or greater do not relax to the more stable
form during the expansion in a pulsed jet but remain frozen at the populations
determined by the sample's temperature just prior to the supersonic expansion ( 8 ).
Since it is expected that the N-N internal rotation barrier (between OMG and OEG
forms) will be greater than this 400 cm 'l, it is likely that both forms are present in the
FT beam. However, searches on the FT spectrometer to find transitions from the
OEG species were not carried out for this study.
For the MetEtNO compound, four a-type R-branch pileups were observed
between 18 and 40 GHz on the Hewlett-Packard Stark cell spectrometer. The centers
o f these pileups was estimated to be 22280, 26700, 31250 and 35700 MHz
corresponding to B+C=4458 MHz. This B+C value agrees well with the calculated
value for the OMG conformer (B+C=4540 MHz). The low K_i transitions displaced
from the pileups were identified and assigned. These lines were measured in the high
resolution mode on the HP spectrometer. Many o f these transitions appear as barely
resolved multiplets of two, three, or four lines o f comparable intensity.
Their
experimental uncertainties are about 0.2 MHz. The frequencies above 18 GHz listed
in Table 2 are those which fit the model best and are usually the lowest frequency
components o f the multiplet. The spectrum is very dense and congested and none of
the lines are very intense. Many lines remain unassigned.
Preliminary rotational constants were determined from the assigned low K_i
lines from the Stark cell spectrometer. Spectra were predicted from these constants at
lower frequencies for higher resolution characterization with the pulsed beam Fourier
66
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Transform spectrometer. The low J lines observed between 8.5 and 14 GHz on the
FT spectrometer are listed first in Table 2. Each transition is split into many hyperfine
components by the two nitrogen nuclei.
In addition, duplicate sets o f these
quadrupole components were observed for five o f the transitions due to splittings from
the low internal rotation barrier o f the N-methyl group. The duplicate sets overlapped
to some extent but the quadrupole assignment could be unambiguously assigned by
comparing the observed splitting patterns with those predicted using quadrupole
coupling constants determined in dimethylnitrosamine ( 6 ). The determined rotational
constants and quadrupole coupling constants are reported in Table 3. The overlap
caused by the N-methyl internal rotation splitting could also be accurately determined.
The frequencies listed in Table 2 are the hypothetical unsplit line centers o f the Aintemal-rotation-states. The frequencies o f the individual resolved quadrupole and
internal rotation components are listed in a table as Supplementary Material.
The conformational structure o f this assigned species is designated OMG and
shown in Figure 1. This assignment is clear from the information reported in Table 1.
The observed and calculated second moments show an excellent agreement for only
one conformer, the OMG conformer. We choose to discuss the structures in terms o f
second moments instead o f the more conventional rotational constants for simplicity.
The second moment o f a rigid molecule is the sum o f the mass o f each atom times the
square of its coordinate along that axis. The second moment is largest along the
molecular a axis and the second moment along the c axis can be as small as zero if the
molecule were planar, Le., no atom with a nonzero c coordinate. The values o f Pec
found in molecules which only have a terminal methyl group out o f the plane o f the
rest of the molecule are about 15-20 amu
67
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Splittings due to the N-methyl group's internal rotation in MetEtNO were
analyzed using a program written by David Eggers (9).
The program uses a
Hamiltonian based on the Principal Axis Method (PAM). The Hamiltonian used in this
simple analysis makes the approximation that the internal rotor axis lies in a plane
formed by two o f the principal axes. The internal rotation barrier was assumed to
have 3-fold symmetry. Our geometrical model predicts that the methyl top axis lies
about
12
degrees from the ab plane but that angle, o f necessity, was set to zero in our
analysis. The observed splittings are listed in Table 4 along with those calculated
assuming V 3 is either 300 cm"l or 320 cm'V
The level o f agreement between
observed and calculated values is adequate for our simple model and indicates that the
barrier is 310 +/_ 30 cm"*.
The quadrupole and internal rotation splitting analysis was carried out some
time after the spectral observations were made on the FT spectrometer. The authors’
major focus is the structural analysis o f the conformational isomers.
Since the
spectrometer's sample manifold was not designed for handling toxic or carcinogenic
compounds, further measurements to specify the internal rotation barrier more
accurately and to refine the spectroscopic constants were not carried out.
N-Methyl-N-propylnitrosamine (MetProNO)
Empirical force field calculations (7) predict eight stable conformations for
MetProNO. Descriptions o f these different conformations, their relative energies, and
the predicted values for their second moments are presented in Table 5. There are
three degrees o f conformational freedom in MetProNO: the orientation o f the NO
group (either on the Methyl side or the Propyl side) and the N-N-C-C and N-C-C-C
68
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
dihedral angles.
The two lowest energy conformers diagrammed in Figure 2 are
labeled OMGA (O on the Methyl side with the N-N-C-C and N-C-C-C dihedral angles
Gauche and Anti, respectively) and OMGG'. In the latter conformer the superscript
minus sign indicates that the sense o f the first gauche dihedral angle is opposite to the
sense o f the next. Both the OMGA and the OMGG" conformers are nearly prolate
symmetric tops
(k =
-0.95 and -0.84, respectively) and are expected to exhibit
rotational spectra with a-type R-branch pileups at intervals o f B+ C= 2523 and 3312
MHz, respectively.
The lH NMR spectrum of MetProNO displays a superposition o f spectra from
two conformational isomers with an intensity ratio at room temperature o f 4:1 just as
in MetEtNO.
The two conformers present in the NMR spectrum correspond to
different configurations about the N-N bond. This is the only bond in the molecule
with a sufficiently high barrier to internal rotation to display resolved conformers by
NMR at room temperature. Microwave experiments require resolution on the order of
10 kHz while the NMR spectra require resolution o f the order o f 1 Hz. Therefore,
microwave spectroscopy can identify species with four orders o f magnitude shorter
lifetimes than can NMR spectroscopy.
The two conformers
identified in our
microwave study o f MetProNO; OMGA and OMGG", are both contained within the
majority species in the NMR spectrum. Species corresponding to the weaker NMR
spectrum with the nitrosyl oxygen on the propyl side should exist but their microwave
spectra have not yet been identified.
Low resolution spectra o f MetProNO were measured on the Hewlett-Packard
spectrometer at room temperature. The spectrum was extremely dense with many
weak lines and no strong lines. However, four broad a-type R-branch pileups could be
69
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
identified at 22450, 25000, 34850, and 37450 MHz consistent with B+C = 2490 MHz.
As shown in Table 5, this value corresponds to only one conformer predicted by the
empirical force calculations; OMGA with B+C=2523 MHz. Attempts to identify low
K -l transitions displaced from the pileups were unsuccessful.
Both the partition
function and the volatility of MetProNO limited experiments on the HP spectrometer
to only low resolution.
Spectral predictions were made to guide searches on the FT spectrometer
using the B+C value obtained from the low resolution experiments. Transitions were
found which are consistent with the OMGA conformer shown in Figure 2.
The
transitions appear as complex, partially resolved multiplets due to quadrupolar splitting
by the two
nuclei. These multiplets appear as duplicate sets due to splitting by
the internal rotation o f the N-methyl group. The complex quadrupole splitting pattern
was analyzed using the MetEtNO quadrupole results as a guide. The frequencies of
the resolved quadrupole and internal rotation components are tabulated, assigned and
listed in the Supplementary Material. The frequencies listed in Table
6
are the A-state
hypothetical unsplit line centers determined after analysis o f the quadrupole splittings.
Since many o f the quadrupole components are not well resolved and are further
complicated by overlap from methyl internal rotation splitting, the line centers are not
well determined resulting in uncertainties o f about 15 kHz. The determined rotational
constants and quadrupole coupling constants are reported in Table 7. Analysis o f the
methyl group internal rotation barrier is discussed later.
While collecting data for the OMGA conformer o f MetProNO on the FT
spectrometer, several transitions were found which did not fit this model. However,
they displayed the quadrupole and internal rotation splittings observed in the assigned
70
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
OMGA conformer,
compound.
so we were confident that they belonged to the MetProNO
These lines (plus others found later) were assigned to the other low
energy conformer; OMGG', predicted by the calculations. Analysis o f the quadrupole
splittings was carried out as before. The frequencies o f the resolved quadrupole and
internal rotation components are tabulated, assigned and listed in the Supplementary
Material. The hypothetical unsplit line centers o f the A-state lines are reported in
Table 6 . The determined rotational constants and quadrupole coupling constants are
listed in Table 7.
The conformational assignment o f the two observed conformers o f MetProNO
is clear from the information presented in Table 5. The relative energies, predicted
second moments, and predicted B+C values for the various possible conformational
isomers indicate that the two spectra observed are due to the OMGA and the OMGG'
conformations.
The barrier to internal rotation of the N-methyl group in MetProNO was
determined with the Eggers1 program described earlier. For the OMGA conformer, a
model that fits the rotational constants has the methyl group internal rotor axis lying at
an angle o f 8 ° with respect to the ab plane. For the OMGG" conformer, a model
which fits the rotational constants requires the methyl rotor axis to lie 40 degrees from
the ab plane.
As with the MetEtNO compound, we calculated internal rotation
splittings as a function o f 3-fold barrier height using a program which sets the angle
between the methyl internal rotation axis and the ab plane to zero. Therefore, this
analysis is appropriate for the OMGA conformer, but is not useful for the OMGG'
conformer.
Table 4 displays the observed and calculated values o f the splittings
between the A- and E-states for the OMGA conformers. From these data we estimate
71
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
the internal rotational barrier to be 320 +/- 30 cm '1. Only one A-E-state splitting for
the OMGG' conformer was observed, 0.530 MHz for the 312-211 transition.
The known barriers to internal rotation o f N-methyl groups in nitrosarnines are
tabulated in Table 8 . The barriers determined here for the methyl ethyl compound and
for the OMGA conformer o f the methyl propyl compound are all about 300 cm '1. It
is puzzling that this value is twice the value found for the barrier in the
dimethylnitrosamine for the methyl group syn to the nitrosyl oxygen 146 cm '1, and
half the value for the methyl in the anti position, 737 cm '1. In the isoelectronic
compound, N,N-dimethylformamide, Heineking and Dreizler
methyl barrier is 370 cm'
1
in the syn position and 770 cm'
1
determined that the
in the anti position (10).
Structural Conclusions
According to the empirical force field calculations, the most stable
conformations of MetEtNO and MetProNO have nonplanar (gauche) configurations of
the N-N-C-C chain. Heavy atom planar structures would have Pec values o f about 4.5
and 6.0 amu
for MetEtNO and for MetProNO, respectively. The hydrogen atoms
from each methylene or methyl group contributes about 1.5 amu
to the value of
Pec- The observed Pee value for the OMG conformer o f MetEtNO and the OMGA
and OMGG' conformer of MetProNO are 17.64, 40.88, and 19.34 amu A^,
respectively. Therefore, none of these species has a heavy atom planar structure.
The pattern o f second moments observed in MetEtNO (Table 1) and
MetProNO (Table 5) is compatible only with structures having the N-N-C-C chain in
the gauche configuration. If one assumes a given set o f bond lengths and bond angles,
the N-N-C-C dihedral angle can be determined from the rotational constants for each
72
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
species assigned in this microwave study.
The determined dihedral angle values
depend upon the assumptions for the bond lengths and angles. From the calculations
using different sets o f assumed structural parameters we estimate that the N-N-C-C
dihedral is about 110° in all the observed MetEtNO and MetProNO conformers.
The NMR spectra indicated that other conformers exist in both nitrosamine
compounds which have not yet been assigned, hi MetEtNO at room temperature
about
20
% o f the sample exists in a conformation unassigned in the microwave
spectrum, probably the OEG conformer.
In MetProNO, both the OMGA and the
OMGG" conformers are contained within the dominate species which may still contain
other OMXX conformers.
73
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
References
1.
R Preussmann and B. W. Stewart, in "Chemical Carcinogens, 2nd Ed.", C. E.
Searle, Ed. ACS Monograph 182, Wash., D. C., (1984) Vol. 2, pp.643-828.
2.
T. Polonski, M. J. Milewska, and A. Katrusiak, "X-Ray study o f 2-phenyl-Nnitroso pyrrolidine", J. Amer. Chem. Soc., 115 (1993) 11410-7.
3.
V. Priya, N. Shamala, V. A. Viswamitra, U. P. Senthil Kumar, and R.
Jeyaraman, "Structure ofN-nitroso-r-2,c-7-diphenylhexahydro-l,4-diazepin5-one", Acta Cryst. C41 (1992) 1048-51.
4.
K. Sekido, K. Okamoto, and S. Hirokawa, "Structure o f 1,4dinitrosopiperazine", Acta Cryst. C41 (1985) 741-3.
5.
B. Krebs and J. Mandt, "N-Nitroso Dimethylamine at 143 K", Chem. Ber., 108
(1975)1130-7.
6
.
7.
8
.
A. Guamieri and R. Nicolaisen, Z. Naturforsch., 34a (1979) 108.
HyperChemTM Release 3 for Windows, Autodesk, lac. 1993
R.S. Ruof£ T.D. Klots, T. Emilsson, andH.S. Gutowsky, "Relaxation o f
conformers and isomers in seeded supersonic jets o f inert gases."
J.Chem.Phys. 93, 1990
9.
David Eggers, private communication.
10.
N. Heineking and H. Dreizler, "Nuclear quadrupole hyperfine structure and
methyl torsional fine structure in the rotational spectra o f N,Ndimethylformamide and N-nitrosodimethylamine", Z. Naturforsch. 48a (1993)
570-7.
74
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Table 1. Empirical Force Field Predictions and Observed Values of Stable
Conformers of Methyl Ethyl Nitrosamine (MetEtNO).
C alcu lation s
C onform er
Relative Enerev
kcal/mol
p
B+ C
MHz
Eaa
*
Phh*
amu A2
Pec
OMG
0.0
4540
187
57
18
OEG
0.5
5690
132
98
16
OMA
1.8
4890
169
85
5
OEA
3.6
5500
44
96
5
O b servation s
C onform er
OMG
Relative E nergy
kcal/mol
B ±_C.
MHz
4458
Eaa*------- Ebb*----- Ecc*
amu
191.81
55.93
17.64
*Paa = ( lb + Ic - la )/2, etc. For a rigid molecule, Paa = SmiafA etc.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Table 2. Assigned Rotational Transitions of Methyl Ethyl Nitrosamine(MetEtNO)
Transition
v(obs)/MHz
Av(obs-calc)/!
-Ooo
2 o2 - loi
2 i 2 - 1 11
2 n - 1 io
322 - 2 2 1
303 - 2 0 2
3 12 ■2 n
4l3 * 3 12
422 * 321
505 - 4(34
5 15 - 4 j 4
514 - 4 13
524 - 423
523 422
8909.498*
8883.430*
8532.940*
9278.840*
13358.859*
13269.505*
13903.720*
18509.89
18013.60
21838.02
21242.17
23089.31
22214.57
22640.47
22350.80
26007.75
25440.52
26616.14
26818.59
26864.79
30113.07
29618.24
32125.86
30995.99
32048.32
31306.70
34178.04
33776.03
36558.44
36785.30
35997.12
38223.06
37916.02
39677.96
0.000
0.003
0.003
0.000
0.000
-0.003
0.000
-0.06
-0.26
111
-
532-431
6 0 6 - 5(35
6 l6 -5 i5
625 524
634 - 533
633 - 532
7(37 * 6 0 6
7 l7 -6 i6
7 16 ~6l5
726 " 6 2 5
725 6 2 4
735 - 6 3 4
8(38 - 7()7
8 1 8 - 7 i7
8 1 7 ~ 7 16
826 - 725
835 *734
9(39 - 8(38
9 i9 - 8 1 8
928 - 8 2 7
-
-
-
0 .0 1
0 .0 1
0 .0 1
0 .0 2
-0.04
-0 . 1 2
0.16
-0 . 2 1
-0.13
0.26
-0 . 1 2
-0.58
-0 . 2 0
0 .0 1
0 .0 2
0 .0 1
-0.34
-0.05
-0.15
0.27
0 .0 1
-
0.01
0.25
0.17
0.17
’’'Hypothetical unsplit central frequency determined from analysis of
quadrupolar
splittings. Only the transitions of the A-states of the methyl internal rotation are listed
here. (See Table 4 for A-E state internal rotation splittings.) Uncertainty of these
lines is about 0.005 MHz. The other lines have uncertainties of about 0.2 MHz. In
the least squares fitting of these lines, they were weighted according to the
appropriate line uncertainties.
76
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Table 3. Rotational, Centrifugal Distortion and Quadrupole Coupling Constants of
the OMG Conformer of Methyl Ethyl Nitrosamine (MetEtNO).
A/MHz
B/MHz
C/MHz
Dj/MHz
D jk /MH z
K
5Caa
%bb
%cc
6869.491(5)
2412.952(1)
2040.000(1)
0.00112(3)
-0.0052(2)
-0.84556
Nitroso N
1-602(2)
-4.752(2)
(3.150)*
Amino N
1.397(2)
2.240(3)
(-3.637)*
*Since %aa + %bb + %cc = 0, only %aa and Xbb are independently determined.
Reproduced with permission o f the copyright owner. Further reproduction prohibited without permission.
Table 4. N-Methyl Group Internal Rotation (All frequencies in MHz)
M etEtNO
OMG conform er
Transition
111 - ooo
202 - 101
212-111
2 11 - 110
VA
VA-VE
observed
8909.498
8883.430
8532.940
9278.840
313-212
322-221 13358.859
303 - 202 13269.505
312-211 13903.720
322-221
404 - 303
413 - 312
423 - 322
505 - 404
515-414
0.478
1.103
0.268
1.718
*
1.460
*
__
—
M etProN O
OMGA conform er
va -ve
VA-VE
calculated
320cm" *
300cm- 1
0.470
1.105
0.320 '
1.651
VA-VE
VA
observed
0.297
0.814
0.278
1.234
-0.075
0.004
0.345
0.476
0.351
0.431
0.742
*
0.500
-7.643
0.602
0.945
-6.323
0.701
0.299
5.352
0.446
0.704
5.582
0.521
0.235
7276.035
-7.643
1.463
2.229
VA-VE
VA-VE
calculated
300cm- 1 320cm
-5.352
1.084
1.781
7458.519
7468.121
9933.544
10206.548
9955.595
12399.196
12119.099
—
0.210
*These E-state transitions lie outside the spectral range scanned and therefore were not observed.
**This E-state was not resolved from the A-state.
Table 5. Empirical Force Field Predictions and Observed Values of Stable
Conformers of Methyl Propyl Nitrosamine (MetProNO).
Calculations
Conformer
Relative Energy
kcal/mol
B+C
MHz
Eaa*
Phh*
amu A2
p *
ICC
OMGG"
0.0
3312
247
79
40
OMGA
0.1
2523
361
63
20
OPGA
0.5
3010
323
62
62
OMGG
0.5
0.5
3177
3889
264
189
70
115
40
38
0.8
3667
210
108
33
OMAA
0.8
2605
346
85
7
OMAG
1.2
3337
244
90
33
OPGG"
OPGG
Observations
C onform er
Relative Energy
kcal/mol
B + C________ Pan*______Pbb*___ Ecc*
MHz
amu A2
OMGG'
3294
248.16
78.68
40.88
OMGA
2489
366.98
60.88
19.34
*Paa = ( lb + Ic - la )/2, etc. For a rigid molecule, Paa = Smjai^, etc.
79
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Table 6. Assigned Rotational Transitions of Methyl Propyl Nitrosamine(MetProNO)
OMGA Conformer
Transition
3 0 3 -2 0 2
3i3 - 2i2
322 - 221
4 04 - 3 03
413
- 3 12
4 23 - 322
5o5 - 4 o4
5i5-4i4
v/MHz
obs-calc/MHz
7458.519
7276.035
7468.121
9933.544
10206.548
9955.595
12399.196
12119.099
0.028
-0.034
0.022
-0.002
-0.008
-0.012
-0.005
0.014
10179.769
9884.092
12751.052
13558.278
13169.545
15919.528
-0.010
0.038
-0.011
0.007
-0.028
0.008
OMGG" Conformer
312 - 2 n
3 22 - 221
4 j4 -3i3
4i3 - 312
4 23 - 3 22
5i5-4i4
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Table 7. Rotational and Quadrupole Coupling Constants of Two Conformers of
Methyl Propyl Nitrosamine (MetProNO)
OMGA Conformer
A/MHz
B/MHz
C/MHz
D jk /M H z
K
Xaa/MHz
Xbb/MHz
Xcc/MHz
OMGG" Conformer
A/MHz
B/MHz
C/MHz
D jk /M H z
k
Xaa/MHz
Xbb/MHz
Xcc/MHz
6307.(5)
1308.177(6)
1181.171(4)
-0 .002( 1)
-0.9504
Nitroso N
1.39(1)
-4.90(5)
(3.51)*
Amino N
0.92(2)
2.86(9)
(-3.78)*
4254.(6)
1748.485(9)
1546.041(13)
-0.20(2)
-0.8491
Nitroso N
0.37(2)
3.26(14)
(-3.63)*
Amino N
1.42(1)
-4.48(16)
(3.06)*
*See footnote to Table 3.
81
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Table 8. Methyl Group Internal Rotation Barriers in N-Methyl Nitrosarnines.
O
N
I
N.
CH,
R
Barrier/cm’1
Ref.
310+/-30
This work
320 +/- 30
This work
CH3 syn to 0
146 +/-1
10
CH3 anti to O
737 +/- 2
10
Compound
MetEtNO (R = Ethyl)
OMG conformer
MetProNO (R = Propyl)
OMGA Conformer
DiMetNO (R = Methyl)
82
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Supplementary Material
Seven A-state and E-state lines with a total o f 29 quadrupole split
components o f MetProNO were simultaneously analyzed.
OMGG" conformer.
A=
4230.0000
B=
1748.5630
C=
1546.0780 MHZ
CHIAA1
0.3746 M H z+/- 0.0166 Change
0.0000
CHIBB1
3.2623 M H z+/- 0.1433 Change
0.0000
CHIAA2
1.4216 M H z+/- 0.0133 Change
0.0000
CHIBB2
-4.4753 M H z+/- 0.1563 Change
0.0000
1.Unsplit frequency
9836.3342 M H z+/- 0.0054 Change
0.0000
2.Unsplit frequency
9884.0924 M H z+/- 0.0026 Change
0.0000
3.Unsplit frequency 10179.7690 M H z+/- 0.0032 Change
0.0000
4.Unsplit frequency 10180.2994 M H z+/- 0.0038 Change
0.0000
5.Unsplit frequency 13169.5455 M H z+/- 0.0035 Change
0.0000
6.Unsplit frequency 12751.0525 MHz +/- 0.0035 Change
0.0000
7.Unsplit frequency 15919.5250 M H z+/- 0.0052 Change
0.0000
Final fit to observed transitions:
Transition 3( 0, 3) - 2( 0, 2) Fitted unsplit frequency:
F" I"
F
r
Obs.(MHz)
Split(obs)
9836.3342 MHz
Obs-Calc
3.0 0.0 2.0 0.0
9836.4000
0.0658
0.0101
3.0 2.0 2.0 2.0
9836.3400
0.0058
-0.0101
85
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Transition 3( 2 ,2 ) - 2( 2, 1) Fitted unsplit frequency:
F" I"
F
]r
Obs.(MHz)
Split(obs)
Obs-Calc
3.0 2.0 2.0 2.0
9884.5050
0.4126
0.0015
4.0 2.0 3.0 2.0
9884.4350
0.3426
-0.0033
4.0 1.0 3.0 1.0
9884.0650
-0.0274
-0.0022
5.0 2.0 4.0 2.0
9883.9650
-0.1274
0.0009
3.0 2.0 2.0 1.0
9883.9200
-0.1724
-0.0018
2.0 2.0 1.0 2.0
9883.7750
-0.3174
0.0017
3.0 1.0 2.0 0.0
9883.7250
-0.3674
0.0014
1.0 2.0 0.0 2.0
9883.6450
-0.4474
0.0017
9884.0924 MHz
Transition 3 ( 1 , 2 ) - 2 ( 1 ,1 ) Fitted unsplit frequency: 10179.7690 MHz
F" I"
F
r
Obs.(MHz)
Split(obs)
Obs-Calc
4.0 2.0 3.0 2.0
10179.8800
0.1110
-0.0071
2.0 2.0
1.0 2.0
10179.8320
0.0630
0.0018
2.0 1.0 1.0 1.0
10179.7850
0.0160
-0.0055
3.0 0.0 2.0 0.0
10179.7550
-0.0140
0.0067
5.0 2.0 4.0 2.0
10179.7220
-0.0470
0.0040
Transition 3( 1,2 )- 2( 1, 1) Fitted unsplit frequency: 10180.2994 MHz
F" I"
F
r
Obs.(MHz)
Split(obs)
Obs-Calc
4.0 2.0 3.0 2.0
10180.4100
0.1106
-0.0074
2.0 2.0 1.0 2.0
10180.3600
0.0606
-0.0006
3.0 0.0 2.0 0.0
10180.2850
-0.0144
0.0063
5.0 2.0 4.0 2.0
10180.2500
-0.0494
0.0017
86
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Transition 4( 2, 3) - 3( 2, 2) Fitted unsplit frequency:
F"
I"
F r
Ob s. (MHz)
13169.5455 MHz
Split(obs) Obs-Calc
4.0 2.0 3.0 2.0
13169.7300
0.1845
0.0122
5.0 1.0 4.0 1.0
13169.5450
-0.0005
-0.0035
4.0 1.0 3.0 2.0
13169.4650
-0.0805
-0.0004
2.0 2.0 1.0 2.0
13169.4100
-0.1355
-0.0084
Transition 4( 1, 4) - 3 ( 1 , 3 ) Fitted unsplit frequency:
F" I"
F r
Obs.(MHz)
12751.0525 MHz
Split(obs) Obs-Calc
5.0 2.0 4.0 2.0
12751.1250
0.0725
0.0025
3.0 2.0 2.0 2.0
12751.0750
0.0225
0.0034
6.0 2.0 5.0 2.0
12751.0350
-0.0175
0.0054
5.0 1.0 4.0 1.0
12750.9900
-0.0625
-0.0114
Transition 5( 1, 5) - 4 ( 1 , 4 ) Fitted unsplit frequency:
F"
I"
F r
Obs.(MHz)
15919.5250 MHz
Split(obs) Obs-Calc
6.0 2.0 5.0 2.0
15919.5950
0.0700
0.0015
7.0 2.0 6.0 2.0
15919.5100
-0.0150
-0.0015
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Correlation Matrix
CHIAAl CHIBB1 CHIAA2 CHIBB2
CfflAAl
1.0000
CHEBB1 -0.3703 1.0000
CH1AA2 -0.3826 0.2883 1.0000
CHIBB2
0.3599-0.7891-0.2947 1.0000
1.Freq.
0.0394 0.2094-0.1153-0.2937 1.0000
2.Freq.
0.0074 0.0403 0.2476-0.0434-0.0291 1.0000
3.Freq.
0.1241-0.1843-0.1081 0.1975-0.0428-0.0169 1.0000
4.Freq.
0.1238-0.3546-0.0888 0.2926-0.0826-0.0112 0.0661 1.0000
5.Freq. -0.0492 -0.0059 0.0477 0.0068 -0.0168 0.0081 -0.0027 0.0033 1.0000
6.Freq.
0.0029 0.0324-0.0147-0.0355 0.0168-0.0040-0.0056-0.0123-0.0021 1.0000
7.Freq.
0.0952-0.2455-0.0550 0.3053-0.0978-0.0047 0.0586 0.09150.0043-0.0120 1.0000
Quadrupole Coupling Constants
ChiAA(l)=
Chi AA (2) =
0.3746
ChiBB(l)=
1.4216 Ch iB B ( 2)=
3.2623
C b i C C ( l ) = -3.6369
-4.4753 C hiC C( 2)=
3.0538
88
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Supplementary Material
Quadrupole analysis of N-Methyl-N-n-Propyl-Nitrosamine (MetProNO). Splittings
o f 12 lines from both A and E methyl group internal rotation states
with a total of 50 components have been simultaneously analyzed.
OMGA conformer.
A=
6296.0000
B=
1308.1740
C=
1181.1890 MHZ
CHIAA1
0.9182 M H z+/- 0.0185
CHTOB1
CHIAA2
2.8629 M H z+/- 0.0911
1.3879 M H z+/- 0.0138
CHEBB2
-4.8950 M H z+/- 0.0463
1.Unsplit frequency
7458.1741 MHz +/- 0.0031
E-state
2.Unsplit frequency
7458.5191 MHz+/- 0.0031
A-state
3.Unsplit frequency
7276.0351 MHz+/- 0.0041
A-state
4. Unsplit frequency
7275.8938 M H z+/- 0.0047
E-state
5.Unsplit frequency
7468.1209 MHz +/- 0.0032
A-state
6.Unsplit frequency
9933.1125 MHz +/- 0.0031
E-state
7.Unsplit frequency
9933.5437 MHz +/- 0.0031
A-state
8.Unsplit frequency
9955.5949 M H z+/- 0.0028
A-state
9.Unspht frequency 10205.8263 MHz +/- 0.0028
E-state
lO.Unsplit frequency 10206.5485 MHz +/- 0.0028
A-state
11.Unsplit frequency 12398.6964 MHz +/- 0.0028
E-state
12.Unsplit frequency 12399.1964 MHz +/- 0.0028
A-state
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Final fit to observed transitions:
Transition 3( 0, 3) - 2( 0, 2) Fitted xmsplit frequency:
F" I"
f
r
Obs.(MHz)
Split(obs)
Obs-Calc
3.0 0.0 2.0 0.0
7458.2350
0.0609
-0.0005
3.0 2.0 2.0 2.0
7458.2100
0.0359
0.0075
3.0 1.0 2.0 1.0
7458.1650
-0.0091
-0.0073
5.0 2.0 4.0 2.0
7458.1500
-0.0241
0.0003
Transition 3( 0, 3) - 2( 0, 2) Fitted unsplit frequency:
F" I"
F r
Obs.(MHz)
Split(obs)
Obs-Calc
3.0 0.0 2.0 0.0
7458.5800
0.0609
-0.0005
3.0 2.0 2.0 2.0
7458.5550
0.0359
0.0075
3.0 1.0 2.0 1.0
7458.5100
-0.0091
-0.0073
5.0 2.0 4.0 2.0
7458.4950
-0.0241
0.0003
Transition 3( 1, 3) - 2( 1,2) Fitted unsplit frequency:
F" I"
F r
Obs.(MHz)
Split(obs)
Obs-Calc
3.0 1.0 2.0 1.0
7276.3450
0.3099
-0.0002
4.0 2.0 3.0 2.0
7276.1450
0.1099
0.0014
3.0 2.0 2.0 2.0
7275.7200
-0.3151
-0.0011
7458.1741MHz
7458.5191 MHz
7276.0351 MHz
90
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Transition 3 ( 1 ,3 ) - 2 (1 ,2 ) Fitted unsplit frequency:
F" I"
f
:r
Obs.(MHz)
Split(obs)
Obs-Calc
3.0 1.0 2.0 1.0
7276.2100
0.3162
0.0061
5.0 2.0 4.0 2.0
7275.8370
-0.0568
-0.0061
7275.8938 MHz
Transition 3( 2, 2) - 2 ( 2 , 1 ) Fitted unsplit frequency:
F" I"
F
r
Obs.(MHz)
Split(obs)
Obs-Calc
3.0 0.0 2.0 2.0
7468.6180
0.4971
-0.0081
4.0 1.0 3.0 1.0
7468.1950
0.0741
0.0107
5.0 2.0 4.0 2.0
7467.9510
-0.1699
-0.0052
3.0 0.0 2.0 0.0
7467.7270
-0.3939
0.0025
Transition 4( 0, 4) - 3( 0, 3) Fitted unsplit frequency:
F" I"
F
r
Obs.(MHz)
Split(obs)
Obs-Calc
2.0 2.0 1.0 2.0
9933.1750
0.0625
0.0063
3.0 2.0 2.0 2.0
9933.1650
0.0525
0.0032
5.0 2.0 4.0 2.0
9933.1050
-0.0075
0.0001
6.0 2.0 5.0 2.0
9933.0900
-0.0225
-0.0095
Transition 4( 0, 4) - 3( 0, 3) Fitted unsplit frequency:
F" I"
F
r
Obs.(MHz)
Split(obs)
Obs-Calc
2.0 2.0 1.0 2.0
9933.6100
0.0663
0.0100
3.0 2.0 2.0 2.0
9933.5950
0.0513
0.0019
5.0 2.0 4.0 2.0
9933.5350
-0.0087
-0.0012
6.0 2.0 5.0 2.0
9933.5200
-0.0237
91
-0.0108
7468.1209 MHz
9933.1125 MHz
9933.5437 MHz
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Transition 4( 2, 3) - 3( 2 ,2 ) Fitted unsplit frequency:
F" I"
F r
Obs.(MHz)
Split(obs)
Obs-Calc
5.0 2.0 4.0 2.0
9955.7050
0.1101
-0.0022
5.0 1.0 4.0 1.0
9955.6300
0.0351
0.0016
3.0 2.0 2.0 2.0
9955.5750
-0.0199
0.0049
6.0 2.0 5.0 2.0
9955.5100
-0.0849
-0.0015
4.0 0.0 3.0 0.0
9955.4650
-0.1299
-0.0027
9955.5949 MHz
Transition 4( 1, 3) - 3 ( 1 , 2 ) Fitted unsplit frequency: 10205.:
F" I"
f
r
Obs.(MHz)
Split(obs)
Obs-Calc
4.0 1.0 3.0 1.0
10205.9650
0.1387
-0.0101
3.0 2.0 2.0 2.0
10205.9140
0.0877
0.0018
5.0 2.0 4.0 2.0
10205.8780
0.0517
0.0087
3.0 1.0 2.0 1.0
10205.8150
-0.0113
0.0032
5.0 1.0 4.0 1.0
10205.7950
-0.0313
-0.0036
Transition 4( 1, 3) - 3( 1, 2) Fitted unsplit frequency: 10206.
F" I"
f
r
Obs.(MHz)
Split(obs)
Obs-Calc
4.0 1.0 3.0 1.0
10206.6930
0.1445
-0.0043
3.0 2.0 2.0 2.0
10206.6300
0.0815
-0.0044
5.0 2.0 4.0 2.0
10206.6000
0.0515
0.0085
3.0 1.0 2.0 1.0
10206.5400
-0.0085
0.0060
5.0 1.0 4.0 1.0
10206.5150
-0.0335
-0.0058
92
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Transition 5 ( 0 ,5 ) - 4 (0 ,4 ) Fitted unsplit frequency: 12398.6964 MHz
F” I” F
F
Obs.(MHz)
Split(obs)
Obs-Calc
6.0 2.0 5.0 2.0
12398.7450
0.0486
0.0000
5.0 1.0 4.0 1.0
12398.7200
0.0236
0.0027
7.0 2.0 6.0 2.0
12398.6900
-0.0064
-0.0002
4.0 2.0 3.0 2.0
12398.6550
-0.0414
0.0002
6.0 1.0 5.0 1.0
12398.6250
-0.0714
-0.0027
Transition 5( 0, 5) - 4( 0, 4) Fitted unsplit frequency: 12399.1964 MHz
F" I"
F
T
Obs.(MHz)
Split(obs)
Obs-Calc
6.0 2.0 5.0 2.0
12399.2450
0.0486
0.0000
5.0 1.0 4.0 1.0
12399.2200
0.0236
0.0027
7.0 2.0 6.0 2.0
12399.1900
-0.0064
-0.0002
4.0 2.0 3.0 2.0
12399.1550
-0.0414
0.0002
6.0 1.0 5.0 1.0
12399.1250
-0.0714
-0.0027
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Correlation Matrix
CHIAA1 CHIBB1 CHIAA2 CHIBB2
CHIAA1
1.0000
CHIBB1 -0.2799 1.0000
CHIAA2 -0.1144 0.3116 1.0000
CHIBB2 0.0418 0.1329-0.3610 1.0000
£
1.Freq.
-0.0399 0.0307-0.0192 0.0256 1.0000
2.Freq.
-0.0399 0.0307-0.0192 0.0256 0.0031 1.0000
3.Freq.
-0.0551 -0.4108-0.0806-0.2049-0.0098-0.0098 1.0000
4.Freq.
0.1294-0.1168-0.0707 0.2947-0.0019-0.0019-0.0136 1.0000
5.Freq.
-0.2389 0.0965 0.1321 -0.0478 0.0070 0.0070 0.0040-0.0369
1.0000
6.Freq.
-0.0450 0.0516-0.0311 0.0419 0.0044 0.0044-0.0202-0.0017
0.0069 1.0000
7.Freq.
-0.0450 0.0516-0.0311 0.0419 0.0044 0.0044-0.0202-0.0017
0.0069 0.0066 1.0000
8.Freq.
-0.0460 0.0352 0.0884-0.0289-0.0002-0.0002-0.0047-0.0094 0.0198-0.0010-0.0010
1.0000
9.Freq.
0.0208-0.1102-0.0857 0.0856-0.0003-0.0003 0.0332 0.0419-0.0138-0.0010 -0.0010-0.0075 1.0000
10.Freq.
0.0208-0.1102-0.0857 0.0856-0.0003-0.0003 0.0332 0.0419 -0.0138-0.0010-0.0010-0.0075
11.Freq.
0.0516 0.0557-0.0495-0.0014 0.0010 0.0010-0.0320-0.0128-0.0170
0.0032 0.0032-0.0063 -0.0074-0.0074 1.0000
12.Freq.
0.0516 0.0557-0.0495-0.0014 0.0010 0.0010-0.0320-0.0128-0.0170
0.0032 0.0032-0.006 0-0.0074-0.0074 0.0150 1.0000
0.0229 1.0000
Quadrupole Coupling Constants
C hiAA(l)=
0.9182
ChiBB(l)=
2.8629
Chi AA (2) =
1.3879
Ch iB B( 2) = -4.8950
C h i C C ( l ) = -3.7811
ChiCC(2) =
3.5071
Reproduced with permission o f the copyright owner. Further reproduction prohibited without permission.
Supplementary M aterial
Analysis o f 14N quadrupolar splittings o f N-methyl-N-ethyl-nitrosamine.(MetEtNO)
A COMBINATION OF 10 LINES WITH 101 COMPONENTS INCLUDING BOTH
A-STATE AND E-STATE TRANSITIONS FROM METHYL INTERNAL
ROTATION WERE SIMULTANEOUSLY
FIT. OM G conformer.
A=
6869.4910
B=
2412.9520
C=
2040.0010 MHZ
Final iteration: Fitted parameters
CHIAAl
1.3966 M H z+/- 0.0023
CHIBB1
2.2404 M H z+/- 0.0027
CHIAA2
1.6022 M H z+/- 0.0023
CHIBB2
-4.7524 MHz +/- 0.0022
1. UNSPLIT FREQUENCY
8909.4983 MHZ +/- 0.0014
A-SJATE
2.UNSPLIT FREQUENCY
8909.0194 MHZ +/- 0.0016
E-STATE
3. UN SPLIT FREQUENCY
8883.4295 MHZ +/- 0.0010
A-STATE
4.UNSPLIT FREQUENCY
8882.3066 MHZ +/- 0.0011
E-STATE
5.UNSPLIT FREQUENCY
8532.9401 M H Z+/- 0.0010
A-STATE
6.UNSPLIT FREQUENCY
8532.6717 MHZ +/- 0.0010
E-STATE
7.UNSPLIT FREQUENCY
9278.8398 MHZ +/- 0.0012
A-STATE
8. UN SPLIT FREQUENCY
9277.1223 MHZ +/- 0.0012
E-STATE
9.UNSPLIT FREQUENCY 13358.8589 M H Z+/- 0.0013
A-STATE
10.UNSPLIT FREQUENCY 13903.7233 MHZ +/- 0.0022
A-STATE
96
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Final fit to observed transitions:
Transition 1( 1, 1) - 0( 0, 0) Fitted unsplit frequency:
P'
I"
f
r
Ob s.(MHz)
!Split(obs)
OBS-CALC
1.0 2.0 2.0 2.0
8911.9730
2.4747
-0.0010
2.0 1.0 1.0 1.0
8910.1800
0.6817
0.0052
3.0 2.0 2.0 2.0
8909.6220
0.1237
-0.0019
1.0 0.0 0.0 0.0
8909.5200
0.0217
-0.0012
0.0 1.0 1.0 1.0
8908.8690
-0.6293
-0.0013
2.0 2.0 2.0 2.0
8908.3200
-1.1783
0.0006
1.0 1.0 2.0 2.0
8907.7530
-1.7453
-0.0003
Transition 1( 1,1) - 0( 0, 0) Fitted unsplit frequency:
F" I"
F
r
Obs.(MHz)
Split(obs)
8911.4900
2.4706
-0.0050
2.0 1.0 1.0 1.0
8909.6980
0.6786
0.0021
3.0 2.0 2.0 2.0
8909.1500
0.1306
0.0050
0.0 1.0 1.0 1.0
8908.3900
-0.6294
-0.0014
2.0 2.0 2.0 2.0
8907.8400
-1.1794
-0.0004
1.0 1.0 2.0 2.0
8907.2740
-1.7454
-0.0004
Transition 2( 0, 2) - 1( 0,1) Fitted unsplit frequency:
F
r
Obs.(MHz)
Split(obs)
8909J
OBS-CALC
1.0 2.0 2.0 2.0
F" I"
8909.4983 MHz
8883.
OBS-CALC
2.0 2.0 3.0 2.0
8884.2100
0.7805
-0.0043
1.0 1.0 1.0 1.0
8884.1760
0.7465
0.0028
1.0 2.0 1.0 2.0
8884.0450
0.6155
0.0005
3.0 2.0 3.0 2.0
8884.0100
0.5805
97
0.0062
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
2.0 0.0 1.0 2.0
8883.9300
0.5005
0.0002
0.0 2.0 1.0 2.0
8883.6870
0.2575
-0.0013
3.0 1.0 2.0 1.0
8883.4480
0.0185
-0.0034
4.0 2.0 3.0 2.0
8883.3720
-0.0575
0.0005
3.0 2.0 2.0 2.0
8883.3180
-0.1115
-0.0095
2.0 0.0 3.0 2.0
8883.0930
-0.3365
-0.0003
1.0 1.0 0.0 1.0
8883.0520
-0.3775
0.0007
2.0 0.0 1.0 0.0
8882.4890
-0.9405
0.0047
2.0 0.0 2.0 2.0
8882.4150
-1.0145
-0.0020
0.0 2.0 1.0 0.0
8882.2480
-1.1815
0.0052
Transition 2( 0, 2) - 1( 0, 1) Fitted unsplit frequency:
pn pi
f
:r
Obs.(MHz)
Split(obs)
OBS-CALC
2.0 2.0 3.0 2.0
8883.0930
0.7864
0.0016
1.0 1.0 1.0 1.0
8883.0520
0.7454
0.0017
3.0 2.0 2.0 1.0
8882.6600
0.3534
0.0024
3.0 1.0 2.0 1.0
8882.3290
0.0224
0.0005
4.0 2.0 3.0 2.0
8882.2480
-0.0586
-0.0007
3.0 2.0 2.0 2.0
8882.1990
-0.1076
-0.0056
2.0 1.0 2.0 1.0
8881.8800
-0.4266
0.0034
1.0 2.0 1.0 0.0
8881.4760
-0.8306
-0.0001
1.0 2.0 2.0 2.0
8881.4080
-0.8986
-0.0007
2.0 0.0 1.0 0.0
8881.3570
-0.9496
-0.0044
2.0 0.0 2.0 2.0
8881.2960
-1.0106
0.0019
0.0 2.0 1.0 0.0
8881.1200
-1.1866
0.0001
Transition 2( 1,2) -
1( 1, 1)
F" I"
F
r
1.0 1.0 2.0 2.0
Fitted unsplit frequency:
Obs.(MHz)
8535.5880
Split(obs)
2.6479
98
8882.:
8532.
OBS-CALC
-0.0018
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
2.0 2.0 2.0 2.0
8535.0650
2.1249
-0.0043
0.0 2.0 1.0 1.0
8534.5670
1.6269
0.0036
3.0 2.0 3.0 2.0
8533.8880
0.9479
0.0012
2.0 1.0 1.0 1.0
8533.7070
0.7669
0.0063
3.0 2.0 2.0 1.0
8533.3380
0.3979
0.0022
1.0 2.0 1.0 1.0
8533.2120
0.2719
-0.0024
3.0 1.0 2.0 2.0
8533.1340
0.1939
0.0002
4.0 2.0 3.0 2.0
8532.7730
-0.1671
-0.0068
1.0 2.0 0.0 1.0
8532.1000
-0.8401
0.0026
1.0 1.0 1.0 2.0
8531.9370
-1.0031
0.0018
2.0 2.0 1.0 2.0
8531.4180
-1.5221
0.0033
2.0 1.0 2.0 1.0
8531.2790
-1.6611
-0.0002
1.0 2.0 2.0 1.0
8530.7900
-2.1501
-0.0029
0.0 2.0 1.0 2.0
8530.3400
-2.6001
-0.0028
Transition 2( 1 , 2 ) - 1( 1, 1) Fitted unsplit frequency:
F" I"
P
]P
Obs.(MHz)
Split(obs)
8532.6717 MHz
OBS-CAL
2.0 2.0 2.0 2.0
8534.8000
2.1283
-0.0009
2.0 0.0 1.0 1.0
8534.4180
1.7463
0.0018
0.0 2.0 1.0 1.0
8534.2940
1.6223
-0.0010
2.0 1.0 1.0 1.0
8533.4320
0.7603
-0.0002
3.0 2.0 2.0 1.0
8533.0710
0.3993
0.0036
1.0 2.0 1.0 1.0
8532.9450
0.2733
-0.0010
3.0 1.0 2.0 2.0
8532.8700
0.1983
0.0047
2.0 0.0 1.0 0.0
8532.6450
-0.0267
-0.0032
4.0 2.0 3.0 2.0
8532.5130
-0.1587
0.0017
1.0 2.0 0.0 1.0
8531.8320
-0.8397
0.0030
1.0 1.0 1.0 2.0
8531.6730
-0.9987
0.0063
3.0 1.0 3.0 2.0
8531.5580
-1.1137
-0.0028
2.0 2.0 1.0 2.0
8531.1380
-1.5337
-0.0083
2.0 1.0 2.0 1.0
8531.0070
-1.6647
99
-0.0037
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Transition 2( 1, 1)- 1( 1,0) Fitted unsplit frequency:
F" I"
f
r
Ob s. (MHz)
iSplit(obs)
9278.8398 MHz
OBS-CAL
2.0 2.0 2.0 2.0
9280.8280
1.9882
-0.0042
1.0 1.0 0.0 1.0
9280.5080
1.6682
0.0003
3.0 2.0 3.0 2.0
9280.1100
1.2702
0.0022
2.0 1.0 1.0 1.0
9279.5590
0.7192
0.0020
3.0 2.0 2.0 1.0
9279.2980
0.4582
-0.0004
1.0 2.0 1.0 1.0
9279.1980
0.3582
0.0028
3.0 1.0 2.0 2.0
9278.9240
0.0842
-0.0031
2.0 0.0 1.0 0.0
9278.7890
-0.0508
0.0057
4.0 2.0 3.0 2.0
9278.6350
-0.2048
-0.0010
1.0 1.0 1.0 2.0
9278.3380
-0.5018
-0.0005
1.0 2.0 2.0 2.0
9278.2210
-0.6188
-0.0037
Transition 2( 1, 1) - 1( 1, 0) Fitted unsplit frequency:
F" I"
f
:r
Obs.(MHz)
Split(obs)
OBS-CAL
2.0 2.0 2.0 2.0
9279.1090
1.9867
-0.0057
1.0 1.0 0.0 1.0
9278.7890
1.6667
-0.0012
3.0 2.0 2.0 1.0
9277.5810
0.4587
0.0000
1.0 2.0 1.0 1.0
9277.4760
0.3537
-0.0017
3.0 1.0 2.0 2.0
9277.2170
0.0947
0.0073
4.0 2.0 3.0 2.0
9276.9190
-0.2033
0.0004
1.0 1.0 1.0 2.0
9276.6200
-0.5023
-0.0011
2.0 2.0 1.0 2.0
9276.1400
-0.9823
0.0039
1.0 2.0 0.0 1.0
9275.6920
-1.4303
-0.0059
2.0 1.0 2.0 1.0
9275.1150
-2.0073
0.0108
0.0 2.0 1.0 2.0
9274.4400
-2.6823
-0.0069
100
Reproduced with permission o f the copyright owner. Further reproduction prohibited without permission.
Transition 3 ( 2 , 2 ) - 2(2 ,1 ) Fitted unsplit frequency: 13358.8589 MHz
F" I"
F
r
Obs.(MHz)
Split(obs)
OBS-CALI
3.0 2.0 2.0 2.0
13359.5130
0.6541
0.0009
4.0 2.0 3.0 2.0
13359.2940
0.4351
0.0037
2.0 1.0 1.0 1.0
13359.2360
0.3771
-0.0004
4.0 1.0 3.0 1.0
13358.9600
0.1011
-0.0030
5.0 2.0 4.0 2.0
13358.6400
-0.2189
-0.0047
3.0 1.0 2.0 1.0
13358.4850
-0.3739
-0.0016
3.0 0.0 2.0 0.0
13358.3700
-0.4889
0.0062
1.0 2.0 0.0 2.0
13358.1080
-0.7509
-0.0012
Transition 3( 1, 2) - 2( 1, 1) Fitted unsplit frequency: 13903.7233 MHz
F" I"
F T
Obs.(MHz)
Split(obs)
OBS-CALC
3.0 2.0 3.0 1.0
13905.5410
1.8177
-0.0004
4.0 2.0 3.0 2.0
13903.8190
0.0957
0.0030
2.0 2.0 2.0 1.0
13903.5100
-0.2133
-0.0026
101
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Correlation Matrix
CHIAA1 CHIBB1 CHIAA2 CHIBB2
CHIAA1
CHIBB1
1.0000
-0.5066 1.0000
CHIAA2 -0.2726 0.1857 1.0000
CHIBB2 -0.1254 0.2565-0.1598 1.0000
1.Freq.
0.0162-0.0322-0.0014-0.0288 1.0000
2.Freq.
0.0228-0.0453-0.0025-0.0383 0.0020 1.0000
3.Freq.
0.0464-0.0146-0.0036-0.0002 0.0004 0.0006 1.0000
§ 4.Freq.
0.1388-0.0551 0.1519-0.0520 0.0026 0.0036 0.0084 1.0000
5.Freq.
0.0154 0.0645 0.0453 0.0452-0.0027-0.0037 0.0022 0.0130 1.0000
6.Freq.
0.1077-0.0697-0.0612 0.0183 0.0014
0.0021 0.0047 0.0084 -0.0001 1.0000
7.Freq. -0.0829-0.0371 0.0628 0.0889-0.0010-0.0011 -0.0037-0.0034-0.0021 -0.0053 1.0000
8.Freq.
0.0992-0.0545-0.1322-0.1072 0.0038
0.0051 0.0029-0.0083-0.0090 0.0105-0.0273 1.0000
9.Freq.
0.0208-0.0099 0.0070-0.0053 0.0004 0.0005 0.0011 0.0055
0.0010 0.0018-0.0012 0.0006 1.0000
10.Freq. -0.0431 0.0597-0.0330 0.1259-0.0043-0.0059-0.0014-0.0145 0.0051 -0.0006 0.0096-0.0123-0.0015 1.0000
Quadrupole Coupling Constants
Chi AA(1) =
1.3966
C h iB B ( l) =
2.2404
Chi AA (2) =
1.6022
C h iB B ( 2 )= -4.7524
C h iC C ( l) = -3.6370
C h iC C ( 2 )=
3.1502
Reproduced with permission o f the copyright owner. Further reproduction prohibited without permission.
Section III:
Van der Waals Complexes
104
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Introduction
This section of the thesis is composed o f three chapters covering four different
van der Waals complexes.
All studies were performed on the Fourier transform
microwave spectrometer. Chapter four, titled The Structural Determination o f the
OCS—HBr van der Waals Complex presents the completed analyses o f the OCS—HBr
molecule. Five separate isotopic species were assigned. This structural and dynamical
information completed the OCS—HX (X=F,C1,CN) studies. A modified version o f the
chapter will be submitted for publication in the coming months.
Early Analysis of the HBr—DBr van der Waals Complex. Chapter 5, provides
the preliminary results for the asymmetric hydrogen halide dimer, HBr—DBr.
The
transitions for this complex were originally unassigned lines from the OCS—HBr
investigation. Although many experiments have been performed on both the HF—DF
and HC1--DC1 complexes, no studies had been done on HBr—DBr until now. When
completed, this project will provide further insight into the well-studied (HX ) 2
complexes which are still puzzling.
An addendum to the laboratory notebook, presented in Chapter 6, is titled
Unassianed Transition Frequencies with Unknown Molecular Carriers. Herein, two
sets o f unassigned frequencies from separate projects are covered. Both have been
tentatively attributed to the complexes H jO —(HC1)2 and NO--H 2 O, respectively.
Suggestions for further experiments are given and past work detailed.
105
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
C hapter 4: A Structural Determination of the OCS—HBr
van der Waals Complex.
106
Reproduced with permission o f the copyright owner. Further reproduction prohibited without permission.
A bstract
The structure o f the OCS—HBr van der Waals complex has been studied by
Fourier transform microwave spectroscopy. The ground state complex is hydrogen
bound and quasi linear with SCO—HBr atomic ordering. Spectra from five different
isotopomers were observed and assigned. The wide amplitude bending angle o f the
hydrogen bromide was calculated from the nuclear quadrupole coupling constant, Xaa,
to be 25.24°. Second order quadrupole effects, centrifugal distortion in X,
spin-
rotation interactions, and spin-spin interactions were all included in the Hamiltonian.
The following spectroscopic constants have been determined for the
O C S —H ^ B r
isotopomer: B = 488.7948(4) MHz, D j = 2.167(6) kHz, H j = 1.19(3) Hz, Xaa =
387.14(1) MHz, Dx = 6.79(14) kHz, CBr = 0.55(13) kHz, Daa = 7.7 (1.2) kHz.
Introduction
The study o f two analogous series o f weakly bound dimers, CO 2 --HX and
OCS—HX, has been ongoing since the early eighties. Initially, striking similarities were
shown between the isoelectronic carbon dioxide and carbonyl sulfide bonding within
van der Waals complexes with hydrogen halides. Microwave studies found both CO2 -HX
( X = F ,C 1 ) 1 ’2 , 4
OCS—HX (X=F,C1)2>3,5 to be quasi linear hydrogen bound
systems. Comparable large amplitude zero point bending motions o f the HX moiety
were observed, and a short r(O - H) distance was associated with an unusually small
force constant in both CO2 --HX and OCS—HX cases.
The ability o f van der Waals complexes to form in more than one
conformation is a relatively unexplored area.
HCN, often thought o f as a quasi
hydrogen halide, exhibits two distinct structures when complexed with CO2 and OCS.
107
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
The CO2 --HCN complex was determined to be T-shaped with the basic nitrogen of
the HCN bonded to the carbon o f the CO2 . 6
However, by changing the carrier gas
and thereby changing the nozzle expansion conditions, a linear, hydrogen bound CO2 -HCN structure was assigned by Klots et al7. This tendency to form more than one
van der Waals "isomer" for a given complex is also seen in N 2 O—H F ^ . However,
Legon and co-workers^ found only one isomer in the OCS—HCN system which was
collinear and hydrogen bound as expected from the prior hydrogen halide work.
The structural discrepancy between the bonding o f CO 2 and OCS to HCN
provides an extreme test of the predictive models o f van der Waals interactions.
Within the CO2 --HX series itself one can use electrostatics to identify two plausible
isomers. The electric quadrupole moment of CO2 could interact favorably with the
dipole moment o f HX by either having the nucleophilic halide adjacent to the
electrophilic carbon or by having the electrophilic hydrogen interact with the
nucleophilic oxygen. In the case o f the OCS—HX series, only one electrostatically
favorable structure exists because unlike CO2 , OCS is a polar molecule. The dipoledipole interaction o f the OCS and the HX would align the complex in the linear
geometry SCO—HX as seen in the X=F,CL,and CN cases.
Recent studies have completed the CO2 --HX (X=F,Cl,CN,Br) series by
determining the structure o f the CO2 —HBr complex. The ground state was found,
both in the infrared9>10 and in the microwave* *, to be T-shaped with the bromine of
the HBr bonding to the carbon o f the CO2 . Unlike the findings o f the CO 2 --HCN
system, only one isomer was observed even though extensive searching for the linear
conformation was carried out. **
108
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
The work presented here extends the OCS—HX (X=F,Cl,HCN,HBr) series by
investigating the structure of the OCS-HBr van der Waals complex with the use o f a
Fourier transform microwave spectrometer. Sharpe et al.*3 concluded, using high
resolution infrared spectroscopy, the geometry o f the complex is linear but were
unable to definitively assign the atomic ordering. A microwave investigation could
thoroughly determine the structure and add the dynamical information that is so
interesting in these CO2 --HX and OCS—HX molecules. It may also prove the
existence of a second isomer, the T-shaped structure, as seen in the CO2 —HBr. We
report at this time only the linear structure with atomic ordering SCO—HBr. However,
several transitions are unassigned and may be due to the T-shaped isomer.
Experimental
Ground state rotational spectra o f five isotopomers o f OCS-H Br were
observed and transition frequencies were measured with an accuracy o f 1 kHz using a
pulsed-nozzle, Fourier transform microwave spectrometer as described in Section I.
The range of the spectrometer is 4-26.5 GHz but for this study data was collected only
from 5-12 GHz due to the rapid decrease in signal as J increased.
The gas mixture used, which was o f critical importance, was neon as the main
carrier at 92.8% with 0.2% OCS, 2% H(D)Br, and 5% Ar combined to 10 atm. The
34S isotopomer was observed in natural abundance at 4%. Alterations in the mixture
resulted in dramatic changes in the signal-to-noise ratios o f the rotational transitions.
This finding follows the CO2 —HCN** and CO2 —HBr* * work which suggests another
low lying conformer may be produced in favor o f the expected conformer when
changes in gas mixture occur.
109
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
In order to obtain the high resolution necessary to resolve the hyperfine
components observed in this system, the spectrometer was operated with the
supersonic nozzle coaxial to the axis o f the Fabry-Perot cavity as detailed further in
Section I. The following is a brief overview o f the specifics o f this experiment. A
pulse o f the gas mixture is introduced into the Fabry-Perot cavity, coaxially, for 0.6
ms. The backing pressure was typically 3 atmospheres using a nozzle diameter o f 0.5
mm. In order to produce appropriate line shapes, as shown in Figure 1, the delay time
between the pulse o f gas and the pulse o f microwave power was 1.3 ms. The
microwave excitation pulse lasting 0.9 ps then stimulates the molecules present in the
cavity. Before collecting the emission of the molecules, a delay, on the order o f 40 ps,
is necessary to ensure the dissipation o f the original microwave pulse. The typically
300 ps free induction decay of the molecules is then gathered for 1000 ps and
digitized. Collecting the free induction decay (FID) for 1000 ps o f total time allows
the discrete FFT algorithm to generate a Fourier transform o f this data with a point to
point resolution o f 2 kHz, Avresolution = 2/t, where t = the total collection time. A few
thousand sweeps were averaged for each transition to obtain a suitable signal-to-noise
ratio. For each o f these gas pulses, the cavity noise resulting from a microwave pulse
with no gas pulse is subtracted in order to eliminate superious signals from cavity
imperfections.
Following the collection o f the time domain signal, a Fast Fourier
transform is performed to obtain the desired power spectrum. Line widths (FWHM)
on the order o f 5 kHz were observed. Figure 2 provides an example o f a typical
transition collected for this study. To obtain the frequency o f a single transition, the
average o f the doublet is added to the frequency o f the pumping microwaves. Again,
further insight can be found in Section I o f this thesis.
110
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Initial work on the OCS—HBr complex began with the nozzle perpendicular to
the Fabry-Perot cavity. With guidance from the early infrared work* * searches began
for a-type rotational transitions of OCS—HBr. No transitions were observed. As a
matter of fact, the well known Ar—HBr transitions^ were not seen for several days
due to the time required to "season the line" with the HBr. The fact that reactive
systems need a substantial amount o f time to equilibrate in the instrument prior to
exhibiting a rotational spectra has been noticed in other laboratories with a FTM
spectrometer and is called the process o f "seasoning the line". With the line seasoned,
and with updated rotational constants from Sharpe et al., searching resumed with a
new mixture and with the nozzle coaxial to the cavity. The new mixture is the one
described above with neon as the main carrier gas. The results obtained are presented
in the following section.
Results
Spectroscopic Constants
Spectra from five different isotopic species o f the OCS--HBr van der Waals
complex were obtained and assigned to a-type, K= 0 rotational energy levels o f a
linear molecule. The spectrum is also consistent with a slightly non-linear equilibrium
geometry. The resultant complex in the non-linear case would be a nearly prolate
asymmetric top with a large value for the rotational constant A. The energy levels
associated with K_j>0 would be negligibly populated and transitions originating from
these states would not be observed provided A is sufficiently large.
The measured microwave frequencies o f the
O C S —H ^ B r
species are listed in Tables 1 and 2, the frequencies for the
and
O C S —H
O C S —D ^ B r
and
111
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
^B r
O CS—
D8*Br are
listedin Tables3and 4,
andthe frequencies
isotopomer arelisted in Tables 5and6. FiveJ +1 <-
for the OC34S--H79Br
J transitions were observed for
all isotopomers. Each single rotational transition is split into ten resolvable hyperfine
components; seven resulting from the bromine quadrupole coupling constant, and
three resulting from the interacting spins o f the bromine and the hydrogen.
The coupling scheme used throughout this paper is as follows: J + Ij = Fj and
3
2
1
2
Fj + 12 = F where I x = — = I Br and I 2 = —= I H. The strong A F j= +1 components of
the hyperfine structure were observed for all o f the isotopomers, while the weaker
AFj= 0 transitions were gathered only for the two main species OCS--H79Br and
OCS--H8lBr.
As the significantly weaker AFj= 0 transitions are the furthest
displaced in frequency from the hypothetical unsplit line center, the measurement of
these transitions allows for the determination o f the quadrupole coupling constant,
Xaa, to a higher precision.
The spectroscopic constants were determined in a two step process. The first
step involved analyzing the spectra using a Hamiltonian which includes only end-overend rotation and electronic hyperfine components. Secondly, consideration was given
to magnetic interactions, specifically that o f nuclear spin-spin (spin o f the bromine with
the spin o f the hydrogen,
and spin-rotation (spin o f bromine with the end-
over-end rotation of the molecule; CBrIg r»J).
The energy expression initially used to fit the spectra is given in equation (1).
Included in this electronic component o f the energy expression are terms involving the
rotational constant B, where B = ~ (B 0 + C0); the centrifugal distortion constant, Z)j;
the sextic distortion constant Hj; the Br nuclear quadrupole coupling constant, Xaa,
112
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
through second order; and the centrifugal distortion o f Xaa, D y , also through second
order. _
E = B(J +1) - Dj[J(J + 1)]2 + Hj[J{J + 1)]3
-
[X aa
+ D zJ(J + l)]f(I,J,F) + [Xaa + D *J
B (J + l)] g(IyJ,F)
(1)
The first four terms in expression (1) were fit to the data by the use o f a linear
least squares procedure. The constants determined from the least squares fit were then
used to determine the fifth term in the energy expression. With the last term included
as a numerical constant, the procedure is performed again. This iteration procedure
converged after two cycles. The determined spectroscopic constants are reported in
Table 7.
The quadrupole coupling constant of OCS—HBr, Xaa , is the projection o f the
quadrupole coupling constant of the HBr monomer along the a axis o f the complex.
The tighter the wide amplitude bending o f the HBr system about its center o f mass
along the a axis, the larger the value obtained for Xaa. The structural ramification are
discussed later. Furthermore, the effective quadrupole coupling constant o f OCS—HBr
is Xaa + Dx J(J+1), where Dx is the positive centrifugal distortion constant.
This
suggests that as J increases, Xaa effective also increases, and therefore, the wide
amplitude bending about the a axis gets smaller.
At this point in the analysis, only the AFj= 1 transitions were included in the
fitting procedure because as mention earlier, the AFj= 0 transitions were further split
and weaker than the AFj= 1 transitions. When it was noticed that the quadrupole
coupling constant was not as precisely determined as it should be, collection o f the
data from the AF^= 0 transitions began. Immediately, an unexpected splitting was
observed in these transitions not seen in the AFi= 1 transitions. Another quantum
113
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
number was necessary to explain the new splitting. By considering the nuclear spinspin interaction, Daa, an additional quantum number enters the analysis. Furthermore,
the spin-rotation interaction, Cjjr, was included as its effect on the spectra is on the
same order o f magnitude as the effect o f Daa.
The nuclear spin-spin interaction, which involves the spin o f bromine (Ijjr=3/2)
interacting with the spin of the hydrogen (Ifj=l/2), brings a new quantum number, Ijj ,
into the spectroscopic analysis which may cause an expected single transition to split
into two. A scaling approximation was performed using the following equation to
estimate the value o f the interaction constant, Daa:
where D0 is the spin-spin interaction of the monomer, H B r^ . From this formula we
observe that the spin-spin interaction constant; Daa, a second rank tensor, projects in
the same manner as the quadrupole coupling constant, Xaa. Equation (2) gives an
estimate of 7.5 kHz for the spin-spin interaction constant o f the OCS—H ^ B r species.
To actually determine the spin-spin interaction present in the OCS—HBr complex the
following energy equation must be added to the energy expression o f equation (1):
E*
+ ! > - J<J +1> ~ 7
(3)
This equation was deduced by suitable adjustments to a more general form presented
in the appendix o f Gordy & C o o k ^ . The third from the last factor is a "six J" symbol
and next to last term is a "nine J" symbol, which are frequently used terms in angular
momentum coupling theory
in Table 7 the spectroscopic results, which are in very
good agreement with the estimates, are given for the spin-spin constant o f the two
114
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
main isotopomers, OCS--H79Br and OCS~H81Br. Information for the other species;
OCS—D79Br, OCS—D8 *Br, OC^^S—H79Br, is not available as their AF}= 0
transitions, where the splitting due to Daa is observed, were not collected. For these
cases the constant was set to zero and was not involved in the fitting procedure.
Using another scaling approximation, an estimation o f the spin-rotation
constant can also be calculated^. The spin o f bromine nucleus (1=3/2) interacting
with the overall rotation of the molecule (J) causes a small shift in the expected
transition frequency. The spin-rotation interaction o f OCS—HBr can be estimated by
the following equation:
(4)
where B is the rotational constant for OCS—HBr, c0 and b0 are constants determined
for the free HBr m onom er^, spin-rotation and rotation, respectively. This formula
estimates the spin-rotation interaction o f the OCS—H79Br complex to be 0.52 kHz.
To spectroscopically determine the constant the following energy expression must be
added to the energy expression given in equation (1):
E.'sr =i|£ FX(F1+ 1
)
-Ij-
(5)
The determined spin-rotation constants are listed in Table 7 for only the main
isotopomers; OCS—H79Br and OCS—H8 *Br. Both determined values are in good
agreement with the scaling calculation.
The weak AF^= 0 transitions were not
gathered for the other species; OCS—D79Br, OCS—
Brand O C ^ S —H79Br,
therefore CBr was not determined for these isotopomers. The constant for these
115
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
species was set to 0.55 kHz which is the value found for the main isotopomers and is
close to the estimated value of 0.52 kHz.
Molecular Geometry and Dynamics
From the determined spectroscopic constants, a structural and dynamical
examination of the OCS--HBr complex can be performed. As mentioned previously,
the microwave spectra suggests that the complex is quasi-linear. Furthermore, the a
axis of the OCS--HBr complex, which connects the center o f mass points o f the two
individual moieties, lies almost directly along the a axis o f each o f these monomer
species. For the structural analysis, it was assumed that all three a axis coincide. Also
assumed in the structural analysis was that the structural and electronic properties of
the OCS and HBr moieties remain unchanged by complexation.
In particular, the
bond lengths (i.e., 0=C, C=S, and H-Br) and the electronic field gradient about the
bromine nucleus.
Spectroscopic constants from five isotopomers o f OCS—HBr enabled a
definitive atomic ordering determination. The atomic ordering o f the complex is
S-C-O—H-Br, as seen in Figure 2, and as was anticipated by analogy from the
structures o f HF--OCS3'4 and HC1—OCS5. In this case the Kraichman equations were
unable to unambiguously assign the atomic ordering because only one rotational
constant was determined.
From the spectroscopically determined quadrupole coupling constant, Xaa> ° f
the OCS—HBr van der Waals complex, the wide amplitude bending angle o f the
hydrogen,S, can be calculated using the following formula:
116
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Xaa = ^ X 0 (3COS2 S - 1 )
(6 )
where the %q is the quadrupole coupling constant o f the H(D)Br
be seen in equation (6),
m onom er^ .
As can
is the projection o f Xo fr°m the free H(D)Br onto the a
axis of the complex as a second rank tensor. The calculated values o f the expectation
angles are listed in Table 8 for all five isotopic species. At equilibrium, it is believed
that this angle is zero such that the van der Waals bond formed between the oxygen of
OCS and the hydrogen o f HBr, lies directly along the a axis o f the complex.
The molecular structure was found by fitting the determined B and 0 to the
structural parameters Rem and Ro -h -
results are reported in Table 8. A
significant deuterium effect is noted in this complex, as the wide amplitude bending
angle tightens almost three degrees for deuterium substitution. More significantly, the
deuterated species accomodates a shorter van der Waals bond than do the normal
hydrogen isotopomers.
Conclusions
The OCS—HBr van der Waals complex has been observed in the microwave
region o f the spectrum. A linear-type spectra was obtained for five isotopomers o f the
complex. This structure is consistent with the other hydrogen halide studies, OCS—HX
(X=F,C1), and with the quasi hydrogen halide HCN studies.
We are particularly grateful to Steven Sharpe for providing ongoing
unpublished results o f his infrared investigation o f the OCS—HBr van der Waals
complex.
117
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
References
1. F. A. Baiocchi, T. A. Dixon, C. H. Joyner and W. Klemperer, J. Chem Phys. 74,
6544 (1981).
2. G. T. Fraser, A. S. Pine, R. D. Suenram, D. C. Dayton and R. E. Miller, J. Chem.
Phys. 90, 1330 (1989).
3. A. C. Legon, L. C. Willoughby, J. Mol. Struct. 131, 159 (1985).
4. Robert S. Altman, Mark D. Marshall, and W. Klemperer, J. Chem. Phys. 77,4344
(1982)
5. Elizabeth J. Goodwin and A. C. Legon, J. Chem. Soc., Faraday Trans. 2. 81,1709
(1985)
6 .K .R . Leopold, G. T. Fraser, and W. Klemperer, J. Chem. Phys. 80, 1039 (1984).
7. A. I. Jaman and A. C. Legon, J. Mol. Struct. 158,205 (1987).
8. T. D. Klots, R. S. Ruofl; H. S. Gutowsky, J. Chem. Phys. 90,4216 (1989).
9. S. W. Sharpe, Y. P. Zeng, C. Wittig, and R. A. Beaudet, J. Chem Phys. 92,943
(1990).
10. Y. P. Zeng, S. W. Sharpe, S. K. Shin, C. Wittig, and R. A. Beaudet, J. Chem
Phys. 97, 5392 (1992).
11. J. K. Rice, R. D. Suenram, F. J. Lovas, G. T. Fraser, and W. J. Lafferty, Ohio
State Symposium on Molecular Spectroscopy, 1990 and further private
communications.
12. C. M. Lovejoy, and D. J. Nesbitt, J. Chem Phys. 87, 1450 (1987).
C. H. Joyner, T. A. Dixon, F. A. Biocchi, and W. Klemperer, J. Chem Phys.
74, 6550 (1981).
13. T. A. Hu and S. W. Sharpe, Ohio State Symposium on Molecular Spectroscopy,
1993 and many further private communications.
118
Reproduced with permission o f the copyright owner. Further reproduction prohibited without permission.
X
JO
1E L
B
0
CJ
»_>
ffl
f f1i
I
CO
o
o
<o
+->
O
P
C/5
T3
<o
C
<o
O
Q
0)
u
S
DD
119
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
FID fron punp freq. 8786.6 KHz
0.2
8.15
£ e.es
-0 .8 S
-0.1S
280
400
1808
688
tine / nlcroeeconds
J = 9 - 8, U > FI = 15/2 - 13/2
end Cb) 17/2 - 15/2
«
a
u
«
4*
■
0
C
4*
c
n
b.
8
108
288
308
488
560
frequency offset / kHz
Figure 2: Typical Rotational Transition of OCS-HBr
120
Reproduced with permission o f the copyright owner. Further reproduction prohibited without permission.
Table 1. Microwave frequencies for OCS—H ^ B r and OCS—H ^ B r.
j - r
F j - F - 'i
F - F"
6o - 5o
9/2 - 9/2
5 -5
OCS-H79Br/MHz
Dev./kHz
OCS-H«Br/MHz
Dev./ kHz
5769.3815
7.8
5727.2645
7.5
5727.268
5.0
60 ' 50
9/2 - 9/2
4 -4
5769.388
8.7
6o - 5o
1 1 /2 -1 1 /2
6 -6
5840.5595
0.2
5786.537
1.2
6o - 5o
1 1 /2 -1 1 /2
5 -5
5840.5675
-0.3
5786.546
1.3
60 - 50
1 3 /2 -1 1 /2
6-5& 7-6
5862.297
3.4
5804.8575
2.7
60 ‘ 50
1 5 /2 -1 3 /2
8- 7& 7-6
5862.317
1.3
5804.870
1.8
5.2
5808.382
3.0
60 ‘ 50
1 1 /2 -9 /2
5-4& 6-5
5866.478
60 - 50
9 /2 - 7 /2
4 - 3 & 5 -4
5866.568
-1.0
5808.444
-0.3
60 ‘ 50
13/2 - 13/2
7 -7
5959.292
2.8
5885.899
2.5
6o-5o
1 3 /2 -1 3 /2
6 -6
5959.3045
2.4
-2.8
2.2
-3.9
70 ’ 60
1 1 /2 -1 1 /2
6 -6
6745.1295
70 ‘ 60
1 1 /2 -1 1 /2
5 -5
6745.1355
-4.9
6693.5435
-2.9
70 ' 60
13/2 - 13/2
7 -7
6820.448
-5.9
6756.311
-4.4
70 ’ 60
1 3 /2 -1 3 /2
6 -6
6820.4565
-5.9
6756.3195
-5.0
70 - 6 0
15/2 - 13/2
7-6& 8-7
6839.186
-4.0
6772.103
-3.7
70 ‘ 60
17/2 - 15/2
9- 8& 8-7
6839.191
-4.0
6772.103
-4.6
70 ‘ 60
1 3 /2 -1 1 /2
6-5&7-6
6842.1855
-2.9
6774.6315
-2.5
70 - 60
1 1 /2 -9 /2
5-4& 6-5
6842.226
-5.7
6774.657
-4.2
70 - 60
15/2 - 15/2
8 -8
6936.162
-1.8
6853.1335
-1.9
70 ‘ 60
1 5 /2 - 15/2
7 -7
6936.1735
-2.3
6853.1465
-1.8
80 ' 70
13/2 - 13/2
7 -7
7720.8805
-1.7
7659.7765
-2.4
00
o
1
o
5885.9125
6693.536
13/2 - 13/2
6 -6
7720.888
-0.7
7659.7845
-1.3
V 70
15/2 - 15/2
8 -8
7799.184
-3.9
7725.0725
-3.1
7725.082
-2.7
80 - 70
15/2 - 15/2
7 -7
7799.193
-3.4
80 - 70
19/2 - 17/2
10 - 9 & 9 - 8
7815.655
-2.1
7738.9525
-1.4
80 - 70
17/2 - 15/2
8-7&9- 8
7815.6625
-1.3
7738.9605
-1.1
80 - 70
15/2 - 13/2
7- 6& 8-7
7817.9225
-1.5
7740.866
-1.0
80 ‘ 70
1 3 /2 -1 1 /2
8 - 7 & 9 -8
7817.9375
-0.8
7740.873
-0.7
80 ‘ 70
17/2 - 17/2
9 -9
7912.6335
0.7
7819.9895
0.1
80 - ?0
1 7 /2 -7 /2
8 -8
7912.645
0.7
7820.002
0.1
* The coupling scheme used throughout this paper is: 7 + /, = F, and Fx+ I2 = F where
121
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Table 2. Microwave frequencies for OCS—H ^ B r and OCS—H ^ B r
continued from Table 1.
OCS~H79Br/
MHz
8696.454
F i-F " i
9 < r 8o
15/2 - 15/2
8 -8
90 ’ 80
15/2 - 15/2
7 -7
8696.461
9 < r 8o
17/2 - 17/2
9 -9
9 o - 80
17/2-17/2
8 -8
9 q - 80
21/2 - 19/2
Dev./
kHz
3.7
OCS~H81Br/
MHz
8625.8245
Dev./
kHz
3.1
3.8
8777.021
1.7
8693.0315
2.1
8777.030
2.2
8693.0415
3.0
11-10 & 10-9
8791.7165
4.2
8705.4155
4.1
19/2 - 17/2
9 - 8 & 10-9
8791.732
4.1
8705.4295
3.7
9o * 8o
15/2 - 13/2
7 - 6& 8 - 7
8793.498
5.8
8706.913
3.4
8706.919
3.6
0
00
1
8625.8325
On
4.1
o
J -J '
F - F"
90 * 80
17/2-15/2
8 - 7& 9 - 8
8793.498
2.9
90 ‘ 80
19/2- 19/2
1 0 -1 0
8888.709
5.2
8786.467
5.2
90 ■ 80
19/2 - 19/2
9 -9
8888.720
5.1
8786.477
3.4
100 - 9 0
17/2 - 17/2
9 -9
9671.746
-0.1
9591.5855
-0.7
100 - 9 0
17/2 - 17/2
8 -8
9671.753
0.1
9591.5935
0.0
100 - 9 0
19/2 -19 /2
1 0 -1 0
9754.0935
-3.4
9660.2955
-2.7
10q - 9 0
19/2 - 19/2
9 -9
9754.1025
-2.9
9660.3055
-1.8
100 - 9 0
2 3 /2 -2 1 /2
12-11 & 11-10
9767.3625
-0.5
9671.4775
-0.6
100 - 9 0
21/2- 19/2
10-9 & 11-10
9767.3835
-1.8
9671.496
-1.7
-0.5
9672.680
-0.3
100 - 9 0
17/2-15/2
8- 7& 9- 8
9768.7905
100 - 9 0
19/2- 17/2
9 - 8 & 10-9
9768.804
-1.6
9672.6935
-1.2
100 - 9 0
21/2 - 21/2
11 - 11
9864.3755
-1.5
9752.5465
-1.7
io 0 - 90
21/2 - 21/2
1 0 -1 0
9864.386
-1.7
9752.5575
-2.1
122
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Table 3. Microwave frequencies for OCS—D ^ B r and OCS—D ^ B r.
O C S-D 79Br/
MHz
Dev./
kHz
OCS~D81Br/
MHz
Dev./
kHz
J -J '
P i-P i
p - F"
6 0 -5 o
9/2 - 9/2
5 -5
60 -5 o
9/2 - 9/2
4 -4
60 -S o
11/2 - 11/2
6 -6
5832.195
-0.1
60 - 5 o
11/2 - 11/2
5 -5
5832.195
-0.1
60 - 5o
1 3 /2 -1 1 /2
6 -5 & 7 - 6
5855.2955
0.6
5798.3385
0.6
60 -5 o
15/2 - 13/2
8 - 7 & 7 -6
5855.3245
0.4
5798.3565
-0.2
60 -5 o
1 1 /2 -9 /2
5-4&6-5
5859.7385
1.3
5802.0865
0.5
5859.849
- 1.8
5802.163
- 1.1
6 0 - 5o
9/2 - 7/2
4 -3 & 5 - 4
60 "5o
13/2 - 13/2
7 -7
5884.809
-2.7
60 ' 50
13/2 - 13/2
6 -6
5884.8145
2.8
70 ' 60
11/2 - 11/2
6 -6
70 “ 60
11/2 - 11/2
5 -5
70 ' 60
13/2 - 13/2
7 -7
70 ‘ 60
13/2 - 13/2
6-6
7o - 6o
15/2 - 13/2
7- 6&8- 7
6764.6445
-0.9
70 ‘ 60
17/2 - 15/2
9- 8& 8- 7
6831.1915
1.1
6764.6515
0.9
7c r 6o
13/2- 11/2
6-5&7-6
6834.368
0.5
6767.335
1.0
70 ‘ 60
1 1 /2 -9 /2
5 - 4& 6 - 5
6834.420
-1.7
6767.369
-0.9
7o - 6o
15/2 - 15/2
8 -8
70 ‘ 60
15/2 - 15/2
7 -7
80 - 70
13/2 - 13/2
7 -7
80 - 70
13/2 - 13/2
6-6
8 0 - 70
15/2 - 15/2
8 -8
8o ■ 70
15/2 - 15/2
7 -7
0.2
0.1
-0.6
0.8
6831.1795
-0.7
1.4
7730.5785
80 - 70
7806.669
-1.0
7730.5825
8 0 - 70
15/2 - 13/2
7 - 6 & 8- 7
7809.0735
0.5
7732.6085
8 0 - 70
1 3 /2 -1 1 /2
7809.0945
-0.2
7732.6225
8 0 - 70
17/2 -1 7 /2
8- 7 & 9- 8
9-9
8-8
0
1
7806.669
8 - 7 & 9 -8
0
0
1
10 - 9 & 9 - 8
00
0
00
19/2 - 17/2
17/2 - 15/2
17/2 - 7/2
123
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Table 4. Microwave frequencies from OCS—D ^ B r and OCS—D ^ B r continued
from Table 3.
OCS~D79Br/
MHz
Dev.
kHz
O C S-
Dev.
kHz
J -J '
F ' l " F"i
F - F"
9()-8o
15/2- 15/2
8 -8
9 0 ‘ 80
15/2- 15/2
7 -7
9 0 ‘ 80
17/2- 17/2
9 -9
9 o - 80
17/2-17/2
8 -8
9 o ’ 80
21/2 - 19/2
11-10 & 10-9
8781.759
0.0
8696.1375
-0.4
9 o - 80
19/2- 17/2
9 - 8 & 10-9
8781.771
0.2
8696.149
0.0
9 o - 8o
15/2 - 13/2
7-6&8-7
8783.653
0.4
8697.733
0.5
9 o - 80
17/2 - 15/2
8- 7&9- 8
8783.649
-0.8
8697.733
-0.6
9 o - 80
19/2 - 19/2
1 0 -1 0
9 ( ) - 80
19/2 - 19/2
9 -9
100 - 9 0
17/2 - 17/2
9 -9
100 - 9 0
D 81Br/
MHz
17/2-17/2
8 -8
100 - 9 0
19/2 - 19/2
1 0 -1 0
100 - 9o
100 - 9 0
19/2 - 19/2
23/2 - 21/2
12-11& 11-10
9756.4535
0.4
9661.315
0.0
100 - 9 0
21/2 - 19/2
10-9 & 11-10
9756.4715
-0.5
9661.331
-0.4
100 ‘ 9o
100 - 9 0
17/2-15/2
8- 7&9- 8
9757.972
-0.3
9662.5945
-0.1
19/2- 17/2
9 - 8 & 10-9
9757.9825
0.3
9662.6055
0.6
100 - 9 0
21/2 - 21/2
1 1 -1 1
100 - 9 0
21/2 - 21/2
1 0 -1 0
9 -9 .
124
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Table 5. Microwave frequencies for O C ^ S —H ^ B r.
F j - F ”,
pi _ pti
OC 34 S -H 79 Br/MHz
Dev./kHz
5 -5
60 -5()
9/2 - 9/2
4 -4
60 -5o
11/2 - 11/2
6 -6
60 - 5o
11/ 2 - 11/2
5 -5
60 -5 o
13/2-11/2
6 -5 & 7 - 6
5685.221
3.2
60 - 5o
15/2 - 13/2
8- 7& 7- 6
5685.243
1.5
60 - 5o
11/2-9/2
5- 4&6- 5
5689.394
3.9
6 0 - 5o
9/2 - 7/2
4-3&5-4
5689.4885
- 0.6
60 - 5o
13/2 - 13/2
7 -7
6 0 -5o
13/2 - 13/2
6 -6
7 o -6 o
11/2 - 11/2
6 -6
7 < r 60
11/2 - 11/2
5 -5
? o - 6o
7 0 ' 60
13/2 - 13/2
7 -7
13/2 - 13/2
6 -6
? 0 - 60
15/2 - 13/2
7- 6& 8-7
6632.668
-3.3
7 o ‘ 60
17/2- 15/2
9- 8&8- 7
6632.676
-1.7
7 o " 60
13/2-11/2
6- 5&7- 6
6635.662
-2.6
7( r 60
7 o -6 o
11/2-9/2
5-4&6-5
6635.707
-3.2
15/2 - 15/2
8-8
7 0 ‘ 60
15/2-15/2
7 -7
8 0 - 7o
13/2 - 13/2
7 -7
80 -7 o
13/2 - 13/2
6 -6
80 - 7o
15/2 - 15/2
8-8
80 - 7o
15/2 - 15/2
7 -7
80 -7 o
19/2- 17/2
10 - 9 & 9 - 8
7579.7215
-1.5
8 0 -7 o
17/2- 15/2
8- 7&9- 8
7579.728
- 0.2
8 0 - 7o
15/2 - 13/2
7- 6&8- 7
7581.984
-0.9
8q -7()
13/2- 11/2
8- 7&9- 8
7582.0005
17/2- 17/2
9 -9
17/2-7/2
8 -8
0
1
60 -5o
9/2 - 9/2
00
0
J ' - J 1'
8 0 - 7o
125
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Table 6. Microwave frequencies for OC34S—H79Br continued from Table 5.
obs.calc./
kHz
OC34S H79Br/
MHz
J' - J"
F 'j - F ” !
F' - F"
9o- 8 0
9 o - 80
15/2 - 15/2
8 -8
15/2 - 15/2
7 -7
9o - 8q
17/2 - 17/2
9 -9
9 o -8 o
17/2 - 17/2
8 -8
9o - 8o
9 o - 8o
21/2 - 19/2
11-10 & 10-9
8526.3895
2.9
19/2 - 17/2
9 - 8 & 10-9
8526.4025
1.8
9o - 8o
9q - 8q
15/2 - 13/2
7- 6 & 8- 7
8528.167
3.1
17/2 - 15/2
8- 7 &9- 8
8528.167
1.9
9 o - 80
19/2 - 19/2
10- 10
9 0 “ 80
19/2 - 19/2
9 -9
100 - 9o
17/2 - 17/2
9 -9
100 - 9o
10q - 9q
17/2 - 17/2
8 -8
19/2 - 19/2
10- 10
10q - 9o
19/2 - 19/2
9 -9
10o - 9q
23/2 - 21/2
12-11& 11-10
9472.667
-1.0
10o - 9o
10q - 9q
21/2 - 19/2
10-9 & 11-10
9472.6865
-2.3
17/2 - 15/2
8- 7 &9- 8
9474.094
0.0
10q - 9q
19/2 - 17/2
9 - 8 & 10-9
9474.1065
-0.3
10q - 9q
21/2 - 21/2
11-11
IOq " 9q
21/2-21/2
10- 10
•
|
126
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Table 7. Spectroscopic constants
O C S -H 79Br
O C S-H 81Br
O C S-D 79Br
O C S-D 81Br
O C ^ S —H79Br
B/ MHz
488.7948(4)
483.9845(3)
488.1910(2)
483.4215(2)
474.0228(4)
D j/ kHz
2.167(6)
2.095(5)
1.717(2)
1.677(3)
1.932(6)
H j/H z
1.19(3)
1.10(2)
0.20(1)
0.19(1)
0.87(3)
Xaa/M H z
387.14(1)
323.560(8)
413.05(1)
345.209(6)
386.88(15)
D x/kH z
6.79(14)
5.58(12)
6.4(1)
5.2(1)
6.5(3)
B aa/ kHz
7.7(1.2)
8.2(1.0)
___ *
*
*
C bA H z
0.55(13)
0.55(11)
0.55**
0.55**
0.55**
* Set to zero, see text for further explanation
** See text for further explanation
127
Reproduced with permission o f the copyright owner. Further reproduction prohibited without permission.
Species
cm.J A
8/°
^O -H ^
SCO -
25.24
5.263
2.385
SCO - H 8lB r
25.23
5.264
2.385
SCO - ©79Br
22.62
5.247
2.347
SCO - D 8lB r
22.61
5.248
2.347
34SCO - H 79fir
25.28
5.296
2.386
Table 8: Structural Parameters of OCS—HBr
128
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Species
k sf
Nm4
&s/cm“l
SCO - H 79Br
0.008812
86.07673
0.4448
14.85
SCO - H ^ B r
0.008814
86.07184
0.4515
14.88
SCO - D 79Br
0.006944
54.49953
0.5601
16.61
SCO - D ^ B r
0.006957
54.53305
0.5630
16.57
34SCO - H79Br
0.00876
85.82361
0.4629
15.01
Table 9: Dynamical Information
129
Reproduced with permission o f the copyright owner. Further reproduction prohibited without permission.
Chapter 5: Early Analysis of the HBr—DBr van der Waals Complex.
130
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
A bstract
The microwave spectrum o f the HBr—DBr van der Waals complex has been
observed using a Fourier transform microwave spectrometer. We present here the
first spectroscopic evidence of a dimer formed by two hydrogen/deuterium bromides.
Data was gathered for the two main isotopomers, H ^ B r—D ^ B r and H ^ B r—D ^ B r,
and also for the two mixed species, H ^ B r—D ^ B r and H ^ B r—D ^ B r.
in the
present early stages of the analysis, the mixed species are indistinguishable.
The
structure is heavy atom linear (F—DF) and exhibits large amplitude bending motions.
Quadrupole coupling effects o f three nuclear spins (Ig r=3/2, Ig r=3/2, Ip)=l)
complicate the spectra. The higher energy conformer with the hydrogen involved in
the bonding, DBr—HBr, has not yet been observed. Much more work, both
experimental and theoretical, needs to be done in order to thoroughly understand this
complex.
Introduction
The van der Waals complex formed between two identical hydrogen halides is
one o f the most extensively studied systems in the field o f molecular spectroscopy.
Homogeneous dimers such as (H F ^ * ^ , (HC1)2^“^ , and (HCN)2 ^ “^ have been
extensively investigated both theoretically and experimentally in the microwave and
infrared regions of the spectrum The lack o f information on (HBr) 2 results from the
fact that its infrared frequencies are technically difficult to access. However, work is
underway in other laboratories to gather this spectra^.
The complexes (HF ) 2 and (HC1)2 have two equivalent structures (Figure 1)
which are connected via quantum mechanical tunneling through a barrier which can be
131
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
cast as a one dimensional motion.
This motion, the breaking and making o f a
hydrogen bond, cannot occur in classical mechanics due to energy considerations but
is allowed by the wave nature o f quantum mechanics. A standard potential energy
curve o f two equal minima separated by a large potential energy hump is shown in
Figure 2. In their ground vibrational state neither the (HF) 2 nor the (HC1)2 have
enough energy to climb over the barrier and rearrange into the other configuration.
Penetration through the barrier, "tunneling1', is the only way o f moving between
conformers.
The interaction between the two wells results in a splitting o f the
degenerate energy levels.
The eigenfunctions must be either symmetric, g, or
antisymmetric, u, with r e je c t to this tunneling motion.
Just as in the ammonia molecule, the inversion motion dominates the
microwave spectra o f (HF) 2 with the rotational constants acting as a perturbation on
the tunneling states*. In principle, this inversion motion which exchanges the non­
bonding hydrogen for the bonding hydrogen, is a vibrational motion. Usually
vibrations produce frequencies in the infrared, but the potential barrier between the
two equivalent configurations (HFacceptor-HdonorF and
slows
down the inversion such that the frequency now lies in the microwave.
Figure 3 shows a correlation diagram for the K=0 energy levels o f a symmetric
dimer. The tunneling frequency, v, for the K=0 states in (HF) 2 is 19,747.248 MHz
while the rotational constant, B0, is 6,494.963 MHz-*. The two K=0 allowed
transitions for J = 0 -» l are observed at 2B0 ± v, or 32,737.174 MHz and 6,757.322
MHz, and involve a change in both the rotational and tunneling stated The observed
microwave spectra* could not be analyzed with a semi-rigid rotor Hamiltonian, but the
allowed transitions do permit the evaluation o f both the tunneling frequency and the
132
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
effective rotational constant. The symmetrical isotopomer (DF) 2 was found to exhibit
quite similar spectra, however the rotational constant is larger than the tunneling
frequency by a factor of four thereby producing a microwave spectrum dominated by
rotational constants and merely perturbed by the inversion m o tio n ^ .
The microwave spectra o f the (HC1)2 van der Waals complex does not exist
due to the rapid tunneling motion between the equivalent forms o f nearly orthogonally
oriented monomer units^. Neither o f the allowed J = 0 -> 1 transitions o f (HC1)2 are
observable in the microwave region o f the spectrum. An effective rotational constant
of 1,944.588 MHz coupled to a tunneling frequency o f 463,975.390 MHz produce
transitions for the K=0, J = 0 -> 1 at 460,086.214 MHz and 467,864.565 M H z^. The
fact that the tunneling frequency in (HC1)2 is on the same order o f magnitude as the
HC1 monomer (21 cm-1) suggests nearly free internal rotation o f the two subunits as
opposed to the more localized behavior observed in ( H F ^ V
The HCN molecule is often considered a quasi-hydrogen halide. It is therefore
plausible to mention the HCN dimer in our summery o f the hydrogen halide dimers.
Microwave structural
d e t e r m i n a t i o n s ^ - 17
found the complex to be hydrogen bound
and linear, unlike the (HF ) 2 and (HC1)2 whose structures are much closer to an "L"
shape. Further, no tunneling splitting was observed. This is due to the heavy reduced
mass o f the inversion motion, Le., the inversion motion involves the geared rotation
about the center of mass of each hydrogen halide. For the true hydrogen halides, the
center of mass is practically on the halide atom itself and therefore only the lightweight
hydrogen atom does the moving. In the (HCN) 2 case, the center o f mass o f the HCN
moiety is between the carbon and the nitrogen. Therefore to pivot about the center of
mass to exchange hydrogen donor-acceptor would require both the hydrogen and the
133
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
carbon to be involved in the motion. Any tunneling motion that occurs in the HCN
dimer is unresolved in microwave experiments.
So far we have only discussed the homodimers o f the hydrogen halides. Of
course, mixed species exist such as HF—DF and H ^ C l—H ^ C l. In these cases the
exchange symmetry is broken and the inversion operator no longer commutes with the
fall Hamiltonian and thus the wave&nctions are no longer perfectly symmetric or
antisymmetric. The spectroscopy o f solved asymmetric hydrogen halide dimers,
including HF—DF, H -^ C l-H ^ C l,
HC1—DC1, has added to the understanding o f
symmetric dimers and farthered the knowledge o f the potential surface.
Experiments and theory have shown that the size o f the energy difference
between the two inversion conformers determines the tunneling capabilities o f the
c o m p le x ^
**» *2,14
When the heavy atom is isotopically substituted as in
H -^ C 1 ~
H-^CL, the zero point energy difference between the configuration with the
being
the acceptor and 3 7 q being the acceptor is quite small*®. The result is that very
similar tunneling patterns are observed for mixed heavy atom dimers. Physically, this
means that the ^ C V ^ C l isotopically induced zero point shifts in the two minima are
small with respect to the inversion splitting, and hence the large amplitude, internal
rotor dynamics are qualitatively identical in the homodimer and in the heterodimer
species. Similarly we see the tunneling value for the heterodimer H ^ C l—H ^ C l is
almost exactly the average o f the homodimer values*®.
The story is quite different for the mixed dimer species involving a deuterium
The first microwave report on HF/DF dimers came from the Klemperer group in the
early seventies*. The observed spectrum could be completely assigned with a semi­
rigid rotor Hamiltonian to the deuterium bonded species, HF—DF.
Spectroscopic
134
Reproduced with permission o f the copyright owner. Further reproduction prohibited without permission.
evidence for neither the tunneling splittings nor the other configuration, DF—HF was
found. Later microwave studies^* found the higher energy conformer; DF—HF, also
localized in a semi-rigid rotor "L" structure with large amplitude bending. However,
absent was evidence for any tunneling motions. A recent calculation1* by Suhm and
Quack found a zero point energy difference between the two isomers to be 65 c m 'l
This may explain why tunneling does not occur in the HF/DF dimers. When the two
zero point energies are equal, the perturbation o f the levels resulting from the
interaction o f the two sides o f the well allowed by the finite barrier between them is
sizable. But, when the zero point energies are quite different, as in the HF/DF dimers,
the effect o f the interaction through the finite barrier is quite small. These two cases
are shown in Figure 4.
A recent infrared study characterized both the HC1—DC1 and DC1—HC1
configurations of the HC1/DC1 dimer1'*. They were able to determine relative binding
energies and isomer inversion rates. The HC1—DC1 conformer was determined to be 16
±4 cm-1 more stable than the DC1--HCL The high degree o f wave function localization
in these complexes is due to both a significant difference between the zero point
energies of the two configurations and a large barrier to inversion1*. It is interesting to
note, however, that the energy differences and estimated barriers to inversion in
HC1/DC1 is four times smaller than in HF/DF dimer.
The HC1—DC1 complex has also been studied in the microwave region o f the
spectrum1^. Howard et. a /.1^ observed pure rotational transitions in the ground
vibrational state o f the hydrogen bound dimer. However, due to low Boltzmann
factors in the cold beam environment, the higher energy, hydrogen bound dimer, DC1HC1, was not detected.
135
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
In a continuation o f the asymmetric hydrogen halide dimer research, we
present here the first evidence o f a dimer formed between HBr and DBr. To date, we
have only identified the deuterium bonded HBr—DBr van der Waals complex. No
concrete structural information has yet been analyzed, but extensive experimental and
theoretical work is underway in our laboratory to further understand the HBr—DBr
complex and perhaps observe the hydrogen bound DBr—HBr complex.
Experimental
As mentioned in Chapter 4, many transitions obtained during the OCS—HBr
study are unassigned. Some of these lines were later found to belong to the OCS—DBr
complex, even though the transitions were originally observed in a mixture nominally
composed o f only OCS and HBr. To check the validity o f this statement the Ar—DBr
complex was searched for and found in the tank o f argon and a naturally occurring
isotopic mixture o f HBr. A DBr impurity existed in our HBr tank, perhaps beyond
natural isotopic abundance (0.015%) o f deuterium.
Several unassigned transitions remained after the OCS—H/DBr complexes
were solved. Through the process o f elimination, the molecular carrier of these
transitions was determined to be HBr—DBr. The proof was relatively simple. The
nearly symmetric prolate top HBr—DBr complex would exhibit a microwave spectra
with transitions separated by 2 JB , where B is the effective rotational constant.
Therefore, once a transition was observed and tentatively assigned a J value, others
could be quickly calculated and scanned for. Also aiding this process is the abundant
information on the other hydrogen halide homo and heterodimers. A first guess at the
structure was done by following what was observed for the HF—DF and HC1—DC1
136
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
c o m p le x e s * ’
Additionally, the pattern o f the transitions was such that at least two
nuclear spins were contained in the complex. The bromine isotopomers o f HBr—DBr
were found by considering the change is mass and recalculating B.
The mixture which gave the best signal to noise ratio o f the rotational
transitions was « 1% DBr in an argon carrier gas. The DBr tank is infested with HBr,
o f course, providing the perfect environment for the complex to form. The nozzle was
oriented perpendicularly to the microwaves to gather most of the data. In order to
complete this project, data must be gathered with the nozzle coaxial to the microwaves
due to the increased sensitivity this mode o f operation achieves. This added precision
will allow for the analysis of the three quadrupole coupling constants, spin-spin
interactions, and spin-rotation interactions.
Results and Discussion
Although it is very early in the analysis, we are confident the molecular carrier
of the transitions observed is HBr—DBr. Transitions were collected for the different
isotopomers; H 7 9 B r-H 7 9 Br, H 8 1 B r-H 8 1 Br, and H 7 9 B r-H 81Br & H 8 1 B r-H 7 9 Br,
and the frequencies are listed in Tables 1, 2 and 3 respectively. The two mixed
isotopomers H 7 9 Br—H8*Br and H 8 *Br—H79Br are indistinguishable at this point in
the analysis and are presented as one table. It is quite obvious by the number o f
hyperfine components that more than one nuclear spin is involved in the complex. In
fact, we see barely resolved additional splittings that may be due to the spin-rotation
and spin-spin interactions. These factors are expected to play a significant role in the
analysis once quality high resolution data has been gathered.
137
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
At this point in the project we are able to estimate three possible effective
rotational constants for the isotopically substituted species. They are (B =) 741.886,
723.761 and 732.767 MHz for the H 7 9 B r-H 7 9 Br, H 8 1 B r-H 8 1 Br, and H 7 9 B rH8^Br & H 8 ^Br--H7 9 Br, respectively. Quantitative fitting has not been performed,
therefore only one Br—Br bond distance can be reported; 4.136 A , for all species. This
is somewhat shorter than the ab initio result for (HBr) 2 which suggests a bond length
o f 4.48 A 20.
This theoretical paper also overestimated the bond distance o f the
(HC1) 2 by 0.30 A.
The dynamical structural information for the HBr—DBr complex will be
determined from the two quadrupole coupling constants o f HBr and DBr. To start
such an analysis, one may use the values o f Xbb fr°m the HBr moiety^l and Xaa from
the DBr moiety^^. The a axis orientation o f the hydrogen bromides must be expressed
in the principal axes of the complex. For monatomic HBr/DBr, the a' axis lies along
the H/D--Br bond and the b' and c' axis are perpendicular to the bond. This means the
a ' axis of the donor (DBr) and the b’ axis o f the acceptor (HBr) he along the complex's
a axis.
Preliminary fits have indicated the probable H-Br--D angle to be ~ 90°
whereas the Br—D-Br angle is close to 0°. This agrees well with the structure o f HC1—
DQ14 and the ab initio results^ 9 for the (HBr)2.
In conclusion, we have observed microwave transitions due to the hydrogen
halide dimer HBr—DBr. The structural and dynamical analysis is underway. The
hydrogen bonded dimer (DBr—HBr) has not been observed, however extensive
searches for this configuration have not yet been carried out. We expect this study to
further the already extensive work on the hydrogen halide dimers, including both the
symmetric and the mixed dimers. The nuclear spin o f the bromine and its equally
138
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
abundant isotopes make the dimers o f HBr/DBr particularly interesting. This research
would be greatly enhanced by either an infrared and/or high quality theoretical
investigations.
References
1. T .R Dyke, B. J. Howard, W. Klemperer, J. Chem Phys. 56, 2442 (1972)
2. B.J. Howard, T .R Dyke, W. Klemperer, J. Chem. Phys. 81, 5417 (1984)
3. H.S. Gutowsky, Carl Chuang, John D. Keen, T.D. Klots, Tryggvi Emilsson, J.
hem Phys. 83 2070 (1985)
4. A.S. Pine, W. J. Lafferty, B.J. Howard, J. Chem Phys. 81 2939 (1984)
5. D. W. Micheal, C.E. Dystra, and J.M. Lisy, J. Chem Phys. 81, 5998 (1984)
6 . P. Jensen, P .R Bunker, A.Karpfen, M. Kofranek, H. Lischka, J. Chem Phys. 93,
266 (1990)
7. A. S. Pine, B. J. Howard, J. Chem Phys. 84,590 (1986)
8 . Z. Latajka, S. Scheiner, Chem Phys. 122,413 (1988)
9. N. Ohashi, A.S. Pine, J. Chem Phys. 81,73 (1984)
10. M.D. Schuder, C.M. Lovejoy, R Lascola, D.J. Nesbitt, J. Chem Phys. 99,4346
1993)
11. M. D. Schuder, C.M. Lovejoy, D.D. Nelson,Jr., D.J.Nesbitt, J. Chem Phys. 91,
418(1989)
12. G.A. Blake, KX. Busarow, R C . Cohen, K B . Laughlin, Y.T. Lee, R J. Saykally,
. Chem Phys. 89,6577 (1988)
13. M.D. Schuder, D.D. Nelson,Jr., D.J. Nesbitt, J. Chem Phys. 99, 5045 (1993)
14. M. D. Schuder, D.J. Nesbitt, J. Chem Phys. 100, 7250 (1994)
15. Brian Howard, private communication.
16. A.C. Legon, L.C. Willoughby, A.D. Buckingham, Chem Phys. Lett. 102, 126
1983).
17. R S . Ruofif T. Emilsson, C. Chaung, T.D. Klots, H.S. Gutowsky, Chem Phys.
ett. 138, 553 (1987).
18. K W. Junks, R E . Miller, J. Chem Phys. 88,6157 (1988)
19. David Nesbitt, private communication.
20. Y. Hannachi, B. Silvi, J. Mol. Struct. (Theochem), 200, 483 (1989)
21. O.B. Dabbousi, W.I. Meerts, F.H. DeLeeuw, A. Dymanus, Chem Phys. 2, 473
(1973)
22. F.A. van Duk, A. Dymanus, Chem Phys. 6 ,474 (1974)
139
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
X
140
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
H -X
The g and u energy levels for the one-dim ensional potential
energy surface o f sym m etric hydrogen halide dimers. Only
the ground sym m etric (g) wave function is shown.
Figure 2
141
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
I
I
I
I
J
Correlation diagram for the K=0
energy levels o f the symmetric
dimers as a function o f the
tunneling
frequency, u. The
allowed transitions are shown as
solid arrows and observed at
frequencies 2B±u. Note these
transitions involve a change in
both the rotational and tunneling
state. The dotted and the dashed
lines
represent
both
the
forbidden pure rotational and
the pure inversion transitions
respectively.
Figure 3
142
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Sketches of two one-dimensional potential energy surfaces.
Diagram A displays the resultant energy levels arising from equal minima.
Diagram B shows alterations to the levels arising from unequal minima.
Solid lines show starting energy levels before the interaction.
Dotted lines are the final energy levels after the interaction.
Note that energy level spacings are exaggerated for clarity.
Figure 4
143
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Table 1. Transition Frequencies o fH ^ B r—D ^ B r.
J . . - f. .
Kft-i
K&.
J. .
- f. .
Kfc.
606 - 505
u/M Hz
8897.710
5930.410
606 - 505
8898.625
404 - 303
5931.335
606 " 505
8898.960
404 " 303
5931.630
606 - 505
8899.300
404 ' 303
5932.010
606 - 505
8899.475
404 - 303
5932.515
606 ■ 505
8899.785
404 - 303
5933.345
606 - 505
8900.095
404 - 303
5937.820
606 - 505
8901.285
404 ' 303
5938.230
606 - 505
8901.880
404 ‘ 303
5939.685
606 - 505
8902.020
404 - 303
5945.135
606 - 505
8902.425
606 - 505
8903.135
404 - 303
u/M Hz
5927.580
404 ' 303
505 - 404
7413.230
606 - 505
8903.280
505 - 404
7414.765
606 - 505
505 - 404
7415.445
8904.650
•
505 - 404
505 - 404
7415.785
505 - 404
7416.325
505 • 404
7416.805
505 - 404
7417.035
505 - 404
7419.645
505 - 404
7419.925
505 - 404
7420.820
505 - 404
7421.245
505 * 404
7421.815
505 * 404
7423.745
7415.815
144
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Table 2. Transition Frequencies o f H ^ B r - D ^ B r .
J . . -
J..
J .
-
J. .
606 “ 505
u/M Hz
8680.985
5786.195
606 - 505
8681.745
404 - 303
5786.960
606 - 505
8682.035
404 “ 303
5787.210
606 " 505
8682.315
404 - 303
5787.535
606 ' 505
8682.455
404 - 303
5787.950
606 - 505
8682.980
404 - 303
5788.650
606 - 505
8683.970
404 " 303
5792.445
606 “ 505
8684.080
404 - 303
5793.970
606 - 505
8684.485
404 - 303
5795.890
606 - 505
8684.610
404 - 303
5798.510
606 ‘ 505
8684.840
606 - 505
8684.940
404 ‘ 303
u/M Hz
5783.835
404 - 303
505 - 404
7232.905
606 ‘ 505
8685.545
505 - 404
7234.190
606 - 505
8685.655
505 - 404
7234.765
606 - 505
8686.790
505 - 404
7235.060
505 - 404
7235.490
505 - 404
7235.890
505 - 404
7235.920
505 - 404
7236.075
505 - 404
7237.160
505 - 404
7238.295
505 " 404
7238.530
505 - 404
7239.520
505 “ 404
7239.620.
505 - 404
7240.300
505 - 404
7241.705
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Table 3. Transition Frequencies o
f H ^ B r —D
J’ . . - J’ . .
J K.,K+1
K.,K+|
Frequencies/MHz
4f)4 - 3m
404 - 3m
4n4 - 3m
4n4 - 3m
404 - 3m
4n4 - 3m
4n4 - 3m
4 o4 - 3m
4n4 - 3m
4n4 - 3m
4 o4 - 3m
4 o4 - 3m
4 o4 - 3m
4 o4 - 3m
4o4 - 3m
4<m - 3m
404 - 3m
4 o4 - 3m
5855.285
5856.060
5858.205
5858.405
5858.480
5859.240
5859.475
5859.590
5860.095
5860.440
5860.725
5861.250
5864.215
5865.175
5867.520
5870.095
5871.465
5872.180
50S - 404
50S - 404
50S - 4ft4
50S - 404
5ns - 4n4
50S - 4fl4
50S - 404
50S - 4fl4
So4) - 404
50S - 4(14
5ns - 4 o4
50S - 404
5ns - 4(m
5OS * 4(14
7322.780
7323.295
7324.475
7325.040
7325.125
7325.265
7325.345
7325.785
7325.840
7326.005
7326.200
7326.470
7327.205
7328.360
7328.465
7328.640
7329.065
7329.780
7329.990
7330.485
7330.855
7331.055
7332.540
7332.920
50S - 4(14
50S - 404
50S - 4(14
50S - 4(14
50S - 4 04
5ns - 4(14
50S - 404
5(1-5 - 4(14
50S - 4(14
5ns * 4n4
^ B r o r H
^K.,K+, "
^ B r - D
j k :,k +1
^ B
r
Frequencies/M Hz
6nfi - 5os
6nfi - 5ns
6nfi - 5ns
6nfi-5os
6nfi - 5ns
6ns - 5ns
6nfi - 5ns
6nfi - 5ns
6nfi - 5ns
6nfi - 5ns
60 fi-5OS
6nfi - 5ns
6nfi - 5ns
6nfi - 5ns
606 - 5ns
6nfi - 5ns
6nfi - 5ns
6n« - 5ns
606 - 5ns
6nfi - 5ns
fyifi - 5ns
6ofi - 5ns
6n« - 5ns
6nfi - 5ns
6ofi - 5ns
6nfi - 5ns
6nfi - 5ns
606 - 5ns
8789.480
8789.530
8790.135
8790.235
8790.415
8790.565
8790.745
8790.925
8791.205
8791.285
8791.465
8791.520
8791.570
8792.140
8792.290
8792.620
8792.925
8793.170
8793.255
8793.255
8793.680
8793.915
8794.075
8794.145
8794.670
8794.760
8795.560
8795.870
707 - 6nfi
7n7 - 6nfi
707 * 60fi
7n7 - 6nfi
10254.785
10255.150
10255.435
10255.595
10255.785
10255.930
10256.100
10256.325
10256.515
10257.520
10257.650
10257.845
707 * 606
707 * 6n«
7n7 - 6n«
707 - 60fi
707 * 6(lfi
707 - 6nfi
707 - 6Ofi
707 - 6nfi
146
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
C hapter 6 ; Unassigned Transition Frequencies with Unknown
Molecular Carriers.
147
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Introduction
This chapter of the thesis is meant to be a permanent record o f transition
frequencies that are not yet unambiguously assigned to a molecular carrier.
Hypothesis exist about the origin o f these unassigned transitions, but time has not
permitted a thorough experimental evaluation.
As an addendum to the laboratory
notebook, this chapter will lead future researchers quickly through the history and
findings o f these projects, and suggest additional tests that need to be performed.
Unlike the molecular beam electric resonance spectrometer also used in the
Novick group, the newly constructed Fourier transform microwave spectrometer is
not equipped with a mass detection system. Despite the numerous advantages o f the
new spectrometer, the lack of a mass detector leaves the FTM spectrometer unable to
place a molecular "name tag" on the transitions. For example, in a simple sample tank
of argon and one percent carbonyl sulfide, transitions can be observed originating from
the monomer species; DCS*, and from the following five van der Waals complexes;
A r-O C S2, Ar2 --OCS3, 0CS--H 2 0 4, A r-H 2 0 5, and (H2 0 )26.
Water is typically an unwanted impurity that contaminates all FTM
spectrometers. To control the water contamination, a desiccating filter can be placed
on the argon tank; a known moisture source, or directly onto the instrument line. As
this method has proved to be inadequate, it is more insightful to monitor the amount of
water in the system on a given day by checking the strength o f transitions due to
previously solved water complexes. It has been noted that, when possible, checking
transitions due to complexes between water and hydrogen halides, i.e., H 2 0 —HBr2,
H 2 0--H Q 8, is more useful than checking the Ar—H 2 0 ^ or (H 2 0 )2^ lines, due to
transition strength considerations.
148
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Further complications in the assignment o f observed transitions to molecular
carriers arise from the ability of "sticky" molecules to remain in the input tubing and
pulsed nozzle body long after their use is discontinued. One such reactive molecule,
hydrogen bromide, can be observed in complexes such as Ar—HBr^ and HBr—H^O^
for days after being intentionally added to the tubing. One way to deal with this
dilemma is to pump out the input line thoroughly (at least overnight) between projects.
Also, by physically removing the line from the spectrometer, cleaning it with a solvent,
and fully drying it, one can be confident that the adhesive molecules are removed.
Presented here are two different sets o f frequencies originating from two
different sample tanks. A summary o f when and how the transitions were obtained is
given, and the suspected molecular carrier is discussed. One project has been
thoroughly researched and the detailed findings are presented in the first part o f this
chapter. The second project is in the infancy o f its research, so a shorter summary is
given. Techniques are suggested to further test the presented hypothesis. As research
continues on these projects, this information could possibly aid in the structural
determination o f two new van der Waals complexes.
(E bO W H C lb ?
While searching for rotational transitions from the PF3 --HCI van der Waals
complex, lines were found around 7800 MHz. To be certain the transitions were
caused by a PF 3 containing species, the frequencies were scanned again with a sample
tank composed (nominally) o f only argon and HC1.
The transitions were still
observable even though PF 3 was not in the gas mixture. Although it was feasible to
decide at this point that the transitions were not due to a PF3 containing species, such
149
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
as PF3 —HCL, it was important to remember that PF3 may be a sticky molecule and
therefore still be present within the spectrometer.
After pumping on the input line overnight, the previously found transitions
were checked and confirmed to be the same strength with the argon and HC1 sample
tank.
As an assurance that the PF 3 had been completely removed from the
spectrometer, rotational transitions due to both the monomer species, PF 3 ^, and the
van der Waals complex Ar—PF3 ^
were inspected. Whereas the Ar-PF3 transitions
were not observable, the monomer lines due to PF3 , which are usually extremely
strong, were just barely visible. No significant change in the signal to noise ratio o f the
unassigned lines occurred when the sample tank was switched from argon, hydrogen
chloride, and phosphorous triflouride to the tank missing the PF 3 , therefore with
confidence we make the claim that the molecular carrier o f these transitions does not
involve PF 3 .
To prove the molecular carrier o f the unassigned transitions involved HC1 in
some fashion, a sample tank of only Ar and PF3 was used to scan the lines. The
unassigned transitions were not observed from the Ar/PF3 sample tank. To ensure no
HC1 remained in the tubing, frequencies for the Ar--HClH van der Waals complex
were searched for but not found. Therefore the molecular carrier o f the unassigned
transitions was limited to one which contains at least one HC1 molecule but no PF 3
molecules.
The observed transitions around 7800 MHz must be due to a molecule from
the sample tank composed o f Ar and HC1, (and any impurities). Many different van
der Waals complexes have been solved from a such a tank. O f course the original van
der Waals complex, Ar—HCl, was solved in the early seventies by Novick et.al.H.
150
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
The transitions origination from Ar~HCl were used to tune the instrument for this
project (see Section I for a definition o f "tune"). The structures o f higher order argon
complexes such as At 2 ~HC1 and AJ3 —HC1 have been determined by the Gutowsky
group 12-13
Lines from AT2 --HC1 were observed but required quite different
instrument settings for optimization.
The unassigned frequencies were compared
against all the Axn—HC1 frequencies, but no matches were found.
A sample tank o f neon and hydrogen chloride was used to check the
unassigned lines. A significant decrease in the signal to noise ratio o f these lines
occurred when neon was used as the carrier gas for the hydrogen chloride. This could
be explained in three ways with three different conclusions being drawn. First, neon in
general does not form van der Waals complexes nearly as readily as argon in a pulsed
supersonic nozzle.
This may suggest that the molecular carrier o f the unassigned
transition is due to a higher order complex, Le., a trimer, which require very
specialized conditions in order to form. Secondly, the dramatic decrease in signal to
noise ratio when changing carrier gas from argon to neon could indicate that argon is
involved in the complex giving rise to these transitions. This seemed somewhat
unlikely since several argon complexes with HC1 have been solved and their
frequencies checked against those found. However, argon could not be discarded as a
component in the molecular carrier of the unassigned transitions. Finally, the switch in
carrier gas from argon to neon would effect the amount o f water impurity that enters
the instrument. It has been noted that the argon tank is quite "wet" with respect to the
neon tank. Therefore, it is possible that the complex causing the unassigned transitions
contains a water molecule.
151
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Returning to the Ar/HCl sample tank, transitions from the EhjO-HCl^ van der
Waals complex were checked to see if, in fact, a water impurity was present in the
sample. The transitions were found to be quite strong and clearly visible in only one
cycle o f the gas pulse and microwaves.
The conditions and timings o f the FTM
spectrometer were changed and set to optimize these H 2 O--HCI transitions.
The
unassigned transitions were then examined with these new conditions and found to be
stronger than when originally observed. An increase in the signal to noise ratio o f the
unassigned transitions resulting from the optimization o f the H 2 O--HCI transitions
suggests that perhaps the molecular carrier of the lines contains a water molecule.
To verify this hypothesis, an additional experiment was performed. Drops of
water were intentionally added to the input tubing, insuring that a substantial and
constant amount o f H 2 O was present in the mixture.. Scanning for the unassigned
transitions revealed a significant decrease in the signal to noise ratio, with the signal
eventually disappearing entirely. This seemingly paradoxical result was eclipsed by the
disappearance o f the H 2 O--HCI transitions
The unexpected disappearance o f the
lines can be explained, however, by the change in the sample ratios o f HC1 and H 2 O
caused by the addition o f EfyO drops. This hypersensitivity to mixture ratios for van
der Waals complex formation has been noted in previous experiments. Therefore, no
new information was gained from this test.
After the input tubing had been dried such that the original amount o f water
impurity was restored, a thorough scan o f the area around the transitions was possible.
Many more transitions were found. Over thirty five frequencies were measured within
a 20 MHz region and are listed with their respective strengths in Table 1. The number
o f lines found within such a small region suggests that we are observing hyperfine
152
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
components o f a single transition. It is also likely that more than one nuclear spin of
greater than one-half is giving rise to this hyperfine structure. As noted earlier, HC1 is
likely to be a constituent of the molecular carrier causing these lines. The spin of a
single 35C1 (1=3/2), Le., one hydrogen chloride, would not produce enough hyperfine
components to explain the number observed. However, if two chlorines, i.e., two
hydrogen chlorides, were in the complex one might expect to see a comparable
hyperfine structure to that observed. Even still, thirty eight hyperfine components are
not fully explained by the presence o f merely two chlorines. Further splitting may
result from spin-rotation interaction, spin-spin interaction, or internal rotation.
Another possible explanation for the abundance o f components is the overlap o f two
separate transitions or two different isotopomers.
If we compile what we have learned about the molecular carrier o f these thirtyplus, closely spaced, unassigned lines, we find that the possible ingredients o f the
complex are argon, hydrogen chloride, and water. Furthermore, hyperfine structures
suggest that at least two HCl's are present. Both the dimer and the trimer o f HC1 have
been studied in the infrared and neither have ground state transitions in the microwave
region of the spectrum ^-15
Through the process of elimination, the molecular
carrier may be either Ar—(HC1)2 , H 2 O—(HC1)2 , or some unknown impurity. Neither
trimer mentioned has been studied spectroscopically.
Rotational spectroscopy of trimers is a relatively new area, but one general rule
has been noticed. The general structure o f the dimers within the trimers typically
remains unchanged. Luckily the dimer structures o f Ar—HCl, (HC1)2 , and H 2 O—HC1
have been solved. Therefore, a hypothesis as to the trimer structure; either Ar—(HC1)2
or H2O—(HC1)2, can easily be made.
153
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
The hydrogen chloride dimer has been thoroughly studied and its "L" shaped,
hydrogen bound structure is well understood. As mentioned in chapter 5 o f this thesis,
an internal rotation occurs within this complex as the hydrogen involved in the van der
Waals bond rapidly exchanges with the hydrogen not involved in bonding.
This
tunneling motion dominates the rotational spectrum o f (HC1)2 - However, if the non­
bonding hydrogen were to interact with another species (Ar or H 2 O) the change in
potential would quench the inversion motion and the resultant trimer would exhibit
typical asymmetric top microwave spectra.
O f the two proposed trimers, the one which satisfies the "dimers within
trimers" rule and quenches the (HC1)2 inversion by interacting with the non-bonding
hydrogen is H2Q—(HC1)2- The proposed structure is shown in Figure 1 . Using the
dimer geometry as a guide, simple calculations were performed on the trimer structure
to obtain rotational constants. The determined rotational constants for the H 2 O-(HC1 ) 2 van der Waals complex are as follows:
A = 4461 MHz
B = 1428 MHz
C = 1116MHz
These constants predict a 6 -type transition, 2i2-l()l>
t0
centered at 7808 MHz.
The center frequency o f the observed hyperfine components is 7800.020 MHz. It is
possible that unassigned components belong to the
2
i 2 -l()l transition in H 2 O—
(HC1)2- T o check the validity o f this statement, more transitions must be observed
which agree with these calculated rotational constants. The transition to be scanned
for next is the 1h-Oqo at 5576 MHz. This line is also a 6 -type and depends only on
the .4 and C rotational constants, just as the observed 2 j2 -lo i transition does.
154
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
In conclusion, it is likely that transitions due to the H 2 O--(H C1)2 van der
Waals complex have been observed. Reasonable doubt remains however, as only one
transition has been measured. The hyperfine pattern observed suggests the presence of
two hydrogen chlorides. As these lines respond to experimental conditions in the same
m anne r as the H 2 O --H C I transitions do, it is likely that water is also involved in the
molecular carrier. Further scanning needs to be performed, hi particular, the
transition at 5576 MHz should be searched for first.
11
i-Oqq
The quadrupole assignment
should be guided by the Xaa value o f the horizontal hydrogen chloride and the Xbb
value o f the vertical HC1. The water molecule is known to internally rotate, so one
should be aware o f this if extra splitting is observed.
(N O l—flEfoOl?
The study o f van der Waals complexes involving an open-shell partner such as
NO has become one o f the most exciting and challenging areas o f microwave
spectroscopy. Only a handful o f such complexes have been investigated and even
fewer are spectroscopically understood.
Nitric oxide is one o f the open-shell
molecules involved most frequently in the solved complexes, i.e., Ar—N O ^ , N O N O ^ , NO—(NO)2 * 8 and NO—H F ^ .
As part o f a collaboration with Professor
Karen Peterson, a co-principle investigator o f the FTM spectrometer, searches began
for transitions o f the NO--H 2 O complex.
The combination o f the available
spectroscopic information on NO complexes and Professor Peterson's plethora of
knowledge about H 2 O complexes enabled this project to begin.
Initially, a hypothesis was made for the structure o f the NO—H 2 O van der
Waals complex. By comparison with the solved N 2 O—^ 0 ^ 0 complex, the oxygen of
155
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
the water molecule was assumed to weakly bound to the nitrogen o f the nitric oxide
open-shell molecule. Furthermore, the distance used for the N—O van der Waals bond
was chosen to be the distance determined in the N 2 O--H 2 O com plex^. Rotational
constants were calculated for this heavy-atom-linear structure, and searching began for
the 2q2_101 transitions predicted at 12400 MHz.
For this investigation, water was intentionally added to the input tubing in large
quantities. This had never been done before in our laboratory, but Professor Peterson
assured us that the amount o f naturally occurring water impurity was not enough to
form the NO--H 2 O complex. Therefore, a 5% mixture o f NO in Ar was bubbled
through approximately 20 ml o f H 2 O at a backing pressure o f 2 atm. This visibly
placed water throughout the length o f the input tubing. The sample mixture allowed
for transitions from the following complexes to be verified: Ar—N O ^ , (NO)2 ^ , Ar—
(H 2 0 )2 ^. However, a waiting period was required to observe any o f the NO
complexes due to the reactivity o f this open-shell molecule. Frequently pumping out
the input tubing was necessary to maintain the strengths o f these known lines. The
tune o f the instrument was set to optimize the Ar—N O ^ transitions and minimize the
(NO)2 ^
transitions. With the nozzle perpendicular to the Fabiy-Perot cavity,
scanning began at 12000 MHz for the predicted 12400 MHz transition o f the NO—
H2 O complex.
Nozzle problems abounded in this experiment. All too often, scanning had to
be stopped so that the poppet could be replaced and the pulsed nozzle body could be
cleaned. It was after such an exercise that scanning resumed and three surprisingly
strong transitions were observed around 13068 MHz. Their frequencies were cross
referenced with the frequencies o f the solved complexes in the mixture, but no
156
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
matches were found. To test the unassigned lines' dependency on the presence o f NO,
the gas tank composed of argon and nitric oxide was replaced with a tank o f pure
argon.
The transitions were checked for, but not found.
The test for water
involvement in the complex was inconclusive as it is almost impossible in a short
amount o f time to make the drenched system water-free. Therefore, it was known that
the unassigned transitions at 13068 MHz were due to a complex composed o f NO and
perhaps H 2 O.
Returning to the original Ar/NO mixture and the initial conditions, the
unassigned lines were examined again. Their strengths had severely decreased and
continued to do so as time passed. Pumping out the input line made the transitions
even weaker and finally they completely disappeared. This lead to the conclusion that
the transitions were due to an impurity of some kind.
To limit the possibilities, the cleaning solvent for the pulsed nozzle body was
discussed.
As it turns out, methanol was used, which has strong ground state
rotational transitions in the microwave region of the spectrum.
However, the
transitions at 13068 MHz did not match frequencies of the monomer, CH3 OH2 I, or
of the solved complexes, i.e., with argon, Ar--CH 3 0 H 2 2 5 or with itself (Q H ^O H ^S .
Furthermore, the transitions seemed to respond to the presence on NO. Following
days o f frustration, the cause o f the unassigned transitions was determined to be a
reaction product of NO and methanol; cis-methyl nitrite, CH3 QNO, whose microwave
spectrum was assigned in 1980^4.
As the time available for this experiment was quickly coming to a close, it was
decided that an ab initio calculation o f the structure was necessary. One o f the best
van der Waals complex theoreticians is Dr. F.M. Tau working for Professor William
157
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Klemperer at Harvard University.
Dr. Tau agreed to perform a quick ab initio
geometry optimization o f the NO--H 2 O
c o m p le x ^ .
The resultant structure, shown in
Figure 2, is hydrogen bound with an eight degree offset from NO—HO linearity. New
rotational constants were then calculated for this structure and the search for the 2 q2 *
lo i transition o f the NO—H 2 O complex started again but this time at 11000 MHz.
Transitions were quickly found near the predicted frequency, however their
molecular carrier has yet to be determined.
The lines were relatively strong, and
several were found within a short frequency span.
A complete listing o f the
unassigned transition frequencies found while working on this project is given in Table
2. The first test was to again cross reference the unassigned frequencies with the
frequencies o f the solved complexes in the sample tank. Some matches were found, as
this region o f the microwave spectrum is rich with assigned lines from A r-N O ^ and
(NO)2 ^ . However, most of the lines remain unassigned.
To determine if the argon carrier gas was involved in the complex giving rise
to the unassigned transitions, a 5% NO sample was made with neon as the carrier.
Just as in the previously discussed project, this test was inconclusive. The strengths of
the lines appeared to be affected, but because whether argon is a member of the
complex, or argon is a better complexer than neon, is unknown. If argon is an agent in
the cause o f the transitions, then it is likely that the complex is Ar—(NO ) 2 (an
unstudied complex), due to the fact that the NO dimer is quite stable and its
microwave transitions are strong in the sample tank.
Next, the check for changes in the unassigned line strengths resulting from the
removal of intentional water yielded little information. A thorough drying o f the line is
necessary before a conclusive result can be obtained from this test. Conversely, the
158
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
test for NO was clear; the transitions disappear when NO is not involved in the sample
tank. Furthermore, the percentage o f NO in the gas sample affects the strength o f the
transitions. The 1% mixture (1% NO in 99% Ar) normally used for the van der Waals
studies produces a decrease in the signal to noise ratio o f the unassigned transitions
when compared to the 5% mixture.
In conclusion, the molecular carrier o f the unassigned transitions is suspected
to be NO—H 2 O. More experiments are necessary to confirm the components o f the
complex and to determine its structure. The first trial for the presented hypothesis is
to search for the 3 q3-2q2 transitions at 16600 MHz. That test will require the antenna
to be changed to allow for higher frequencies to be accessed.
Furthermore, it is
unlikely that the transition will be found near the predicted frequency. The simple
model used in these preliminary predictions ignores the electronic interactions found in
open-shell molecules that can in fact, greatly affect the rotational spectrum. Also, a
more qualitative examination of the water dependence on the strength o f the
unassigned transitions needs to be performed. The results presented here, no matter
how seemingly inconclusive, will lead any future research on complexes involving
open-shell molecules.
References
1. F.J. Lovas, R.D. Suenram, J. Chem. Phys. 87,2014 (1987)
2. J.A. Shea, W.G. Read, E.J. Campbell, J. Chem. Phys. 79, 2562 (1983)
3. Y. Xu, M.C.L. Gerry, J.P. Connelly, B.J. Howard, J. Chem. Phys. 98, 2735 (1993)
4. T. Ogata, Y. Ohshima, Y. Endo, Ohio Symposium RG'
8
(1993)
5. G.T. Fraser, F.J. Lovas, R.D. Suenram, K. Matsumura, J. Mol. Spec. 144, 97
1990)
159
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
6
. L.H. Coudert, F.J. Lovas, R.D. Suenram, J.T. Hougen, J. Chem. Phys. 87, 6290
1987)
7. A.C. Legon, A.P. Suckley, Chem. Phys. Lett. 150, 153 (1988)
8.
A.C. Legon, L.C. Willoughby, Chem. Phys. Lett. 95, 449 (1983)
9. E. Hirota, Y. Morino, J. Mol. Spec. 33,460 (1970)
10. A. Taleb-Bendiab, M.S. Labarge, L.L. Lohr, R.C. Taylor, K.W. Hilhg, R.L.
uczkowski, R. Bohn, J. Chem Phys. 90, 6949 (1989)
11. S.E. Novick, P. Davies, S.J. Harris, W. Klemperer, J. Chem Phys. 59, 2273
1973)
12. T.D. Klots, C. Chuang, R.S. RuofT, T. Emilsson, H.S. Gutowsky, J. Chem Phys.
6,
5315 (1987)
13. T.D. Klots, R.S. RuofF, C. Chuang, T. Emilsson, H.S. Gutowsky, J. Chem Phys.
7, 4383 (1987)
14. M.D. Schuder, C.M. Lovejoy, D.D. Nelson, D.J. Neshitt, J. Chem Phys. 91, 4418
(1989)
15. J. Han, Z. Wang, A.L. McIntosh, R.R. Lucchese, J.W. Bevan, J. Chem Phys.
100, 7101 (1994)
16. P.D.A. Mills, C.M. Western, B.J. Howard, J. Phys. Chem 90, 3331 (1986)
17. C.M. Western, P.R.R. Langridge-Smith, B.J. Howard, S.E. Novick, Molecular
Physics 44, 145 (1981) and S.G. Kukolich, J.Amer.Chem. Soc. 104, 4715
(1982)
18. A.H. Brittain, A.P. Cox, R.L. Kuczkowski, Trans. Farad. Soc. 65, 1963 (1969)
and S.G. Kukolich, J. Amer. Chem Soc. 104, 6927 (1982)
19. W.M. Fawzy, G.T. Fraser, J.T. Hougen, A.S. Pine, J. Chem Phys. 93, 2992
(1990)
20. D. Yaron, W. Klemperer, K.I. Peterson, Ohio Symposium (1989)
21
. meoh
22. R.D. Suenram, F.J. Lovas, G.T. Fraser, J.Z. Gilles, C.W. Gilles, M. Onda, J. Mol.
Spect. 137, 127 (1989)
23. F.J. Lovas, R.D. Suenram, M.Y. Tretyakov, S.P. Belov W. Stahl, Ohio
Symposium TC05 (1993)
24. P.N. Ghosh, A. Bauder, H.H. Gunthard, Chem Phys. 53, 39 (1980)
25. calculation tau
160
Reproduced with permission o f the copyright owner. Further reproduction prohibited without permission.
Figure 1: Proposed Tentative Structure of
H2 O—(HCI)2 trimer
161
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Figure 2: Proposed Tentative Structure of NO--H2 O
162
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Table 1. Transition Frequencies Found in a Sample Tank Nominally Composed of
Argon and HC1. Possible Molecular Carrier: (H 2 O)—(HC1 ) 2
Frequency/
Strength*/
Frequency/
Strength*/
MHz
HV
MHz
uv
7779.020
2400
7795.050
7780.010
1606
7795.150
15800
*
7780.160
2142
7795.800
4800
7781.630
7798.520
1600
7783.900
1992
*
7799.700
2000
7784.670
3080
7802.120
5200
7786.570
5280
7804.460
6400
7787.180
6600
7805.630
2400
7787.700
6160
7806.940
2800
7787.810
6160
7808.270
16800
7789.290
880
7809.700
246
7789.410
1200
7810.660
8900
7791.520
17000
7811.920
728
7791.910
12600
7812.990**
5270
7792.880**
10800
7814.130
4000
7793.020
10800
7814.480**
1200
7794.040
15250
7819.970
4000
7794.210
1200
7820.550
2800
7794.870
7900
7822.380
187
I
*
These lines are quite weak and difficult to measure, i.e., they may not be "real".
**
These lines are unusually broad.
*
These strengths are peak intentsities o f the Fourier transform when the number
o f scans o f the FID is 500.
163
Reproduced with permission o f the copyright owner. Further reproduction prohibited without permission.
Table 2: Transition Frequencies Possibly Due to the NO—1^0 Complex (in MHz)
9142.770
11060.620
11088.015
10694.115
11061.070
11116.310
10694.750
11061.410
11116.405
10694.875
11061.470
11126.250
10695.390
11062.020
11148.240
10697.925
11062.070
11264.770
10698.451
11062.280
11266.635
10698.210
11068.935
11266.850
11023.515
11069.330
11267.365
11056.775
11069.545
11267.935
11057.170
11069.595
11268.865
11057.390
11069.815
13254.680
11057.435
11070.645
13254.725
11057.645
11071.060
13254.765
11057.775
11071.500
13255.215
11058.885
11072.170
13255.350
11058.460
11072.205
13255.380
11058.885
11072.240
13255.405
11059.310
11072.520
13255.580
11059.990
11073.675
13255.650
11060.035
11074.205
13255.750
11060.340
11074.275
11060.560
11087.880
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Документ
Категория
Без категории
Просмотров
0
Размер файла
5 941 Кб
Теги
sdewsdweddes
1/--страниц
Пожаловаться на содержимое документа