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Determination of discharge products using chirped-pulse Fourier transform microwave spectroscopy

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Graduate School ETD Form 9
(Revised 12/07)
PURDUE UNIVERSITY
GRADUATE SCHOOL
Thesis/Dissertation Acceptance
This is to certify that the thesis/dissertation prepared
By Amanda Jo Shirar
Entitled DETERMINATION OF DISCHARGE PRODUCTS USING CHIRPED-PULSE
FOURIER TRANSFORM MICROWAVE SPECTROSCOPY
For the degree of Doctor of Philosophy
Is approved by the final examining committee:
Brian C. Dian
Chair
Paul Wenthold
Timothy S. Zwier
Adam Wasserman
To the best of my knowledge and as understood by the student in the Research Integrity and
Copyright Disclaimer (Graduate School Form 20), this thesis/dissertation adheres to the provisions of
Purdue University’s “Policy on Integrity in Research” and the use of copyrighted material.
Brian C. Dian
Approved by Major Professor(s): ____________________________________
____________________________________
Approved by: R. E. Wild
11/16/2011
Head of the Graduate Program
Date
Graduate School Form 20
(Revised 10/07)
PURDUE UNIVERSITY
GRADUATE SCHOOL
Research Integrity and Copyright Disclaimer
Title of Thesis/Dissertation:
DETERMINATION OF DISCHARGE PRODUCTS USING CHIRPED-PULSE
FOURIER TRANSFORM MICROWAVE SPECTROSCOPY
Doctor of Philosophy
For the degree of ________________________________________________________________
I certify that in the preparation of this thesis, I have observed the provisions of Purdue University
Executive Memorandum No. C-22, September 6, 1991, Policy on Integrity in Research.*
Further, I certify that this work is free of plagiarism and all materials appearing in this
thesis/dissertation have been properly quoted and attributed.
I certify that all copyrighted material incorporated into this thesis/dissertation is in compliance with
the United States’ copyright law and that I have received written permission from the copyright
owners for my use of their work, which is beyond the scope of the law. I agree to indemnify and save
harmless Purdue University from any and all claims that may be asserted or that may arise from any
copyright violation.
Amanda Jo Shirar
________________________________
Signature of Candidate
11/16/2011
________________________________
Date
*Located at http://www.purdue.edu/policies/pages/teach_res_outreach/c_22.html
DETERMINATION OF DISCHARGE PRODUCTS USING CHIRPED-PULSE
FOURIER TRANSFORM MICROWAVE SPECTROSCOPY
A Dissertation
Submitted to the Faculty
of
Purdue University
by
Amanda Jo Shirar
In Partial Fulfillment of the
Requirements for the Degree
of
Doctor of Philosophy
December 2011
Purdue University
West Lafayette, Indiana
UMI Number: 3506464
All rights reserved
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a note will indicate the deletion.
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ii
ACKNOWLEDGEMENTS
I would like to thank all the people that have helped me get to where I am today. I
thank my adviser, Dr. Brian Dian, who actively recruited me when I first got to Purdue
and gave me the chance to build his lab. You challenged me in ways I did not expect and
drove me to be better than I was. I also want to thank Tim Zwier and his research group.
It was nice to know you were always there to let us “borrow” supplies. Thank you Tim,
for your advice and support when it was needed.
Next I have to thank those who were in my research lab along the way. Firstly,
this would include Kelly and Giana who came in with me. It was great starting out with
you girls and I wouldn’t change that. I’ll never forget that first trip to OSU, and I mean
those manicures we got, not the presentations we gave. I also want to thank Jasper, who
was instrumental in showing us the ropes when we got here. To David, thanks for all the
music education and for helping me with all that theory that I sometimes struggle with.
I have to give a shout-out to the “A-Team,” without whom I would have never
survived graduate school. It all started out in the attic of Wetherill, which had to have
been one of the best semesters I had while I was here. So many wonderful memories
were created with you guys and I loved how the group grew and changed. A list of things
I will never forget: John Santa poster, corn maze, Harry Potter midnight showings,
Thursday dinners, A-Team Thanksgiving, girlie movies with cocktails, and alllll the
weddings. Giana: Edward will always be better than Jacob. Kelly: you still have the
cutest sneeze I’ve ever heard. Ken: thanks for those tickets to the Purdue/OSU game.
Thank you guys so much for a life outside of the lab.
I want to also thank my church family, both here in Lafayette and in Indy. I
definitely felt the support of both congregations. It was nice to always know that you
were wondering how my life was going and I could feel your prayers when I needed
iii
them. To CUMC, I thank the praise band in particular for giving me a musical outlet
during my graduate career. Sometimes I just needed to pound on a keyboard and rock
out.
Without the love and support of my family, I would have never gotten to this
point. I say thank you to my extended family in Lafayette for looking out for me and
making sure to invite me for some real dinners. I thank my closer family, who never
doubted that I would get to the end of my degree. Thanks Mumsie and Daddio for always
believing in me and reminding me that I can always do whatever I set my mind to doing.
I appreciated all the cash you threw my way when you suspected I needed it and for
watching Shadow when I went to conferences. I always appreciated knowing you were
there for me, in whatever way you were needed.
iv
TABLE OF CONTENTS
Page
LIST OF TABLES ............................................................................................................ vii
LIST OF FIGURES .............................................................................................................x
ABSTRACT ..................................................................................................................... xiii
CHAPTER 1: INTRODUCTION ........................................................................................1
Atmospheric Relevance and Environmental Impact................................................1
Previous Experimental Methods ..............................................................................4
Rotational Spectroscopy As A Detector ..................................................................5
Molecules of Interest................................................................................................6
References ..............................................................................................................10
CHAPTER 2: EXPERIMENTAL METHODS .................................................................16
Chirped Pulse Fourier Transform Microwave Spectrometer .................................16
Theory .................................................................................................................16
Phase Stability and Timing .................................................................................17
Signal Generation................................................................................................18
Chamber ..............................................................................................................24
Molecular Interaction ..........................................................................................26
Coherence Detection ...........................................................................................27
Electric Discharge ..................................................................................................27
Data Processing......................................................................................................29
Ab Initio Calculations ............................................................................................31
References ..............................................................................................................33
v
Page
CHAPTER 3: GROUND STATE SPECTRUM OF
METHYLVINYLKETONE ..................................................................................34
Introduction ............................................................................................................34
Experimental ..........................................................................................................36
Microwave Spectrum .............................................................................................37
References ..............................................................................................................52
CHAPTER 4: INITIAL DISCHARGE EXPERIMENTS .................................................54
Introduction ............................................................................................................54
Experimental Setup and Assignments ...................................................................55
2,3-Dihydrofuran ...................................................................................................57
Previous Experiments .........................................................................................57
Discharge Spectrum ............................................................................................58
2,5-Dihydrofuran ...................................................................................................66
Previous Experiments .........................................................................................66
Discharge Spectrum ............................................................................................67
1,3-Butadiene .........................................................................................................71
Previous Experiments .........................................................................................71
Discharge Spectra ...............................................................................................73
Additional Research ...............................................................................................80
References ..............................................................................................................82
CHAPTER 5: DISCHARGE REACTIONS OF ISOPRENE............................................86
Introduction ............................................................................................................86
Experimental ..........................................................................................................90
Results ....................................................................................................................92
Discussion ..............................................................................................................98
Hydrocarbon Species ..........................................................................................98
C4H6O Species ..................................................................................................101
Other Oxygenated Products ..............................................................................104
References ............................................................................................................107
CHAPTER 6: PRODUCTS IN A BUTANOL DISCHARGE ........................................111
Introduction ..........................................................................................................111
Experimental Setup ..............................................................................................112
1-Butanol..............................................................................................................114
Previous Experiments .......................................................................................114
Discharge Spectrum ..........................................................................................117
2-Butanol..............................................................................................................123
vi
Page
Previous Experiments .......................................................................................123
Discharge Spectrum ..........................................................................................124
Comparison of Butanol Isomers ..........................................................................130
References ............................................................................................................133
APPENDICES
Appendix A ..........................................................................................................136
Appendix B ..........................................................................................................152
Appendix C ..........................................................................................................155
Appendix D ..........................................................................................................157
Appendix E ..........................................................................................................159
Appendix F...........................................................................................................163
Appendix G ..........................................................................................................172
VITA ................................................................................................................................238
PUBLICATIONS.............................................................................................................239
vii
LIST OF TABLES
Table
Page
3.1
List of JB95 parameters used to fit the methyl vinyl ketone conformers ..............42
3.2
List of XIAM parameters used in the global fit of the methyl vinyl ketone
conformers .............................................................................................................43
3.3
Comparison of rotational data, dipole moment, and relative energies for
experimental, ab initio calculated, and published values for both conformers
of methyl vinyl ketone. ..........................................................................................44
3.4
Isotopic assignments for methyl vinyl ketone .......................................................48
3.5
Comparison of the molecular structure according to ab initio predictions and
Kraitchman analysis ...............................................................................................49
6.1
A summary of previously reported branching ratios for hydrogen abstraction
from 1-butanol. ....................................................................................................116
Appendix Table
A1.
A-state transitions for ap-methyl vinyl ketone (MHz). ........................................136
A2.
E-state transitions for ap-methyl vinyl ketone (MHz). ........................................138
A3.
A-state transitions for 13C1 ap-methyl vinyl ketone (MHz). ................................140
A4.
E-state transitions for 13C1 ap-methyl vinyl ketone (MHz). ................................140
A5.
A-state transitions for 13C2 ap-methyl vinyl ketone (MHz). ................................141
A6.
E-state transitions for 13C2 ap-methyl vinyl ketone (MHz). ................................141
A7.
A-state transitions for 13C3 ap-methyl vinyl ketone (MHz). ................................142
A8.
E-state transitions for 13C3 ap-methyl vinyl ketone (MHz). ................................142
viii
Appendix Table
A9.
Page
A-state transitions for 13C4 ap-methyl vinyl ketone (MHz). ................................143
A10. E-state transitions for 13C4 ap-methyl vinyl ketone (MHz). ................................143
A11. A-state transitions for 18O ap-methyl vinyl ketone (MHz). .................................144
A12. A-state transitions for sp-methyl vinyl ketone (MHz). ........................................144
A13. E-state transitions for sp-methyl vinyl ketone (MHz). ........................................145
A14. A-state transitions for 13C1 sp-methyl vinyl ketone (MHz). ................................145
A15. E-state transitions for 13C1 sp-methyl vinyl ketone (MHz). ................................146
A16. A-state transitions for 13C2 sp-methyl vinyl ketone (MHz). ................................146
A17. E-state transitions for 13C2 sp-methyl vinyl ketone (MHz). ................................147
A18. A-state transitions for 13C3 sp-methyl vinyl ketone (MHz). ................................147
A19. E-state transitions for 13C3 sp-methyl vinyl ketone (MHz). ................................148
A20. A-state transitions for 13C4 sp-methyl vinyl ketone (MHz). ................................148
A21. E-state transitions for 13C4 sp-methyl vinyl ketone (MHz). ................................149
A22. Unscaled harmonic vibrational frequencies determined by ab initio
calculations ..........................................................................................................149
B23. The assigned transitions in the discharge spectrum of 2,3-dihydrofuran ..............152
C24. The assigned transitions in the discharge spectrum of 2,5-dihydrofuran ..............155
D25. The assigned transitions in the discharge spectrum of 1,3-butadiene ...................157
D26. The assigned transitions in the discharge spectrum of 1,3-butadiene and
molecular oxygen .................................................................................................157
E27. The assigned transitions in the discharge spectrum of isoprene............................159
E28. The assigned transitions in the discharge spectrum of isoprene and molecular
oxygen ..................................................................................................................160
ix
Appendix Table
Page
F29. The assigned transitions in the discharge spectrum of 1-butanol ..........................163
F30. The assigned transitions in the discharge spectrum of 2-butanol ..........................164
F31. The preliminary spectral parameters for three conformers of butanal ..................166
F32. The assigned transitions for the cis-trans conformer of butanal ...........................167
F33. The assigned transitions for the trans-gauche conformer of butanal ....................167
F34. The assigned transitions for the cis-gauche conformer of butanal ........................169
G35. A list of previously reported rotational constants and microwave frequency
transitions of molecules related to the discharge spectra .....................................172
x
LIST OF FIGURES
Figure
Page
2.1
The circuit used to generate the chirped frequency polarizing pulse by mixing
with a 13 GHz phase-locked dielectric oscillator ..................................................19
2.2
The circuit used to generate the chirped frequency polarizing pulse by
quadrupling the pulse generated by the arbitrary waveform generator .................20
2.3
Characteristics of the chirped pulse generated by the 13 GHz mixing circuit.
A) The 1 Ps pulse in the time domain. B) The frequency spectrum from 8.018.0 GHz. C) A frequency resolved optical gate (FROG) showcasing when
the frequency components appear during the chirped pulse. The intensity
range is -10dB down on log scale in the Z-dimension...........................................22
2.4
Characteristics of the chirped pulse generated by the quadrupler circuit.
A) The 1 Ps pulse in the time domain. B) The frequency spectrum from 7.518.5 GHz. C) A frequency resolved optical gate (FROG) showcasing when
the frequency components appear during the chirped pulse. The intensity
range is -10dB down on log scale in the Z-dimension...........................................23
2.5
The stainless steel vacuum chamber viewed from the top (A) and the side (B).
The chamber is divided into three sections: 1) Laser Entrance 2) Microwave
Cavity and 3) Broadband Spectrometer .................................................................25
2.6
The microwave circuit required to amplify and down-convert the molecular
free induction decay ...............................................................................................28
2.7
The pulsed discharge nozzle ..................................................................................30
3.1
The two conformers of methyl vinyl ketone (MVK) designated as
antiperiplanar (ap) and synperiplanar (sp) .............................................................35
3.2
The microwave spectrum of methyl vinyl ketone (MVK). The inset
demonstrates the signal-to-noise achieved with the CP-FTMW spectrometer.
The dashed line in the inset indicates the average peak height of isotopic
species measured in natural abundance for ap-MVK ............................................41
xi
Figure
Page
4.1
Comparison of the discharge spectrum of 2,3-dihydrofuran obtained from
CP-FTMW spectrometer (top) and known frequencies (bottom) ..........................59
4.2
List of molecules identified in the discharge spectrum of 2,3-dihydrofuran .........60
4.3
Relative zero point corrected energies for the four conformers of
crotonaldehyde. Conformers related to the aldehyde group are identified by
d- and s- respectively. The lowest energy structure, the s-trans conformer
of d-trans-crotonaldehyde, is set at 0 kJ/mol .........................................................62
4.4
Comparison of the discharge spectrum of 2,5-dihydrofuran obtained from
CP-FTMW spectrometer (top) and known frequencies (bottom) ..........................68
4.5
List of molecules identified in the discharge spectrum of 2,5-dihydrofuran .........69
4.6
Comparison of the discharge spectrum of 1,3-butadiene obtained from
CP-FTMW spectrometer (top) and known frequencies (bottom) ..........................74
4.7
Comparison of the discharge spectrum of 1,3-butadiene and O2 obtained
from CP-FTMW spectrometer (top) and known frequencies (bottom) .................76
4.8
List of molecules identified in the discharge spectra of 1,3-butadiene..................77
5.1
Simplified mechanism of the isoprene and hydroxyl radical reaction. The
primary products are methyl vinyl ketone (MVK), 3-methylfuran (3-MF),
methacrolein (MAC) and formaldehyde. ...............................................................88
5.2
The discharge spectrum of pure isoprene comparing the experimental data
(top) and the previously known transitions (below). The intensities of the
assigned transitions are set to the values obtained from the experimental
data .........................................................................................................................93
5.3
The discharge spectrum of isoprene in the presence of molecular oxygen
comparing the experimental data (top) and the previously known transitions
(below). The intensities of the assigned transitions are set the values obtained
from the experimental data ....................................................................................95
5.4
A list of products observed in the discharge spectra, including the parent
molecule isoprene ..................................................................................................96
5.5
A graph demonstrating the relative energies of the four observed C4H6O
isomers in the discharge spectrum of oxygenated isoprene .................................102
xii
Figure
Page
6.1
The discharge spectrum of 1-butanol comparing the experimental data (top)
and known frequencies (bottom). The intensity scale has been set to observe
the smaller discharge products; the formaldehyde peak is seven times larger
than the largest peak seen here.............................................................................118
6.2
Product list for the discharge spectrum of 1-butanol ...........................................119
6.3
The discharge spectrum of 2-butanol comparing the experimental data (top)
and known frequencies (bottom) .........................................................................125
6.4
Product list for the discharge spectrum of 2-butanol ...........................................126
xiii
ABSTRACT
Shirar, Amanda Jo. Ph.D., Purdue University, December 2011. Determination of
Discharge Products Using Chirped-Pulse Fourier Transform Microwave Spectroscopy.
Major Professor: Brian C. Dian.
The purpose of this study is to study products generated in reactions initiated by
an electric discharge. A chirped-pulse Fourier transform microwave (CP-FTMW)
spectrometer is used as the molecular detector, allowing for discrimination between
structural and conformational isomers. The ground state spectrum of methyl vinyl ketone
is presented first to demonstrate the capabilities of the CP-FTMW instrument. Two
conformers were identified and heavy atom isotopic species were observed in natural
abundance. Furthermore, the presence of a methyl rotor resulted in A-E frequency
splitting for each species that were clearly resolved in the microwave spectrum. The first
set of discharge experiments included three different molecules and set out to
characterize and optimize the additional parameters necessary with the discharge
apparatus. The initial molecules of interest were 2,3- and 2,5-dihydrofuran because these
isomers were known to have different decomposition reactions. It is shown that the
discharge spectra for both molecules are unique and isomer-specific chemistry occurs for
these molecules. There was also interest in conducting a bimolecular experiment, which
led to studying the oxidation of 1,3-butadiene. Once these initial studies were complete,
focus shifted to more environmentally relevant molecules. The next molecule studied was
isoprene because it is the largest biogenic emission in the world and its atmospheric
oxidation is an extremely important reaction. In addition to assigning previously known
products, several new molecular species were observed in the reaction of isoprene and
molecular oxygen. Another important area of environmental chemistry is the study of
xiv
biofuels, such as butanol. Two isomers of butanol were studied, 1- and 2-butanol, to
determine differences in reaction chemistry. There are unique molecular species observed
in each discharge spectrum. This research demonstrates that CP-FTMW spectroscopy has
distinct advantages for product determination of discharge chemistry and can be used to
study a variety of molecules and reactions.
1
CHAPTER 1: INTRODUCTION
Atmospheric Relevance and Environmental Impact
There is a clear need to understand the dynamic nature of earth’s atmosphere and
to recognize the implications of this changing chemistry, where influential molecular
species can originate from anthropological and naturally occurring processes. Even
during the mid-to-late 19th century,1-3 scientists questioned how molecules like carbon
dioxide (CO2) affect the atmosphere. This research, begun long ago on a simple
molecule, remains important today due to its continuing impact on the environment.4,5
Though experiments had been focused on the dominant components of the atmosphere,
the 1970’s and 1980’s introduced a shift toward studying trace gases such as CFCl3, CH4
and N2O.6-8 This research strongly correlated with government regulations like the Clean
Air Act of 1963 and the 1970 Extension that established National Ambient Air Quality
Standards.9 During this time Hansen et al.10 reported a global warming of 0.4qC over the
past century. Hansen et al. also used a model based on CO2 levels to predict future global
temperatures and the impact this would have on the environment. The consequences
included rising sea levels, areas of drought and shifting climate zones. A more recent
study conducted by Davis et al.11 estimated the global temperature in 2060 based only on
currently existing CO2 emitting infrastructure; the result was a global warming of 1.3qC
compared to pre-industrial values.
Current global simulations no longer focus solely on CO2 concentrations. Models
often base atmospheric conditions on the concentrations of the hydroxyl radical (OH) or
ozone (O3) because these species are highly sensitive to the composition of the
atmosphere. A prime example is the Master Chemical Mechanism (MCM),12 which
studies the kinetics and mechanisms that lead to atmospheric pollutants. This model
includes 4,351 organic species that participate in 12,691 reactions and 46 inorganic
2
reactions. Models have significantly improved in the last 10 years due to numerous
observational studies testing simulations and the publication of multiple models that
allow for comparisons.13
There are numerous complications to consider when modeling the atmosphere.
A significant problem is the number of volatile organic compounds (VOC) that are
present in the atmosphere. The number of detected VOC in 1978 was 606, which rose to
2,851 in 1986 and was estimated in 2007 to be between 104-105 species.14 Another
difficulty is regional specificity that becomes a factor when comparing models with
experimentally observed concentrations. For example, air and ship traffic has increased
significantly in recent years and this chemistry impacts the environment differently than
land-based emissions.13 It is also difficult to account for developing countries, who were
responsible for 73% of CO2 emissions growth in 2004.15 Another problem is
incorporating the lifetimes of atmospheric molecules, which can range from minutes16 to
years.17
The largest impact on atmospheric chemistry has been from anthropogenic
influences, particularly fuel emissions. A study in 1998 estimated a 63% increase in
global ozone (O3) since the beginning of industrial era.18 Many molecules can be
considered airborne pollutants, but the largest contributor is CO2.10 The second largest
pollutant is soot.19-22 The composition of soot varies and can include numerous
molecules, many of which are carcinogens.23 The impact of soot on the atmosphere is
often underestimated because additional interactions with aerosols and byproducts are
often neglected.24 Another large class of environmentally harmful molecules is polycyclic
aromatic hydrocarbons (PAH)22,25-28 and is classified as an EPA priority pollutant.29
Carbonyls30 are also extremely important in atmospheric chemistry because they produce
free radicals after undergoing photolysis31,32 and react quickly with hydroxyl (OH)
radicals.33 Not only are carbonyls released into the atmosphere as emissions, they are
generated from hydrocarbon species reacting with OH radicals.34
Since fuel sources have a significant impact on atmospheric chemistry, much is
known about the kinetics, mechanisms and product distributions formed by the
combustion of traditional fuel sources. Examples of studied fossil fuel and petroleum
3
components include methane,35-37 1,3-butadiene,38,39 octane40,41 and more complex
molecules like naphthenes.42-44 The consumption of global oil reserves is continuing at a
rapid pace and companies are looking for additional sources located in unusual places.
Unfortunately, technology regarding the extraction of non-traditional oil is not keeping
pace with demand.45 There has been fresh interest in biofuels from the general public, due
in part to the recent disasters of the Deepwater Horizon oil spill and the Fukushima
nuclear power plan.46 Based on the diminishing supply and increasing price of oil, there
has also been attention from companies and governments in alternative fuels. For
example, Dupont and British Petroleum (BP) have partnered to start a biofuel plant that
generates butanol.47,48 The United States Department of Energy plans to replace 30% of
liquid petroleum with biofuel by 202549 and the European Union is replacing 10% of
transport fuel with renewable sources by 2020.50
Biofuels are an attractive alternative to traditional fuels largely because they are
“carbon neutral;” the plants used to generate biofuels consume CO2 until they are
harvested. Ethanol has several environmental benefits,51 but there is discussion about the
economic repercussions of its production.52,53 The viability of ethanol as a fuel has been
studied extensively and much is known about decomposition reactions,54-56 gasolineethanol blends56-59 and direct ethanol fuel cells.54,60 However, ethanol is not the only
candidate for biofuels and there is a push to use longer chain alcohols.48 Recently butanol
has been gaining attention because of its higher energy density47,61 and readiness to use in
existing engines and fuel infrastructure.62 The general public often assumes that biofuels
are more environmentally friendly than traditional oil, but there are some factors that
affect biofuel atmospheric impact. The production costs need to be considered including
energy input, fertilization, processing and transportation. It is possible to create more
emissions during harvesting and processing than the biofuel conserves creating an overall
negative effect.50,52 Additionally, biofuels are responsible for emissions that include
different molecules than tradition hydrocarbon fuels. Ethanol and butanol contain a
source of oxygen that leads to carbonyls47 and the fertilizer needed to grow the
agricultural resources releases dangerous nitrous oxide.63
4
Though anthropogenic sources are responsible for large changes in our
atmosphere, they are not the only sources of disturbance. Naturally occurring emissions
also impact atmospheric composition. The hydrocarbon isoprene is emitted by terrestrial
plant foliage at an estimated global rate of 500-700 Tg/year.64 The effects of isoprene are
greatest in regions with low emissions from human sources or large areas of tropical
forests such as the Amazon and underdeveloped or equatorial countries.64,65 The
oxidation of isoprene also needs to be included in global simulations. These models
continue to improve as more data is included from additional observation, improved
kinetic rates and new atmospheric species and reactions.
Previous Experimental Methods
Many of the reactions occurring in atmospheric chemistry are initiated by radicals
that lead to molecular decomposition or recombination reactions. Pyrolysis is one of the
earliest techniques used to study decomposition reactions and details about temperature
dependent mechanisms are obtained in these studies.66-68 When studying fuels, flame
sources are often used to initiate reactions.61,69,70 In this case, O2 is the co-reactant, and
oxidative processes dominate these environments. Information obtained from these
studies include ignition delay and concentration profiles dependent on the height above
the flame.71 Electrical discharges are also used to generate radicals. For example, the
propargyl-propargyl radical recombination to form benzene has been initiated using a
discharge source.72 The dynamic environment of the discharge can also create molecules
not often observed in normal laboratory conditions and therefore used to study unusual
astrological molecules like C3H2 and C5H2.73-75
Identifying the molecular species generated by these reactions is the first step in
the analysis of the experiment. Most laboratory analysis is based on the separation of
mass. Unfortunately, it is impossible to distinguish between isomers on that basis, which
leads to ambiguity when trying to identify new species. One tactic that has been taken to
provide isomer-specific detection is the use of electron bombardment or photoionization
to distinguish isomers based on their different ionization thresholds or fragmentation
5
behavior. The more common practice is to use electron ionization or resonance enhanced
multiphoton ionization (REMPI) to measure energy-dependent ionization cross sections
that are unique to each isomer. Recently, the unique and powerful attributes of
synchrotron light sources have been used to generate tunable, vacuum-ultraviolet
radiation that can be scanned to obtain photoionization cross sections over wide
wavelength regions. Several groups are using the synchotron in conjunction with flame
experiments to map out isomeric content in a range of fuels.76 However, synchrotrons are
rare and research time is limited. Two examples are the Advanced Light Source (ALS) of
the Lawrence Berkeley National Laboratory and the National Synchrotron Radiation
Laboratory (NSRL) in Hefei, China.71
Some difficulties arise from using ionization energy to identify isomeric species.
There is a strong chance of fragmentation at higher energies and many molecules have an
ionization energy potential at higher values where it is difficult to identify the
introduction of a new species. The spectra generated in these experiments can also be
difficult to interpret, especially when several isomers with similar ionization potentials
exist. This makes for ambiguous assignments of molecular species. In addition, while
structural isomers may be identified in these experiments, the instrumentation still has the
limitation of being unable to distinguish between multiple molecular conformers. A
shape-sensitive technique is needed to clearly identify samples containing multiple
isomers and conformers.
Rotational Spectroscopy As A Detector
Rotational spectroscopy, also known as microwave spectroscopy, is in many ways
an ideal shape-sensitive detector. Since the spectrum obtained is dependent on the
permanent dipole moment of a molecule, any difference in atomic position for each
isomer will generate a completely unique spectrum.77 Microwave spectroscopy has been
used to determine the shapes of small species like methanol78,79 or more complex
biomolecules such as histamine80 that has four observed conformers. Traditional
microwave spectroscopy involves scanning for signal over a broad frequency range in a
6
stark cell81-83 or Balle-Flygare cavity84-86, and collecting a large frequency spectrum takes
several hours. However in recent years, electronics have advanced and new
instrumentation can generate and process large bandwidth pulses called chirped pulses.
This technology has been applied to traditional microwave spectroscopy to create a
chirped-pulse Fourier transform microwave (CP-FTMW) spectrometer.87 With this
spectrometer, a single broadband microwave pulse is used instead of scanning and a 10
GHz bandwidth spectrum can be collected in minutes instead of hours. The publication
that introduced this spectrometer compared a 14-hour cavity scan with 10,000 averages
of an 11 GHz broadband spectrum acquired in only 17 minutes and observed no
difference in sensitivity.87 The sensitivity of the CP-FTMW spectrometer is an adjustable
parameter based on the number of averages in a spectrum.
However microwave spectroscopy does have limitations, and the largest
disadvantage is that the technique is blind to molecules without a permanent dipole
moment. The inability to detect molecules like benzene or acetylene is important to note
here because many of the molecular mixtures generated in the discharge will likely
contain these molecules. However, there are many more advantages that are provided by
using a CP-FTMW spectrometer and this research will demonstrate that microwave
spectroscopy is complementary to other detectors that use mass or fluorescence signals as
a means of identification.
Molecules of Interest
The research here concentrates on the application of a CP-FTMW spectrometer as
a shape-sensitive detection method to identify multiple conformers in a pure sample or
complex mixtures generated by an electrical discharge. A ground state spectrum of a pure
molecule is used to demonstrate the sensitivity and abilities of broadband microwave
spectroscopy. All of the discharge studies have some relevance to environmental
chemistry. The molecules of interest include fuel combustion products, a biogenic
emission, and a promising candidate for biofuel production. The use of an electrical
7
discharge to initiate reactions allowed new molecular products to be generated in a highly
reactive environment.
The experimental methods are described in detail in Chapter 2. The initial theory
of CP-FTMW spectroscopy is explained first. Next the different components of the
spectrometer are explained including signal generation, molecular interaction and
coherence detection. The process by which we process and analyze molecular signal is
also discussed. Several ab initio calculations were completed for this research and
include energy optimization and torsional analysis.
Chapter 3 demonstrates the ability of the CP-FTMW spectrometer to identify
multiple conformers in the ground state spectrum of methyl vinyl ketone (MVK). Early
microwave studies88,89 of MVK observed only a single conformer while IR studies90,91
had observed a second stable conformer. In the process of identifying products in a
discharge spectrum of oxygenated isoprene, this second conformer was observed and a
more thorough study was conducted on the ground state spectrum of MVK. Not only
were two conformers observed, but multiple isotopic species were assigned in natural
abundance and the structure was determined using Kraitchman analysis.
The purpose of Chapter 4 is to optimize the discharge experiment and
demonstrate the different applications of this technique. In order to characterize the
microwave spectrometer and optimize discharge parameters, the first molecule studied
needed to have a well-known mechanism. Previously reported products from pyrolytic or
flame experiments are used as a guide to anticipate molecules generated in the discharge
environment. Conclusions made in those experiments typically focus on lowest energy
threshold pathways and discharge chemistry does not always follow the same reaction
schemes. Those publications simply provide a starting point for analysis of the discharge
spectra of each molecular system. 2,3-Dihydrofuran (2,3-DHF) has been studied with
pyrolysis experiments and involves interesting chemistry as it decomposes.92,93 A large
percentage of reacted 2,3-DHF isomerizes into cyclopropanecarboxyaldehyde (CPCA)
and crotonaldehyde.92 Both of these products have multiple conformers and there is
discussion on which conformers are generated.93 Another isomer of 2,3-DHF is 2,5-DHF,
which was the second molecule studied. This isomer is much closer in shape than CPCA
8
or crotonaldehyde. There was interest to determine whether there would be a difference
between the discharge spectra of these isomers.
Also presented in Chapter 4 are the discharge experiments of 1,3-Butadiene (1,3BD). Like the previous molecules, it is a well-studied system allowing for much
comparison.68,72,94 It is also a zero-background technique because 1,3-BD has no
permanent dipole. 1,3-BD decomposition reactions are often studied for the ability to
generate large polycyclic hydrocarbons from radical recombination products.72 Unlike the
dihydrofurans, 1,3-BD is a hydrocarbon. This allowed for two different experiments, the
unimolecular decomposition of 1,3-BD and the bimolecular reaction of 1,3-BD and
molecular oxygen (O2). Research is conducted on oxidation of 1,3-BD because of its
impact in atmospheric chemistry.39,95 The discharge experiment of 1,3-BD and O2 is also
directly analogous to combustion reactions and this chemistry is important to
understanding this hydrocarbon fuel.96
Chapter 5 presents the research conducted on isoprene. The experiments followed
the same procedure as 1,3-BD and the electric discharge decomposition of pure isoprene
was studied initially. A thorough thermal study had been published97 on isoprene, but
additional products were identified in the unimolecular discharge spectrum. Most of the
research on isoprene has focused on oxidation mechanisms because this reaction scheme
is so prevalent in the atmosphere. There is general agreement in the community that the
main products of isoprene oxidation are methyl vinyl ketone (MVK), methacrolein
(MAC), and formaldehyde.98-101 These three molecules were observed along with other
lesser-known products. Once again, additional molecules were identified in the discharge
spectrum that had not been previously published.
Finally, the research of 1- and 2-butanol is presented in Chapter 6. Butanol is
different than the other experiments because less is known about the reactions and
kinetics. In 2009, Hansen et al.71 noted that not much research had been conducted on the
combustion chemistry of larger alcohols like butanol. This molecule has only been a
focus during the last few years when its potential as a biofuel became known. One
complication of this molecule is the number of isomers and conformers. There are four
isomers (1-butanol, 2-butanol, iso-butanol, tert-butanol) and these isomers have multiple
9
conformers.102-105 There have been a few publications detailing the product distribution
from each of these isomers, but there are some contradictory results.47,106-108 The research
here has focused on 1-butanol and 2-butanol because these are the most similar
conformers with both molecules containing a linear carbon chain. Since both isomers
contain a source of oxygen, only the unimolecular reactions were studied. Their
respective discharge spectra are unique and offer new species in each reaction that have
yet to be reported.
Each of these molecules has offered a different problem to study and has a unique
story to tell. The products observed in the discharge spectra will continue filling in
missing pieces of model simulations that are important in the way we plan for the future.
These molecules and their products already exist in our atmosphere and are impacting the
way our world is changing. The more that is known about these reactions, the more
prepared scientists will be. The research presented here has offered new information but
has also posed new questions. This is simply one step towards a more complete
understanding of our atmosphere and our world.
10
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16
CHAPTER 2: EXPERIMENTAL METHODS
Chirped Pulse Fourier Transform Microwave Spectrometer
Theory
The chirped-pulse Fourier transform microwave spectrometer (CP-FTMW) is
adapted from an instrument built at the University of Virginia and detailed in a review by
Brown et al.1 The primary advantage of the CP-FTMW spectrometer over typical
microwave cavity instruments is the use of a frequency sweep, or chirped, excitation
pulse instead of a single frequency sine wave. Creating a broadband polarizing pulse
enables a large frequency bandwidth spectrum to be acquired in a relatively short amount
of time (~30 min). The electric field, E(t), produced by a chirped pulse is defined by the
peak electric field, Emax, the initial frequency parameter, ω0, and the linear sweep rate, α:
E(t) = E max e
1
i(ω 0 t + αt 2 )
2
.
(1)
The instantaneous frequency can be determined by
ω inst =
d⎛
1 2⎞
⎜ω 0 t + α t ⎟ = ω 0 + α t .
⎠
dt ⎝
2
(2)
The sweep range of the pulse, Δω, is a function of the sweep rate and the pulse duration,
tpulse:
Δω = α ⋅ t pulse .
(3)
As seen in these equations, there is a separation of duration and bandwidth for chirped
pulses. This fact changes the experimental conditions when compared to a transform
limited pulse experiment; chirped pulse instruments can independently control the pulse
power and frequency bandwidth. Another advantage of a linear frequency sweep is
17
apparent in the relationship between molecular signal strength and the bandwidth of the
polarizing pulse. It has been shown1,2 that the molecular signal, S, is dependent on the
frequency, ω, the transition dipole moment, μ, the electric field, Epulse, and the population
difference, ΔN0:
⎛π ⎞
S ∝ ω ⋅ μ ⋅ E pulse ⋅ ΔN 0 ⋅ ⎜ ⎟
⎝α ⎠
2
1
2
.
(4)
It can be seen from this relationship that the signal decreases with the square root of the
bandwidth for a fixed pulse duration. In contrast, the molecular signal from a transform
limited pulse decreases linearly as a function of bandwidth. Therefore, less power is
required for a chirped pulse compared to a transform limited pulse of equal bandwidth.
Traditional cavity experiments are highly sensitive over a narrow frequency
bandwidth, but must be scanned to record a large bandwidth spectrum. The scanning
process is time-consuming and transitions can be overlooked. High sensitivity in the CPFTMW spectrometer is achieved by signal averaging. Collecting 10,000 averages is
relatively quick (~35 min), every molecular frequency is recorded simultaneously and the
sensitivity is comparable to the cavity technique.1
Phase Stability and Timing
In order to average the molecular signal in the time domain, phase stability is
essential. The entire experiment is set at a repetition rate of 10 Hz to match the frequency
of the laser system, which is not used in these experiments. A Rb-disciplined crystal
oscillator operating at 10 MHz (Stanford Research Systems FS725) is connected to the 12
GHz oscilloscope and drives a phase-locked loop (Wenzel Associates 501- 10137B). The
10 MHz signal is phase-locked to an oven controlled quartz oscillator, then up-converted
to 100 MHz. This combination provides a phase noise of -125 dBc/Hz and is connected
to the phase-locked dielectric resonator oscillators (PLDRO, 13.0 and 18.9 GHz), the
arbitrary waveform generator and the Masterclock. The 20-output Masterclock (Thales
Laser) controls the timing of the arbitrary waveform generator, pulsed valve, the electric
discharge and an additional delay generator. The timing of the 200 W amplifier and back-
18
end switch are controlled with TTL pulses from a four-channel digital delay/pulse
generator (Stanford Research Systems model DG535). Most timing for the components
remains unchanged between experiments; only the pulsed valve and electric discharge are
optimized at the outset of each experiment. Additionally, a power supply constructed by
the Purdue Amy Facility provides power to the PLDROs, amplifiers (excluding 200 W),
quadrupler, quartz oscillator and the switch.
Signal Generation
All polarizing microwave pulses were produced using an arbitrary waveform
generator (arb, Tektronix AWG7101, 10 GS/s). The chirped pulse waveforms (≤ 5 GHz)
consisted of a linear frequency sweep covering a bandwidth of interest and had an output
intensity of 1 V peak to peak. Pulse frequencies varied based on the relative microwave
circuitry and design of the experiment. The pulses were transmitted through a 5 GHz low
pass filter (Lorch 10LP-5000-S, ±164 MHz) to reject any high frequency harmonics. The
chirped pulse generated by the arb also had an inconsistent electric field that decreased in
intensity at higher frequencies. This uneven pulse was balanced with an amplifier (Minicircuits ZX60-6013E-S+ 6000MHz, +14.2 dB gain) that also increased the power to
12.25 dBm.
At this point, two different circuit designs were used to convert the polarizing
pulse to the frequency and bandwidth of interest. The first method (Figure 2.1) consisted
of mixing a 5 GHz bandwidth pulse with a 13.0 GHz sine wave generated by a phaselocked dielectric resonant oscillator (PLDRO Microwave Dynamics, PLO-2000-13.00,
phase noise -102 dBc/Hz). The power emitted from the PLDRO was 20.0 dBm, but was
attenuated to 16 dBm to not overdrive the mixer. A 13.0 GHz cavity filter (Lorch 6CF713000/100-S, ±62.23 MHz) was applied to the sine pulse prior to mixing to ensure a
clean carrier signal. The 13.0 GHz sine wave was then mixed (Miteq, TB0440LW1) with
19
Figure 2.1. The circuit used to generate the chirped frequency polarizing pulse by mixing
with a 13 GHz phase-locked dielectric oscillator.
20
Figure 2.2. The circuit used to generate the chirped frequency polarizing pulse by
quadrupling the pulse generated by the arbitrary waveform generator.
21
the chirped pulse (0.1-5.0 GHz) to create an 8.0-18.0 GHz polarizing pulse. The resulting
mixed pulse had a signal power of 5.60 dBm. A 13.0 GHz cavity notch filter (Lorch
6BR6-13000/100-S, ±164 MHz) was inserted to remove the large signal from the
PLDRO. A different initial waveform was used for the second circuit (Figure 2.2). Rather
than mixing, the bandwidth of the original chirped pulse (1.875-4.625 GHz) was
increased by a factor of four using a quadrupler (Phase One PS06-0161) and resulted in a
final polarizing pulse of 7.5-18.5 GHz. The quadrupler circuit generates a higher power
microwave pulse at 16.08 dBm.
The CP-FTMW spectrometer was originally built with the 13 GHz mixing circuit
and as a result, early experiments used a polarizing pulse of 8.0-18.0 GHz. The
quadrupler was obtained later to increase the bandwidth of the polarizing pulse. The
characteristics of both polarizing pulses are displayed in Figures 2.3 and 2.4. The top and
middle panels show a 1 μs pulse in the time and frequency domain. In the bottom panel
of each figure is a time-frequency analysis using a variable window Fourier transform
method. This image demonstrates at what time interval the frequencies are emitted during
the polarizing pulse. Both pulses contain harmonic distortion from the arb, but the
quadrupler introduces more noise. There is an inherent 2 GHz frequency that is
constantly produced and any noise is propagated four times instead of twice, as with the
mixing circuit. Another difference between the circuits is that the quadrupler pulse has a
higher power output. Currently the mixing circuit is primarily used in experiments
requiring a small bandwidth polarizing pulse and the quadrupler is used to acquire large
bandwidth spectra. In the case of discharge experiments, where signal levels are
significantly small, it is advantageous to use the polarizing pulse with the highest power.
The benefits of more power and increased bandwidth make the quadrupler circuit the best
system for discharge experiments.
After the appropriate polarizing pulses were converted, subsequent components
controlled the power of the signal. A step attenuator (Weinschel AF117A-69-11) was
used as the most direct control of signal intensity and a 200 W amplifier (Amplifier
Research 200T8G18A, 7.5-18.0 GHz) provided the primary power for the polarizing
22
Figure 2.3. Characteristics of the chirped pulse generated by the 13 GHz mixing circuit.
A) The 1 μs pulse in the time domain. B) The frequency spectrum from 8.0-18.0 GHz.
C) A frequency resolved optical gate (FROG) showcasing when the frequency
components appear during the chirped pulse. The intensity range is -10dB down on log
scale in the Z-dimension.
23
Figure 2.4. Characteristics of the chirped pulse generated by the quadrupler circuit.
A) The 1 μs pulse in the time domain. B) The frequency spectrum from 7.5-18.5 GHz.
C) A frequency resolved optical gate (FROG) showcasing when the frequency
components appear during the chirped pulse. The intensity range is -10dB down on log
scale in the Z-dimension.
24
pulse. An isolator (Ditom Microwave Inc., DMI 6018) was placed between the attenuator
and amplifier to protect the microwave components preceding this step. A 24-in. jacketed
flexible waveguide (Western Test Systems WRD-750) was used to bridge the space
between the amplifier and the vacuum chamber. To couple the microwave field to the
chamber and maintain vacuum, double-ridge waveguide bulkheads (Western Test
Systems WRD-750) were combined with WRD-750 pressure windows (Advanced
Technical Materials 750-230-C3-G3).
Chamber
The stainless steel vacuum chamber used in the microwave experiments is
presented in Figure 2.5. It is composed of three compartments: 1) laser entrance, 2)
microwave cavity and 3) broadband spectrometer. The third section was used to acquire
the ground state microwave and discharge spectra. This portion of the chamber is lined
with Eccosorb microwave absorber backed with metal shielding (Emerson & Cuming
HR-25/ML) to reduce spurious reflection noise from the polarizing microwave pulse. The
entire chamber is evacuated through two diffusion pumps (Varian VHS 10) located
beneath sections 1 and 2. The second pump is also fitted with a water baffle (Varian
F8600310). These diffusions pumps are supported with a roughing pump (Alcatel 2063)
and roots blower (BOC Edwards EH 500). The base pressure of the chamber reaches 5 x
10-6 Torr with a working pressure of 5 x 10-5 (2 x 10-4) Torr using the 1 mm (2 mm) valve
nozzle.
The molecules studied here are liquid at room temperature and most samples were
converted into a gas sample using a freeze-pump-thaw method. However if the vapor
pressure was too low, the molecule was used in its liquid form and deposited in a small
stainless steel container filled with cotton. This sample was then placed inside the
vacuum chamber prior to the molecular valve. Specific volumes differed with
experiments, but most samples had a concentration of 2% in a buffer gas. When a ground
state spectrum was taken, the sample was seeded with He/Ne (70/30) to increase
25
Figure 2.5. The stainless steel vacuum chamber viewed from the top (A) and the side (B).
The chamber is divided into three sections: 1) Laser Entrance 2) Microwave Cavity and
3) Broadband Spectrometer.
26
molecular collisional cooling through a supersonic expansion. When conducting
discharge experiments, a different buffer gas was used to induce the formation of highly
reactive species via molecular collisions. Ar is often used in discharge experiments to
create radicals and ions through energy transfer processes3-6 and was chosen here as the
buffer gas. The sample was introduced into the chamber using a Series 9 general valve (1
or 2 mm orifice) that is controlled by a pulsed valve driver (Purdue Amy Facility). The
backing pressure of the nozzle ranged between 2-40 psi and was optimized for each
experiment. The duration of the molecular pulse was approximately 2 ms.
Molecular Interaction
The polarizing pulse was broadcast into the vacuum chamber using a microwave
horn antenna with a gain enhancer (Amplifier Research model AT4004, 8-18 GHz). The
horn is 4.5 in. wide and indicates the size of the interaction region between the
microwave pulse and the sample molecules. The pulsed valve is placed perpendicular to
the polarizing pulse to minimize Doppler effects and fitted with adjustable arms to
optimize the interaction region. Upon entering the vacuum the sample undergoes
supersonic expansion, which cools the molecules to the ground vibrational state. By
fitting experimental rotational transition intensities with the spectral fitting program
SPCAT,7 the temperature in the supersonic expansion has been calculated to be
approximately 2.5 K. After interaction with the polarizing microwave pulse, there is a
molecular emission, or free induction decay (FID), that has a dephasing (T2) time of
about 10 μs. The signal is collected by an identical receiving horn placed opposite to the
broadcast antenna. This receiving horn is attached to an adjustable XYZ stage (see Figure
2.5.a) that was constructed to maximize the coupling between the microwave horns. No
observable difference was noted when repositioned in the X and Y dimensions. An
optimal location on the Z axis was determined based on the intensity of the microwave
signal and remained set for all experiments.
27
Coherence Detection
The molecular FID is weak and in order to detect and digitize the signal, it must
be amplified and down-converted (Figure 2.6). The primary component after the chamber
is a low noise amplifier (LNA) that needs protection from high power signals. Excess
power from the polarizing pulse is contained using a p-i-n diode limiter (Advanced
Control Components ACLM 4619F-C36-1K), which is rated to 1 kW peak power for a
1μs pulse. A reflective single-pole single-throw (SPST) switch (Advanced Technical
Materials S1517D isolation 80 dB, 2-18.0 GHz) is also used as protection. The switch
remains closed during the polarizing pulse and the initial noise decay of the system, a
total duration of approximately 2 μs. The molecular FID is then amplified with the highgain (+45 dB) LNA (Miteq AMF-6F-06001800-15-10P). In order to digitize the 8.0-18.0
GHz free inductive decay with an oscilloscope, the signal needs to be down-converted to
a dc-12.0 GHz frequency regime. An 18.9 GHz sine pulse from a PLDRO (Microwave
Dynamics PLO-2000-18.90, phase noise -96 dBc/Hz, 13.6 dBm) is filtered (Lorch 7CF718900/100-S, ±50 MHz) to provide a single carrier pulse. The 18.9 GHz signal mixed
(Miteq TB0440LW1) with the FID creates two side bands (0.9-10.9 and 26.9-36.9 GHz).
A 12.0 GHz low pass filter rejects the upper sideband (Lorch 7LA-12000-S, ±92.54
MHz) and any frequency offsets are corrected with a DC block (MCL 15542 BLK-18).
Finally, the signal is recorded and digitized with a 12 GHz oscilloscope (Tektronix
TDS6124C, 40 GS/s).
Electric Discharge
The electric discharge apparatus is based on work by Newby et al.,8 who adapted
the design from previous experiments.9-11 The electric discharge pulse is generated with a
high voltage pulse modulator (DEI PVM-4210) containing a positive and negative
channel. These channels are tunable up to 950 V and working voltages are typically set at
28
Figure 2.6. The microwave circuit required to amplify and down-convert the molecular
free induction decay.
29
+300 and -500 V. Safe high voltage (SHV) cables connect the modulator outputs to the
input channels on the chamber and a ballast resistor (10 kΩ) is located between them.
Figure 2.7 shows the pulsed discharge nozzle configuration located within the chamber.
Two aluminum electrodes (2 mm orifice diameter) are held in a Delrin extension that is
fitted to the face of the pulsed valve. The electrodes are separated by a 1.5 mm Delrin
insulated spacer. The discharge pulse is initiated by a discharge timing control box
(Purdue Amy Facility). The potential duration of the discharge pulse is 0.040-1000 μs
and has been set at 1 ms for these experiments.
Data Processing
After acquiring the molecular free inductive decay in the time domain, the data is
converted to the frequency domain. The two methods employed in this lab to accomplish
this step include online or offline processing. An online fast Fourier transform is
completed on the 12 GHz oscilloscope itself; both frequency and time domain data files
are saved. To convert the time domain offline, a MathCAD program is used to apply the
fast Fourier transform (fft). The normalization factor for the Fourier transform on the
oscilloscope is N-1, where N is the number of data points. Conversely, the MathCAD
program uses a normalization of N-1/2, which makes it difficult to compare the absolute
signal intensities of the data if both methods are used for a given experiment. For a
molecular FID with a 10 or 20 μs duration, the final frequency resolution is nominally 40
or 20 kHz respectively. The FID is zero-padded before and after the data to center the
signal in the digital filter. For both methods a Kaiser-Bessel12 window function is applied
to suppress side lobes (69-70dB) and increase the full width half maximum linewidth.
The 3dB bandwidth of this window is 1.72 bins, which correlates to a resolution
bandwidth of 172 kHz (86 kHz) for a 10 μs (20 μs) gate duration.
In addition to acquiring a molecular FID, a background spectrum is necessary to
improve the signal-to-noise of the data. There are two main sources of noise in the CPFTMW spectrometer. The first is due to the residual ringdown of the polarizing pulse.
30
Figure 2.7. The pulsed discharge nozzle.
31
Much of this comes from reflections propagating in the stainless steel chamber that have
not been completely suppressed with the microwave foam. The other source of noise in
the experiment comes from spurious peaks from electronic components. The spectrum is
processed using a threshold/replace MathCAD routine instead of a straight subtraction
between the background and molecular spectra, which would result in incomplete
subtraction due to the variation in absolute intensity. A threshold intensity is selected on
the background frequency spectrum slightly above the baseline; any peak above this
threshold that is also present in the molecular spectrum is removed. The microwave field
is not constant across the entire frequency range and the noise floor is more intense at
high frequencies due to inconsistent amplification from the LNA. To account for the
differences in the baseline, the program divides the spectrum into thirds in order to set
distinct and appropriate thresholds. The program applies the routine to the separate
spectra and recombines them to regenerate the entire broadband spectrum. The final S/N
ratio of the largest methyl vinyl ketone transition (100,000 averages) is 10000:1 and
represents the typical sensitivity achieved with the CP-FTMW spectrometer.
Ab Initio Calculations
All ab initio calculations were completed using the Gaussian03 suite.13 Three
different levels of theory were used in these experiments: Hartree-Fock (HF, 6311++G(d,p)), density functional (DFT, B3LYP/6-311++G(d,p)) and second order
Møller-Plesset perturbation theory (MP2, 6-311++G(d,p)). Many of the molecules
studied here have multiple conformational isomers and it was necessary to determine the
most energetically favorable species for each molecule that would be observed in the
microwave spectrum. Optimization calculations were completed for each conformer and
the relative energies were determined. Complementary vibrational analysis determined
whether the species of interest was a stationary state by verifying that all vibrational
frequencies were positive. This calculation also provided the zero-point correction for
each optimized structure.
32
Relaxed potential energy scans were also performed on several molecules. After
selecting an appropriate dihedral angle, the angle is rotated in a fixed step size (4° or 10°)
and the structure is optimized at each point. This calculation was utilized in two different
situations. First, to ensure that all probable conformers of a particular molecule were
included, this torsional analysis was completed on many of the heavy atom backbones. If
an additional conformer was observed, this calculation provided the relative energies of
the species as well as the barrier to isomerization. Second, this technique was also applied
to calculate the barrier to internal rotation of a methyl rotor and determine the V3 value
associated with this energy potential.
33
References
(1)
Brown, G. G.; Dian, B. C.; Douglass, K. O.; Geyer, S. M.; Shipman, S. T.; Pate,
B. H. Rev. Sci. Inst. 2008, 79, 053103.
(2)
McGurk, J. C.; Schmalz, T. G.; Flygare, W. H. J. Chem. Phys. 1974, 60, 41814188.
(3)
Remy, J.; Biennier, L.; Salama, F. Plasma Sources Sci. Technol. 2003, 12, 295301.
(4)
Broks, B. H. P.; Brok, W. J. M.; Remy, J.; van der Mullen, J. J. A. M.; Benidar,
A.; Biennier, L.; Salama, F. Phys. Rev. E 2005, 71, 036409(036401-036408).
(5)
Al-Jalal, A.; Khan, M. J. App. Phys. 2006, 99, 033302.
(6)
Seo, H.; Kim, J.-H.; Shin, Y.-H.; Chung, K.-H. J. App. Phys. 2004, 96, 60396044.
(7)
Pickett, H. M. J. Mol. Spec. 1991, 148, 371-377.
(8)
Newby, J.; Stearns, J.; Liu, C.; Zwier, T. J. Phys. Chem. A 2007, 111, 1091410927.
(9)
Xin, J.; Fan, H. Y.; Ionescu, I.; Annesley, C.; Reid, S. A. J. Mol. Spec. 2003, 219,
37-44.
(10)
Harper, W. W.; Clouthier, D. J. J. Chem. Phys. 1997, 106, 9461-9473.
(11)
Güthe, F.; Ding, H.; Pino, T.; Maier, J. P. Chem. Phys. 2001, 269, 347-355.
(12)
Elliot, D. F. Handbook of Digital Signal Processing Engineering Applications;
Academic: San Diego, 1987.
(13)
Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.;
Cheeseman, J. R.; Montgomery Jr., J. A.; Vreven, T.; Kudin, K. N.; Burant, J. C.;
et al. Gaussian 03, Revision C.02; Gaussian, Inc.: Wallingford, CT, 2004.
34
CHAPTER 3: GROUND STATE SPECTRUM OF METHYL VINYL KETONE
Introduction
The simplest α,β-unsaturated ketone, methyl vinyl ketone (MVK, 3-buten-2-one),
is important in atmospheric chemistry as a primary product of isoprene oxidation. Early
laboratory studies discovered significant yields of MVK from the reaction of isoprene
with the hydroxyl radical1 or ozone2. MVK was later identified in the atmosphere at
concentrations strongly correlated with biogenic isoprene.3 MVK reacts quickly with the
hydroxyl radical to generate other carbonyls including methylglyoxal and formaldehyde
that contribute to the additional destruction of ozone.4 Accurately determining the
structure of MVK will improve analyses of its chemistry in the atmosphere.
The microwave spectrum of MVK was first measured in 1965 by Foster et al.5 in
the region of 7-33 GHz. MVK was identified in the antiperiplanar configuration (ap,
Figure 3.1) and A-E frequency doublets were observed arising from hindered internal
rotation of the methyl group. The authors were able to determine a V3 value of 437(7)
cm-1 for the barrier to internal rotation from this splitting. The microwave spectrum was
revisited at higher frequencies (26.4-40.0 GHz) in 1987 where rotational transitions were
determined from the ground and first excited torsional states of ap-MVK. A slightly
smaller V3 value of 424(7) cm-1 was reported for internal methyl rotation.6 No evidence
for an additional conformation was reported in either study.
Infrared studies, however, have confirmed a second stable conformation,
synperiplanar (sp, Figure 3.1), in the liquid and gas phases.7 From the measured
enthalpies, sp-MVK was found to be 2.36 kJ/mol higher in energy than ap-MVK. Ab
initio calculations have predicted similar energies (0.12-2.36 kJ/mol) depending on the
level of theory used.7,8 This small energy difference suggests that the sp conformer could
35
Figure 3.1. The two conformers of methyl vinyl ketone (MVK) designated as
antiperiplanar (ap) and synperiplanar (sp).
36
be detected in the MVK rotational spectrum with a chirped-pulse Fourier transform
microwave (CP-FTMW) spectrometer. The rotational spectrum of both the ap- and spMVK conformers is presented here, as well as assignments for the isotopic species
(heavy atoms only) in natural abundance.
Experimental
The CP-FTMW spectrometer has previously9 been described in detail and will
only be summarized here. Methyl vinyl ketone was obtained from Sigma-Aldrich (99%
pure) and used without further purification. Approximately 1 mL of sample was
introduced into a tank using a freeze-pump-thaw method and balanced to approximately 7
bar with a 30/70% He/Ne mixture. An arbitrary waveform generator with a 10 GS/s
sampling rate (Tektronix AWG 7101) produced a 1 μs microwave chirped pulse (1.8754.625 GHz). The pulse was then quadrupled (Phase One) and the power increased with a
200 W amplifier (Amplifier Research) to create an 11 GHz bandwidth pulse (7.5-18.5
GHz). This polarizing pulse was then broadcast with a microwave horn into a vacuum
chamber with a typical base pressure of 1 x 10-6 Torr. The sample was introduced into the
chamber with a Series 9 General Valve (2 mm orifice) at a backing pressure of 1 bar
resulting in a supersonic expansion that cooled the molecules to an estimated rotational
temperature of 4 K. The microwave pulse induced a macroscopic polarization and the
resulting free inductive decay was collected with a second microwave horn. The
molecular signal passed through a p-i-n diode limiter (Advanced Control Components)
and a switch (single pole single throw, Advanced Technical Materials) before entering a
low noise amplifier (+45 dB gain, Miteq). The molecular signal was then mixed with an
18.9 GHz phase locked dielectric resonator oscillator (Microwave Dynamics) and the
lower sideband was isolated and digitized at 40 GS/s for 20 μs with a 12 GHz
oscilloscope (Tektronix TDS6124C). In order to signal average, the entire system was
synchronized with a 100 MHz phase locked loop (Wenzel Associates) driven by a 10
MHz Rb-disciplined crystal oscillator (Stanford Research Systems) and the timing was
controlled with a 20-output Masterclock (Thales Laser). A Kaiser-Bessel digital filter was
37
applied to the time domain signal to suppress side lobes of the rotational transitions in the
frequency domain. The Fourier transform of the free induction decay yielded a nominally
11 GHz bandwidth frequency spectrum with 20 kHz resolution. The resolution was
interpolated to 5 kHz offline in the spectral fitting program JB95.10 A signal-to-noise
ratio of 10000:1 on the strongest transition was achieved with 100,000 time domain
averages. The ground state microwave spectrum and background spectrum (to remove
spurious microwave resonances) were recorded in a combined 15 hours.
Microwave Spectrum
An effective Hamiltonian describing the torsion-rotation interaction for each Aand E-state symmetry species is implemented in the spectral fitting program JB95.10 The
fitting parameters in the least squares procedure are based on the perturbative torsionrotation Hamiltonian. For high V3 barriers to internal rotation, the torsion-rotation cross
terms are treated as perturbations. The Hamiltonian is factored through successive Van
Vleck transformations into submatricies describing A- and E-internal rotation states
separately for each torsional level.11 In the principal axis frame, this method is
straightforward and suitable for molecules in torsional states near the bottom of the
potential well with V3 barriers greater than approximately 100 cm-1.12 Published
microwave5,6 and IR/Raman13 data have reported V3 barrier heights between 420-437
cm-1 for ap-MVK. Furthermore, all recorded lines originated from the ground torsional
state (vt = 0 for both symmetry species) under the experimental conditions in this lab.
The effective Hamiltonian used to fit the A-states is of the form14
H Aeff = AA Pˆa2 + B A Pˆb2 + C A Pˆc2 + H cd ,
(1)
38
where
AA = ARR + FW00( 2 ) ρ a2
B A = BRR + FW00( 2 ) ρ b2
(2)
C A = C RR ,
and H cd describes 4th order distortion. In the preceding equation, W00( 2 ) is the second order
perturbation coefficient of the A-state in the ground torsional level. ARR , etc. are the
unperturbed rigid rotor constants in the absence of internal rotation. F and ρ g (g = a,b)
are functions of the methyl rotor inertial moment, I φ , the direction cosines of the top with
respect to the principal axis, λ g , and the principal moments of inertia I g :
F=
F
2
= R
2rI φ
r
r = 1−
∑λ
2
g
g = a ,b
ρ g = λg
Iφ
Ig
(3)
Iφ
(4)
Ig
.
(5)
The (a b) plane of symmetry in both conformers simplifies terms in the Hamiltonians
since the c-principal axis is normal to the methyl rotor axis ( λc = 0). ρ c is therefore zero,
indicating C RR is unperturbed by internal rotation.
The E-state effective Hamiltonian includes linear operators that describe the
angular momentum of methyl hydrogen tunneling,14
H Eeff = AE Pˆa2 + BE Pˆb2 + C E Pˆc2 +
∑D
g
Pˆg + H cd
(6)
g = a ,b
with the operator constants
AE = ARR + FW0(±21) ρ a2
BE = BRR + FW0(±21) ρ b2
(7)
C E = C RR
D g = FW0±(11) ρ g .
(8)
39
W0±(11) and W0±( 21) are the 1st and 2nd order perturbation coefficients, respectively, for the
ground torsional level of the E-state. The A- and E-state perturbation coefficients are
tabulated elsewhere 11 as a function of the reduced barrier parameter, s , which is in turn
related to V3 by
s=
4 V3
.
9 F
(9)
The V3 barrier is thus obtained by determining s from the perturbation coefficients and
fixing FR to the appropriate value ( I φ is assumed 3.18 μÅ2 from Ref. 11). Perturbation
coefficients, rigid rotor constants, and direction cosines are obtained iteratively from the
best-fit parameters of the A- and E-state effective Hamiltonians in a method described by
Lavrich et al.14 This procedure, generalized for the case of multiple internal rotors, is
implemented in a program integrated with the JB95 GUI.
An additional torsional analysis was performed with the internal rotor program
XIAM.15 The precision of the V3 barrier becomes harder to determine in the principal
axis frame (PAF) as the barrier increases and the torsion-rotation cross terms rapidly
decrease to zero. Improved internal rotation parameters such as λ g and V3 may be
obtained in a number of methods employing Rho-axis frame (RAF) Hamiltonians.12 A
new coordinate axis system is defined in the RAF that is related to the PAF by a rotation
of z-axis parallel to the ρ vector. The contributions of ρ x and ρ y are eliminated from
the ρ vector ( ρ x = ρ y = 0 ) in the RAF, permitting the separation of the Hamiltonian into
purely torsional and rotational components. In XIAM, the pseudorigid rotor
PAF
Hamiltonian, H rot
, is expressed in the PAF
PAF
H rot
= APˆa2 + BPˆb2 + CPˆc2 + H cd .
(10)
Global rotational parameters A , B , C , and H cd differ from those of Equations 1 and 6 in
that they simultaneously describe A- and E-state transitions. The torsional Hamiltonian in
the RAF contains internal rotation terms that may be directly varied in the fitting
procedure:
40
1
RAF
H tor
= F ( pˆ φ + ρPˆz ) 2 + V3 (1 − cos(3φ )) .
2
(11)
Here, φ is the internal rotation angle and p̂φ is the conjugate momentum operator. The
final rotational parameters are expressed in the PAF by transforming Equation 11 back
into the PAF and adding the contributions to the rotational constants of Equation 10.16
The microwave spectrum of methyl vinyl ketone in the region of 6.0-18.9 GHz is
presented in Figure 3.2. Using the reported rotational constants of Fantoni et al.6 the Astate lines of ap-MVK were identified in our spectrum and the corresponding E-state
transitions were fit to the operator constants of Equation 6 in JB95. Several new lines
were included in the fit of the A- and E-states of ap-MVK, refining the assignments of
Foster et al.5 at higher resolution. Quartic distortion terms were required in both effective
Hamiltonians to reduce the root-mean-square error. Improved values of all fitted
parameters were obtained by including the previously measured high frequency
transitions (19 to 40 GHz) with their appropriate experimental weights into the analysis.
The quantum number assignments from JB95 were used as input for a global analysis in
XIAM. The fit parameters from JB95 and XIAM are reported in Tables 3.1 and 3.2,
respectively.17
Though no preceding microwave studies have identified a second MVK
conformer, experimental IR data has provided evidence for a stable conformation of
MVK (synperiplanar of Figure 1) that is 2.36 kJ/mol higher in energy than ap-MVK. Ab
initio calculations were performed to estimate conformational energies using the
Gaussian suite18 and are reported in Table 3.3. Zero-point corrected energies predicted
sp-MVK to be the most stable conformer by 0.44 and 0.07 kJ/mol, respectively, at the
Hartree-Fock (HF/6-311++G(d,p)) and density functional (B3LYP/6-311++G(d,p)) levels
of theory in contradiction with experimental results.7 Second-order Møller-Plesset
perturbation theory (MP2/6-311++G(d,p)) calculations verified the experimental data
with ap-MVK as the lower energy conformer by 1.16 kJ/mol. Calculated vibrational
mode frequencies of each optimized geometry were positive, indicating that each
conformer was a minimum energy structure.17 Regardless of the computational accuracy,
41
Figure 3.2. The microwave spectrum of methyl vinyl ketone (MVK). The inset
demonstrates the signal-to-noise achieved with the CP-FTMW spectrometer. The dashed
line in the inset indicates the average peak height of isotopic species measured in natural
abundance for ap-MVK.
42
Table 3.1. List of JB95a parameters used to fit the methyl vinyl ketone conformers.
43
Table 3.2. List of XIAMa parameters used in the global fit of the methyl vinyl ketone
conformers.
44
Table 3.3. Comparison of rotational data, dipole moment, and relative energies for
experimental, ab initio calculated, and published values for both conformers of methyl
vinyl ketone.
45
both theory and IR data offer strong evidence that the sp conformer should be observed in
the MVK microwave spectrum.
After assigning all possible lines to ap-MVK, unassigned doublets up to three
orders of magnitude above the noise floor were attributed to sp-MVK. Based on the A-E
splitting of each doublet, the V3 barrier was expected to be similar but slightly lower than
that of ap-MVK. This was anticipated since steric hindrance to internal rotation is
increased when the methyl and methylene groups are in close proximity as in the ap
conformation. A relaxed potential energy scan (RPES) about the methyl torsional
coordinate at the HF level of theory (6-311++G(d,p)) predicted V3 values of 375 and 398
cm-1 for sp- and ap-MVK, respectively. DFT calculations (B3LYP/6-311++G(d,p))
underestimated the barrier of the known ap conformer by nearly 50% and predicted apMVK to be the higher barrier configuration (Table 3.3). Using the HF results, initial
rotational constants for the A- and E-internal rotation species of sp-MVK were generated
from Equations 2 and 7. The optimized geometry of sp-MVK was used to predict the
unperturbed rigid rotor rotational constants as well as F and ρ in a method described by
Pitzer.19 The reduced barrier parameter, s , was determined from F and V3 (Equation 9),
and perturbation coefficients were estimated from Herschbach’s tables.11 By simulating
the spectrum with the perturbed rotational constants, ambiguity in the relative frequency
order of the A-E symmetry doublets was eliminated in the early stages of the fit. The fit
parameters of sp-MVK from JB95 and XIAM are also reported in Tables 3.1 and 3.2,
respectively.17
The internal rotation analysis in JB95 gave a V3 barrier height of 435(6) cm-1 for
ap-MVK with our data set, and 433(4) cm-1 when high frequency transitions from Refs. 5
and 6 were included in the fit. The most precise V3 barrier height, 433.8(1) cm-1, was
obtained with the combined data in XIAM by fixing FR to 5.3 cm-1 and floating V3 and
the angle between the top axis and a-principal axis, θ a . A direct comparison with
previously reported results (Table 3.3) is not straightforward, since the assumptions
regarding the reduced moment, F , are different in all three models. The V3 barrier of
424(7) cm-1 reported by Fantoni et al.6 was obtained by fixing the reduced moment, F .
46
In this study, only I φ was fixed, allowing the structural variables of F to be varied in the
fit. Similar assumptions were made by Foster et al. who obtained a more directly
comparable V3 barrier of 437(7) cm-1 with a slightly smaller fixed value of I φ (3.164
μÅ2).5
The V3 barrier height of sp-MVK was determined to be 375(5) cm-1 in JB95.
Including 4th order corrections into the effective Hamiltonians to account for the lower
barrier did not improve the precision of V3 . It is anticipated that incorporating data from
higher J transitions is necessary to reduce the uncertainty with this method. A value of
376.6(2) cm-1 was obtained with XIAM by floating the internal rotation parameters
FR , θ a , and V3 . In contrast to ap-MVK, the quality of the fit increased 5-fold by
varying FR . The variables FR and V3 are highly correlated, particularly in high barrier
cases. It is suspected that due to the lower barrier of sp-MVK, more information is
carried in the FR parameter and thus it was necessary to vary in the fit. Tables 3.1-3.3
summarize internal rotation parameters from this study, calculations, and literature.
The high signal-to-noise in the MVK rotational spectrum made it possible to
resolve isotopic species in natural abundance, particularly carbon-13 substituted isotopes
(1.1% natural abundance). The unperturbed rigid rotor constants of the isotopologues
were calculated with the same computational accuracy as the unsubstituted parent
species, and rotational constants for the A- and E-internal rotation species were predicted
with Equations 2 and 7 assuming the same barrier parameters as the parent conformers.
The perturbed rotational constants were then scaled by the difference between the
calculated and experimental values of the parent species, increasing the accuracy of the
isotopically substituted predictions by 1-2 orders of magnitude. The a- and b-type lines of
the 13C isotopes of ap-MVK were appreciable in intensity (Table 3.3) resulting in 15-17
line assignments per internal rotor state. Owing to the large signal of the parent species,
10 A-state lines of 18O substituted ap-MVK were also detected. While there were several
suspected E-state transitions, a reliable set of parameters was not obtained.
The sp-MVK rotational spectrum, in contrast, consists primarily of a b-type
transition moment (Table 3.3). Consequently, the low intensity a-type lines of the sp-
47
MVK
13
C isotopologues were difficult to detect; only 10-13 lines were assigned per
internal rotor state. We were unable to assign 18O substituted transitions due to the lower
signal intensity of the parent species and the relative abundance of this isotope (0.2%
natural abundance). The quantum number assignments of the A-E symmetry doublets
were confirmed by noting the similar magnitude of doublet splitting with respect to the
parent conformer. In general, it was only possible to float combinations of several
distortion constants in the fits of the isotopologues. Rotational constants and 4th order
distortion terms resulting in the lowest residual error are given in Table 3.4.
With heavy atom substitution data, structural information on the frame of apMVK and carbon backbone of sp-MVK was acquired through Kraitchman analysis.20
Vibrational contributions from the methyl torsion contaminate both A- and E-state
effective rotational constants. In order to obtain the most accurate substitution structures
(rs), these effects may be removed by calculating the unperturbed rigid rotor constants
from Equations 2 and 7 in the iterative process of Lavrich et al.14 Since the Db crossterms were not well-determined due to the low J transitions in the fits of the isotopically
substituted species, an approximate expression for the rigid rotor constants was used
instead:21
approx
ARR
=
1
(AA + 2 AE ).
3
(12)
approx
A similar relation holds for BRR
. To account for small differences on the order of 1 kHz
between C A and C E , the average was taken to calculate C RR . Equation 12 is an
approximation that neglects denominator corrections accounting for the small differences
in the rotational energy levels after Van Vleck transformations. However, a systematic
analysis on the equilibrium structure of cis-methyl formate ( V3 = 372.67 cm-1) found that
inclusion of denominator corrections induced a negligible effect on the value of the
unperturbed rigid rotor constants.21
The errors of the rs structures in Table 3.5 are propagated from the rigid rotor
constants, although the calculated precision is greater than the zero-point fluctuations of
the molecule. Small changes in the pseudoinertial defect (the difference between the
equilibrium and effective principal moments estimating the vibration-rotation
Table 3.4. Isotopic assignments for methyl vinyl ketone.
48
49
Table 3.5. Comparison of the molecular structure according to ab initio
predictions and Kraitchmana analysis.
50
contribution to rs) upon isotopic substitution lead to systematic errors in the rs
coordinates.22 Corrections with Costain’s method lead to more physically meaningful
uncertainties that are comparable with calculated equilibrium structures.23,24 The adjusted
error in Table 3.5 is given by δas/Å = 0.0015/|as|, where as is the Kraitchman coordinate.
The larger Costain errors observed in the substitution structures involving the C2 and C4
atoms of both conformers are a consequence of the close proximity of the atoms to the aprincipal axis (0.06 to 0.1 Å).23 Costain’s results give errors less than 0.01 Å when
coordinates are greater than 0.15 Å from the principal axis; it may be possible to reduce
this uncertainty by double-isotopic substitution.25
Calculated and observed bond lengths and angles generally concurred at all levels
of theory in this study with the differences between calculated and experimental values
being attributed to the modest basis sets used (Table 3.3). Without E-state rotational
constants for the
18
O substituted ap conformer, rigid rotor constants could not be
determined. Because of the high barrier, rotational constants for the A-internal rotation
species are expected to be within 0.1% of the rigid rotor values. For these reasons,
18
O
substituted coordinates were included in the Kraitchman analysis and are in reasonable
agreement with theory. One notable disagreement between calculated and experimental
values is in the rC1C2 methyl rotor axis bond of ap-MVK. At all levels of theory, the
predicted rC1C2 bond length is typical for a carbon-carbon single bond. Within Costain’s
error, the experimental bond length is shorter by nearly 2 pm. Ab initio calculations less
accurately predicted the V3 barrier associated with the RPES about this bond, suggesting
the deviations from experimental values are correlated.
It has been suggested that a non-planar species of MVK exists,7 but a RPES
conducted about the C2-C3 bond did not reveal any additional minima. Though low in
intensity, approximately 550 lines were unassigned in the spectrum with many appearing
to be A-E symmetry doublets. We attempted to find vibrationally excited satellite states
corresponding to skeletal and methyl torsional motion using the rotational constants of
Fantoni et al.6 No such lines were found. Efficient vibrational cooling in the supersonic
jet diminishes population in these low energy modes and accordingly we do not generally
observe vibrational satellites with our spectrometer. From the suspected torsional
51
splitting of the unidentified peaks, dimerization or other clusters with the backing gas in
the supersonic jet may account for the remaining lines in the spectrum.
The microwave spectrum of methyl vinyl ketone was measured using a CPFTMW spectrometer. In addition to the previously observed ap-MVK conformer, a
second conformer, sp-MVK, was assigned. A torsional analysis in JB95 gave V3 barriers
of 433(4) and 375(5) cm-1 for ap- and sp-MVK, respectively, with the former barrier
including previously measured high frequency transitions. The internal rotor program
XIAM was used to determine more precise barriers of 433.8(1) and 376.6(2) cm-1 for apand sp-MVK, respectively. The observed differences in precision were found to be a
reflection of the models used to interpret the data rather than the resolution of the
measurements. The high signal-to-noise of the CP-FTMW spectrometer resolved MVK
isotopologues in natural abundance. Kraitchman analysis determined the molecular
structure using approximate rigid rotor rotational constants. Ab initio calculations were
also completed that resulted in several discrepancies with the experimental data
concerning relative energies, barrier heights, and structural parameters, particularly for
the ap conformer.
On a final note, the high resolution, high signal-to-noise, and broad bandwidth of
the CP-FTMW spectrometer makes it an ideal apparatus for obtaining substitution
structures of carbon-based gas phase molecules. By signal averaging, detection of
isotopologues in natural abundance requires no additional searching for these transitions
in the measurement process, nor does it necessitate the synthesis of additional molecular
samples. Though weaker in intensity, the improved predictions for isotopic rotational
constants significantly expedite the assignment process of low intensity transitions and
hence the additional transitions are efficiently identified in a single broadband spectrum.
52
References
(1)
Tuazon, E. C.; Atkinson, R. Int. J. Chem. Kinet. 1990, 22, 1221-1236.
(2)
Kamens, R. M.; Gery, M. W.; Jeffries, H. E.; Jackson, M.; Cole, E. I. Int. J.
Chem. Kinet. 1982, 14, 955-975.
(3)
Pierotti, D.; Wofsy, S. C.; Jacob, D. J. Geophys. Res. 1990, 95, 1871-1881.
(4)
Tuazon, E. C.; Atkinson, R. Int. J. Chem. Kinet. 1989, 21, 1141-1152.
(5)
Foster, P. D.; Rao, V. M.; Curl, Jr., R. F. J. Chem. Phys. 1965, 43, 1064-1066.
(6)
Fantoni, A. C.; Caminati, W.; Meyer, R. Chem. Phys. Lett. 1987, 133, 27-33.
(7)
Bowles, A. J.; George, W. O.; Maddams, W. F. J. Chem. Soc. B 1969, 810-818.
(8)
García, J. I.; Maryoral, J. A.; Slavatella, L.; Assfeld, X.; Ruiz- López, M. F. J.
Mol. Struct. 1996, 362, 187-197.
(9)
Shirar, A. J.; Wilcox, D. S.; Hotopp, K. M.; Storck, G. L.; Kleiner, I.; Dian, B. C.
J. Phys. Chem. A 2010, 114, 12187-12194.
(10)
Plusquellic, D. F.; Suenram, R. D.; Maté, B.; Jensen, J. O.; Samuels, A. C. J.
Chem. Phys. 2001, 115, 3057-3067.
(11)
Herschbach, D. J. Chem. Phys. 1959, 31, 91-108.
(12)
Kleiner, I. J. Mol. Spec. 2010, 260, 1-18.
(13)
Durig, J. R.; Little, T. S. J. Chem. Phys. 1981, 75, 3660-3668.
(14)
Lavrich, R.; Plusquellic, D.; Suenram, R.; Fraser, G.; Walker, A.; Tubergen, M. J.
Chem. Phys. 2003, 118, 1253-1265.
(15)
Hartwig, H.; Dreizler, H. Z. Naturforsch 1996, 51a, 923-932.
(16)
Gillies, J.; Gillies, C.; Grabow, J.-U.; Hartwig, H.; Block, E. J. Phys. Chem. 1996,
100, 18708-18717.
(17)
Information., Assigned rotational transitions and calculated vibrational
frequencies for ap- and sp-MVK are given in the Appendix A.
(18)
Frisch, M. J.; et al. Gaussian 03, Revision C.02; Gaussian, Inc.: Wallingford, CT,
2004.
(19)
Pitzer, K. S. J. Chem. Phys. 1946, 14, 239-243.
53
(20)
Kraitchman, J. Am. J. Phys. 1953, 21, 17-24.
(21)
Demaison, J.; Margulès, L.; Kleiner, I.; Császár, A. J. Mol. Spec. 2010, 259, 7079.
(22)
Harmony, M.; Laurie, V.; Kuczkowski, R.; Schwendeman, R.; Ramsay, D.;
Lovas, F.; Lafferty, W.; Maki, A. J. Phys. Chem. Ref. Data 1979, 8, 619-721.
(23)
Costain, C. C. J. Chem. Phys. 1958, 29, 864-874.
(24)
Costain, C. C. Trans. Am. Crystallogr. Assoc. 1966, 2, 157-164.
(25)
Wiedenmann, K.; Botskor, I.; Rudolph, H. J. Mol. Spec. 1985, 113, 186-195.
54
CHAPTER 4: INITIAL DISCHARGE EXPERIMENTS
Introduction
The first three molecular systems studied with the discharge CP-FTMW
spectrometer all offered a unique characteristic to the experimental analysis. The first
molecular system needed to have an established mechanism to test the validity of the
experiment. 2,3-Dihydrofuran (2,3-DHF) fits this profile and several thermal studies have
detailed decomposition products at a range of temperatures.1,2 Theoretical calculations
have also been completed on the two dominant decomposition mechanisms, hydrogen
elimination3 and isomerization.4 Although the theoretical study by Dubnikova and
Lifshitz4 discusses which conformers should be observed, these conclusions have not
been confirmed experimentally. Knowing the expected products of 2,3-DHF reactions
allowed the discharge experiment to be justified as an expedient method for product
identification and also presented the opportunity for disagreement. A CP-FTMW
discharge spectrum has confirmed the existence of multiple conformers and new
conclusions have been made.
2,5-Dihydrofuran (2,5-DHF) was the second molecule studied because of its
relationship to 2,3-DHF. The two isomers have been shown to have different
chemistry2,3,5 despite their similar structures and there was interest to ascertain whether
this precedent would continue in a discharge environment. Decomposition of 2,3-DHF
predominantly proceeds via a ring opening mechanism followed by isomerization to
cyclopropanecarboxaldehyde,2 whereas 2,5-DHF primarily undergoes dehydration to
furan.5 The microwave spectra clearly indicates the differences between the discharge
reactions of 2,3- and 2,5-DHF.
Once these unimolecular reactions had been studied, another system was found to
study bimolecular reactions. 1,3-Butadiene (1,3-BD) was chosen because dissociation
55
reactions are known6,7 and the mechanisms for oxidation have been extensively studied.810
Two different discharge experiments were conducted for this molecule: the simple
decomposition of 1,3-BD and the bimolecular reaction of 1,3-BD and molecular oxygen
(O2). The introduction of oxygenated species in the second experiment clearly affected
the discharge spectrum. Additionally, 1,3-BD has no permanent dipole moment and
resulted in a zero-background experiment.11,12 For this system, the parent molecule was
not observed with the microwave spectrometer and rotational transitions only appeared
when the electrical discharge was activated and new products were generated.
Experimental Setup and Assignments
All of the molecules were obtained from Aldrich (2,3-DHF, 99%; 2,5-DHF, 97%;
1,3-BD, +99%). For each molecule, a saturated sample tank was made using a freezepump-thaw method. The tanks were filled with Ar and resulted in a final concentration of
approximately 2% of the parent molecule. An equivalent amount of oxygen was added
for the 1,3-BD bimolecular reaction to obtain a final 1,3-BD/O2/Ar molecular
concentration of 2%/2%/96%. Both microwave circuits were used to generate the
polarizing pulses for these discharge spectra. For the 2,5-DHF and the oxygenated 1,3BD experiments, the initial pulse was mixed with 13 GHz. The quadrupler circuit was
used for the 2,3-DHF and 1,3-BD molecules.
After the initial discharge spectrum of 2,3-DHF was obtained, additional
experimentation was conducted by varying parameters to gain an understanding of the
discharge environment. Several discharge spectra were recorded with different
concentrations of 2,3-DHF resulting in signal intensities increasing with higher density
sample tanks. Therefore all gas tanks in the final analysis were saturated with the parent
molecule. The positive and negative voltage channels of the discharge were varied
independently over a large range of values; no significant dependence was observed for
signal intensities. The voltages were set at +300 V and -500 V, which remained constant
for all experiments. Lastly, the spacing between the electrodes was tested at distances of
56
1, 1.5 and 2 mm. A 1.5 mm spacer was determined to produce the best discharge
spectrum and was used consistently throughout the study.
The rotational free inductive decay was digitized for 10 μs, yielding a nominal
experimental resolution of 40 kHz. The ground state spectrum of each parent molecule
was subtracted from its respective discharge spectrum to observe only newly generated
rotational lines. The resolution was improved to 4 kHz by interpolating the spectrum in
JB95,13 which assisted in determining accurate frequencies for rotational transitions.
Experimental frequencies that differ from known values by more than 40 kHz are due to
distorted line shapes. These lines represent experimentally unresolved transitions arising
from methyl rotor torsional barriers or highly prolate molecules. Full lists of assigned
molecules for these discharge experiments are listed in Appendixes B-D. Ab initio
calculations were performed using Gaussian0314 density functional theory with the basis
set B3LYP/6-31++G(d,p). Zero point corrected energies were calculated for the multiple
conformational isomers of crotonaldehyde and vibrational frequencies were used to
verify the stability of each conformer. A relaxed potential energy scan was conducted to
determine the potential energy surface for the torsion between cis and trans
cyclopropanecarboxaldehyde (CPCA). The barrier height was determined as well as the
difference in energy between the two conformers.
The presence of oxygen in an electrical discharge environment will introduce
several additional species compared to a purely hydrocarbon discharge. Discharge
experiments by Paulson et al.15 used molecular oxygen to generate many species of
oxygen: O(3P), O(1D), O-, O+, and O2-. Furthermore, this experiment also observed
secondary reactions that formed O3. This chemistry is particularly influential in the 1,3BD/O2 experiment. However, even the presence of oxygen on molecules like 2,3- and
2,5-DHF will initiate similar chemistry. Several of the mechanisms mentioned here
explaining the presence of certain molecules will include some of these exotic oxygen
species.
57
2,3-Dihydrofuran
Previous Experiments
Two different pyrolysis experiments of 2,3-DHF have been reported in the
literature. In the first study, both cyclopropanecarboxaldehyde (CPCA) and
crotonaldehyde molecules were identified by their melting points at various temperatures
(670-820 K).1 In the late 1980s, Lifshitz et al.2 revisited this study of 2,3-DHF pyrolysis
behind reflected shocks in a single-pulse tube at increased temperatures (up to 1300 K).
Major reaction pathways and rate constants for certain unimolecular reactions were
determined and additional pyrolytic products were detected. Identification of products
was derived from retention times and GC/MS, but the authors could not distinguish
between individual conformers. Contrary to experimental results obtained for
tetrahyrofuran16, furan17 and 2,5-dihydrofuran5, Lifshitz discovered that the major
reaction path of 2,3-DHF was a unimolecular isomerization process yielding CPCA.
Another favorable unimolecular isomerization to form crotonaldehyde from CPCA at
higher temperatures was also proposed, but the reaction rate could not be determined
because of incomplete separation between GC signals.
According to theory and experiment, the main thermal reaction of 2,3-DHF is the
furan ring opening to form two isomerization products, CPCA and crotonaldehyde. The
original experimental studies2,18 focused on determining products, but were unable to
identify any contributions from specific conformers. Later, density functional theory
calculations by Dubnikova and Lifshitz4 proposed that both CPCA and crotonaldehyde
are formed by unimolecular isomerization processes proceeding via a concerted
mechanism with a single transition state. These calculations follow a specific reaction
coordinate that leads exclusively to the cis conformer for each reactant.
58
Discharge Spectrum
The majority of the data pertaining to this discharge spectrum of 2,3-DHF has
been published.19 and is located at the end of this thesis. The molecules initially identified
were cis and trans cyclopropanecarboxaldehyde (CPCA), trans-trans crotonaldehyde, cis
and trans acrolein, formaldehyde, propene, propyne and cyclopropenylidene. Since that
publication
three
additional
molecules
have
been
assigned:
1,3-pentadiyne,
vinylacetylene and propynal. The discharge spectrum of 2,3-DHF is presented in Figure
4.1 and a list of molecules showing the conformational structure is given in Figure 4.2.
Approximately 65% of the integrated intensity has been accounted for by the
twelve assigned molecules. There are nearly 80 lines that remain unassigned in this
discharge spectrum. This spectrum contains several lines resulting in doublet peaks,
which implies that these lines are due to molecules containing a methyl rotor. Two trios
of lines at 14.0 and 18.2 GHz appear to have k-stack symmetry indicative of a prolate
molecule. However, it is uncertain whether these trios derive from the same molecule or
several different species. There is a significant peak at 16828 MHz that remains
unassigned despite extensively searching the online databases from the Jet Propulsion
Laboratory20 (JPL) and the Cologne Database for Molecular Spectroscopy21 (CDMS).
Through other experiments involving pure hydrocarbon species, we have determined that
this molecule is a hydrocarbon. It is a single line at high frequency, which suggests a
small molecule. It has been observed in several of the discharge spectra discussed here
and appears nearly as often as cyclopropenylidene. To further characterize this molecule,
it would be beneficial to conduct double resonance IR experiments to identify any
functional groups present.
The discussion here begins with reiterating the analysis given in the article,19 then
extends the discussion to the new assignments. The mechanisms to form CPCA and
crotonaldehyde are isomerization reactions from the parent molecule. The production of
CPCA is initiated by the cleavage of the O1-C2 bond of 2,3-DHF (1). Though theory4
(1)
59
Figure 4.1. Comparison of the discharge spectrum of 2,3-dihydrofuran obtained from CPFTMW spectrometer (top) and known frequencies (bottom).
60
Figure 4.2. List of molecules identified in the discharge spectrum of 2,3-dihydrofuran.
61
only predicts the formation of cis-CPCA, both cis and trans conformers were identified
on our spectrum. Density functional theory calculations of CPCA determined that the
torsional barrier is 26.22 kJ/mol and the trans conformer is 0.68 kJ/mol more stable than
the cis conformer. Microwave work by Volltrauer and Schwendeman22 confirms that
stable forms of cis and trans CPCA exist with an energy difference of 0.121 kJ/mol and a
barrier height of 18.4 kJ/mol.
In regards to crotonaldehyde, the problem is more complicated. The energies and
vibrations of the four possible structures have been calculated (B3LYP/6-31++G(d,p))
and the zero point corrected energies are presented in Figure 4.3. A recent quantumchemical study by Bokareva et al.23 calculated structures and torsional barriers of acrolein
derivatives and their labeling scheme of crotonaldehyde was adopted here (d- indicates
isomers relating to the carbon double bond). It can be seen in Figure 4.3 that both the dtrans conformers are more stable than either d-cis conformer. Dubnikova and Lifshitz4
predicted that 2,3-DHF proceeds directly to the s-cis conformer of d-cis-crotonaldehyde.
In the discharge spectrum, only the trans conformer of d-trans-crotonaldehyde was
recorded. In order to convert from the predicted theoretical structure to the
experimentally observed structure, two separate torsions would need to be applied, to the
aldehyde group and also around the carbon double bond. The torsional barrier for the
aldehyde group is fairly straightforward to calculate and Bokareva et al.23 published
results of ~25 kJ/mol for d-cis conformers and ~36 kJ/mol for d-trans conformers.
Experimentally, only the d-trans molecules have been studied and vibrational analysis24
determined this torsional barrier to be ~68 kJ/mol. Unfortunately, calculating the torsion
around the double bond is much more difficult and has not been studied in
crotonaldehyde. However, many studies involving the double bond in stilbene have been
conducted and the barrier to cis-trans isomerization in stilbene25 is ~187 kJ/mol. This
barrier height gives an estimate of the possible barrier to convert from the cis form of dcis-crotonaldehyde to the trans form of d-trans-crotonaldehyde. Once crotonaldehyde has
been created in the discharge, it likely contains an excess of internal energy that allows
for torsion around a double bond to relax into a lower energy structure.
62
Figure 4.3. Relative zero point corrected energies for the four conformers of
crotonaldehyde. Conformers related to the aldehyde group are identified by d- and srespectively. The lowest energy structure, the s-trans conformer of d-transcrotonaldehyde, is set at 0 kJ/mol.
63
A purely trans assignment was confirmed by obtaining the pure rotational spectrum of cis
and trans crotonaldehyde using our spectrometer system. In the region of 14 GHz of the
discharge spectrum, a feature similar to cis crotonaldehyde appeared, but we were unable
to reasonably fit these transitions. Splitting on K1 bands of these transitions does suggest
that this unassigned molecule has a methyl rotor. The presence of these additional
conformers suggests that the discharge products contain enough internal energy to
isomerize to these conformers subsequent to dissociation. This excess energy could be
due to the discharge itself, or collisions with other excited species. Though we are unable
to determine the origin of this additional energy, the presence of additional conformers
implies the discharge environment provides enough energy to initiate numerous
reactions.
In addition to the aldehydes, both propyne and propene were observed. Several
studies
2,26
propose that propyne is produced from either propene or 2,3-DHF ring
opening. To form propyne directly from 2,3-DHF, a side product of formaldehyde is
necessary (2). The authors were unable to verify this reaction as no formaldehyde was
(2)
observed in the post-shock samples. Unlike those experiments, a strong formaldehyde
signal exists in our discharge spectrum. We are unable to distinguish between the two
mechanisms, but there is now evidence to support direct formation of propyne from 2,3DHF. Additionally, it has been proposed that propene is formed by secondary thermal
decomposition of the aldehyde products instead of a direct decomposition of 2,3-DHF.2
The kinetics derived in the experiment did not agree with a single-step unimolecular
reaction. However, such conclusions cannot be drawn from our data set without
performing isotopic substituted experiments of 2,3-DHF.
Both cis and trans conformers of acrolein have been observed in this discharge
spectrum. Several studies have studied the reaction of allyl radical with atomic oxygen
and observed acrolein as a significant product (3). Initially, acrolein was the only
(3)
64
experimentally observed product for this reaction.27 Further research by a different group
identified additional molecular products, but still detected a 47% yield of acrolein.28 The
most recent data is from a theoretical study of the C3H5O potential energy surface by
FitzPatrick and included conformational differences in product structures.29 FitzPatrick’s
work included many different intermediates and his simulation predicted a more
conservative product yield of 28% for acrolein. Though it appears that the allyl reaction
with O (3P) often generates products other than acrolein, it is still a possible pathway in
the discharge. Additionally, acrolein resulted from the decomposition of crotonaldehyde
(4)
(5)
by reactions 4 and 5 proposed by Lifshitz et al.30 Simply based on statistics, it is much
more likely that acrolein is formed from decomposition of crotonaldehyde rather than a
bimolecular reaction between an oxygen atom and propargyl radical. Two different
conformers of acrolein have been identified in the discharge spectrum. According to
UV31-33 and Raman34 spectroscopy, the trans conformer is lower in energy than cis by
6.98-9.21 kJ/mol. To further understand the chemistry of these two conformers, the 2,3DHF discharge spectrum was compared to the acrolein ground state spectrum taken with
our CP-FTMW instrument. The relative signal intensities for the conformers are
significantly different in the two spectra. The trans/cis ratio for the 000-101 transition
intensities is 43:1 in the ground state, but 6:1 in the discharge spectrum. This suggests
that the 2,3-DHF discharge is generating a larger amount of cis molecules compared to
the normal ground state spectrum. The barrier to isomerization for these isomers requires
27.99-31.10 kJ/mol (2340-2600 cm-1) of energy,32,33 which could be provided by the
discharge environment.
A product not reported in the pyrolysis study, but assigned in our spectrum, is
cyclopropenylidene (C3H2). Possible precursors to this molecule could be propyne, allene,
acetylene or ethane, which are all significant products of 2,3-DHF pyrolysis.2 Since the
presence of propyne can be confirmed in the discharge, a mechanism initiating from this
molecule is proposed as the primary source of cyclopropenylidene. With the excess of
energy produced in the discharge, propyne can dissociate to form vinylidenecarbene (6).35
65
(6)
Since the most stable conformer of C3H2 is cyclopropenylidene,36,37 vinylidenecarbene
will rearrange to that configuration. Likewise allene, an isomer of propyne, can also form
vinylidenecarbene35 and experiments have shown that allene in a discharge38 generates
cyclopropenylidene. Another possible mechanism begins with a stable cyclopropenyl
cation, C3H3+, that can be created from acetylene and ethane.39-41 Cyclopropenylidene
could then be formed via dissociative recombination39 seen in reaction 7. Unfortunately
(7)
acetylene, ethane, and allene cannot be detected by our CP-FTMW spectrometer due to
their lack of a permanent dipole. The strong signal of this carbene may be due to
contributions from several sources and could be indirect evidence for these molecules.
In the case of vinylacetylene, Lifshitz et al.2 observed this molecule but offered
no direct mechanism from 2,3-DHF. Instead the authors suggested that vinylacetylene
was primarily a decomposition product from 1,3-butadiene (1,3-BD), a reaction which
has been verified by other studies.7,42 The mechanism would proceed via hydrogen
abstraction by a radical such as the methyl radical (8), then further decomposition would
1,3-BD + •CH3 → CH4 + •C4H5 → •H + C4H4
(8)
lead to vinylacetylene. Since 1,3-BD has no permanent dipole moment, we are unable to
verify the presence of this molecule in our discharge, but two separate mechanisms could
lead to its formation. The first mechanism is a recombination of two vinyl radicals (9)
•C2H3 + •C2H3 → C4H6
(9)
and the second process is decomposition from 1-butene that is generated from a methyl
attack of propene (10). There is no evidence for 1-butene in the discharge spectrum of
(10)
2,3-DHF, therefore the first mechanism for 1,3-BD formation is more likely.
66
Another observed product is propynal, which could result from a reaction
involving a propargyl radical (11). Lee et al.43 have studied the various energetic
pathways of the reaction between a propargyl radical and triplet oxygen, O(3P). One of
(11)
the most stable product channels is the formation of propynal and hydrogen, which is 252
kJ/mol lower in energy than the reactants.
The most recently assigned product is the large hydrocarbon 1,3-pentadiyne. A
recent publication by Jamal and Mebel44 gives a reaction pathway to form 1,3-pentadiyne
from possible discharge products. Jamal and Mebel44 conducted an ab initio/RRKM
study of the potential energy surface of C5H5 isomers resulting from a reaction between
propyne and ethynyl radical (12). Of the six isomers studied, 1,3-pentadiyne was a
(12)
primary product with a branching probability of 27-56%.44
2,5-Dihydrofuran
Previous Experiments
Reactions of 2,5-dihydrofuran (2,5-DHF) have also been studied and information
about the products and mechanisms have been reported. Early low temperature
experiments concluded that the main products of thermal decomposition were furan and
hydrogen; no other molecules were observed.45 Later, reactions of Hg photosensitized
2,5-DHF produced primarily propene and hydrogen, but also generated smaller quantities
of furan, 2,3-DHF and tetrahydrofuran.46 A study was also conducted by Lifshitz et al.,5
similar to the 2,3-DHF experiments, that reaffirmed the primary products of furan and
hydrogen observed in the low temperature study. However, Lifshitz et al. were also able
to identify six additional products comprised primarily of hydrocarbon species. They did
not observe any of the large carbonyl products that were the focus of their 2,3-DHF
experiments. Several recent studies have focused on the dehydration of 2,5-DHF to
67
furan,3,47 but there is little current research being conducted on the secondary products of
2,5-DHF reactions.
Discharge Spectrum
The discharge spectrum of 2,5-dihydrofuran (2,5-DHF) and the associated
product list are presented in Figures 4.4 and 4.5 respectively. Despite 2,5-DHF being an
isomer of 2,3-DHF with a similar molecular structure, the discharge spectra are unique.
The spectrum for 2,5-DHF is sparser and fewer molecular products have been assigned:
trans-trans crotonaldehyde, trans acrolein, formaldehyde, propyne, propynal and
cyclopropenylidene. Nearly 70% of the integrated intensity has been accounted for and
10 lines remain unidentified. None of the lines appear to have any distinguishing
characteristics that would give insight to the type of molecules that remain. Previous
research of 2,5-DHF reactions has given insight to some unique product mechanisms that
will be discussed here. On the other hand, it is also likely that some molecules will be
generated in a manner similar to the reactions discussed for 2,3-DHF. To our knowledge,
no previous research of 2,5-DHF has discussed the formation of cyclopropenylidene,
propynal or acrolein. Therefore we will assume that the mechanisms in this experiment
are analogous to those presented in the discussion of 2,3-DHF and will not be repeated.
Only the lower energy trans conformer of acrolein was observed in the 2,5-DHF
discharge spectrum and the signal levels for trans acrolein are smaller compared to 2,3DHF experiment. The lack of cis acrolein in the current spectrum could be for two
reasons; the intensity of the signal may simply be too small to observe or the trans
conformer is being created preferentially.
The first issue to discuss is whether 2,5-DHF in a discharge environment will
dehydrate to furan or undergoe a ring opening. To address the first mechanism, there is
no evidence of furan in the discharge spectrum to support this pathway. Conversely, there
is data to support the second mechanism. A ring opening adjacent to the oxygen would
lead to the formation of propyne and formaldehyde (13), two molecules observed in the
68
Figure 4.4. Comparison of the discharge spectrum of 2,5-dihydrofuran obtained from CPFTMW spectrometer (top) and known frequencies (bottom).
69
Figure 4.5. List of molecules identified in the discharge spectrum of 2,5-dihydrofuran.
70
(13)
discharge spectrum of 2,5-DHF. Analogous reactions have been shown to occur in the
pyrolysis of tetrahydrofuran, which decomposes into propene and formaldehyde.16 2,3DHF also reacts to form propyne and formaldehyde as was suggested by Lifshitz et al.2
and discussed in the previous discharge experiment. Yet in the 2,5-DHF decomposition
analysis by Lifshitz et al.,5 the authors determined that the molecule was more likely to
form propene and carbon monoxide due to the propene:propyne product ratio of 4:1.
There is no peak in the discharge spectrum associated with propene, so Reaction 13
would be the dominant mechanism in the discharge. A ring opening mechanism may also
lead to other products. The mechanism for crotonaldehyde (CA) was proposed by Francis
et al.46 who studied mercury photosensitized decomposition of 2,5-DHF. The excited 2,5DHF undergoes a ring opening at the oxygen to create a biradical that rearranges to form
3-butenal (14). As the 3-butenal is still excited, it will further react and isomerize to
crotonaldehyde (15).
(14)
(15)
There are several differences between the 2,3- and 2,5-DHF discharge spectra that
should be pointed out. It was mentioned earlier in the discussion that while both acrolein
conformers were observed in the 2,3-DHF experiment, only the trans conformer was
detected in the 2,5-DHF discharge spectrum. Another noticeable distinction is the CA
intensity in each spectrum; the largest CA transition is almost 7 times larger in 2,3-DHF.
To generate CA from 2,3-DHF directly, there is a ring opening combined with a C3-C2
hydrogen shift.4 Alternatively, CA can also be generated from CPCA through a ring
opening and similar hydrogen shift.4 Therefore in the discharge spectrum of 2,3-DHF,
there are two sources for CA formation. There is no CPCA in the 2,5-DHF discharge
spectrum and the only known pathway to generate CA from 2,5-DHF is described in
71
Reactions 14 and 15. Having multiple methods to create CA in 2,3-DHF may account for
the increased signal in the discharge spectrum compared to that of 2,5-DHF.
The most significant differentiation between the discharge spectra of 2,3- and 2,5DHF is the presence/absence of CPCA. The propensity to generate CPCA is determined
by the structure of the parent molecule and can be attributed to the asymmetry of 2,3DHF. For 2,5-DHF, the experimental O1-C2 and C5-O1 bond lengths are identical at
1.43 Å and perfectly match calculated values (B3LYP/cc-pVDZ).3 The same bonds in
2,3-DHF are calculated to be 1.45 Å for O1-C2 and 1.37 Å for C5-O1. The experimental
values for 2,3-DHF are more symmetric, 1.43 Å and 1.40 Å respectively, and more
closely resemble the bond lengths in 2,5-DHF.3 The symmetric C-O bonds in 2,5-DHF
lead to an equal probability of ring opening at either bond. However the shorter, and
therefore stronger, C5-O1 bond in 2,3-DHF is less likely to be susceptible to cleavage.
However this difference in bond length is small and not the only factor driving
isomerization. The location of unsaturated carbons in the ring is the main element
determining the formation of CPCA. In 2,5-DHF a C3-C5 hydrogen shift would be
necessary to create CPCA; no such mechanism is necessary in 2,3-DHF.
1,3-Butadiene
Previous Experiments
Early experiments of 1,3-butadiene focused on decomposition reactions, but there
was controversy over the initial process. The first proposed mechanism described an
initial dissociation into two radicals (16) that would further react to form ethene and
1,3-BD → 2•C2H3
(16)
acetaldehyde.48 However a model based on this singular theory overestimated the
formation of ethene by 50%.6 A second theory emerged that suggested 1,3-BD first
isomerizes into 1,2-butadiene (1,2-BD) and then dissociates (17). Adding this reaction to
1,2-BD → •CH3 + •C3H3
(17)
72
models to account for large CH4 signals resolved many differences between laboratory
results and predicted product ratios.7 Two separate isomerization pathways have been
explored, a difficult C3-C1 H-shift or a two-step process involving a C2-C1 H-shift to
form a vinylidene (18). The latter mechanism is more favorable, but deuterium
(18)
scrambling from isotopically labeled 1,3-BD pyrolysis experiments concluded that a
mixture of the two pathways occurs.42 The isotopologue study also determined that the CC single bond in 1,3-BD is unusually strong due to the conjugated system of the
molecule, which leads to a more energetically favorable rearrangement instead of a
cleavage at the C-C single bond. In addition to decomposition experiments, 1,3-BD has
also been studied due to its importance in interstellar chemistry and the formation of
polycyclic aromatic hydrocarbons (PAHs). These large PAH species, such as
ethylbenzene and 3-phenylpropyne, are generated from the recombination of radicals in a
1,3-BD discharge environment.49
The presence of 1,3-BD in the atmosphere has also led to oxidation studies using
the hydroxyl radical, ozone or NOx species (x = 0, 1, 2). The first detailed kinetic study
was published by Laskin et al.9 in 2000 and continues to agree with experimental data.50
When reacting with a hydroxyl radical, there are four reactions around which
mechanisms are constructed (19-22). Two channels involve the addition of the hydroxyl
(19)
(20)
(21)
(22)
radical and the other two channels involve a hydrogen abstraction of 1,3-BD to form
water. Several studies have been conducted to determine the primary channel. It has been
determined that 87% of reactions involving 1,3-BD and •OH will proceed through
73
reaction 19,51 which is also the most energetically favorable.10 Furthermore, recent
research has been questioning whether any isomer-specific chemistry is occurring.52,53
The first isomeric study by Greenwald et al.52 focused on demonstrating the
experimental procedure to generate the initial β-hydroxyradical seen in Reaction 20. This
was assumed to be the minor hydroxyl addition channel for 1,3-BD since the αhydroxyradical in Reaction 19 is more stable by 25.08 kJ/mol.52 The kinetic study
determined that a significant portion of the β-hydroxyradical population quickly
(23)
isomerized via cyclization to α-hydroxyradicals (23). Another study by Ghosh et al.53
focused on the main addition channel (19) and observed a new isomeric species that will
form 4-hydroxy-2-butenal.
There has been general agreement on the yields of the primary oxidation products:
55-64% acrolein,51,54,55 64% formaldehyde51,55 and 21-23% 4-hydroxybutenal.51,55 With
this consensus, the percentage of carbon accounted for has increased from 60% as of
19998 to 80-90% in 200755 and 2010.51 Furthermore, experiments involving ozone rather
than the hydroxyl radical do not differ in their product formation.8
Discharge Spectra
Due to the fact that 1,3-BD has no permanent dipole moment, the discharge
experiment is a zero background process. For this reason, we are unable to verify which
conformer is present, trans or cis 1,3-BD. However, it is assumed that trans 1,3-BD is the
dominant species since cis 1,3-BD is 11.7 kJ/mol higher in energy.10 As discussed earlier,
several experiments have confirmed that 1,3-BD often undergoes isomerization to 1,2BD. Therefore it is possible that both molecules are present in the discharge and
discussions about the reaction mechanisms will include both molecules. The discharge
spectrum for 1,3-BD is rather sparse (Figure 4.6), with only a few lines representing the
three identified products: propyne, vinylacetylene and cyclopropenylidene. Though a few
74
Figure 4.6. Comparison of the discharge spectrum of 1,3-butadiene obtained from CPFTMW spectrometer (top) and known frequencies (bottom).
75
lines remain unassigned, 72% of the integrated intensity has been accounted for. These
unidentified peaks have no unusual characteristics to aid in the spectral analysis. None of
the research conducted on 1,3-BD decomposition has discussed the formation of
cyclopropenylidene and no reactions have been proposed. However, the mechanisms for
cyclopropenylidene formation described in the discussion of 2,3-DHF would apply here.
Vinylacetylene
has
been
observed
in
several
1,3-BD
decomposition
experiments.7,42,56 The mechanism proposed by Hidaka et al.7 involves two successive
hydrogen abstractions (24). The first abstraction could be initiated by either a methyl
1,3BD + •CH3 → CH4 + •C4H5 → •H + C4H4
(24)
radical or propargyl radical; both species are likely present in the discharge. This
mechanism was confirmed later by Chambreau et al.42 in an isotopic study of 1,3-BD.
Simultaneous hydrogen abstraction is also possible via 1,1-H2 elimination from 1,3-BD
or 1,2-BD, but the energy barriers to initiate these reactions are significantly high, 360
kJ/mol and 375 kJ/mol respectively.57
In the case of propyne, several different pathways have been proposed. Propyne is
generated directly from 1,3-BD via a reaction with a hydrogen radical (25).9 A similar
(25)
reaction occurs with 1,2-BD or 2-butyne.7 Another likely source of propyne is the
propargyl radical, which will recombine with a hydrogen radical (26).6,58 It is also
(26)
possible for 1,3-BD to undergo a hydrogen abstraction by a propargyl radical to generate
propyne, but this is less likely to occur.42 Isomerization reactions between propyne and
allene also need to be considered, but Laskin et al.9 determined that propyne
concentration was twice as large as allene. Lastly, propyne is generated from hydrogen
abstraction of propene.6 However no propene was observed in the spectrum and
discredits this mechanism as a viable source in the discharge.
The discharge spectrum of 1,3-BD in the presence of molecular oxygen is
presented in Figure 4.7. This spectrum contains the same hydrocarbon species observed
in the previous 1,3-BD experiment (see Figure 4.8) and the amount of observed
76
Figure 4.7. Comparison of the discharge spectrum of 1,3-butadiene and O2 obtained from
CP-FTMW spectrometer (top) and known frequencies (bottom).
77
Figure 4.8. List of molecules identified in the discharge spectra of 1,3-butadiene.
78
hydrocarbon species is reduced by 50% when molecular oxygen was introduced to the
discharge environment. Three oxygenated compounds have been assigned in the 1,3BD/O2 spectrum: trans-trans crotonaldehyde, trans acrolein, and formaldehyde. The six
identified molecules are responsible for nearly 80% of the integrated intensity. Only five
significant lines remain unassigned in the oxygenated discharge spectrum. Although
these lines do not have any interesting features, one line near 17.2 GHz was also present
in the 1,3-BD discharge spectrum. Therefore we can conclude that this peak is from a
hydrocarbon molecule.
There are two prominent crotonaldehyde mechanisms that have been proposed in
1,3-BD experiments. The first pathway, proposed by Brezinsky et al.,59 involves the
addition of an oxygen atom to the double bond. This process generates a biradical that
will form 2-butenal (27) and isomerize into crotonaldehyde (28). Laskin et al.9 disagreed
(27)
(28)
with this mechanism and offered an alternative pathway in a more recent, comprehensive
model. Laskin et al. included both biradical isomers formed by oxidation of the terminal
carbon and concluded that either 2,5-DHF (29) or vinyloxirane (30) will be formed, not
(29)
(30)
3-butenal. The authors proposed that the decomposition of 2,5-DHF contributed to the
crotonaldehyde signal, a reaction that has been confirmed in our 2,5-DHF discharge
experiment. The pathway to create crotonaldehyde from vinyloxirane is more
complicated. At low temperature vinyloxirane undergoes rapid ring expansion to form
2,3-DHF (31),60 which has a noticeable crotonaldehyde signal in a discharge
(31)
79
environment. Attempting to determine which mechanism occurs in the discharge of 1,3BD is difficult, but certain conclusions can be drawn. There is no evidence of 2,3- or 2,5DHF in the 1,3-BD discharge spectrum, which indicates that pathways including these
molecules are unlikely. Furthermore cyclopropanecarboxaldehyde (CPCA), a signature
product of 2,3-DHF reactions, was not detected in the 1,3-BD discharge spectrum. The
identification of crotonaldehyde also provides a mechanism for other molecules, such as
the formation of acrolein.30 However acrolein is likely generated directly from 1,3-BD
oxidation since it is a major product in several experimental studies.
Unlike crotonaldehyde mechanisms that focused on atomic oxygen as a reactant,
acrolein reactions involve either the hydroxyl radical or ozone. In the case of •OH
mechanisms, nitrous oxide is included in the reaction scheme to more accurately
represent atmospheric conditions.8,51,52,54,55,61-63 Though •OH may attack either the
primary or secondary carbon to form a hydroxy alkyl radical, the reactions follow similar
pathways (32, 33). The molecular oxygen addition to this initial radical creates a peroxy
(32)
(33)
radical that further reacts in the presence of NO to form an alkoxy radical. This unstable
species decomposes to acrolein and also regenerates a hydroxy alkyl radical. It is worth
noting that these mechanisms closely resemble isoprene oxidation discussed later in
Chapter 5. By fitting the kinetics of hydroxyl radical cycling generated in these two
reactions, previous studies have determined that 87% of molecules proceed through
Reaction 32.51 Since these reactions end with identical products, the two pathways are
indistinguishable in the discharge spectrum.
Experiments focusing on 1,3-BD and ozone have also been conducted,8,64 though
not to the same extent as the hydroxyl radical reaction. The central reaction is an ozone
attack of the double bond creating a moleozonide that quickly decomposes to a Criegee
80
(34)
biradical and acrolein (34). Work published by Liu et al.8 concluded that there is little
difference between the reaction of 1,3-BD with the •OH or with O3; both processes
primarily generate acrolein and formaldehyde. Only trans acrolein has been observed in
this discharge spectrum. A discussion of the conformational energies for acrolein was
given in earlier in the discharge of 2,3-DHF.
A significant formaldehyde peak is observed in the discharge experiment because
this molecule appears as a side product of acrolein formation. The end product radicals
•CH2OH + O2 → •HO2 + H2CO
(35)
seen in Reactions 32 and 33 will interact with molecular oxygen to create formaldehyde
(35).54,55 Furthermore if 1,3-BD initially decomposes to two ethyl radicals via Reaction
16, a reaction with molecular oxygen generates formaldehyde (36).9
•C2H3 + O2 → CH2O + HCO•
(36)
Additional Research
There are several additional techniques that could be employed to further
characterize the discharge spectra and identify more products. As mentioned earlier
regarding the unassigned hydrocarbon peak at 16828 MHz, introducing an IR laser
system would give new insight. Considerable knowledge would be attained from being
able to determine the functional group present. For hydrocarbon species, one could
observe the types of bonds present and have a better initial starting point when
conjecturing about a new molecular species. In the case of oxygenated species, knowing
whether a molecule is an alcohol, ketone or aldehyde is extremely beneficial.
Additionally, two-dimensional (2D) microwave techniques could also increase the
amount of information that can be obtained from the discharge spectra. This type of
research is directly analogous to other 2D experiments such as those utilizing NMR
spectroscopy.65 Research in our group has shown that this technique can be applied with
81
our CP-FTMW spectrometer to show the connectivity of rotational energy levels.66 Twodimensional spectroscopy has previously been proven to work for narrowband
microwave pulses,67 but the advancement of technology has enabled our research group
to further apply this method in a broadband microwave spectrum. Using this technique to
analyze the discharge spectrum would disentangle the data and determine which
rotational lines were correlated. Knowing that a particular set of frequencies was assigned
to a single molecule would certainly accelerate the data analysis. However, there are
several factors that are inhibiting the ability to use 2D spectroscopy with the discharge
spectra. One problem is the low signal levels associated with the discharge spectra. Often,
the rotational lines due to discharge products only appear after a significant number of
averages (1000-10000.) Combined with the number of steps necessary for a full 2D
experiment, it would take 120-1200 hours to collect the data. Another problem is the
inherent variability of signal intensities present in the discharge experiment. Due to the
capricious nature of the discharge environment at the pulsed nozzle, the intensity of a
particular rotational line will vary from day to day. 2D experiments depend on signal
intensity fluctuations to determine the connectivity of energy levels and it would be
impossible to determine if the change in signal is due to the microwave pulses or the
discharge nozzle. It is likely that a more stable discharge source could be created and
signal levels could be increased. However there is a more practical issue concerning the
microwave spectra. Most of the molecules generated in the discharge have very few lines
in the bandwidth we collect and these rotational transitions are rarely connected in a way
that allows for detection with 2D spectroscopy. It is likely that no information would be
acquired by using this experimental method with the discharge spectra.
82
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Wang, H.; Frenklach, M. Combust. Flame 1997, 110, 173-221.
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Brezinsky, K.; Burke, E. J.; Glassman, I. In Twentieth Symposium (International)
on Combustion; The Combustion Institute: Pittsburth, PA, 1984.
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Crawford, R.; Lutener, S.; Cockcroft, R. Can. J. Chem. 1976, 54, 3364-3376.
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Tuazon, E. C.; Alvarado, A.; Aschmann, S. M.; Atkinson, R.; Arey, J. Environ.
Sci. Technol. 1999, 33, 3586-3595.
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Baker, J.; Arey, J.; Atkinson, R. Environ. Sci. Technol. 2005, 39, 4091-4099.
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Vimal, D.; Pacheco, A.; Iyengar, S.; Stevens, P. J. Phys. Chem. A 2008, 112,
7227-7237.
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Grosjean, C.; Grosjean, E.; Williams II, E. Environ. Sci. Technol. 1994, 28, 186196.
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Barbet-Massin, E.; Ricagno, S.; Lewandowski, J. R.; Giorgetti, S.; Bellotti, V.;
Bolognesi, M.; Ernsley, L.; Pintacuda, G. J. Am. Chem. Soc. 2010, 132, 55565557.
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(67)
Vogelsanger, B.; Andrist, M.; Bauder, A. Chem. Phys. Lett. 1988, 144, 180-186.
86
CHAPTER 5: DISCHARGE REACTIONS OF ISOPRENE
Introduction
Since the detection of isoprene (2-methyl-1,3-butadiene) in the atmosphere in
1957,1 considerable research has been focused on the reactions of this biogenic molecule.
Currently, global models predict the annual emission rate of isoprene to be 500-750 Tg/yr
with a projected factor of two increase by the year 2100.2 Approximately 1/3 of annual
global volatile organic compound emissions are credited to isoprene and this significant
quantity contributes to the complicated chemistry of atmospheric molecules. For
example, the reaction with the hydroxyl radical is fast and generates a number of
important carbonyls that impact the environment. These atmospheric carbonyl
compounds create free radicals, react quickly with hydroxyl radicals and influence the
formation of ozone.3 Since isoprene is a dominant emission in our atmosphere, it is
essential to understand reactions involving this molecule.
One of the goals of understanding the complex chemistry of isoprene oxidation is
to incorporate reactions into atmospheric simulations, particularly global concentrations
of the hydroxyl radical. Primary and secondary reactions due to isoprene are a vital
component in these calculations since the hydroxyl radical is difficult to directly detect.
Two models in particular are used to compare theory and atmospheric measurements, the
Master Chemical Mechanism (MCM) and the Mainze Isoprene Mechanism (MIM). The
MCM is composed of over 12,500 organic and inorganic reactions including 4000
species.4 Isoprene has been used to test the validity of the MCM model and predicted
concentrations correlated well with atmospheric measurements taken in a southeastern
U.S. site. A condensed and specialized model of isoprene is the MIM, which uses 195
reactions of 68 species; this model is used primarily for regional and global simulations.5
A 2008 hydroxyl radical measurement made over the Amazon forest revealed a
87
discrepancy with these large calculations, and a new mechanism involving •OH recycling
in an isoprene reaction was proposed that had not been included in the previous models.6
Another study,7 which compared eight published isoprene mechanisms, also concluded
that no current model can accurately reproduce the concentrations observed over the
Amazon forest. Though these models are extensive, they are not complete because certain
products and mechanisms are still missing.
The oxidation of isoprene is complex and largely dictated by the oxygen species
present (atomic oxygen, molecular oxygen, ozone, or OH radical.) Each reactant will
engage in unique mechanisms and therefore change the final products and their
concentrations. Research focused on •OH dominates because isoprene is released during
daylight hours when •OH concentration is highest due to UV radiation.8 The kinetics of
this reaction have been thoroughly investigated, yielding a general consensus that methyl
vinyl ketone (MVK), methacrolein (MAC), and formaldehyde are the dominant products
(see Figure 5.1).
Another important atmospheric species of oxygen is ozone (O3), which also reacts
with isoprene. This reaction generates the same primary products discussed above (MVK,
MAC, formaldehyde) but can also lead to formation of •OH chain reactions, reformation
of O3, and generation of new carbonyl compounds.9 Reaction schemes for isoprene and
ozone are used in data replication of smog chamber experiments10 and are included in
global simulations.4 Low concentrations of atomic oxygen in the atmosphere result in less
interest in this particular species. However the chemistry of this reaction should not be
neglected because atomic oxygen is observed in many laboratory experiments that
replicate atmospheric conditions.11
Isoprene oxidation studies focus on the two dominant oxygen species in the
atmosphere, hydroxyl radical and ozone. The main products are methacrolein, methyl
vinyl ketone and formaldehyde, but the distribution of these molecules varies with each
mechanism. Formaldehyde yields in hydroxyl experiments12 range from 31-34% and
dominate in ozone studies with a product yield13 of 90%. There is a nearly even
distribution of MVK and methacrolein in hydroxyl radical reactions12,14 but a slight
88
Figure 5.1 Simplified mechanism of the isoprene and hydroxyl radical reaction. The
primary products are methyl vinyl ketone (MVK), 3-methylfuran (3-MF), methacrolein
(MAC) and formaldehyde.
89
increase in methacrolein concentration is reported in ozone experiments.13 Regardless of
the mechanism driving isoprene oxidation, the same products are generated.
Methacrolein, MVK and formaldehyde account for a large percentage of products, but
several publications include a category to address carbonyls that remain unidentified.
These molecules are responsible for 15-35% of the product, depending on the model
used.12,15-17 Previous studies have not characterized these products any further than
determining functional groups or molecular mass. Microwave spectroscopy is sensitive to
the shape of the molecule, therefore more information can be discovered about these
unidentified carbonyls.
Research surrounding isoprene has primarily involved atmospheric chemistry that
includes both oxygen and nitrogen as principal reactants. However, it is also worthwhile
to understand the unimolecular reactions of isoprene, both its initial dissociation products
and recombination mechanisms. An early isoprene pyrolysis experiment18 detected
several gas-phase thermal decomposition products, although most of the sample formed a
tar consisting of large polyaromatic hydrocarbons such as xylene and styrene. More
recently, Weber and Zhang19 focused on initial decomposition products of isoprene by
combining flash pyrolysis and a supersonic expansion to quench reactions and isolate
products. Molecules were assigned by mass spectrometry and energy calculations were
performed to predict the most likely structural isomers of a given mass. Though many
different hydrocarbon species were identified by mass analysis, the method was unable to
discern which isomers were present as decomposition products.
While much is known about the isoprene reaction scheme, there is still
information to learn about its chemistry. The primary products of isoprene oxidation have
been identified, but there is ambiguity regarding supplemental products. An early FTIR
spectrum of an isoprene/O3 reaction revealed that ~30% of the consumed isoprene was
converted into unknown products.9 A 1990 kinetics study of isoprene and •OH in the
presence of NO2 determined that MVK, MAC, formaldehyde and 3-methylfuran only
account for ~55% of the consumed isoprene.20 Identification of the remaining compounds
is still an active area of study. Research conducted in 2004 categorized ~30% of
unknown product into C5-carbonyl and C4- and C5-hydroxycarbonyls.16 In 2006, a
90
photooxidation study of terpenes assigned unidentified product (~20% of the consumed
carbon) according to mass.21 Many of these newer studies are still assigning products
with mass spectrometry and the probability of multiple isomers within a designated mass
channel is still a substantial limitation.
Motivated by the need to identify the remaining 20-30% of unidentified products
of isoprene oxidation, we present here first results from a chirped-pulse Fourier transform
microwave (CP-FTMW) study of the discharge products formed in an isoprene/O2
mixture. An electric discharge source is an excellent method to generate highly reactive
molecules through Penning ionization and microwave spectroscopy is an ideal detector
for identifying complicated mixtures. The ability to distinguish between not only
structural isomers but also conformational isomers provides a significant amount of
information about the chemistry involved in discharge reactions. Considering the
discrepancies between model predictions and experimental data of isoprene reactions,
identifying new products is still important. The simple unimolecular discharge spectrum
of isoprene allowed understanding of initial hydrocarbon decomposition reactions.
Molecular oxygen was then added in a second experiment to simulate more
environmentally relevant oxygenated species.
Experimental
A description of the discharge source and the CP-FTMW spectrometer has been
previously reported in detail22 and only relevant details will be described here. Isoprene
was purchased from Aldrich (99%) and converted into a gas sample using a freeze-pumpthaw method. Approximately 1.5 mL of isoprene was used to saturate the gas tank and
then filled to 100 psi with Ar carrier gas. For experiments including oxygen, an
equivalent amount of molecular oxygen (commercial grade, 99.5% pure) was added to
the tank to obtain a 1:1 ratio between reactants. The final concentrations for Isoprene/Ar
and Isoprene/O2/Ar were 2%/98% and 2%/2%/96% respectively. The gas sample was
introduced into a vacuum chamber using a discharge nozzle created by combining a
Series 9 General Valve with an extension that housed two aluminum electrodes separated
91
by a 1.5 mm Delrin insulator. Working voltages were set at +300 V and -500 V to create
the discharge pulse.
A 1 μs chirped pulse (1.875-4.625 GHz) was quadrupled and amplified with a 200
W amplifier. The 11 GHz bandwidth pulse (7.5-18.5 GHz) was broadcast into the
vacuum chamber to interact with the molecular sample. A 10 μs free inductive decay
(FID) was collected and the signal was increased using a low noise amplifier. The time
domain signal was recorded and digitized with a 12 GHz oscilloscope in sets of 10,000
averages and a Labview program was used to record multiple sets of averages. The
multiple time domain signals were averaged and a Fourier transform was applied to
generate the 11 GHz broadband discharge spectrum. Initial experimental resolution from
a 10 μs FID was 40 kHz, but interpolation in JB9523 calculated a final resolution of 4 kHz
to more accurately determine the frequency of rotational transitions.
Products in the discharge spectrum were identified using several methods. The
most accurate method referenced individual ground state spectra obtained with our
spectrometer. The rotational lines of several molecules in the discharge were compared
directly to their respective frequencies recorded with our CP-FTMW spectrometer. Other
molecules were determined based on published frequencies. If no published transitions
existed in the appropriate frequency range, previously reported rotational constants were
used in the spectral fitting program JB95 to determine the frequencies of rotational
transitions that fell within our spectral range.23
To understand the chemistry involved with the discharge spectra, theoretical
calculations were needed. Since many of the known products have several stable
conformers, it was important to know which structures were lowest in energy and
therefore most likely to be observed in the discharge spectrum. Structural optimization
calculations were performed to determine the relative energy of possible conformers
present in the discharge. Vibrational analysis confirmed that all frequencies were positive
and identified each conformer as minima. The Gaussian03 suite24 was used to perform all
density functional theory calculations and were carried out at the B3LYP/6-311++G(d,p)
level of theory.
92
Results
The ground state spectrum of isoprene was taken to verify the purity of the
original sample and frequencies recorded with our spectrometer agreed with the
microwave spectrum taken by Lide, Jr. et al.25 Though multiple conformers of isoprene
have been studied,26,27 only the trans conformer was observed with our spectrometer. A
non-planar, gauche conformer exists but is calculated to be 10.2 kJ/mol higher in energy
than the trans conformer26,27 and was not identified in our ground state spectrum.
The discharge spectrum for a 2% isoprene in Ar mixture is presented in Figure 5.2
and six discharge products were identified: propene, propyne, vinylacetylene,
cyclopropenylidene, (E)-trans 1,3-pentadiene and 1,3-pentadiyne. These molecules
account for 61% of the integrated intensity in the discharge spectrum. The first three
species were identified by Weber and Zhang as products in an isoprene pyrolysis
experiment.19 That study also predicted several other species including ethene, acetylene,
methane, allene, and butatriene; however microwave spectroscopy is unable to detect
these products due to their lack of a permanent dipole moment. Previous studies19 have
identified other species in the thermal decomposition of isoprene including the propargyl
(C3H3) and methyl (CH3) radicals. No positive identification of radical species was
observed in our spectrum. This discrepancy between our results and previous studies
could arise from two reasons. Our experiment relies on a pulsed discharge to initiate
molecular dissociation rather than flash pyrolysis used in other work. It is possible that
the local environment of our plasma discharge contains more reactive species than that of
the pyrolysis studies and therefore fewer radicals are cooled intact in our expansion. It is
also possible that small radicals are sufficiently cooled rotationally so that the high J,
high K rotational transitions of small radicals do not appear in our spectral range. The full
lists of assigned molecules for the isoprene experiments are given in Appendix E.
There are approximately 26 lines unassigned in the discharge spectrum of
isoprene. All of these peaks are located in the high frequency end of the spectrum (above
12 GHz), which suggests the presence of small hydrocarbon species. These peaks remain
unidentified despite extensive searching in the literature and spectroscopic databases at
93
Figure 5.2 The discharge spectrum of pure isoprene comparing the experimental data
(top) and the previously known transitions (below). The intensities of the assigned
transitions are set to the values obtained from the experimental data.
94
the Jet Propulsion Laboratory (JPL)28 and the Cologne Database for Molecular
Spectroscopy (CDMS).29 Additionally, many unusual hydrocarbon species have not been
studied with microwave spectroscopy, which has complicated the analysis. The
significant unassigned peak at 16828 GHz was also observed in the oxygenated
experiment and has been labeled as an unassigned hydrocarbon.
To initiate the oxidation of isoprene, molecular oxygen was added. The presence
of oxygen in an electrical discharge environment introduces several new species
compared to a purely hydrocarbon discharge. Isoprene experiments by Paulson et al.30
used a molecular oxygen discharge source to generate many species of oxygen: O(3P),
O(1D), O-, O+, and O2-. Furthermore, Paulson et al. also observed secondary reactions
that formed O3. Although no concentration ratios of the oxygen species were reported,
Paulson et al. mentioned that radical species concentrations at the reaction chamber exit
were negligible due to recombination reactions. We believe the same behavior occurs in
the discharge. Though they are still important in the chemistry of the discharge, no
radicals survive long enough to be observed. Several of the mechanisms mentioned here
include these exotic oxygen molecules, in addition to other radical species, to explain the
existence of the discharge products.
Figure 5.3 displays the discharge spectrum for a 2% isoprene and 2% O2 mixture
balanced in Ar. All hydrocarbon species identified in the unimolecular discharge
spectrum are also present in oxygenated spectrum. Two of the assigned hydrocarbon lines
do not appear in the oxygenated spectrum, and we attribute this simply to signal-to-noise
levels. Hydrocarbon signal levels are reduced in the oxygenated spectrum, and low
intensity transitions are lost. Furthermore, there are seven unidentified lines in the
isoprene/O2 spectrum that were also observed in the simple isoprene/Ar discharge
spectrum. These lines can be labeled as hydrocarbon species. In addition to these
molecules, seven oxygenated species have been assigned: ap and sp-methyl vinyl ketone
(MVK), s-trans methacrolein (MAC), formaldehyde, trans acrolein, trans-trans
crotonaldehyde, and propynal (see Figure 5.4). These thirteen species (49 lines) are
responsible for 66% of the integrated intensity for this discharge spectrum. MVK, MAC
95
Figure 5.3 The discharge spectrum of isoprene in the presence of molecular oxygen
comparing the experimental data (top) and the previously known transitions (below). The
intensities of the assigned transitions are set the values obtained from the experimental
data.
96
Figure 5.4 A list of products observed in the discharge spectra, including the parent
molecule isoprene.
97
and formaldehyde have previously been identified as primary products of isoprene
oxidation.11,13,30 We see no evidence for 3-methylfuran (3-MF) in our discharge spectrum
although it is often identified as a minor product channel in the oxidation of isoprene. The
rotational constants for 3-MF have not been previously reported therefore it is possible it
is present as weak features in our spectrum.
There are approximately 113 unassigned lines in the isoprene/O2 discharge
spectrum that could be attributed to a number of candidates. There are other spectral
features that give insight to the nature of these unidentified species. There are several
peaks with frequency resolved doublet splitting. This behavior could arise from a
molecule that contains a methyl group because the symmetry of a methyl rotor will
separate transitions into A and E species in the microwave spectrum. There is also a set
of three lines that appear to exhibit k-stack splitting indicative of a prolate molecule.
However it is difficult to determine if the trio of lines result from the same species.
Isoprene oxidation also generates hydroxycarbonyls, hydroperoxides and unsaturated
diols.12,14 Many of these molecules have not been studied with microwave spectroscopy
and ab initio calculations are unable to predict rotational constants to a high degree of
accuracy to be useful in assigning the spectra.
A detailed list of assigned rotational transitions is provided in Appendix E. In
some cases the difference between observed and known frequencies for some transitions
is larger than the experimental resolution of 40 kHz. We offer two possible explanations
for this discrepancy. For molecules that contain a methyl moiety, a symmetric top (such
as a methyl rotor) on an asymmetric frame will give rise to a threefold barrier that
generates A and E symmetry species with distinct rotational transitions. The frequency
difference between the transitions is related, in part, to the height of the barrier to internal
rotation. In general, molecules with a higher barrier will produce smaller frequency
splitting that our spectrometer cannot resolve, typically when the barrier is larger than 8
kJ/mol. In the high barrier case, the result is a rotational line that has a doublet peak or
unusual peak shape that complicates assignment.
The second instance involves the prolate top 1,3-pentadiyne. A low frequency tail
appears on the peaks assigned to this molecule that we attribute to unresolved K-stacks.
98
The low frequency tails are consistent with the previously reported frequencies of the Kstacks.31 Additionally, four of the identified molecules are based on a single observed
line. Three of these molecules, propene, propyne and formaldehyde have been previously
reported as products of isoprene reactions and we are confident in these assignments. The
fourth molecule, cyclopropenylidene, is currently a tentative assignment because it is an
exotic species with a single line. However, the error of this transition is below our
experimental resolution of 40 kHz, which makes us confident in the assignment.
Many studies involving isoprene focus on product ratios and concentration
profiles of the reaction products. Obtaining quantitative information relating
concentration to the peak intensities in the discharge spectrum is difficult, particularly for
rotational spectra. The relative intensities of individual peaks often fluctuate significantly
from experiment to experiment. Furthermore, the peak intensities are dependent on a
number of parameters including chamber pressure, rotational temperature and the
temperature of the discharge. However it is possible to get semi-quantitative information
by comparing species observed in both discharge spectra. In this experiment, the
integrated intensity of hydrocarbon species is reduced by 50% when oxygen is introduced
to the discharge environment. From this, we estimate the molecular oxygen discharge
converted half of the hydrocarbon intensity in the previous experiment into oxygenated
species. Lastly, peaks in the discharge spectrum assigned to crotonaldehyde were
compared to the respective ground state spectrum acquired with our spectrometer. The
relative intensity pattern of the transitions was similar and revealed that the rotational
temperature of the discharge experiment is approximately 2.5 K.
Discussion
Hydrocarbon Species
Without a source of oxygen, the first isoprene discharge spectrum contains purely
hydrocarbon species and the mechanisms presented here focus on either decomposition
99
reactions or recombination of radical species. A detailed mechanistic study by Weber and
Zhang19 on the thermal decomposition of isoprene contains numerous pathways that will
be presented here. The first reactions listed here involve a homolysis of the C2-C3 σbond between the conjugated carbons of isoprene. A cleavage of this bond followed by a
1,3 H shift would generate propene and acetylene (1). A similar cleavage and H shift also
leads to formation of allene and ethane (2a). However, allene is not the most stable C3H4
(1)
(2a)
(2b)
isomer; propyne is lower in energy by 1.25 kJ/mol.32 To put this into perspective, our CPFTMW spectrometer has observed both cis and trans conformers of acrolein (ΔE = 7.89
kJ/mol)33 in the discharge spectrum of 2,3-dihydrofuran.22 It is possible to generate
propyne either directly from isoprene (2b) or through a 1,3 H-shift isomerization reaction
occurring between allene and propyne.32 Since allene cannot be detected in our
spectrometer, the mechanism is unclear. Furthermore, the positive identification of
propene implies the existence of acetylene in the discharge, even though this molecule is
also undetected by microwave spectroscopy.
Vinylacetylene is created by methane abstraction of isoprene (3), and is the most
(3)
energetically favorable decomposition reaction of isoprene.19 The study by Weber and
Zhang19 also demonstrates that a number of radicals can be formed in a system containing
excess energy (up to 460 kJ/mol.) The discharge source generates a highly reactive
environment that will create additional radicals not mentioned in the pyrolysis study.
These highly reactive radical species are important in recombination mechanisms and
integral to the understanding of discharge chemistry.
100
Cyclopropenylidene (c-C3H2) is an unusual product and the only molecular ring
assigned in the discharge spectrum. Weber and Zhang19 did not discuss any C3H2
molecules in their study since no peak was detected at this mass channel. c-C3H2 has been
previously assigned with our CP-FTMW spectrometer in the discharge spectrum of 2,3dihydrofuran.22 As discussed in that publication, several different pathways generate this
reactive carbene. One possibility is that c-C3H2 is created directly from allene in a
discharge.34 Additionally vinylidenecarbene, a high energy C3H2 isomer, can be
(4)
generated from propyne35 and rearrange to the more stable c-C3H2 (4). Lastly, c-C3H2 is
also formed from a cyclic C3H3+ cation (5) generated from acetylene.36,37
(5)
Another unexpected molecule observed in the isoprene discharge was (E)-trans
1,3-pentadiene. This molecule is an isomer of isoprene, and may be formed through a
high energy isomerization. In our earlier 2,3-dihydrofuran discharge experiment22 a
similar isomerization reaction was observed between cyclopropanecarboxaldehyde and
crotonaldehyde, which required 241 kJ/mol of energy.38 The highly reactive discharge
environment provided enough excess energy to facilitate this chemistry and could also
initiate an isoprene rearrangement. One possible pathway is to use vinylcyclopropane
(VCP) as an intermediate between 1,3-pentadiene and isoprene. 1,3-pentadiene is more
stable than VCP by 58.8 kJ/mol, but the reaction requires an activation energy of 75.7
kJ/mol.39 Despite several research studies on the isomerization reactions of VCP, no
isoprene is observed as a product.40,41 This implies that rearrangement from isoprene to
1,3-pentadiene would be an entirely downhill process. A ground state microwave
spectrum for VCP has been reported,42 but no transitions are observed in the discharge
spectrum. It is likely that isoprene gains enough energy from the discharge environment
to proceed directly to 1,3-pentadiene. Conversely, 1,3-pentadiene may also result from
recombination reactions (6-7). Recently it has been shown that 1,3-pentadiene is
101
(6)
(7)
generated from a vinyl radical reacting with propene at high temperatures (> 450K).43
When comparing the two possible mechanisms for 1,3-pentadiene formation, basic
statistics dictate that isoprene rearrangement is the dominant pathway rather than a
bimolecular reaction. Only a single 1,3-pentadiene conformer is observed in the
discharge spectrum. The next lowest energy structure is (Z)-trans and HF/6-31G
calculations report44 it to be 6.96 kJ/mol higher in energy. This value corresponds well
with our DFT value of 6.53 kJ/mol at B3LYP/6-311++G(d,p) level of theory.
Another large carbon species observed in the discharge spectrum is 1,3pentadiyne, which is likely generated from a recombination reaction. Jamal and Mebel45
conducted an ab initio/RRKM study of the potential energy surface of C5H5 isomers
resulting from a reaction between propyne and ethynyl radical. Six final products were
(8)
studied and 1,3-pentadiyne (8) was formed with a probability of 27-56%.45 Though there
is no direct evidence for the formation of ethynyl radicals in the discharge, it is formed
during the pyrolysis of acetylene.46
C4H6O Species
Four C4H6O isomers have been identified as a result of isoprene oxidation and the
relative energies of these molecules are displayed in Figure 5.5. In the case of MVK, both
anti-periplanar (ap) and syn-periplanar (sp) conformers were observed. The microwave
spectrum of ap-MVK had been observed by Foster et al.47 in 1965, but the sp conformer
has only recently been assigned with our spectrometer.48 Though there is disagreement
concerning the difference in energy as well as the relative ordering of the conformers
(ΔEsp-ap = 2.36 kJ/mol experimental49, ΔEap-sp = 0.07 kJ/mol calculated48), the energy
difference is sufficiently small to observe both conformers. Conversely, the only species
of MAC detected in the discharge spectrum was the trans conformer. Ab initio
102
Figure 5.5 A graph demonstrating the relative energies of the four observed C4H6O
isomers in the discharge spectrum of oxygenated isoprene.
103
calculations determined that trans-MAC was more stable than the cis conformer by 14.11
kJ/mol and is comparable to the experimental50 value 9.09 kJ/mol. Crotonaldehyde is
more complex and has four conformers due to the torsions about the two double bonds.
Only the most stable species has been observed in the discharge spectrum, the trans-trans
conformer. A conformational analysis of this molecule has been previously discussed
with regards to the 2,3-dihydrofuran discharge spectrum and has the same conclusions
noted here.22
The most studied isoprene reaction is the attack of a hydroxyl radical and the
subsequent branching ratios. A simplified mechanism is presented in Figure 5.1 that
summarizes a large body of work. The radical will attack one of four distinct carbons and
create four different reaction intermediates, α- and β-hydroxyalkyl radicals. Although
several studies have focused on the branching ratio of I:II:III:IV (see Figure 5.1), a recent
study51 has calculated this ratio to be 0.68:0.03:0.01:0.28. The dominant outer channels I
and IV (α-hydroxyalkyl radicals) lead to MVK and MAC respectively. The inner
channels II and III (β-hydroxyalkyl radicals) also generate those main products but
uniquely lead to 3-methylfuran formation. However 3-methylfuran was not detected in
the discharge spectrum and is often not observed by other experimental methods. In fact,
the probability of the inner channels occurring is low enough that these pathways, and
subsequently their products, are often ignored in global simulations.4,5
The hydroxyl radical is not the only form of oxygen in the atmosphere; therefore
studies have been conducted with ozone and atomic oxygen. Ozone will initiate a
cylcloaddition to either double bond in isoprene to create two possible molozonides. The
molozonides, an unstable cyclic organic molecule involving three oxygen atoms, will
further react to form both MVK and MAC.52 Though the same end products are created,
the mechanisms and product ratios are different for reactions with ozone compared to
those with OH radical.22 Conversely, reactions involving atomic oxygen will generate
different products. The atomic oxygen will attack the terminal carbons and
thermodynamically choose the methyl-substituted double bond to form diradical
species.11 These intermediates will either stabilize to form epoxide compounds or
rearrange to form 2-methyl-2-butenal; none of these species have been identified in the
104
discharge spectrum. The mechanisms that involve the hydroxyl radical and ozone both
lead to the same primary products, MVK and MAC. Since it is possible that either
oxygen species is present, we are unable to determine whether a single mechanism exists
or some combination is directing product formation.
As presented in Figure 5.5, a third C4H6O isomer was detected that has been
identified as crotonaldehyde. Despite searching in the literature, no known mechanism
was found to describe the formation of crotonaldehyde from isoprene. As discussed
above with 1,3-pentadiene, high-energy rearrangements are possible in discharge
experiments and crotonaldehyde may be formed through isomerization from MVK or
methacrolein. We also present a mechanism involving radical recombination. A
significant concentration of crotonaldehyde has been observed in a gas phase reaction
study of cis-1,3-pentadiene and hydroxyl radical.53 A mechanism was suggested in that
study which has been applied to trans-1,3-pentadiene and is shown in reactions 9 and 10.
(9)
(10)
Though this mechanism dictates a hydroxyl radical attack on the terminal carbon (C1) of
the terminal double bond, it is also possible to attach the radical to the secondary carbon
(C2) and generate identical products. Once again we suggest that rearrangement, instead
of recombination, is the dominant pathway to crotonaldehyde generation.
Other Oxygenated Products
In addition to the four C4H6O isomers, several smaller oxygenated species were
observed in the discharge spectrum. An oxygenated product that has not been observed in
isoprene experiments is propynal. After searching the literature, the only mechanism to
create this molecule from species in the discharge experiment involves a triplet oxygen
atom (17). Lee et al.54 have studied the various energetic pathways of the reaction
(11)
105
between a propargyl radical and triplet oxygen, O(3P). One of the most stable product
channels is the formation of propynal and hydrogen, which is 252 kJ/mol lower in energy
than the reactants. There is evidence to support the presence of propargyl radicals (C3H3)
in our discharge. In their isoprene decomposition study, Weber and Zhang19 observed a
significant m/e 39 peak correlating to C3H3 and attributed its existence to hydrogen
abstraction of propyne or allene. Observing propyne and cyclopropenylidene in the
discharge spectrum suggests a source of propargyl radical.
A molecule similar to crotonaldehyde that has not been previously observed in
isoprene oxidation reactions is acrolein. The microwave spectra of cis and trans acrolein
have been previously reported55,56 and both conformers have been observed with our
spectrometer. Yet only the trans species was detected in the isoprene/O2 spectrum. We
credit this occurrence to the stability of the trans conformer relative to its cis counterpart
(ΔEcis-trans = 6.98-9.21 kJ/mol using UV33,57,58 and Raman59 spectroscopy.) While no
mechanism was discovered to produce trans acrolein directly from an isoprene oxidation,
acrolein can be formed from other discharge products. For example, although a
mechanism was not given, the same experiment that generated crotonaldehyde from 1,3pentadiene also created acrolein as a product.53 Other publications have observed acrolein
as a significant product from the reaction of allyl radical with atomic oxygen and (12).
Initially, acrolein was the only experimentally observed product for this reaction.60
(12)
Further research identified additional molecular species, but still detected a 47% yield of
acrolein.61 The most recent data from a theoretical study of the C3H5O potential energy
surface by FitzPatrick62 includes conformational differences in product structures.
FitzPatrick’s work incorporated many different intermediates and predicted a more
conservative product yield of 28% for acrolein. Although this bimolecular reaction is a
possible mechanism, it is more likely that acrolein is created from the decomposition of
crotonaldehyde via reactions 13 and 14 proposed by Lifshitz et al.63 A closer look at
(13)
(14)
106
the experiment done by Lifshitz et al. also reveals other products that we observed.
Crotonaldehyde forms a C3H4 radical (15) that will form propyne (16) or propene (17).
(15)
(16)
(17)
In regard to formaldehyde, since many of these mechanisms produce formaldehyde as a
side product, it is difficult to determine the exact reaction that may be occurring. It is
most likely a combination of several different pathways that lead to the formation of this
small compound. We attribute the rather significant signal intensity of formaldehyde to
the reaction scheme presented in Figure 6.1 and the fact that MVK and methacrolein
formation from isoprene also leads to formaldehyde.
We have taken the discharge spectrum of isoprene in the presence and absence of
molecular oxygen and have identified a total of thirteen molecular species. Using a CPFTMW spectrometer, we were able to discriminate between structural isomers and
molecular conformers. Several previously known reaction products were observed in the
decomposition and oxidation of isoprene (vinylacetylene, propene propyne, ap-MVK, spMVK, trans methacrolein, and formaldehyde.) Additionally, new and unexpected species
were assigned in both discharge spectra (trans 1,3-pentadiene, 1,3-pentadiyne,
cyclopropenylidene, trans-trans crotonaldehyde, trans acrolein, and propynal.) We were
able to calculate that when molecular oxygen was introduced into the discharge
environment, the integrated intensity of the hydrocarbon species was reduced by 50%.
The identification of new species in these isoprene experiments will help to develop more
complete mechanisms that are used in global atmospheric simulations and give better
insight to the chemistry occurring in these reactions. There are still a significant number
of unassigned lines in both spectra. As more molecules are studied with microwave
spectroscopy, a larger database of frequency transitions will be available for comparison.
107
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111
CHAPTER 6: PRODUCTS IN A BUTANOL DISCHARGE
Introduction
The interest in biofuels has been increasingly prevalent in recent years resulting
from a demand for alternative energy. In fact, a 50% reduction in greenhouse gases over
the next 40 years will require a quadrupled increase in bioenergy production.1 Likewise
there has been a surge of research regarding the production, performance and
consequences of applying biofuels in our daily lives. While ethanol is currently the most
readily available biofuel, there are several other candidates that may have other
advantages. Biobutanol has recently received attention because it 1) can be used in
different concentrations including its pure form, 2) involves less modification to current
engines, 3) possesses a low vapor pressure, 4) is not hydroscopic, 5) is less corrosive than
ethanol, 6) contains a high energy content and 7) its derivatives may be applicable to
diesel fuels.2 Another advantage of butanol is its ability to be created from a large variety
of plants, including drought resistant species like agave.3 Though butanol is a newer
addition to the energy debate, production of butanol from biomass was industrialized4 as
early as the 1940s and its potential as a biofuel have led large companies, such as Dupont
and BP, to invest in butanol plants.5
However, while there is a considerable amount of research about ethanol, the
kinetics and combustion products of butanol have only recently been studied. Other than
a few early experiments6,7 in the 1950’s that identified a few products, the most
significant research has been published since the turn of the century. There are four
isomers of butanol (1-butanol, 2-butanol, iso-butanol, tert-butanol), and research has
shown that each species will behave differently as a biofuel.8 Although this molecule
exists in multiple isomers, bio-butanol publications usually discuss 1-butanol (nbutanol).9 This is due to the fact that many of the fermentation methods, such as the well
112
known ABE (acetone-butanol-ethanol) process, generate butanol from butanoic acid or
butanal.2 In these reactions the functional group is located on the terminal carbon, which
selectively leads to 1-butanol. However, there has been an emphasis to include the other
isomers as biofuel candidates.
The mechanisms that are shown to be the primary reactions of butanol
decomposition are dehydration, hydrogen abstraction and fission. Intermolecular
dehydration leads to 1-butene in both 1- and 2-butanol systems.5,9-12 Hydrogen
abstraction and fission are often correlated, with the latter process typically following the
former.5 These reactions lead to a variety of molecules including formaldehyde and
propene.10 Many of the predominant species have been identified, but there is
disagreement about what product ratios exist for different experiments.11,13,14
Furthermore, new unexpected products and reactions are still being discovered and
integrated into large mechanistic studies.12
With research on butanol still emerging, it is prudent to expect more reactions and
molecular species to be unearthed. While a study of all four butanol isomers would be
ideal, the research here focuses on the two forms containing a linear carbon chain, 1- and
2-butanol. The focus of this chapter is to use a CP-FTMW spectrometer to determine
products from the discharge reaction of two butanol isomers and analyze any significant
differences between the spectra. Implementing butanol as a biofuel is expected to reduce
CO, hydrocarbon and NOx emissions.2 However, it is also more likely to generate
oxygenated compounds such as dangerous aldehydes, which can also be harmful to the
environment. For instance, the Clean Air Act of 1990 generated the Hazardous Air
Pollutants list and includes both acetaldehyde and formaldehyde.15 If there is a preference
for either isomer to generate aldehydes or hydrocarbons, this information will be
important for biofuel research and understanding emissions.
Experimental Setup
Liquid samples of butanol were obtained from Aldrich (1-butanol, 99%; 2butanol, 99.5%). A gas tank of 2-butanol was prepared similar to other samples
113
mentioned previously; the cylinder was saturated with 2-butanol via a freeze-pump-thaw
procedure and filled with Ar to 100 psi. However the difference in boiling points for 1and 2-butanol, 117°C and 98°C respectively, made it difficult to convert 1-butanol to a
gas cylinder using our standard method. For this reason, a liquid sample was placed prior
to the pulsed valve by injecting approximately 1mL of 1-butanol into a small stainless
steel container packed with cotton. To acquire the broadband discharge spectrum of these
species, the quadrupler circuit was implemented and a set of 100,000 averages was
collected. The full set of assigned lines for the butanol experiments is listed in Appendix
F.
An additional ground state spectrum was acquired in order to verify an
assignment in the discharge spectrum of 1-butanol. Due to the location of the alcohol
substituent on the terminal carbon, butanal is a primary product of 1-butanol
decomposition. Though no published microwave spectrum or rotational constants for
butanal were found, a theoretical conformational study performed by Langley et al.16
identified six conformers. A liquid sample was obtained from Aldrich (99.5%) and
converted into a gas sample using the freeze-pump-thaw method. The tank was saturated
with butanal and filled with He/Ne. Rotational constants were not provided in the
theoretical study, so density functional theory calculations (B3LYP/6-311++G(d,p)) were
performed using the Gaussian03 suite.17 Initial structures were based on proposed
structures given by Langley et al.; optimization and vibrational calculations verified the
stability of each conformer. Of the six possible species, three conformers have been
assigned in the spectral fitting program JB95:18 cis-trans, cis-gauche, and trans-gauche.19
Lists of fitting parameters and assigned transitions for each conformer are given in
Appendix F.
114
1-Butanol
Previous Experiments
Of the four butanol isomers, 1-butanol is most often discussed as a biofuel and
therefore mechanistic research has focused on this molecule. The two earliest studies
included a pyrolysis6 and flame experiment7 published in the mid 1950’s. The pyrolytic
products were largely formaldehyde, carbon monoxide and methane with smaller
concentrations of other hydrocarbon species largely resulting from a fission between the
C1 and C2 carbons (1).6 The flame study by Smith et al.7 observed larger molecular
(1)
species such as 1-butene and propanal, but saw no evidence of formaldehyde. This study
proposed an alternate pathway that consisted of an initial H-abstraction prior to bond
fission.
There is a rather large gap in butanol research until the turn of the century when
the use of butanol as a biofuel was revealed.2 In 2008, Moss et al.5 published a shock tube
study in conjunction with a kinetic model including 1250 reactions of 158 species. The
authors observed butanal, acetaldehyde, ethene, propene and 1-butene as the primary
products, concluding that H-abstraction was the dominant reaction. Two butanol
oxidation studies11,13 emerged in 2009 that concurred with the initial mechanistic
assessment provided by Moss et al.5 However the research by Dagaut et al.13 stated that
certain product concentrations were not in agreement with predictions. Furthermore, the
analysis by Sarathay et al.11 notes that in low oxygen environments, unimolecular
decomposition will dominate.
A larger kinetic model highlighting more enol chemistry was provided by Black
et al.9 in 2010. The conclusions from this study are less certain because the model and
experimental data occasionally disagreed. Additionally, it was unclear whether the
primary mechanism was hydrogen abstraction or fission. A comprehensive model, now
including 3381 reactions of 263 species, has been presented by Harper et al.20 and
115
compares results from several other simulations.5,9,11 Experimental data from this
publication gave evidence of both fission and abstraction mechanisms, closely
resembling the mechanism provided by Black et al.9 Another recent study further
clarified the issue when it was determined that high temperature reactions lead to
unimolecular decomposition and lower temperatures generated radicals formed from
hydrogen abstraction.12
Intermolecular decomposition reactions for 1-butanol can be divided into two
categories. The first is a dehydration mechanism into 1-butene (2), and has only one
(2)
pathway.5,9-12 Though several studies have included this reaction, the likelihood of
dehydration occurring ranges from 2-25%.9,11,12,20 The other category is direct fission and
will occur at any C-C bond, most likely the C1-C2 bond.12 However, there is little
discussion about this group of reactions since fission is more favorable following
hydrogen abstraction. Numerous publications have focused on the H-abstraction reactions
of 1-butanol, and many have determined branching ratios.5,9,11-13,20 As seen from
Reactions 3-7, five different isomers are created from hydrogen abstraction and there is
(3)
(4)
(5)
(6)
(7)
disagreement concerning the most probable pathway. A compilation of published
branching ratios is given in Table 6.1. Although many of the numbers vary greatly, one
pattern emerges; the least likely product channels are I and V. Due to the large variability
in branching ratios, there is subsequent disagreement on the final product ratios.
116
Table 6.1. A summary of previously reported branching ratios for hydrogen abstraction
from 1-butanol.
Reference
Black et al.9
Sarathay et al.11
Moss et al.5
Dagaut et al.13
Weber et al.12
Harper et al.20
I
II
4%
8%
24%
7%
69%
22%
11%
25%
20%
6%
8%
Reaction
III
13%
16%
7.30%
22%
15%
17%
IV
V
19%
16%
7.30%
22%
22%
18%
12%
13%
14%
14%
12%
117
Discharge Spectrum
Prior to obtaining the discharge spectrum, a ground state spectrum of 1-butanol
was acquired with the CP-FTMW spectrometer. By comparing the spectrum with
previously reported rotational constants,21 it was determined that three conformers were
present: Tgt, Gtg, and Ttt. Due to the large number of possible conformers of 1-butanol,
there is still ambiguity concerning the relative energy values of the optimized
structures.21-23 Therefore it is difficult to speculate whether the lowest energy structures
are the ones observed in our ground state spectrum. The discharge spectrum of 1-butanol
and the associated product list are presented in Figures 6.1 and 6.2 respectively. A total of
nine products have been assigned: cis-trans butanal, cis-gauche butanal, skew 1-butene,
trans acrolein, propynal, propene, propyne, cyclopropenylidene and formaldehyde. These
molecules account for 74% of the integrated intensity of the discharge spectrum and
approximately 43 remain unassigned. The peak at 16828 MHz is an unidentified
hydrocarbon and has been previously discussed in Chapter 4. The remaining lines do not
have any distinguishing characteristics, such as doublet splittings, that would indicate
certain molecular properties.
With a highly reactive discharge environment, there are many possible
mechanisms to direct product formation. The pathways presented here will fall under two
categories. One set consists of first order reactions that generate products from the parent
molecule, 1-butanol. The other category includes second order reactions. These
mechanisms will be initiated with products that have been identified in the discharge
spectra or have been observed in other experiments. Mechanisms to generate
cyclopropenylidene have been discussed in Chapter 4 and are applicable for this system.
The presence of oxygen in the discharge allows for many types of oxygen species to
exist, which is also mentioned in Chapter 4.
Despite the source of oxygen present in the parent molecule, there are several
hydrocarbon species that are generated in the 1-butanol discharge spectrum. The largest
is skew 1-butene and two pathways exist to create this molecule from 1-BuOH.
Intermolecular dehydration is one method (8), but there is disagreement as to how
118
Figure 6.1 The discharge spectrum of 1-butanol comparing the experimental data (top)
and known frequencies (bottom). The intensity scale has been set to observe the smaller
discharge products; the formaldehyde peak is seven times larger than the largest peak
seen here.
119
Figure 6.2 Product list for the discharge spectrum of 1-butanol.
120
(8)
prevalent this reaction is.5,9-12,20 The second mechanism is a two-step process. First the 2hydroxyl butyl radical is generated from Reaction 5, then a C-O bond fission (9) will
(9)
occur.5,9,11,13,20,24 There is also discussion about the final product percentage due to this
reaction and ranges from 5-20%. In two different flame studies, the peak C4H8 mole
fraction due to 1-butanol occurs closer to the burner compared to other butanol isomers;
this indicates that a different mechanism is occurring for 1-butanol.24,25 Since C4H8
species are generated from dehydration reactions for 2-, iso- and tert-butanol,5,24,25 this
suggests that the two-step mechanism is primarily responsible for generating 1-butene
from 1-butanol. Two different conformers have been identified in the ground state
microwave spectrum of 1-butene, cis and skew.26 The skew species is only slightly more
stable than the cis conformer, with calculations predicting a difference of 0.81-3.46
kJ/mol and microwave data giving a value of 0.62 kJ/mol.27 With such a small energy
difference, it is difficult to suggest a reason for not observing this second conformer. To
get a better understanding of the intensity ratios for these species and compare with the
discharge spectrum, a ground state spectrum of 1-butene would need to be collected with
our spectrometer.
There are three C3 hydrocarbons produced in the 1-butanol discharge spectrum:
propene, propyne and cyclopropenylidene. Propene is either generated directly from 1BuOH or from a radical. The former reaction (10) is less likely to occur because although
(10)
a direct fission is mentioned in several instances, no quantitative results have been
determined.6,12,20 Alternatively, a C1-C2 bond fission (11) of the 3-hydroxybutyl radical
(11)
generated from Reaction 6 is frequently mentioned in the literature5,24 and will lead to
propene in 93-97% of reactions.9,11,13,20 The propensity of this reaction to occur is
increased by the fact that the C1-C2 is the weakest C-C bond in 1-butanol due to the
electron withdrawing group.9 Having assigned propene in the discharge, it seems likely
121
that propyne would be present since large propyne signals have been previously detected
in propene flame experiments.28,29 Another possible mechanism to form propyne is a
recombination reaction30 involving a propargyl radical (12), which has been previously
(12)
detected in butanol studies.24,31 Propyne may also be formed by isomerization of the
slightly higher energy allene species. Only recently have experiments been able to
distinguish between these isomers to extract possible product ratios. Yang et al.31 were
able to fit a photoionization efficiency spectrum (m/z = 40) with a product distribution of
33% allene/67% propyne, which concurs with mole ratios determined in the Oßwald et
al.24 flame study.
The largest oxygenated compound in the discharge spectrum is butanal. In the
literature, there is disagreement as to the source of this product. One possibility is
stabilization (13) from the radical I generated in Reaction 3. However, the fate of this
(13)
radical is undetermined. One report11 suggests that this radical will lead to butanal in
100% of reactions, while another13 only predicts a product percentage of 46%. Several
other publications conclude that this radical will decompose to propane and
formaldehyde.9,20,24 The discharge spectrum clearly has a formaldehyde signal, so either
reaction is a viable pathway. Another likely source of butanal is from radical II as seen in
Reaction 14. Two sources say that this radical will proceed to butanal at a rate of 33(14)
38%.5,20 All other schemes predict that this radical fractures to generate acetaldehyde and
ethene.9,11,13,24 No evidence of acetaldehyde exists in the discharge spectrum and suggests
that if this radical is present, it produces butanal. A total of four conformers have been
assigned in the ground state spectrum of butanal that was taken with our CP-FTMW
spectrometer. However, only two conformers are observed in the 1-butanol discharge
spectrum: cis-trans (ωCCC’O/ωCCCC’, 0°/180°) and cis-gauche (ωCCC’O/ωCCCC’,
122
6°/ 73°). According to our calculations, the lowest energy structure is cis-trans and the
cis-gauche conformer is 2.13 kJ/mol higher in energy. Though the relative energies
published by Langley et al.16 (0.63-1.67 kJ/mol) are slightly smaller than our calculated
values, the relative ordering is identical.
The case of trans acrolein is interesting due to the lack of discussion in butanol
experiments. Many combustion or flame experiments distinguish molecules based on
mass then use ionization potentials to further discriminate between isomers. Several
studies on 1-butanol mention a mass channel at 56, which corresponds with acrolein.
However, the products are assigned to other molecules: 1-butene, 2-butene, 2-methyl-1propene or methylketene.24,31 No form of acrolein is ever mentioned. For this reason, no
one has proposed a mechanism for production directly from butanol. Here we present a
reaction of an allyl radical and oxygen atom to create acrolein (15). The existence of an
(15)
allyl radical in 1-butanol reactions has been confirmed in numerous publications10,24,31
and a theoretical study32 used density functional theory calculations to show that the
products of Reaction 15 are more stable than the reactants by 63.9 kcal/mol. The authors
also employed kinetics to confirm that this is the dominant pathway. Both of these
calculations agree with an earlier experimental study by Slagle et al.33 that only observed
acrolein as a product. A more recent FTIR study by Hoyermann et al.34 measured
additional products from this reaction including formaldehyde, acetylene and ethene. The
authors were also able to determine product ratios for the mechanisms and verified that
acrolein was the major product (47%). The only acrolein species observed in this
discharge spectrum is trans and a discussion of the relative energies of the two
predominant conformers is discussed in Chapter 4.
Another molecule never discussed in butanol reactions is propynal. The products
in the mass channel corresponding to propynal, m/z = 54, are solely attributed to 1,3butadiene, 1,2-butadiene, 1-butyne, 2-butyne or porpadienal.24,31 However, Van Geem et
al.14 admitted their model underestimated the maximum concentration of this channel by
a factor of five and suggested that an important reaction may be missing. Since no
mechanism has been suggested to generate propynal directly from butanol, we present a
123
reaction involving radicals detected in other butanol experiments. The propargyl radical
has been previously observed in flame experiments24,31 and will form propynal if
combined with an oxygen atom (16).35
(16)
The intensity of the formaldehyde transition is considerably larger than any other
line in the discharge spectrum (Figure 6.1). Many of the reactions that lead to C3
hydrocarbons generate formaldehyde as a side product. The C1-C2 bond fission in
Reaction 10 directly produces formaldehyde6 and the radical created from a similar
•CH2OH → CH2O + •H
process in Reaction 11 will lead to this formaldehyde as well (17).
(17)
5,9,11,13,24
Additionally,
the formation of propane observed in Reactions 18 and 19 will make formaldehyde.9,13,24
(18)
(19)
The peak assigned to formaldehyde accounts for nearly 30% of the integrated intensity
over the entire discharge spectrum. Further discussion on the importance of this feature is
given at the end of this chapter.
2-Butanol
Previous Experiments
Compared to 1-butanol, fewer experiments have been published concerning 2butanol. One of the most referenced butanol articles was published by Moss et al.,5 which
compared shock-tube data with a kinetic model containing 1250 reactions with 158
species. From their simulations, Moss et al. concluded that 61% of 2-butanol was
converted into 1-butene through a dehydration mechanism. Later in 2010, a flame study
by Grana et al.10 agreed that dehydration was a significant pathway, but only accounted
for 40% of decomposition. That same year, Black et al.9 published another model
containing 1400 reactions and 234 species using experimental data from Moss et al. for
124
comparison and debated whether abstraction or fission was the primary mechanism. The
model and pyrolysis data by Van Geem et al.14 finally shed some light on the
mechanisms. With an even larger set of reactions, and still using Moss et al.5 for
comparison, Van Geem et al.14 concluded that mechanisms differ with experimental
conditions; pyrolysis initiates hydrogen abstraction while flames result in fission and
dehydration reactions. This is a clear indication that further sets of data, reactions, and
products will lead to increased understanding of chemical mechanisms. Furthermore, Van
Geem et al. acknowledged that one particular data set was poorly described by their
simulation and proposed that an additional pathway was missing.
Discharge Spectrum
At the onset of this experiment, a ground state microwave spectrum of 2-butanol
was recorded. As with 1-butanol, there are multiple conformers of 2-butanol that have
been observed in a microwave spectrum.36,37 Comparing the published rotational
constants with data collected with our CP-FTMW spectrometer, only three conformers
have been detected in the ground state spectrum of 2-butanol: e-ga, m-ag, and m-gg.
There is disagreement on the predicted energies of the optimized structures and the
relative ordering of the conformers changes with different calculations;37-40 therefore it is
difficult to discuss the significance of observing only these particular conformers.
The discharge spectrum of 2-Butanol is presented in Figure 6.3. A total of 11
different products have been assigned and are listed in Figure 6.4: trans 2-butanone, skew
1-butene, syn propanal, gauche propanal, trans acrolein, propynal, propene, propyne,
cyclopropenylidene, acetaldehyde, and formaldehyde. These species are responsible for
53% of the discharge spectrum’s integrated intensity. Similar to the 1-butanol discharge
spectrum, there is an unidentified hydrocarbon peak at 16828 MHz. Approximately 42
lines remain unassigned. Unlike the 1-butanol discharge spectrum, certain lines have
notable characteristics. Several sets of lines have doublet splitting that would imply the
125
Figure 6.3 The discharge spectrum of 2-butanol comparing the experimental data (top)
and known frequencies (bottom).
126
6.4 Product list for the discharge spectrum of 2-butanol.
127
presence of a methyl rotor on a molecule. There are also examples of multiplet peaks that
show characteristics similar to radical species. However the frequency spacing and
intensity patterns are sufficiently different enough to suggest that these features are
products of the averaging method of the spectrometer and are in fact noise peaks.
Furthermore, many of the unassigned peaks are located at the high frequency end of the
spectrum (>17 GHz) and this information leads us to believe that these lines are due to
smaller molecules. There are reactions directly involving 2-butanol and others that derive
from secondary products. We are assuming the reactions for cyclopropenylidene, acrolein
and propynal are similar to those presented for the 1-butanol discharge since these
reactions are based on molecules generated in the discharge and not directly dependent
on the parent species.
Skew 1-butene is the largest hydrocarbon assigned in the 2-butanol discharge
spectrum. Like the 1-butanol experiment, only one conformer was observed. Though this
molecule is present in both discharge spectra, there are different mechanisms involved in
the formation of this species. There is conflicting evidence in the literature about this
molecule’s presence in 2-butanol reactions. In two studies, 2-butene is proposed to
dominate over the 1-butene isomer.24,31 However, Moss et al.5 concluded that 61% of 2butanol reacted to form 1-butene, while only 5% formed 2-butene. Unfortunately, we
cannot comment on the presence of 2-butene in the discharge spectrum. Though there is
debate over concentrations, there is a consensus that 1-butene is formed through
intramolecular dehydration (20). 1-Butene is the only hydrocarbon mentioned here that
(20)
includes 2-butanol in the mechanism. The other hydrocarbon products are generated from
other products or radical species. As with the 1-butanol discharge spectrum, only the
lowest energy skew conformer was observed.
Propene is often observed in 2-butanol experiments, but mechanisms directly
from 2- butanol have either been difficult to find24 or simply not discussed.31 Recently
however, a butanol isomer study14 suggested that propene is generated indirectly from 1butene decomposition (21), which had been confirmed in 1958.41 The propyne signal in
128
(21)
2-butanol is due to the same mechanisms present in the 1-butanol discharge; propyne is
generated from reactions of propene,28,29 propargyl radical24,30,31 and allene. The
photoionization efficiency spectrum of C3H4 by Yang et al.31 is identical for both 1- and
2-butanol, leading to the same isomeric product ratio of 33% allene/67% propyne.
Similarly, the flame study by Oßwald et al.24 has the same peak mole fractions of allene
and propyne for both butanol isomers.
The largest product observed in the discharge spectrum of 2-butanol is trans 2butanone, which is a primary product of this isomer. 2-Butanone is often referred to as a
fingerprint of 2-butanol because no other butanol isomers generate this species in large
quantities, if at all.10,14,24 Two mechanisms are presented in the literature and differ only
in regard to the location of initial hydrogen extraction. The first scheme was proposed by
Moss et al.5 and involves initially removing hydrogen from the carbon chain, then the
(22)
hydroxyl group (22). A second possible mechanism, suggested by Grana et al.,10 reverses
the order (23). The pyrolysis study by Van Geem et al.14 determined that most of the
(23)
2-butanone is created via Reaction 22 and does not mention the second mechanism. An
additional pathway is presented in their scheme (24), though it is not considered to be
(24)
a contributing factor. Only the trans conformer of 2-butanone has been identified in the
previously reported microwave spectrum.42 A second gauche, or skew, conformer has
been observed in the gas phase, but is 9.20 kJ/mol higher in energy.43
One molecule that was difficult to assign was propanal. We are positive that the
syn conformer is present, but we are more uncertain about the gauche species. Several
microwave studies have published transition frequencies for both conformers, but
assignments differ up to 300 kHz.44,45 For this reason, the CDMS database was used for
129
comparison with the discharge spectrum.46 Each conformer has a different problem that
complicates the accuracy of assigned transitions. For syn propanal, the methyl rotor V3
value (796.6 cm-1) is sufficiently high to observe frequency splitting without complete
resolution between peaks; the splitting for the 101-110 transition is only 110 kHz.
However, five discharge peaks match well with published frequencies and confirm the
existence of this molecule as a product. The gauche conformer is more difficult to assign.
Previously reported data reveals that the gauche propanal is 3.76 kJ/mol higher in energy
than the syn conformer,44 which suggests that this molecule may be seen in the discharge.
A problem arises in the ground state spectrum of this species, which is said to
occasionally split transitions into doublets.44,45 Unlike the syn conformer, this splitting is
not due to a methyl rotor and was tested by recording the microwave spectrum of
CD3CH2CHO.44 If the methyl group were responsible for the doublet peaks, propanal
with a -CD3 top would have significantly reduced splitting compared to the natural
species, which was not the case. Instead the phenomenon is the result of two equivalent
gauche conformers and the coupling between the torsional momentum and the overall
angular momentum. We observe two peaks near one of the reported transitions (-0.08
MHz, +0.49 MHz). It is possible that these lines are gauche propanal or two entirely
different molecules. Unfortunately these are the only lines to compare with gauche
propanal, which is why this remains a tentative assignment.
When considering syn propanal as a product of 2-butanol reactions, there is some
debate in the literature. In certain isomeric studies, propanal was an observed product for
1-butanol and iso-butanol, but not for 2-butanol.24,31 The kinetic modeling study by Grana
et al.10 revealed a more complicated situation. They did not include propanal in their
experimental analysis, but a mechanism from 2-butanol (25) is proposed and incorporated
(25)
into their kinetic model. A few months later, Van Geem et al.14 included a second
mechanism from a different radical (26). Propanal is not often mentioned in the
(26)
130
discussion of butanol products because another C4H6O isomer, acetone, is more
prevalent.14,24,31 The ground state spectrum of pure acetone has been recorded with our
spectrometer, but we see no evidence of acetone in either of the discharge spectra.
A product often observed in 2-butanol experiments is acetaldehyde. Oßwald et
al.24 detected large signals in a flame study, and Yang et al.31 concluded that 80% of the
mass in the m/z = 44 channel was due to acetaldehyde. There are several reaction
schemes mentioned in the literature. The first one consists of 2-butanol undergoing an
hydroxyl-H abstraction followed by a β–scission to generate acetaldehyde and an ethyl
(27)
radical (27).10,24 The second mechanism involves an initial dissociation followed by an
(28)
hydrogen abstraction (28).25 Van Geem et al.14 included both mechanisms in their
analysis, but also mentioned a third pathway. The reaction is similar to Reaction 27 and
only differs in regard to the location of the initial hydrogen abstraction (29).
(29)
As suggested by the number of acetaldehyde mechanisms proposed, smaller
products often originate from many different sources. In the case of formaldehyde, a
number of reactions generate this molecule as a side product and we will not attempt to
determine the origin of this particular product. Interestingly the study by Van Geem et
al.14 observed no experimental evidence of formaldehyde, nor was it predicted by their
pyrolysis model.
Comparison of Butanol Isomers
When conducting isomeric decomposition studies, molecules often appear that are
a result of a single isomer and are labeled as signatures of that isomer. In the case of 1butanol, butanal is considered a signature molecule and is not observed in other butanol
131
isomer reactions.14,24,31 Though it is present in 1-butanol experiments, there is
disagreement on the final concentrations and predicted product yields range from 0.066.57%.5,9,11,20 The analysis from our experiments agrees with the previous data’s claim
that butanal is a signature of 1-butanol reactions; it is not observed in the 2-butanol
discharge spectrum. In order to observe butanal in the 2-butanol experiment, a difficult
rearrangement involving the oxygen atom or a separate bimolecular reaction would be
necessary.
2-Butanone is known as a signature of 2-butanol decomposition. It is present in 2butanol experiments, occasionally seen with tert-butanol and never in 1-butanol
studies.14,24,31 The discharge experiment concurs with this data; 2-butanone is observed in
the discharge spectrum of 2-butanol, but not 1-butanol. Like the case of butanal and 1butanol, a significant rearrangement would need to occur in order to see this species in an
environment lacking 2-butanol. It makes sense that both 1- and 2-butanol isomers would
generate a ketone functional group from the hydroxy substituent previously existing on
the molecule.
One molecule that many butanol studies agree upon is acetaldehyde. This
molecule is typically observed in all isomers and many concur that there is twice as much
acetaldehyde in 1-butanol reactions than 2-butanol reactions.14,24,31 Despite this accord in
the published data, the discharge spectra reveal a different situation. There are
appreciable acetaldehyde peaks in the discharge spectrum of 2-butanol, but there is no
evidence of this molecule in the 1-butanol experiment. The only publication to support
the discharge data is by McEnally et al.,25 who observed a considerably larger mole
fraction of acetaldehyde in 2-butanol decomposition compared to the other butanol
isomers. They attributed this large acetaldehyde signal to fission of the weak C2-C3 bond
in 2-butanol.
In the case of formaldehyde, there is disagreement in the literature concerning its
presence in butanol experiments. Most studies state that formaldehyde is present in 1butanol studies, but it is not always observed in 2-butanol reactions.10,14,20,31 When
formaldehyde is observed in both systems, the concentration in 1-butanol reactions is on
average twice that of 2-butanol experiments.24 Simulations also appear to be incomplete.
132
The study by Harper et al.20 included three other mechanisms5,9,11 and all four models
underestimated the formaldehyde signal in 1-butanol pyrolysis. There is a clear difference
in the formaldehyde peak when comparing the discharge spectra. The signal in the 2butanol spectrum is comparable to other peaks, while formaldehyde completely
dominates the 1-butanol spectrum. A direct comparison of the signal intensities shows
that the signal increases by a factor of 22 between the spectra. The large presence of
formaldehyde in 1-butanol is likely due to the alcohol substituent being located on the
terminal carbon rather than a secondary carbon. It is important to focus on formaldehyde
as a discharge product because it is on the list of Hazardous Air Pollutants that was
created in the Clean Air Act of 1990.15 That this molecule is a primary decomposition
product of a biofuel candidate is cause for concern.
From this analysis, it is clear that there is a considerable difference in the product
distribution from 1- and 2-butanol discharge experiments. Butanol is being considered as
a biofuel and it is essential to note the emissions from these reactions. Despite both parent
molecules being alcohols, no other assigned molecular species contained a hydroxyl
substituent. Additionally, no radical species were identified in the discharge spectrum. In
fact, many of the observed products are aldehydes or ketones. Aldehydes, such as
acetaldehyde and formaldehyde, are significant air toxins25,47 and we need to find fuel
sources with the lowest possible emissions. Although 1- and 2-butanol have different
discharge spectra, both molecules have a large percentage of aldehyde products that will
impact the environment.
133
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APPENDICES
136
Appendix A: Tables for Methyl Vinyl Ketone
Table A1. A-state transitions for ap-methyl vinyl ketone (MHz).
J'
Ka'
Kc'
J"
1
2
3
5
6
3
2
3
6
8
6
4
2
4
3
7
5
2
2
4
2
7
6
3
2
2
4
3
5
7
3
6
4
16
12
4
19
4
0
1
1
2
3
1
0
1
2
4
2
1
1
2
2
3
2
0
2
1
1
2
2
0
1
2
1
1
1
2
0
3
0
5
4
2
6
3
1
1
2
4
3
2
2
2
4
5
5
3
2
2
1
5
3
2
0
3
1
6
4
3
2
1
3
3
4
5
3
3
4
11
8
3
13
2
0
2
3
4
5
2
1
3
6
7
5
4
1
4
3
6
5
1
2
4
1
6
6
2
1
2
3
2
5
7
2
6
3
16
12
3
19
3
Ka" Kc"
0
0
1
3
4
2
1
0
2
5
3
1
1
1
1
4
1
0
1
0
1
3
1
1
0
1
2
1
0
1
0
2
0
5
3
2
6
3
0
2
3
1
2
1
1
3
5
2
2
4
1
3
2
2
4
1
1
4
0
3
5
2
1
2
2
2
5
6
2
4
3
12
9
2
14
1
Observeda
JB95b omcc
XIAMd omc
7219.864(5)
7571.104(5)
7946.144(5)
7947.204(5)
9231.774(5)
9481.619(5)
9526.979(5)
10326.169(5)
11554.305(5)
12089.170(5)
12937.185(5)
13093.035(5)
13110.510(5)
13163.075(5)
13355.405(5)
14105.815(5)
14139.415(5)
14194.025(5)
14246.770(5)
14473.580(5)
15768.905(5)
16284.771(5)
16606.275(5)
17144.106(5)
17777.5(3)
17988.666(5)
18773.541(5)
19524.0(3)
19894.7(3)
20714.9(3)
20727.8(3)
20891.3(3)
26803.73(5)
26855.30(5)
28049.55(5)
28686.40(5)
28913.61(5)
29295.76(5)
-0.013
-0.009
0.000
-0.001
0.021
0.002
-0.011
-0.002
0.010
-0.014
-0.003
0.012
-0.014
0.006
-0.011
-0.002
0.001
-0.014
-0.012
0.008
0.000
0.012
0.004
-0.007
-0.073
-0.001
0.019
-0.140
0.251
-0.003
0.153
0.294
-0.213
-0.049
-0.002
-0.067
-0.068
0.318
-0.013
-0.001
0.005
-0.029
0.003
-0.014
-0.018
0.004
0.014
-0.022
-0.038
0.017
-0.016
0.017
0.001
-0.033
0.011
-0.015
0.002
0.016
-0.001
-0.031
0.014
-0.014
-0.069
0.016
0.002
-0.144
0.257
0.003
0.150
0.302
-0.219
0.009
-0.007
-0.075
0.050
0.308
137
4
22
4
4
5
13
25
5
11
14
5
7
28
14
10
13
5
31
15
34
40
37
6
5
12
5
17
5
5
5
4
5
4
16
6
5
14
6
6
5
17
6
6
18
5
3
7
2
1
0
4
8
1
3
5
0
1
9
4
4
5
1
10
5
11
13
12
4
2
5
4
5
4
4
3
4
4
4
5
4
3
4
0
1
1
6
0
1
6
2
1
15
2
3
5
9
17
5
8
9
5
6
19
10
6
8
5
21
10
23
27
25
2
4
7
1
12
2
1
3
0
2
1
11
3
2
10
6
6
4
11
6
6
12
3
3
22
3
3
4
13
25
4
11
14
4
7
28
14
9
13
4
31
15
34
40
37
6
4
12
5
17
4
4
4
4
5
4
16
6
4
14
5
5
4
17
5
5
18
4
3
7
2
1
1
3
8
1
2
4
0
0
9
4
5
4
0
10
4
11
13
12
3
2
4
3
5
4
4
3
3
3
3
4
3
3
3
1
1
1
5
0
0
5
2
0
16
1
2
4
10
18
4
9
10
4
7
20
11
5
9
4
22
11
24
28
26
3
3
8
2
13
1
0
2
1
3
2
12
4
1
11
5
5
3
12
5
5
13
2
29465.20(5)
30567.33(5)
30758.97(5)
30951.32(5)
31236.39(5)
31823.90(5)
31858.86(5)
31956.27(5)
32142.13(5)
32345.36(5)
32616.96(5)
32641.74(5)
32823.99(5)
32974.46(5)
33130.93(5)
33218.92(5)
33337.08(5)
33494.53(5)
33522.99(5)
33898.98(5)
34014.76(5)
34064.07(5)
35013.85(5)
35554.47(5)
35598.03(5)
36175.76(5)
36258.15(5)
36642.62(5)
36664.74(5)
36684.30(5)
36759.57(5)
36913.59(5)
36955.42(5)
36993.08(5)
37015.81(5)
37248.45(5)
37550.28(5)
37656.57(5)
38005.08(5)
38037.81(5)
38328.45(5)
38376.51(5)
38725.24(5)
38937.25(5)
39014.30(5)
-0.023
-0.138
-0.027
-0.024
-0.055
0.054
-0.193
-0.196
0.100
0.162
-0.034
0.350
-0.284
0.063
not fite
0.038
0.066
-0.212
0.098
-0.175
0.352
-0.007
-0.099
-0.036
0.071
-0.060
0.000
0.094
-0.046
-0.093
0.068
0.024
-0.013
0.018
0.077
-0.018
-0.058
-0.018
-0.175
-0.061
0.100
-0.099
-0.035
0.053
0.084
-0.032
0.056
-0.034
-0.030
-0.067
0.050
0.082
-0.206
0.091
0.114
-0.043
0.338
0.054
0.065
not fite
-0.027
0.058
0.137
0.068
0.089
-0.067
0.022
-0.108
-0.050
-0.005
-0.066
0.030
0.076
-0.064
-0.109
0.066
0.020
-0.015
0.001
0.070
-0.033
-0.073
-0.034
-0.189
-0.072
-0.019
-0.112
-0.048
-0.024
0.071
138
8
1
7
8
0
8
39038.66(5)
0.127
0.099
12 3
9 12
2
10
39061.35(5)
0.186
0.154
16 6
10 16
5
11
39879.69(5)
0.115
-0.040
a
Experimental uncertainty specified in parenthesis. 5 kHz data was recorded in this study.
Transitions with 50 and 300 kHz resolution were taken from Fantoni et al.1 and Foster et
al.,2 respectively.
b
Plusquellic et al.3
c
Observed minus calculated
d
Hartwig et al.4
e
Not included in fit. Observed minus calculated deviations greater than 1 MHz.
Table A2. E-state transitions for ap-methyl vinyl ketone (MHz).
J'
K a'
K c'
J"
K a"
K c"
Observeda
JB95b omcc
XIAMd omc
1
2
3
5
8
6
3
2
3
6
1
8
6
4
2
4
3
7
5
2
2
4
5
2
7
6
3
2
4
3
5
7
0
1
1
2
3
3
1
0
1
2
1
4
2
1
1
2
2
3
2
0
2
1
2
1
2
2
0
2
1
1
1
2
1
1
2
4
5
3
2
2
2
4
1
5
5
3
2
2
1
5
3
2
0
3
3
1
6
4
3
1
3
3
4
5
0
2
3
4
8
5
2
1
3
6
0
7
5
4
1
4
3
6
5
1
2
4
4
1
6
6
2
2
3
2
5
7
0
0
1
3
3
4
2
1
0
2
0
5
3
1
1
1
1
4
1
0
1
0
3
1
3
1
1
1
2
1
0
1
0
2
3
1
6
2
1
1
3
5
0
2
2
4
1
3
2
2
4
1
1
4
2
0
3
5
2
2
2
2
5
6
7219.514(5)
7570.589(5)
7944.084(5)
7945.384(5)
8808.444(5)
9232.234(5)
9479.499(5)
9526.139(5)
10324.745(5)
11549.755(5)
11886.860(5)
12075.195(5)
12936.600(5)
13089.710(5)
13110.180(5)
13163.270(5)
13356.095(5)
14101.575(5)
14138.595(5)
14193.465(5)
14247.615(5)
14470.830(5)
14851.820(5)
15767.870(5)
16286.005(5)
16603.840(5)
17143.046(5)
17988.301(5)
18770.596(5)
19524.0(3)
19890.3(3)
20710.5(3)
-0.011
-0.005
-0.007
-0.011
-0.005
-0.005
0.000
-0.010
-0.002
0.005
0.007
-0.001
-0.002
0.010
-0.011
-0.001
-0.005
0.002
-0.003
-0.012
-0.008
0.009
0.020
-0.002
-0.004
-0.002
-0.007
0.001
0.024
0.291
0.075
0.104
-0.008
-0.006
-0.017
0.027
-0.015
0.008
0.000
-0.006
-0.008
-0.017
0.013
0.005
0.028
-0.012
-0.003
0.001
-0.005
0.049
-0.004
-0.005
-0.014
-0.008
0.032
0.004
0.029
-0.011
0.001
0.003
0.027
0.303
0.039
0.073
139
3
0
3
2
0
2
20727.8(3)
not fit f
not fit f
6
3
3
6
2
4
20892.9(3)
0.053
0.047
4
0
4
3
0
3
26803.38(5)
-0.156
-0.139
16
5
11
16
5
12
26837.45(5)
-0.112
-0.166
12
4
8
12
3
9
28045.99(5)
-0.043
-0.044
4
2
3
3
2
2
28685.36(5)
0.134
0.142
19
6
13
19
6
14
28891.55(5)
-0.347
-0.332
22
7
15
22
7
16
30541.35(5)
-0.600
-0.440
4
2
2
3
2
1
30756.82(5)
0.039
0.046
4
1
3
3
1
2
30949.62(5)
0.010
0.015
5
0
5
4
1
4
31235.65(5)
0.068
0.087
13
4
9
13
3
10
31816.33(5)
-0.039
-0.076
5
1
5
4
1
4
31955.87(5)
-0.146
-0.123
e
10
32346.12(5)
-0.024
-0.017
14
5
9
14
4
5
0
5
4
0
4
32616.72(5)
0.017
0.040
7
1
6
7
0
7
32635.04(5)
0.039
-0.065
14
4
10
14
4
11
32958.49(5)
0.144
-0.010
13
5
8
13
4
9
33222.91(5)
-0.033
-0.061
31
10
21
31
10
22
33461.84(5)
0.266
not fit f
15
5
10
15
4
11
33519.54(5)
0.030
0.046
5
2
4
4
2
3
35552.85(5)
-0.249
-0.241
12
5
7
12
4
8
35603.71(5)
0.087
0.021
17
5
12
17
5
13
36237.63(5)
-0.042
-0.198
5
3
3
4
3
2
36682.99(5)
0.226
0.224
16
5
11
16
4
12
36984.57(5)
-0.101
-0.116
5
3
2
4
3
1
37245.78(5)
0.009
0.023
14
4
10
14
3
11
37538.98(5)
-0.040
-0.149
6
0
6
5
1
5
37656.00(5)
0.080
0.107
5
1
4
4
1
3
38036.12(5)
0.013
0.017
17
6
11
17
5
12
38330.95(5)
0.054
0.053
6
0
6
5
0
5
38376.49(5)
0.136
0.167
18
6
12
18
5
13
38934.58(5)
0.088
0.129
8
1
7
8
0
8
39031.63(5)
0.114
-0.041
12
3
9
12
2
10
39050.08(5)
0.197
0.027
16
6
10
16
5
11
39886.03(5)
0.160
0.089
a
Experimental uncertainty specified in parenthesis. 5 kHz data was recorded in this study. Transitions
with 50 and 300 kHz resolution were taken from Refs 1 and 2, respectively.
b
Ref 3
c
Observed minus calculated
d
Ref 4
e
We believe a typographical error was made in Ref 6. Based on predictions and in accordance with the
corresponding A-state transition, Ka" was changed from 5 to 4.
f
Not included in fit. Observed minus calculated deviations greater than 1 MHz.
140
Table A3. A-state transitions for 13C1 ap-methyl vinyl ketone (MHz).
J' Ka' Kc' J" Ka" Kc"
1
0
1
0
0
0
2
1
1
2
0
2
3
1
2
3
1
3
2
0
2
1
1
1
3
1
2
2
2
1
3
1
2
3
0
3
4
2
2
4
1
3
3
2
1
3
1
2
2
1
2
1
1
1
2
2
0
2
1
1
2
0
2
1
0
1
4
1
3
4
0
4
2
1
1
1
1
0
3
0
3
2
1
2
2
2
1
2
1
2
a
5 kHz resolution
b
Ref 3
c
Observed minus calculated
Observeda
7159.524
7361.034
8078.609
9662.109
10150.219
10208.215
12541.890
12601.420
12966.955
13436.490
14051.920
14479.900
15671.100
17167.576
17225.576
JB95b omcc
-0.005
-0.011
-0.005
-0.008
0.000
0.004
0.007
-0.014
-0.002
-0.001
0.004
-0.001
0.010
-0.001
0.008
Table A4. E-state transitions for 13C1 ap-methyl vinyl ketone (MHz).
J' Ka' Kc' J" Ka" Kc"
1
0
1
0
0
0
2
1
1
2
0
2
3
1
2
3
1
3
2
0
2
1
1
1
3
1
2
2
2
1
3
1
2
3
0
3
1
1
1
0
0
0
3
2
1
3
1
2
2
1
2
1
1
1
4
1
3
4
1
4
2
2
0
2
1
1
2
0
2
1
0
1
4
1
3
4
0
4
2
1
1
1
1
0
3
0
3
2
1
2
2
2
1
2
1
2
a
5 kHz resolution
b
Ref 3
c
Observed minus calculated
Observeda
7159.184
7360.534
8076.594
9661.309
10148.159
10206.790
11549.275
12602.040
12966.635
13286.405
13437.295
14051.380
14477.165
15670.090
17166.581
17225.186
JB95b omcc
-0.012
0.012
-0.007
0.004
-0.008
0.004
-0.001
0.001
0.001
0.010
-0.001
-0.006
-0.009
0.011
0.005
-0.008
141
Table A5. A-state transitions for 13C2 ap-methyl vinyl ketone (MHz).
J' Ka' Kc' J" Ka" Kc"
1
0
1
0
0
0
2
1
1
2
0
2
3
1
2
3
1
3
2
0
2
1
1
1
3
1
2
3
0
3
2
1
2
1
1
1
4
2
2
4
1
3
3
2
1
3
1
2
5
2
3
5
1
4
2
0
2
1
0
1
2
2
0
2
1
1
4
1
3
4
0
4
2
1
1
1
1
0
3
0
3
2
1
2
2
2
1
2
1
2
a
5 kHz resolution
b
Ref 3
c
Observed minus calculated
Observeda
7206.574
7568.269
7918.559
9493.914
10310.729
13088.585
13176.615
13377.255
14136.565
14169.415
14270.160
14439.965
15737.660
17100.161
18000.041
JB95b omcc
-0.002
0.004
-0.002
-0.006
-0.005
-0.003
0.001
-0.001
-0.001
0.000
-0.002
0.002
0.011
0.000
-0.001
Table A6. E-state transitions for 13C2 ap-methyl vinyl ketone (MHz).
J'
Ka' Kc'
J"
Ka"
1
0
1
0
0
2
1
1
2
0
3
1
2
3
1
2
0
2
1
1
3
1
2
3
0
1
1
1
0
0
2
1
2
1
1
4
2
2
4
1
3
2
1
3
1
5
2
3
5
1
2
0
2
1
0
2
2
0
2
1
4
1
3
4
0
2
1
1
1
1
3
0
3
2
1
2
2
1
2
1
a
5 kHz resolution
b
Ref 3
c
Observed minus calculated
Kc"
0
2
3
1
3
0
1
3
2
4
1
1
4
0
2
2
Observeda
7206.214
7567.754
7916.519
9493.079
10309.325
11882.010
13088.255
13176.825
13377.940
14135.770
14168.860
14271.010
14437.220
15736.630
17099.106
17999.686
JB95b omcc
-0.018
0.002
-0.003
0.001
-0.001
-0.003
0.004
0.001
-0.007
0.001
0.001
0.003
0.000
0.011
-0.004
0.002
142
Table A7. A-state transitions for 13C3 ap-methyl vinyl ketone (MHz).
J'
Ka' Kc'
J"
Ka" Kc"
1
0
1
0
0
0
2
1
1
2
0
2
3
1
2
3
1
3
3
1
2
2
2
1
2
0
2
1
1
1
3
1
2
3
0
3
2
1
2
1
1
1
4
2
2
4
1
3
3
2
1
3
1
2
5
2
3
5
1
4
2
0
2
1
0
1
2
2
0
2
1
1
4
1
3
4
0
4
2
1
1
1
1
0
3
0
3
2
1
2
2
2
1
2
1
2
a
5 kHz resolution
b
Ref 3
c
Observed minus calculated
Observeda
7172.924
7519.529
7901.104
9436.834
9469.619
10260.085
13024.125
13066.580
13254.510
14042.290
14101.100
14139.095
14385.400
15667.480
17036.131
17859.401
JB95b omcc
0.003
0.001
0.003
0.003
-0.002
-0.010
-0.004
-0.007
0.008
0.003
-0.004
-0.004
-0.001
0.008
-0.002
0.005
Table A8. E-state transitions for 13C3 ap-methyl vinyl ketone (MHz).
J'
1
2
3
3
2
3
1
2
4
3
5
2
2
4
2
3
Ka'
0
1
1
1
0
1
1
1
2
2
2
0
2
1
1
0
Kc'
1
1
2
2
2
2
1
2
2
1
3
2
0
3
1
3
J"
0
2
3
2
1
3
0
1
4
3
5
1
2
4
1
2
Ka" Kc"
0
0
0
2
1
3
2
1
1
1
0
3
0
0
1
1
1
3
1
2
1
4
0
1
1
1
0
4
1
0
1
2
Observeda
7172.579
7519.014
7899.069
9434.789
9468.809
10258.690
11804.330
13023.800
13066.765
13255.165
14041.420
14100.550
14139.920
14382.700
15666.470
17035.106
JB95b omcc
-0.002
0.006
-0.012
0.000
0.006
-0.002
-0.006
-0.003
0.004
0.000
-0.006
-0.008
0.001
0.008
0.013
-0.002
143
2
2
1
2
1
5 kHz resolution
b
Ref 3
c
Observed minus calculated
2
17859.011
0.001
a
Table A9. A-state transitions for 13C4 ap-methyl vinyl ketone (MHz).
J'
Ka' Kc'
J"
Ka" Kc"
1
0
1
0
0
0
3
1
2
3
1
3
2
1
1
2
0
2
3
1
2
2
2
1
2
0
2
1
1
1
3
1
2
3
0
3
2
1
2
1
1
1
4
2
2
4
1
3
3
2
1
3
1
2
2
0
2
1
0
1
4
1
3
4
0
4
2
2
0
2
1
1
2
1
1
1
1
0
3
0
3
2
1
2
2
2
1
2
1
2
a
5 kHz resolution
b
Ref 3
c
Observed minus calculated
Observeda
7017.494
7525.719
7535.024
8384.374
9016.404
10099.825
12776.725
13396.695
13710.905
13818.515
13969.785
14622.720
15293.200
16465.560
18180.961
JB95b omcc
-0.007
0.008
-0.003
-0.018
-0.005
0.004
0.005
0.004
-0.006
-0.001
0.000
-0.001
0.006
0.016
-0.012
Table A10. E-state transitions for 13C4 ap-methyl vinyl ketone (MHz).
J'
1
2
2
3
1
2
4
3
2
4
2
2
3
2
Ka'
0
1
0
1
1
1
2
2
0
1
2
1
0
1
Kc'
1
1
2
2
1
2
2
1
2
3
0
1
3
2
J"
0
2
1
3
0
1
4
3
1
4
2
1
2
1
Ka" Kc"
0
0
0
2
1
1
0
3
0
0
1
1
1
3
1
2
0
1
0
4
1
1
1
0
1
2
0
1
Observeda
7017.164
7534.549
9015.624
10098.520
11819.540
12776.415
13396.950
13711.590
13817.985
13967.255
14623.540
15292.240
16464.546
17578.766
JB95b omcc
-0.016
0.009
-0.009
0.001
0.004
0.011
0.001
-0.003
-0.005
-0.001
-0.003
0.007
-0.002
0.005
144
2
2
1
2
1
5 kHz resolution
b
Ref 3
c
Observed minus calculated
2
18180.626
-0.004
a
Table A11. A-state transitions for 18O ap-methyl vinyl ketone (MHz).
J'
Ka' Kc'
J"
Ka" Kc"
2
1
1
2
0
2
3
1
2
3
1
3
2
0
2
1
1
1
1
1
1
0
0
0
2
1
2
1
1
1
4
1
3
4
0
4
2
2
0
2
1
1
2
1
1
1
1
0
3
0
3
2
1
2
2
2
1
2
1
2
a
5 kHz resolution
b
Ref 3
c
Observed minus calculated
Observeda
7382.069
7631.239
9136.879
11582.085
12690.045
13978.880
14026.920
15242.650
16519.121
17625.096
JB95b omcc
-0.0175
-0.0043
0.0009
0.0108
-0.0011
0.0077
-0.0026
-0.0084
0.0022
0.0036
Table A12. A-state transitions for sp-methyl vinyl ketone (MHz).
J'
Ka'
Kc'
J"
1
1
2
2
3
4
4
2
6
1
4
2
2
3
5
4
6
5
0
1
0
1
1
1
4
1
2
1
1
0
1
0
2
2
2
1
1
0
2
1
2
3
0
2
4
1
3
2
1
3
3
2
4
4
0
1
1
2
3
3
5
1
5
0
4
1
1
2
5
4
6
5
Ka" Kc"
0
0
1
0
0
2
3
1
3
0
0
0
1
1
1
1
1
0
0
1
1
2
3
2
3
1
3
0
4
1
0
2
4
3
5
5
Observeda
JB95b omcc
XIAMd omc
6917.274
7315.259
7460.174
8506.289
10511.174
12289.055
12345.640
12768.560
13101.895
13166.580
13522.500
13709.465
14900.505
14954.125
16864.196
17055.166
17574.266
17647.081
-0.007
-0.006
-0.001
-0.007
-0.003
-0.001
-0.001
-0.009
-0.001
0.005
0.005
-0.005
0.004
0.010
0.000
-0.005
0.002
-0.002
-0.008
-0.006
-0.004
-0.008
-0.002
-0.003
-0.001
-0.012
0.000
0.003
0.007
-0.008
0.001
0.006
-0.001
-0.005
-0.002
0.002
145
3
2
1
3
1
2
2
2
0
2
1
1
a
5 kHz resolution
b
Ref 3
c
Observed minus calculated
d
Ref 15
17846.381
18872.796
0.000
0.008
0.000
0.008
Table A13. E-state transitions for sp-methyl vinyl ketone (MHz).
J'
Ka'
Kc' J" Ka" Kc"
1
0
1
0
0
0
1
1
0
1
0
1
2
0
2
1
1
1
2
1
1
2
0
2
3
1
2
3
0
3
4
1
3
3
2
2
2
1
2
1
1
1
1
1
1
0
0
0
4
1
3
4
0
4
2
0
2
1
0
1
2
1
1
1
1
0
3
0
3
2
1
2
5
2
3
5
1
4
4
2
2
4
1
3
6
2
4
6
1
5
5
1
4
5
0
5
3
2
1
3
1
2
2
2
0
2
1
1
a
5 kHz resolution
b
Ref 3
c
Observed minus calculated
d
Ref 4
Observeda
6917.094
7312.859
7464.179
8503.039
10507.555
12304.755
12769.120
13162.020
13518.535
13709.095
14899.285
14956.985
16856.941
17048.486
17567.336
17642.646
17843.766
18891.651
JB95b omcc
-0.007
0.021
0.002
-0.004
-0.005
-0.003
0.009
0.000
0.003
0.000
-0.015
0.009
0.013
-0.007
-0.004
0.001
-0.009
0.002
XIAMd omc
-0.008
-0.002
-0.001
-0.004
0.000
-0.004
0.003
0.004
0.000
0.000
0.008
0.013
0.008
-0.009
0.001
-0.003
0.000
0.002
Table A14. A-state transitions for 13C1 sp-methyl vinyl ketone (MHz).
J'
2
1
2
3
1
4
2
3
Ka'
0
1
1
1
1
1
0
0
Kc'
2
0
1
2
1
3
2
3
J"
1
1
2
3
0
4
1
2
Ka" Kc"
1
1
0
1
0
2
0
3
0
0
0
4
0
1
1
2
Observeda
7178.314
7272.064
8421.629
10352.120
13008.275
13248.380
13418.490
14517.345
JB95b omcc
0.001
-0.005
-0.001
0.006
-0.003
-0.002
-0.004
-0.001
146
4
2
2
4
1
3
2
1
3
1
2
1
2
1
0
2
2
0
2
1
a
5 kHz resolution
b
Ref 3
c
Observed minus calculated
3
2
1
1
17030.561
17829.356
18744.471
18838.056
-0.003
0.004
0.005
-0.001
Table A15. E-state transitions for 13C1 sp-methyl vinyl ketone (MHz).
J' Ka' Kc' J" Ka" Kc"
2
0
2
1
1
1
1
1
0
1
0
1
2
1
1
2
0
2
3
1
2
3
0
3
4
1
3
3
2
2
2
1
2
1
1
1
1
1
1
0
0
0
4
1
3
4
0
4
2
0
2
1
0
1
3
0
3
2
1
2
4
2
2
4
1
3
3
2
1
3
1
2
2
1
2
1
0
1
a
5 kHz resolution
b
Ref 3
c
Observed minus calculated
Observeda
7182.379
7269.704
8418.419
10348.559
11639.430
12504.890
13003.710
13244.555
13418.160
14520.280
17023.981
17827.096
18740.651
JB95b omcc
0.000
-0.001
0.006
-0.004
-0.001
0.007
0.001
0.001
0.006
-0.003
-0.001
0.000
-0.007
Table A16. A-state transitions for 13C2 sp-methyl vinyl ketone (MHz).
J' Ka' Kc' J" Ka" Kc"
1
1
0
1
0
1
2
0
2
1
1
1
2
1
1
2
0
2
3
1
2
3
0
3
4
4
1
5
3
2
4
1
3
3
2
2
3
0
3
2
1
2
4
2
2
4
1
3
3
2
1
3
1
2
2
2
0
2
1
1
a
5 kHz resolution
b
Ref 3
c
Observed minus calculated
Observeda
7317.694
7436.689
8505.254
10503.745
12172.220
12228.025
14921.110
17069.276
17862.226
18887.751
JB95b omcc
-0.008
0.003
0.008
-0.003
0.000
0.001
-0.003
0.002
-0.005
0.006
147
Table A17. E-state transitions for 13C2 sp-methyl vinyl ketone (MHz).
J' Ka' Kc' J" Ka" Kc"
2
0
2
1
1
1
2
1
1
2
0
2
3
1
2
3
0
3
4
1
3
3
2
2
1
1
1
0
0
0
4
1
3
4
0
4
2
0
2
1
0
1
3
0
3
2
1
2
4
2
2
4
1
3
5
1
4
5
0
5
3
2
1
3
1
2
a
5 kHz resolution
b
Ref 3
c
Observed minus calculated
Observeda
7440.684
8502.009
10500.139
12244.420
13157.750
13501.185
13690.815
14923.955
17062.636
17612.296
17859.646
JB95b omcc
0.004
-0.003
-0.001
0.001
0.009
-0.005
-0.009
-0.001
0.001
0.004
0.000
Table A18. A-state transitions for 13C3 sp-methyl vinyl ketone (MHz).
J' Ka' Kc' J" Ka" Kc"
1
1
0
1
0
1
2
0
2
1
1
1
2
1
1
2
0
2
3
1
2
3
0
3
1
1
1
0
0
0
4
1
3
4
0
4
2
0
2
1
0
1
3
0
3
2
1
2
4
2
2
4
1
3
3
2
1
3
1
2
2
2
0
2
1
1
2
1
2
1
0
1
a
5 kHz resolution
b
Ref 3
c
Observed minus calculated
Observeda
7238.854
7453.354
8428.814
10433.454
13051.995
13445.470
13628.240
14901.745
16852.551
17631.116
18650.436
18865.096
JB95b omcc
0.003
-0.006
-0.003
-0.002
0.006
0.001
0.012
0.001
-0.003
0.011
-0.007
-0.010
148
Table A19. E-state transitions for 13C3 sp-methyl vinyl ketone (MHz).
J' Ka' Kc' J" Ka" Kc"
1
1
0
1
0
1
2
0
2
1
1
1
2
1
1
2
0
2
3
1
2
3
0
3
1
1
1
0
0
0
4
1
3
4
0
4
2
0
2
1
0
1
3
0
3
2
1
2
4
2
2
4
1
3
3
2
1
3
1
2
2
2
0
2
1
1
2
1
2
1
0
1
a
5 kHz resolution
b
Ref 3
c
Observed minus calculated
Observeda
7236.519
7456.979
8425.669
10429.940
13047.600
13441.610
13627.685
14904.475
16846.106
17628.521
18667.931
18861.161
JB95b omcc
-0.004
0.010
-0.003
0.004
0.015
-0.001
-0.024
0.003
0.000
-0.001
0.001
0.002
Table A20. A-state transitions for 13C4 sp-methyl vinyl ketone (MHz).
J' Ka' Kc' J" Ka" Kc"
2
0
2
1
1
1
1
1
0
1
0
1
2
1
1
2
0
2
3
1
2
3
0
3
2
1
2
1
1
1
1
1
1
0
0
0
2
0
2
1
0
1
3
0
3
2
1
2
4
2
2
4
1
3
3
2
1
3
1
2
2
1
2
1
0
1
a
5 kHz resolution
b
Ref 3
c
Observed minus calculated
Observeda
6977.569
7380.209
8501.239
10377.825
12448.330
13099.300
13347.615
14282.065
17389.421
18213.951
18818.351
JB95b omcc
-0.006
-0.005
0.005
0.002
0.021
-0.003
-0.007
-0.001
-0.005
0.005
-0.005
149
Table A21. E-state transitions for 13C4 sp-methyl vinyl ketone (MHz).
J' Ka' Kc' J" Ka" Kc"
1
1
0
1
0
1
2
1
1
2
0
2
3
1
2
3
0
3
2
1
2
1
1
1
1
1
1
0
0
0
4
1
3
4
0
4
2
0
2
1
0
1
3
0
3
2
1
2
4
2
2
4
1
3
3
2
1
3
1
2
2
1
2
1
0
1
a
5 kHz resolution
b
Ref 3
c
Observed minus calculated
Observeda
7377.874
8498.024
10374.239
12448.875
13094.700
13184.620
13347.260
14284.995
17383.011
18212.161
18814.526
JB95b omcc
-0.004
-0.001
0.002
-0.014
-0.003
-0.001
0.007
0.001
-0.003
0.003
0.007
Table A22. Unscaled harmonic vibrational frequencies determined by ab initio
calculations.
ap-methyl vinyl ketone
HF
a
DFT
b
sp-methyl vinyl ketone
MP2
a
HF
a
DFTb
MP2a
Q1
117.1050
119.3402
93.1687
91.0306
92.8771
35.643
Q2
143.0863
124.1360
125.8956
139.4473
114.5764
126.4213
Q3
297.9304
281.1554
285.0491
286.4181
268.0898
268.4355
Q4
471.5692
431.1754
416.4017
442.3054
415.6327
415.5584
Q5
525.3330
492.3169
486.5123
500.0781
459.9017
441.4896
Q6
577.0507
538.1187
535.9981
647.9987
601.5091
601.5078
Q7
752.0596
694.1815
670.5890
745.1292
682.6043
650.8074
Q8
812.7187
760.5552
779.8717
837.8907
778.6384
792.0098
Q9
1025.6023
945.2175
919.6776
1042.6477
957.5862
914.9614
Q10
1109.2013
990.2727
955.8011
1108.4100
1009.5017
972.7572
Q11
1122.2378
1035.6963
1015.8487
1135.3695
1022.8833
1006.7315
Q12
1143.4080
1046.6031
1043.1099
1148.3966
1043.9406
1039.6522
Q13
1163.5828
1072.1055
1076.1274
1168.8103
1082.5753
1081.5815
Q14
1371.4212
1268.4369
1287.1618
1295.4427
1195.0005
1213.5002
Q15
1409.2854
1307.2642
1307.1884
1429.5782
1326.3964
1324.0302
Q16
1523.5813
1388.8290
1402.8822
1516.5935
1383.3101
1397.0663
Q17
1558.4339
1443.9715
1454.3184
1554.7390
1433.2243
1446.5184
150
Q18
1592.1668
1474.5715
1492.0452
1582.8350
1464.4682
1485.45
Q19
1597.2715
1478.3907
1493.7223
1593.7232
1472.8198
1487.8508
Q20
1823.0356
1677.5701
1663.7924
1818.5076
1664.9131
1666.462
Q21
1963.3072
1743.9957
1727.3692
1968.5308
1766.7623
1753.0236
Q22
3178.6459
3035.4483
3081.2237
3166.9365
3023.7562
3072.1616
Q23
3238.5379
3093.2659
3165.5486
3222.1727
3077.5016
3153.2499
Q24
3285.6281
3140.3114
3185.6361
3284.5175
3137.0827
3181.125
Q25
3296.0269
3143.3709
3204.3691
3293.6137
3142.6197
3202.5044
Q26
3330.9924
3165.1614
3212.1596
3317.7304
3154.2614
3206.0716
Q27
3379.8030
3223.2351
6-311++G(d,p)
b
B3LYP/6-311++G(d,p)
3283.3616
3389.2861
3231.0509
3288.1262
a
151
References
(1)
Fantoni, A. C.; Caminati, W.; Meyer, R. Chem. Phys. Lett. 1987, 133, 27-33.
(2)
Foster, P. D.; Rao, V. M.; Curl, Jr., R. F. J. Chem. Phys. 1965, 43, 1064-1066.
(3)
Plusquellic, D. F.; Suenram, R. D.; Mate, B.; Jensen, J. O.; Samuels, A. C. J.
Chem. Phys. 2001, 115, 3057-3067.
(4)
Hartwig, H.; Dreizler, H. Z. Naturforsch 1996, 51a, 923-932.
152
Appendix B: Discharge Products of 2,3-Dihydrofuran
Twelve molecules were assigned in the discharge spectrum of 2,3-dihydrofuran:
1,3-pentadiyne, vinylacetylene, propene, propyne, cyclopropenylidene, cis and trans
cyclopropanecarboxaldehyde (CPCA), trans-trans crotonaldehyde (CA), cis and trans
acrolein, propynal, and formaldehyde. Lines marked with a * were taken directly from
averaged data set since intensities were unobserved in a single data set. Frequencies
from the averaged set are not as accurate. Superscript numbers refer to publications
listed in the References while superscript letters are footnotes defined at the end of the
table. We have taken the ground state spectrum of several oxygenated molecules
present in this discharge spectrum. The rotational constants have been determined and
compared with previously published values. These molecules include trans-trans CA,1
cis and trans CPCA2 and both acrolein conformers.3,4
Table B23. The assigned transitions in the discharge spectrum of 2,3-dihydrofuran.
Observed
Frequency
(MHz)
7567.36
7844.13
7937.13
8242.13
8262.97
8403.38
8513.02
8623.36
8902.21
9074.80*
9325.80
11143.10
12214.56*
12317.85
12470.69
12604.71
12626.18
12768.76
12934.90
Known
Frequency
(MHz)
7567.40
7844.12
7937.12
8242.12
8262.96
8403.23
8513.00
8623.37
8902.18
9074.74
9325.80
11143.22
12214.47
12317.84
12470.68
12604.68
12626.20
12768.75
12934.88
Molecule
Transition
('Jka,kc)
OMKa
(MHz)
Cis CPCA
Cis CPCA
Cis CPCA
Cis CPCA
Cis CPCA
Trans-Trans CA
Trans-Trans CA
Trans-Trans CA
Trans Acrolein
Vinylacetylene5
Propynal6
Cis Acrolein
1,3-Pentadiyne7
Trans CPCA
Trans CPCA
Trans-Trans CA
Trans CPCA
Trans-Trans CA
Trans-Trans CA
101-110
000-101
303-312
404-413
111-202
111-212
101-202
110-211
000-101
000-101
000-101
000-101
2-3
111-212
101-202
202-313
110-211
202-303
211-312
-0.04
0.01
0.01
0.01
0.01
0.15
0.02
-0.01
0.03
0.06
0.00
-0.12
0.09
0.01
0.01
0.03
-0.02
0.01
0.02
153
14488.50
14488.48
Formaldehyde8
211-212
0.02
15267.26
15267.24
Cis CPCA
000-111
0.02
15328.32
15328.32 b
Cis Acrolein
202-212
0.00
15543.93
15543.92
Cis CPCA
111-212
0.01
15686.08
15686.08
Cis CPCA
101-202
0.00
15832.43
15832.40
Cis CPCA
110-211
0.03
16242.83
16242.80
Cis CPCA
212-303
0.03
16285.96
16285.95
1,3-Pentadiyne7
3-4
0.01
16805.88
16805.90
Trans-Trans CA
313-414
-0.02
16828.32
Unidentified HCc
17023.61
17023.61
Trans-Trans CA
303-404
0.00
9
17091.75
17091.72
Propyne
0-1
0.03
17246.15
17246.18
Trans-Trans CA
312-413
-0.03
17387.60
17387.58
Trans Acrolein
111-212
0.02
17439.54
17439.49
Propene10
000-101
0.05
b
17791.19
17791.19
Cis Acrolein
212-303
0.00
17801.32
17801.30
Trans Acrolein
101-202
0.02
17929.36
17929.34
Cis Acrolein
101-110
0.02
5
18146.64*
18146.58
Vinylacetylene
101-202
0.06
18221.16
18221.16
Trans Acrolein
110-211
0.00
6
18325.64*
18325.56
Propynal
111-212
0.08
18343.16
18343.14
Cyclopropenylidene11
101-110
0.02
18476.00
18475.88
Trans CPCA
212-313
0.12
18650.40*
18650.33
Propynal6
101-202
0.07
18702.50
18702.48
Trans CPCA
202-303
-0.02
a
Observed minus known
b
Known frequencies and quantum numbers predicted using JB9512 and published4
rotational constants.
c
Unidentified hydrocarbon that has been observed in several spectra.
154
References
(1)
Suzuki, M.; Kozima, K. Bull. Chem. Soc. Jpn. 1969, 42, 2183-2186.
(2)
Volltrauer, H. N.; Schwendeman, R. H. J. Chem. Phys. 1971, 54, 260-267.
(3)
Wagner, R.; Fine, J.; Simmons, J.; Goldsten, J. J. Chem. Phys. 1957, 26, 634637.
(4)
Blom, C.; Bauder, A. Chem. Phys. Letters 1982, 88, 55-58.
(5)
Hirose, C. Bull. Chem. Soc. Jpn. 1970, 43, 3695-3698.
(6)
Winnewisser, G. J. Mol. Spec. 1973, 46, 16-24.
(7)
http://www.astro.uni-koeln.de/cdms. Catalog - #64507.
(8)
Bocquet, R.; Demaison, J.; Poteau, L.; Liedtke, M.; Belov, S.; Yamada, K.M.T.;
Winnewisser, G.; Gerke, C.; Gripp, J.; Kohler, T. J. Mol. Spec. 1996, 177, 154159.
(9)
Ware, J. M.; Roberts, J. A. J. Chem. Phys. 1984, 81, 1215-1219.
(10)
Lide Jr., D. R.; Mann, D. E. J. Chem. Phys. 1957, 27, 868-873.
(11)
Matthews, J.; Irvine, W. The Astrophysical Journal 1985, 298, L61-L65.
(12)
Plusquellic, D. F.; Suenram, R. D.; Mate, B.; Jensen, J. O.; Samuels, A. C. J.
Chem. Phys. 2001, 115, 3057-3067.
155
Appendix C: Discharge Products of 2,5-Dihydrofuran
Six molecules have been assigned in the discharge spectrum of 2,5dihydrofuran: propyne, cyclopropenylidene, trans-trans crotonaldehyde (CA), trans
acrolein, propynal and formaldehyde. We have taken the ground state spectrum of
several oxygenated molecules present in this discharge spectrum. The rotational
constants have been determined and compared with previously published values. These
molecules include trans-trans CA1 and trans acrolein.2
Table C24. The assigned transitions in the discharge spectrum of 2,5-dihydrofuran.
Observed
Known
Transition
Frequency
Frequency
Molecule
('Jka,kc)
(MHz)
(MHz)
8902.20
8902.18
Trans Acrolein
000-101
9325.80
9325.80
Propynal3
000-101
4
14488.50
14488.48
Formaldehyde
211-212
16828.23
Unidentified HCb
17023.60
17023.61
Trans-Trans CA
303-404
17091.75
17091.72
Propyne5
0-1
17387.60
17387.58
Trans Acrolein
111-212
17801.30
17801.30
Trans Acrolein
101-202
18221.15
18221.16
Trans Acrolein
110-211
6
18343.15
18343.14
Cyclopropenylidene
101-110
18650.30
18650.33
Propynal
101-202
a
Observed minus known
b
Unidentified hydrocarbon that has been observed in several spectra.
OMKa
(MHz)
0.02
0.00
0.02
-0.01
0.03
0.02
0.00
-0.01
0.01
-0.03
156
References
(1)
Suzuki, M.; Kozima, K. Bull. Chem. Soc. Jpn. 1969, 42, 2183-2186.
(2)
Wagner, R.; Fine, J.; Simmons, J.; Goldsten, J. J. Chem. Phys. 1957, 26, 634637.
(3)
Winnewisser, G. J. Mol. Spec. 1973, 46, 16-24.
(4)
Bocquet, R.; Demaison, J.; Poteau, L.; Liedtke, M.; Belov, S.; Yamada, K.M.T.;
Winnewisser, G.; Gerke, C.; Gripp, J.; Kohler, T. J. Mol. Spec. 1996, 177, 154159.
(5)
Rhee, W.; Roberts, J. J. Mol. Spec. 1987, 126, 356-369.
(6)
Matthews, J.; Irvine, W. The Astrophysical Journal 1985, 298, L61-L65.
157
Appendix D: Discharge Products of 1,3-Butadiene Experiments
Superscript numbers refer to publications given in the References while letters
are defined at the end of the table as footnotes. We have taken the ground state
spectrum of several oxygenated molecules present in this discharge spectrum. The
rotational constants have been determined and compared with previously published
values. These molecules include trans-trans crotonaldehyde (CA),1 and trans
acrolein.2,3
Table D25. The assigned transitions in the discharge spectrum of 1,3-butadiene.
Observed
Known
Transition
Frequency
Frequency
Molecule
('Jka,kc)
(MHz)
(MHz)
9074.72
9074.74
Vinylacetylene4
000-101
16828.22
Unidentified HCb
17091.76
17091.72
Propyne5
0-1
18146.56
18146.58
Vinylacetylene
101-202
18343.16
18343.14
Cyclopropenylidene6
101-110
a
Observed minus known
b
Unidentified hydrocarbon that has been observed in several spectra.
OMKa
(MHz)
-0.02
0.04
-0.02
0.02
Table D26. The assigned transitions in the discharge spectrum of 1,3-butadiene and
molecular oxygen.
Observed
Known
Frequency
Frequency
(MHz)
(MHz)
9074.72
9074.74
14488.48
14488.48
17023.60
17023.61
17091.72
17091.72
17801.32
17801.30
18343.16
18343.14
a
Observed minus known
Molecule
Transition
('Jka,kc)
OMKa
(MHz)
Vinylacetylene
Formaldehyde7
Trans-Trans CA
Propyne
Trans Acrolein
Cyclopropenylidene
000-101
211-212
303-404
0-1
101-202
101-110
-0.02
0.00
-0.01
0.00
0.02
0.02
158
References
(1)
Suzuki, M.; Kozima, K. Bull. Chem. Soc. Jpn. 1969, 42, 2183-2186.
(2)
Wagner, R.; Fine, J.; Simmons, J.; Goldsten, J. J. Chem. Phys. 1957, 26, 634637.
(3)
Blom, C.; Bauder, A. Chem. Phys. Letters 1982, 88, 55-58.
(4)
Hirose, C. Bull. Chem. Soc. Jpn. 1970, 43, 3695-3698.
(5)
Rhee, W.; Roberts, J. J. Mol. Spec. 1987, 126, 356-369.
(6)
Matthews, J.; Irvine, W. The Astrophysical Journal 1985, 298, L61-L65.
(7)
Bocquet, R.; Demaison, J.; Poteau, L.; Liedtke, M.; Belov, S.; Yamada, K.M.T.;
Winnewisser, G.; Gerke, C.; Gripp, J.; Kohler, T. J. Mol. Spec. 1996, 177, 154159.
159
Appendix E: Discharge Products of Isoprene Experiments
Table E27. The assigned transitions in the discharge spectrum of isoprene.
Observed
Known
Transition
OMKa
Frequency
Frequency
Molecule
(MHz)
('Jka,kc)
(MHz)
(MHz)
8143.00
8142.99
1,3-Pentadiyne1
10-20
0.01
8387.12
8387.16
Trans 1,3-Pentadiene2
101-202
-0.04
9074.70
9074.74
Vinylacetylene3
000-101
-0.04
12214.42
12214.47
1,3-Pentadiyne
20-30
-0.05
12579.54
12579.55
Trans 1,3-Pentadiene
202-303
-0.01
16285.90
16285.95
1,3-Pentadiyne
30-40
-0.05
16770.56
16770.61b
Trans 1,3-Pentadiene
303-404
-0.05
c
16828.21
Unidentified HC
17091.74
17091.72
Propyne4
0-1
0.02
5
17439.47
17439.49
Propene
000-101
-0.02
17734.55
17734.53
Vinylacetylene
111-212
0.02
18146.56
18146.58
Vinylacetylene
101-202
-0.02
18343.15
18343.14
Cyclopropenylidene6
101-110
0.01
18564.88
18564.91
Vinylacetylene
110-211
-0.03
a
Observed minus known.
b
The known frequency and quantum numbers were determined by using the rotational
constants from Ref. 2 and the spectral programs JB957 and SPCAT.8
c
This is an unidentified hydrocarbon also present in the isoprene/O2 discharge
spectrum.
We have taken the ground state spectrum of several oxygenated molecules
present in the oxygenated isoprene discharge spectrum. The rotational constants have
been determined and compared with previously published values. These molecules
include antiperiplanar (ap) and synperiplanar (ap) methyl vinyl ketone (MVK),9 cis
and trans acrolein,10,11 trans-trans crotonaldehyde (CA)12 and trans methacrolein.13 The
known frequencies for these molecules are taken directly from the ground state spectra
obtained with our CP-FTMW spectrometer.
160
Table E28. The assigned transitions in the discharge spectrum of isoprene and
molecular oxygen.
Observed
Frequency
(MHz)
7312.83
7315.23
8142.98
8502.99
8506.25
8513.00
8902.20
9074.69
9325.80
12214.39
12579.57
12768.72
13110.16
13110.48
13298.95
13299.23
14193.44
14194.00
14246.80
14247.62
14426.84
14427.25
14488.47
14954.09
14956.98
15767.86
15768.86
16173.94
16174.76
16285.91
16770.58
16805.87
16806.32
16828.20
17023.59
17091.73
17143.02
17144.11
Known
Frequency
(MHz)
7312.86
7315.26
8142.99
8503.04
8506.29
8513.00
8902.18
9074.74
9325.80
12214.47
12579.55
12768.75
13110.18
13110.51
13298.96 b
13299.24 b
14193.47
14194.03
14246.77
14247.62
14426.88
14427.28
14488.48
14954.13
14956.99
15767.87
15768.91
16173.96
16174.80
16285.95
16770.61c
16805.90
16806.32
17023.61
17091.72
17143.05
17144.11
Molecule
sp MVK
sp MVK
1,3-Pentadiyne1
sp MVK
sp MVK
Trans-Trans CA
Trans Acrolein
Vinylacetylene3
Propynal14
1,3-Pentadiyne
Trans 1,3-Pentadiene2
Trans-Trans CA
ap MVK
ap MVK
Trans Methacrolein
Trans Methacrolein
ap MVK
ap MVK
ap MVK
ap MVK
Trans Methacrolein
Trans Methacrolein
Formaldehyde15
sp MVK
sp MVK
ap MVK
ap MVK
Trans Methacrolein
Trans Methacrolein
1,3-Pentadiyne
Trans 1,3-Pentadiene
Trans-Trans CA
Trans-Trans CA
Unidentified HCd
Trans-Trans CA
Propyne4
ap MVK
ap MVK
Transition
('Jka,kc)
OMKa
(MHz)
312-321 (E)
312-321 (A)
10-20
202-211 (E)
202-211 (A)
101-202
000-101
000-101
000-101
20-30
202-303
202-303
111-212 (E)
111-212 (A)
111-212 (E)
111-212 (A)
101-202 (E)
101-202 (A)
211-220 (A)
211-220 (E)
101-202 (E)
101-202 (A)
211-212
312-303 (A)
312-303 (E)
110-211 (E)
110-211 (A)
110-211 (E)
110-211 (A)
30-40
303-404
313-414 (A)
313-414 (E)
-0.03
-0.03
-0.01
-0.05
-0.04
0.00
0.02
-0.05
0.00
-0.08
0.02
-0.03
-0.02
-0.03
-0.01
-0.01
-0.03
-0.03
0.03
0.00
-0.04
-0.03
-0.01
-0.04
-0.01
-0.01
-0.05
-0.02
-0.04
-0.04
-0.03
-0.03
0.00
303-404
0-1
212-303 (E)
212-303 (A)
-0.02
0.01
-0.03
0.00
161
17387.60
17387.58
Trans Acrolein
111-212
0.02
5
17439.44
17439.49
Propene
000-101
-0.05
17734.53
17734.53
Vinylacetylene
111-212
0.00
17777.52
17777.52
ap MVK
101-212
0.00
17801.30
17801.30
Trans Acrolein
101-202
0.00
17843.74
17843.77
sp MVK
312-321 (E)
-0.03
17846.35
17846.38
sp MVK
312-321 (A)
-0.03
18146.54
18146.58
Vinylacetylene
101-202
-0.04
18221.15
18221.16
Trans Acrolein
101-211
-0.01
18325.52
18325.56
Propynal
111-212
-0.04
18343.14
18343.14
Cyclopropenylidene6
101-110
0.00
18650.30
18650.33
Propynal
101-202
-0.03
a
Observed minus known.
b
The known frequency and quantum numbers were determined by using the
rotational constants from Ref. 13 and the spectral program JB95.7
c
The known frequency and quantum numbers were determined by using the
rotational constants from Ref. 2 and the spectral programs JB957 and SPCAT.8
d
This is an unidentified hydrocarbon also present in the isoprene/O2 discharge
spectrum.
162
References
(1)
http://www.astro.uni-koeln.de/cdms. Catalog - #64507.
(2)
Hsu, S. L.; Flygare, W. H. J. Chem. Phys. 1970, 52, 1053-1057.
(3)
Hirose, C. Bull. Chem. Soc. Jpn. 1970, 43, 3695-3698.
(4)
Rhee, W.; Roberts, J. J. Mol. Spec. 1987, 126, 356-369.
(5)
Lide Jr., D. R.; Mann, D. E. J. Chem. Phys. 1957, 27, 868-873.
(6)
Matthews, J.; Irvine, W. The Astrophysical Journal 1985, 298, L61-L65.
(7)
Plusquellic, D. F.; Suenram, R. D.; Mate, B.; Jensen, J. O.; Samuels, A. C. J.
Chem. Phys. 2001, 115, 3057-3067.
(8)
Pickett, H. M. J. Mol. Spec. 1991, 148, 371-377.
(9)
Wilcox, D. S.; Shirar, A. J.; Williams, O. L.; Dian, B. C. Chem. Phys. Lett.
2011, 508, 10-16.
(10)
Wagner, R.; Fine, J.; Simmons, J.; Goldsten, J. J. Chem. Phys. 1957, 26, 634637.
(11)
Blom, C.; Bauder, A. Chem. Phys. Letters 1982, 88, 55-58.
(12)
Suzuki, M.; Kozima, K. Bull. Chem. Soc. Jpn. 1969, 42, 2183-2186.
(13)
Suzuki, M.; Kozima, K. J. Mol. Spec. 1971, 38, 314-321.
(14)
Winnewisser, G. J. Mol. Spec. 1973, 46, 16-24.
(15)
Bocquet, R.; Demaison, J.; Poteau, L.; Liedtke, M.; Belov, S.; Yamada, K.M.T.;
Winnewisser, G.; Gerke, C.; Gripp, J.; Kohler, T. J. Mol. Spec. 1996, 177, 154159.
163
Appendix F: Spectral Assignments Related to Butanol Experiments
The discharge transitions were assigned according to known frequencies that
originate from two sources. The most accurate comparison is with ground state spectra
taken with our CP-FTMW spectrometer. When possible, these spectra were compared
with published rotational constants; this process was conducted for acrolein.1,2 There are
no rotational parameters published for butanal, but the ground state spectrum of butanal
was acquired with our spectrometer. The fitting parameters and assigned transitions of
that spectrum are listed later in this Appendix.3
Table F29. The assigned transitions in the discharge spectrum of 1-butanol.
Observed
Frequency
(MHz)
Known
Frequency
(MHz)
Molecule
Transition
('Jka,kc)
OMKa
(MHz)
8052.84
8052.83
Cis-Gauche Butanal
111-202
0.01
8902.12
8902.18
Trans Acrolein
000-101
-0.06
9271.14
9271.11
Cis-Gauche Butanal
404-413
0.03
9391.79
9391.79
Cis-Trans Butanal
111-212
0.00
9664.60
9664.60
Cis-Trans Butanal
101-202
0.00
9946.57
9946.56
Cis-Trans Butanal
110-211
0.01
11437.11
11437.10
Cis-Gauche Butanal
000-111
0.01
12760.09
12760.09
Cis-Trans Butanal
414-505
0.00
12790.75
12790.74
Cis-Trans Butanal
101-110
0.01
13072.71
13072.70
Cis-Trans Butanal
202-211
0.01
13100.61
13100.59
Cis-Gauche Butanal
514-523
0.02
13474.18
13474.16
Cis-Gauche Butanal
413-422
0.02
13504.17
13504.15
Cis-Trans Butanal
303-312
0.02
14084.86
14084.85
Cis-Trans Butanal
212-313
0.01
14095.21
14095.20
Cis-Trans Butanal
404-413
0.01
14116.71
14116.69
Cis-Gauche Butanal
312-321
0.02
14485.50
14485.49
Cis-Trans Butanal
202-303
0.01
14488.48
14488.48
Formaldehyde 211-212
0.00
14821.17
14821.17
Cis-Gauche Butanal
211-220
0.00
14859.06
14859.05
Cis-Trans Butanal
505-514
0.01
14916.95
14916.94
Cis-Trans Butanal
211-312
0.01
14985.39
14985.38
Cis-Gauche Butanal
212-303
0.01
4
164
15811.59
Cis-Trans Butanal
606-615
0.03
16424.44
16424.43
5
Skew 1-Butene 101-202
0.01
16525.05
16525.00
Skew 1-Butene
110-211
0.05
16739.67
16828.23
16970.95
17091.75
17294.29
16739.67
16970.90
17091.72
17294.29
Cis-Gauche Butanal
Unidentified HCb
Cis-Trans Butanal
Propyne6
Cis-Gauche Butanal
212-221
707-716
0-1
101-212
0.00
0.05
0.03
0.00
17347.98
17347.97
Cis-Trans Butanal
000-111
0.01
17387.63
17387.58
Trans Acrolein
111-212
0.05
17439.47
17439.49
Propene 000-101
-0.02
17801.32
17801.30
Trans Acrolein
101-202
0.02
18149.08
18149.03
Cis-Trans Butanal
515-606
0.05
18343.14
Cyclopropenylidene 101-110
0.02
18650.33
Propynal 101-202
-0.02
15811.62
18343.16
18650.31
a
b
7
8
9
Observed minus known
Unidentified hydrocarbon that has been observed in several spectra.
2-Butanone peaks published by Pierce et al.10 do not have all the lines and those
that are similar are significantly different. All known frequencies here are acquired by
inputting the published rotational constants into JB95. Certain transitions for propanal
have two observed frequencies. The methyl group on this molecule results in A-E
splittings, which are occasionally observed. The differences are not significant enough
to fit any spectral parameters, but they are given for a more complete record of the
discharge spectrum.
Table F30. The assigned transitions in the discharge spectrum of 2-butanol.
Observed
Frequency
(MHz)
Known
Frequency
(MHz)
Molecule
Transition
('Jka,kc)
OMKa
(MHz)
7827.03
7827.02
Trans 2-Butanone
202-211 (E)
0.01
8212.43
8212.45
Skew 1-Butene
000-101
-0.02
8243.44
8243.48
11
Acetaldehyde
111-202
-0.04
8902.16
8902.18
Trans Acrolein
000-101
-0.02
9323.12
9323.17
Trans 2-Butanone
303-312 (A)
-0.05
165
9325.78
9325.80
Propynal
9332.04
9332.08
Trans 2-Butanone
303-312 (E)
-0.04
111-202
0.07
10098.84
10492.44
10492.47
Syn Propanal
000-101
-0.03
10720.59
10720.63
Acetaldehyde
413-4-14
-0.04
11627.12
11627.08
Trans 2-Butanone
404-413 (E)
0.04
11640.43
11640.40
Trans 2-Butanone
404-413 (A)
0.03
12014.96
12014.99
Acetaldehyde
303-212
-0.03
12022.62
12022.70
Trans 2-Butanone
000-111 (E)
-0.08
12070.63
Syn Propanal
101-110
12325.71
12325.68
Trans 2-Butanone
000-111 (A)
0.03
12635.23
12635.24
Acetaldehyde
303-2-12
-0.01
13474.90
Syn Propanal
202-211
13487.36
13487.36
Trans 2-Butanone
212-303 (A)
0.00
13627.70
13627.73
Trans 2-Butanone
212-303 (E)
-0.03
14488.48
14488.48
Formaldehyde
211-212
0.00
15778.70
15778.67
Syn Propanal
303-312
0.03
16424.45
16424.43
Skew 1-Butene
101-202
0.02
16525.04
16525.00
Skew 1-Butene
110-211
0.04
12070.69
13474.81
13475.04
16924.37
-0.05
0.06
-0.09
-0.14
b
16828.24
b
-0.02
10098.91
12070.58
a
Syn Propanal
12
000-101
Unidentified HC
-0.08
16924.94
17091.76
17387.62
17439.47
17664.42
16924.45
Gauche Propanal13
101-202
17091.72
17387.58
17439.49
17664.34
Propyne
Trans Acrolein
Propene
Trans 2-Butanone
0-1
111-212
000-101
101-212 (E)
0.49
0.04
0.04
-0.02
0.08
17801.33
17801.30
Trans Acrolein
101-202
0.03
17818.91
17818.92
Trans 2-Butanone
101-212 (A)
-0.01
18221.17
18221.16
Trans Acrolein
101-211
0.01
18343.17
18343.14
Cyclopropenylidene
101-110
0.03
18650.32
18650.33
Propynal
101-202
-0.01
Observed minus known
Unidentified hydrocarbon that has been observed in several spectra.
166
Table F31. The preliminary spectral parameters for three conformers of butanal.
cis/trans Butanal
(CCC'O/CCCC')
(0°,180°)
Parameters
Publisheda
A (MHz)
B (MHz)
C (MHz)
DK
DJK
DJ
dk
dj
OMCd (MHz)
# Assigned Lines
Relative Energy
(kJ/mol)
ZPEe
0
MM3
MM4
0
0
Parameters
Publisheda
A (MHz)
B (MHz)
C (MHz)
DK
DJK
DJ
dk
dj
OMCd (MHz)
# Assigned Lines
a
Gaussianb
JB95c
Gaussianb
JB95c
15271.6269
2513.7454
2247.4056
15075.473688
2556.133355
2278.457085
6.1306
-5.0218x10-3
6.1191x10-4
7.3299x10-2
1.3969x10-4
0.008060
21
10173.2668
2912.6967
2528.8766
9960.364621
3021.861135
2591.258413
1.8459x10-1
-5.6835x10-2
6.3460x10-3
9.9180x10-3
1.8918x10-3
0.012320
38
0
B3LYP
Relative Energy
(kJ/mol)
ZPEe
B3LYP
MM3
MM4
trans/gauche Butanal
(CCC'O/CCCC')
(-130°,68°)
cis/gauche Butanal
(CCC'O/CCCC')
(6°,73°)
Gaussianb
JB95c
8599.8814
8508.527049
3489.2307
3588.809186
2866.3073
2928.574885
2.3344x10-2
-1.2037x10-2
3.6346x10-3
5.2819x10-3
1.1012x10-3
0.014653
58
1.882483
1.1286
1.6720
0.6270
Langley et al.14
Density function theory (B3LYP/6-311++G(d,p)); Frisch et al.15
c
Plusquellic et al.16
d
Observed Minus Calculated
e
Zero point corrected energies.
b
3.568054
167
Table F32. The assigned transitions for the cis-trans conformer of butanal.
Transition
(Jka,kc)
Observed
Frequency
(MHz)
Calculated
Frequency
(MHz)
OMCa (MHz)
313-404
7452.7608
7452.7664
-0.006
111-212
9391.7898
9391.8028
-0.013
101-202
9664.5978
9664.6038
-0.006
110-211
9946.5588
9946.5600
-0.001
414-505
12760.0859
12760.0769
0.009
101-110
12790.7359
12790.7489
-0.013
202-211
13072.6989
13072.7050
-0.006
303-312
13504.1549
13504.1534
0.002
212-313
14084.8529
14084.8604
-0.007
404-413
14095.1999
14095.1955
0.004
202-303
14485.4889
14485.4842
0.005
221-322
14503.8219
14503.8258
-0.004
220-321
14522.0509
14522.0468
0.004
505-514
14859.0479
14859.0334
0.014
211-312
14916.9409
14916.9325
0.008
606-615
15811.5859
15811.5964
-0.011
707-716
16970.9039
16970.9140
-0.010
000-111
17347.9659
17347.9549
0.011
515-606
18149.0299
18149.0349
-0.005
808-817
18356.1719
18356.1653
0.007
313-414
18774.5759
18774.5710
0.005
a
Observed minus calculated.
Table F33. The assigned transitions for the trans-gauche conformer of butanal.
Transition
(Jka,kc)
Observed
Frequency
(MHz)
Calculated
Frequency
(MHz)
OMCa (MHz)
101-111
6938.4428
6938.4600
-0.017
101-110
7368.9958
7369.0079
-0.012
524-431
7809.0278
7809.0267
0.001
202-211
7819.1468
7819.1535
-0.007
168
a
303-312
8530.0948
8530.0988
-0.004
211-303
8942.1288
8942.1431
-0.014
616-615
8989.5708
8989.5664
0.004
422-514
9411.5128
9411.5131
0.000
404-413
9542.3918
9542.3903
0.002
423-514
9699.9648
9699.9652
0.000
212-303
10233.6868
10233.6961
-0.009
111-212
10795.7558
10795.7609
-0.005
505-514
10904.1478
10904.1423
0.006
101-202
11206.6218
11206.6202
0.002
110-211
11656.7668
11656.7659
0.001
000-111
12551.5618
12551.5541
0.008
606-615
12662.8518
12662.8582
-0.006
000-110
12982.1139
12982.1020
0.012
312-404
13729.0759
13729.0822
-0.006
707-716
14854.1379
14854.1448
-0.007
716-726
15754.5399
15754.5345
0.005
523-615
15806.9139
15806.9073
0.007
212-313
16181.5329
16181.5091
0.024
313-404
16311.3759
16311.3680
0.008
524-615
16471.6649
16471.6649
0.000
202-303
16761.3209
16761.2967
0.024
221-322
16840.0269
16840.0373
-0.010
220-321
16917.3989
16917.4139
-0.015
615-625
17117.3179
17117.3090
0.009
211-312
17472.2389
17472.2420
-0.003
808-817
17490.3599
17490.3591
0.001
101-212
17734.2259
17734.2208
0.005
817-826
17906.4259
17906.4007
0.025
716-725
18031.1069
18031.1448
-0.038
918-927
18120.9499
18120.9427
0.007
413-505
18143.6169
18143.6267
-0.010
5145-524
18321.7039
18321.6911
0.013
615-624
18420.1809
Observed minus calculated.
18420.2020
-0.021
169
Table F34. The assigned transitions for the cis-gauche conformer of butanal.
Transition
(Jka,kc)
Observed
Frequency
(MHz)
Calculated
Frequency
(MHz)
OMCa (MHz)
000-101
6517.3458
6517.3695
-0.0237
110-202
7392.6228
7392.6354
-0.0126
303-312
7496.4318
7496.4482
-0.0164
716-726
7701.4558
7701.4569
-0.0011
431-523
7973.2838
7973.2859
-0.0021
432-523
7998.8218
7998.8224
-0.0006
111-202
8052.8278
8052.8398
-0.0120
404-413
9271.1148
9271.1225
-0.0077
642-735
9357.8418
9357.8264
0.0154
615-625
9445.4928
9445.4962
-0.0034
515-514
9800.0888
9800.0939
-0.0051
643-734
10089.2788
10089.2717
0.0071
422-515
10649.5308
10649.5378
-0.0070
514-524
11101.4448
11101.4562
-0.0114
000-111
11437.0998
11437.1031
-0.0033
423-515
11546.3058
11546.3074
-0.0016
505-514
11713.6298
11713.6315
-0.0017
532-625
11959.3518
11959.3327
0.0191
533-625
12060.4548
12060.4463
0.0085
000-110
12097.3068
12097.3075
-0.0007
111-212
12374.5268
12374.5220
0.0048
854-945
12531.3888
12531.3884
0.0004
413-423
12577.3878
12577.3992
-0.0114
101-202
12972.5769
12972.5733
0.0036
211-303
13004.7959
13004.7971
-0.0012
514-523
13100.5899
13100.6074
-0.0175
321-413
13136.4499
13136.4362
0.0137
615-624
13184.5729
13184.5950
-0.0221
322-413
13443.2009
13443.2348
-0.0339
413-422
13474.1609
13474.1689
-0.0080
616-615
13562.5839
13562.5396
0.0443
110-211
13694.8849
13694.8779
0.0070
170
a
312-322
13809.8919
13809.8947
-0.0028
523-616
13817.2329
13817.2378
-0.0049
716-725
13875.4229
13875.4364
-0.0135
312-321
14116.6919
14116.6933
-0.0014
211-221
14759.0929
14759.0914
0.0015
211-220
14821.1689
14821.1699
-0.0010
606-615
14839.1809
14839.1887
-0.0078
212-303
14985.3769
14985.3573
0.0196
817-826
15285.4319
15285.3947
0.0372
532-624
15698.4179
15698.4316
-0.0137
533-624
15799.5309
15799.5451
-0.0142
524-616
15816.3749
15816.3889
-0.0140
743-836
15911.8929
15911.8909
0.0020
624-717
15946.7919
15946.7964
-0.0045
212-221
16739.6699
16739.6517
0.0182
212-220
16801.7429
16801.7302
0.0127
725-818
17003.4889
17003.4709
0.0180
101-212
17294.2889
17294.2556
0.0333
744-835
17462.5009
17462.5262
-0.0253
918-927
17488.2449
17488.2350
0.0099
633-726
17554.3639
17554.3593
0.0046
313-322
17767.1799
17767.1896
-0.0097
312-404
17982.0209
17982.0071
0.0138
313-321
18073.9829
18073.9882
-0.0053
212-313
18524.5119
18524.5107
0.0012
707-716
18564.8869
Observed minus calculated.
18564.9034
-0.0165
171
References
(1)
Wagner, R.; Fine, J.; Simmons, J.; Goldsten, J. J. Chem. Phys. 1957, 26, 634637.
(2)
Blom, C.; Bauder, A. Chem. Phys. Letters 1982, 88, 55-58.
(3)
Personal Communication with Kelly Hotopp.
(4)
Bocquet, R.; Demaison, J.; Poteau, L.; Liedtke, M.; Belov, S.; Yamada, K.M.T.;
Winnewisser, G.; Gerke, C.; Gripp, J.; Kohler, T. J. Mol. Spec. 1996, 177, 154159.
(5)
Kondo, S.; Hirota, E.; Morino, Y. J. Mol. Spec. 1968, 28, 471-489.
(6)
Rhee, W.; Roberts, J. J. Mol. Spec. 1987, 126, 356-369.
(7)
Lide Jr., D. R.; Mann, D. E. J. Chem. Phys. 1957, 27, 868-873.
(8)
Matthews, J.; Irvine, W. The Astrophysical Journal 1985, 298, L61-L65.
(9)
Winnewisser, G. J. Mol. Spec. 1973, 46, 16-24.
(10)
Pierce, L.; Chang, C. K.; Hayashi, M.; Nelson, R. J. Mol. Spec. 1969, 5, 449457.
(11)
Kleiner, I.; Lovas, F. J.; Godefroid, M. J. Phys. Chem. Ref. Data 1996, 25,
1113-1210.
(12)
http://www.astro.uni-koeln.de/cdms. Catalog - #58505.
(13)
Pickett, H. M.; Scroggin, D. G. J. Chem. Phys. 1974, 61, 3954-3958.
(14)
Langley, C. H.; Lii, J.-H.; Allinger, N. L. J. Comput. Chem. 2001, 22, 13961425.
(15)
Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.;
Cheeseman, J. R.; Montgomery Jr., J. A.; Vreven, T.; Kudin, K. N.; Burant, J.
C.; et al. Gaussian 03, Revision C.02; Gaussian, Inc.: Wallingford, CT, 2004.
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Plusquellic, D. F.; Suenram, R. D.; Mate, B.; Jensen, J. O.; Samuels, A. C. J.
Chem. Phys. 2001, 115, 3057-3067.
172
Appendix G: Database of Published Rotational Spectra
Table G35. A list of previously reported rotational constants and microwave frequency
transitions of molecules related to the discharge spectra.
Frequency
(MHz)
Transition
(Jka,kc)
Molecule
Formula
Mass
(g/mol)
A (MHz)
B (MHz)
C (MHz)
7733.2853
707-717
1-Butanol (Gtg)1
C4H10O
74
12304.9926
2330.5978
2146.2295
8270.1941
606-616
8357.4538
313-404
8754.8479
505-515
8769.2863
111-212
8951.1222
101-202
9138.0229
110-211
9176.1187
404-414
9213.8179
515-423
9251.7711
515-422
9524.5606
303-313
9792.5589
202-212
9974.3948
101-111
10158.7631
101-110
10345.6638
202-211
10630.7487
303-312
11019.6495
404-413
11313.9841
413-505
11377.3104
413-322
11389.9690
413-321
11519.7638
505-514
12140.1033
606-615
12891.0375
707-716
13152.3575
212-313
13157.5150
414-505
13220.8412
414-322
13233.4998
414-321
13420.3557
202-303
13430.4819
221-322
13440.6081
220-321
13705.4406
211-312
13783.9016
808-817
14451.2221
000-111
14635.5904
000-110
15253.0920
514-606
16217.7438
312-221
16220.2761
312-220
173
17323.9318
313-221
1-Butanol (Gtg)
17326.4642
313-220
Cont’d
17533.5725
313-414
17882.0145
303-404
17905.3395
322-423
17912.3093
331-432
17912.4397
330-431
17930.6341
321-422
18018.0079
515-606
18270.9153
312-413
18743.6811
101-212
7993.3004
707-717
8183.7566
313-404
8522.8357
606-616
8800.8578
111-212
8978.7861
101-202
8999.6444
505-515
9161.4550
110-211
9413.2763
404-414
9754.8781
303-313
9770.0176
515-423
9805.5443
515-422
10017.3194
202-212
10195.2477
101-111
10375.5463
101-110
10558.2152
202-211
10836.6502
303-312
11189.4831
413-505
11216.1260
404-413
11703.5782
505-514
11998.6760
413-322
12010.5247
413-321
12307.4712
606-615
12992.3328
414-505
13037.5755
707-716
13199.8149
212-313
13462.2562
202-303
13471.7346
221-322
13481.2130
220-321
13740.6912
211-312
13801.5257
414-322
13813.3743
414-321
13904.6265
808-817
14685.8259
000-111
14866.1245
000-110
1-Butanol
(Tgg')1
C4H10O
74
12530.6861
2335.4384
2155.1398
174
15157.3812
514-606
16845.0519
312-221
16847.4222
312-220
17597.0330
313-414
17861.3150
515-606
17926.8240
313-221
17929.1943
313-220
17938.6347
303-404
17960.4692
322-423
17966.9954
331-432
17967.1123
330-431
17984.1471
321-422
18318.1105
312-413
7899.7646
707-717
8402.2818
606-616
8480.8892
313-404
8853.8329
505-515
8865.0260
111-212
8903.3882
515-423
8935.6191
515-422
9032.7761
101-202
9204.8268
110-211
9244.9129
404-414
9567.4817
303-313
9815.0690
202-211
9982.8191
101-111
10152.7195
101-110
10324.7702
202-211
10586.8670
303-312
10943.7978
404-413
11289.6576
413-322
11300.4067
413-321
11401.8627
505-514
11600.6168
413-505
11968.7621
606-615
12653.4072
707-716
12988.5426
414-322
12999.2916
414-321
13296.2035
212-313
13299.5017
414-505
13543.7908
202-303
13552.3896
221-322
13560.9884
220-321
13805.8877
211-312
14500.2823
000-111
1-Butanol (Tgg')
Cont’d
1-Butanol (Tgg)1
C4H10O
74
12326.5009
2343.6818
2173.7814
175
14670.1827
000-110
15628.0355
514-606
16142.5696
312-221
16144.7199
312-220
17161.9550
313-221
17164.1053
313-220
17725.8020
313-414
18048.3708
303-404
18068.1803
322-423
18074.1028
331-432
18074.2048
330-431
18089.6621
321-422
18176.0654
515-606
18405.3016
312-413
18847.8451
101-212
7877.5509
707-717
8410.2367
606-616
8567.8042
313-404
8890.3978
505-515
8939.9582
111-212
9119.5561
101-202
9136.9391
515-423
9173.5020
515-422
9304.0330
110-211
9307.2939
404-414
9651.8184
303-313
9916.6341
202-212
10096.2320
101-111
10278.2694
101-110
10462.7463
202-211
10744.0224
303-312
11127.5249
404-413
11414.0498
413-322
11426.2443
413-321
11620.3868
505-514
11628.8066
413-505
12231.3066
606-615
12970.3118
707-716
13234.2807
414-322
13246.4753
414-321
13408.4227
212-313
13449.0375
414-505
13673.2383
202-303
13682.9934
221-322
13692.7485
220-321
1-Butanol (Tgg)
Cont’d
1-Butanol (Tgt)1
C4H10O
74
12467.749
2371.5176
2189.4802
176
13954.5145
211-312
1-Butanol (Tgt)
14657.2298
000-111
Cont’d
14839.2672
000-110
15660.6989
514-606
16334.1815
312-221
16336.6210
312-220
17426.3855
313-221
17428.8250
313-220
17875.0982
313-414
18219.6227
303-404
18242.0937
322-423
18248.8094
331-432
18248.9320
330-431
18266.4621
321-422
18390.6879
515-606
18603.1252
312-413
7555.4951
111-212
7652.7134
101-202
7750.7829
110-211
8911.8568
615-707
9100.1799
202-111
9197.8238
202-110
10962.2813
616-707
11332.9772
212-313
11478.0062
202-303
11479.7085
221-322
11481.4108
220-321
11625.9078
211-312
12366.6498
716-808
15100.4356
717-808
15110.1425
313-414
15113.1782
808-818
15302.0226
303-404
15305.9470
322-423
15310.2024
321-422
15476.4384
707-717
15500.7111
312-413
15799.8985
606-616
16081.3158
505-515
16318.7660
404-414
16510.6460
303-313
16655.6750
202-212
16752.8933
101-111
16850.5372
101-110
16948.6067
202-211
1-Butanol (Ttg)1
C4H10O
74
18715
1962.1067
1864.4628
177
17096.5083
303-312
1-Butanol (Ttg)
17295.1969
404-413
Cont’d
17545.9419
505-514
17850.3231
606-615
18210.2242
707-716
18627.8231
808-817
18886.8879
414-515
7600.7723
111-212
7704.5652
101-202
7809.3329
110-211
8975.9997
202-111
9080.2800
202-110
9086.0438
615-707
11275.8105
616-707
11400.8545
212-313
11555.6293
202-303
11557.5789
221-322
11559.5285
220-321
11713.6940
211-312
12539.2231
716-808
14934.7467
808-818
15200.5740
313-414
15320.5845
707-717
15405.2317
303-404
15409.7261
322-423
15414.5996
321-422
15458.7726
717-808
15617.6867
312-413
15664.6134
606-616
15964.2663
505-515
16217.3395
404-414
16421.9972
303-313
16576.7720
202-212
16680.5649
101-111
16784.8452
101-110
16889.6129
202-211
17047.6776
303-312
17260.1326
404-413
17528.4309
505-514
17854.3801
606-615
18240.1339
707-716
18688.1799
808-817
8633.3398
000-101
10223.8596
312-404
12981.4109
313-404
1-Butanol (Ttt)1
C4H10O
74
18658.9682
1978.4033
1874.123
1-Butyne2
C4H6
54
27147.7589
4546.4713
4086.8685
178
16807.0768
111-212
1-Butyne
17259.7411
101-202
Cont’d
17726.2824
110-211
17813.7758
413-505
9305.59
9396.49
11054.39
11055.23
202-211
1-Methylcyclobutene3
C4H8
56
11679.74
4219.48
3292.00
1-Methylcylopropene4
C4H6
54
20556.191
6356.981
5176.421
1,1-Dimethylcyclopropane5
C5H10
70
6135.257
5203.342
3810.509
303-312
14095.49
111-212
14941.76
101-202
15950.48
110-211
11533.51
000-101
11787.68
414-413
15379.86
101-110
16630.94
201-211
18636.95
303-312
7770.4572
212-220
8012.5523
313-312
8094.0359
303-312
8554.5822
322-331
8899.0279
322-330
8920.2657
431-440
9013.8510
000-101
9467.4182
313-322
9557.9018
303-322
9945.7660
000-111
10018.4481
312-331
10240.1636
423-432
10643.5690
432-441
10762.0916
413-432
11338.5990
000-110
12081.9829
414-413
12098.2410
404-413
12520.9940
313-321
12603.9110
414-423
12620.1690
404-423
14906.7408
110-202
15920.6814
505-514
16055.8788
515-524
16299.5738
111-202
16634.8690
111-212
16687.4380
220-303
16768.9216
220-313
17231.4888
101-202
17565.1348
221-313
179
17566.7840
101-212
12744.621
101-202
18855.109
202-303
7764.398
422-515
7829.698
111-202
7940.322
432-523
8231.030
615-625
8330.237
303-312
8439.098
414-413
9135.908
726-725
9187.456
423-515
10212.644
514-524
10336.392
532-625
10733.597
404-413
11363.016
000-111
12012.979
111-212
12037.136
414-423
12169.580
211-303
12212.139
000-110
12531.724
515-514
12761.642
101-202
13115.007
321-413
13321.315
514-523
13460.192
413-422
13588.939
312-322
13608.395
322-413
13771.223
110-211
13904.984
615-624
14024.219
505-514
14082.330
212-321
14716.948
212-303
14795.734
211-221
14896.214
211-220
15243.724
633-726
15388.141
716-725
16463.736
312-404
16944.923
101-212
17233.586
616-615
17343.103
212-221
1,1-DMCP
Cont’d
1,2-Propanediol (gG't)6
1,2-Propanediol (tG'g)6
C3H8O2
76
8393.4003
3648.5661
2778.2963
C3H8O2
76
8572.0553
3640.1063
2790.9666
180
17443.584
212-220
17887.912
817-826
17959.964
212-313
18900.228
202-303
7580.520
211-220
8337.544
404-413
9770.492
212-221
9928.654
212-220
10012.656
001-111
10795.372
000-110
11034.964
313-322
11662.018
110-202
11787.314
313-322
12444.740
111-202
12740.140
414-423
12894.760
422-431
13686.086
321-330
14294.285
111-212
14417.958
322-331
14438.430
322-330
14918.861
101-202
15859.702
110-211
16097.870
220-312
16768.407
101-212
7764.1300
000-101
7775.5300
000-111
10136.3200
000-110
11831.3806
533-532
11831.3832
523-532
11831.9721
423-422
11831.9727
413-422
11832.2698
313-312
11832.2699
303-312
11832.4757
313-322
11832.4758
303-322
11832.5898
423-432
11832.5905
413-432
11832.8220
533-542
11832.8246
523-542
13167.4288
111-202
13167.4700
111-212
13178.8288
101-202
13178.8700
101-212
16565.3026
524-523
16565.3052
524-533
1,2-Propanediol (tG'g)
Cont’d
1,2-Propanediol (tGg')6
C3H8O2
76
6634.7621
4160.6347
3377.9063
1,3-Hexadiene7
C6H8
80
5073.86
5062.46
2701.67
181
16565.3536
414-413
1,3-Hexadiene
16565.3542
414-423
Cont’d
17889.0500
110-211
17923.2500
110-221
18576.4842
212-303
18576.4843
212-313
18576.5254
202-303
18576.5255
202-313
9577.53
111-212
9923.5
101-202
10283.63
110-211
14361.95
212-313
14867.49
202-303
14895.96
221-322
14924.15
220-321
15420.91
211-312
8260.24
111-212
8387.16
101-202
8515.04
110-211
12390.07
212-313
12579.55
202-303
12581.47
221-322
12583.40
220-321
12772.27
211-312
16519.60
313-414
17029.16
312-413
16899.14
202-211
18100.37
515-606
18143.73
404-413
7734.8878
303-312
7750.8085
414-413
9644.8972
514-524
9932.4656
404-413
10760.2155
000-111
11326.5950
413-423
11498.1816
111-212
11513.7849
515-514
11539.9120
000-110
11791.1318
211-303
12187.3186
101-202
12611.8293
413-422
12713.2231
321-413
12754.3806
312-322
13057.5676
110-211
13158.2636
322-413
1,3-Pentadiene
(cis)8
C5H7
55
15600
2659.17
2306.12
1,3-Pentadiene
(trans)8
C5H7
55
28200
2160.61
2033.21
1,4-Pentadiyne9
C5H4
64
19076.77
2859.224
2520.801
2-Butanol
(e-ga)10
C4H10O
74
8080.5979
3459.3327
2679.6247
182
13199.4159
312-321
13863.6990
211-221
2-Butanol
(e-ga)
13954.2716
211-220
Cont’d
14130.2128
212-303
15980.1683
312-404
16119.4423
101-212
16202.7806
212-221
16293.3556
212-220
17193.5838
212-313
17425.8902
313-322
17870.9278
313-321
18062.3362
202-303
7857.2512
111-202
10288.9646
000-111
11093.1609
000-110
11363.0536
111-212
12062.5864
101-202
12971.4325
110-211
14337.6083
212-303
15568.3811
101-212
16982.8973
212-313
17843.4065
202-303
17980.9584
101-211
9785.2187
404-413
12133.0896
101-202
12351.8370
413-422
12481.4119
312-322
12925.2141
312-321
12994.3877
110-211
17125.1513
212-313
17981.6719
202-303
7576.8945
303-312
7789.7553
111-202
10266.8528
413-423
10372.1762
000-111
11186.1730
000-110
11364.1761
111-212
11840.6118
211-303
12072.8270
101-202
12214.3852
312-321
12849.1642
211-221
12954.5292
211-220
12992.1611
110-211
14282.5895
212-303
15291.1431
212-221
2-Butanol
(h-ag)11
C4H10O
74
7649.2575
3443.9171
2639.7225
2-Butanol
(h-ga)11
C4H10O
74
7967.301
3441.3521
2670.3779
2-Butanol
(m-ag)10
C4H10O
74
7734.632
3451.5529
2637.5524
183
15396.5055
212-220
15647.2515
101-212
2-Butanol
(m-ag)
17089.7043
313-321
Cont’d
17857.0109
202-303
18089.2287
101-211
7717.1517
414-413
7752.2734
303-312
9934.5278
404-413
10757.0882
000-111
11424.7930
111-212
11441.5217
413-423
11533.2987
000-110
11654.1731
211-303
12111.8372
101-202
12708.3561
413-422
12864.5965
312-322
12977.2065
110-211
17084.2993
212-313
17952.3242
202-303
9854.1099
212-220
10289.7052
000-110
10304.6619
221-313
10725.2454
110-202
11405.7044
313-321
13405.6056
111-212
13970.0586
303-312
14773.8203
110-211
14972.7603
423-431
9323.10
303-312
11640.49
404-413
12325.78
000-111
14828.80
505-514
16170.78
615-624
17203.30
716-725
17818.98
101-212
18891.30
606-615
8304.2509
111-212
8334.2183
101-202
8364.2095
110-211
12456.3689
212-313
12501.2977
202-303
12546.3068
211-312
16608.4780
313-414
16668.3415
303-404
16728.3952
312-413
2-Butanol
(m-ga)11
C4H10O
74
8094.9464
3438.3702
2662.1479
2-Butanol
(m-gg)10
C4H10O
74
6425.2164
3864.5047
3180.3917
2-Butanone
(trans)12
C4H8O
72
9544.63
3596.85
2746.51
2-Butynal13
C4H4O
68
58759.3217
2098.5472
2068.5679
184
7986.272
111-212
8106.515
101-202
8227.827
110-211
11979.127
212-313
12158.675
201-303
12161.040
221-322
12162.752
220-321
12341.446
211-312
15971.626
313-414
16209.514
303-404
16214.337
322-423
16216.593
331-432
16216.593
330-431
16218.622
321-422
16454.697
312-413
8051.340
101-202
8110.815
1-10-221
8130.275
111-2-11
11099.926
303-212
11684.283
3-12-414
12077.725
202-303
12161.040
212-313
12166.537
221-3-21
12212.413
2-11-3-12
15546.535
4-13-515
16104.911
303-404
16180.173
313-414
16222.686
3-21-4-22
16224.515
322-423
16313.153
3-12-4-13
9482.2790
111-202
11516.1950
303-312
13343.4627
000-111
13968.6687
111-212
15140.6234
101-202
15782.8920
404-413
16139.7098
312-321
16771.7137
110-211
17204.4980
211-220
17688.5854
212-303
10108.5
303-312
12214.6
000-111
12578.0
111-212
13567.9
101-202
14680.8
413-422
2-Butynol (A)14
C4H6O
70
23744
2093.429
1966.358
2-Hydroxy-2propen-1-al15
C3H4O2
72
10201.6867
4543.3353
3141.7866
2-Methyl-1buten-3-yne 16
C5H6
66
9359.58
4013.52
2854.80
2-Butynol (E)14
185
14895.3
110-211
14955.9
514-523
2-Methyl-1buten-3-yne
15235.5
312-321
Cont’d
17924.0
101-212
18767.9
212-313
14312.68
514-523
16489.77
101-212
16787.11
212-313
17084.82
716-725
17841.70
202-303
7831.8642
303-313
8161.0858
202-212
8386.4100
101-111
8616.4000
101-110
8851.0558
202-211
9211.7451
303-312
9708.7563
404-413
9715.6100
111-212
9898.7913
312-221
9903.4571
312-220
9940.9342
101-202
10175.5900
110-211
10355.4292
505-514
10632.9122
312-404
11167.6129
606-615
11278.6721
313-221
11283.3380
313-220
12012.7930
313-404
12162.7575
707-716
13359.2100
000-111
13589.2000
000-110
14570.5279
212-313
14899.7495
202-303
14918.4000
221-322
14937.0505
220-321
15062.5037
413-505
15260.4387
211-312
17361.9896
414-505
18102.0200
101-212
18791.9900
101-211
8094.1484
303-312
8453.4694
321-413
8662.3119
322-413
9587.6648
404-413
10977.3779
211-303
2-Methylfuran17
C5H6O
82
8791.615
3543.278
2565.635
2-Pentene (cis)18
C5H10
70
10987.805
2601.395
2371.405
3-Buten-1-ol
(A)19
C4H8O
72
9185.7309
3330.05915
2742.3666
186
11557.1590
111-212
11635.4016
505-514
11928.0975
000-111
12102.8003
101-202
12515.7901
000-110
12675.4111
615-625
12732.5440
110-211
12740.4556
212-303
14236.4688
514-524
14289.4167
606-615
15353.7305
615-624
15415.3405
422-514
15594.6432
413-423
15635.7906
514-523
15787.4134
312-404
16032.2541
423-514
16211.5568
413-422
16712.7662
312-322
16921.6087
312-321
17310.3350
212-313
17412.8307
101-212
17567.0153
211-221
17609.0665
211-220
18050.4860
202-303
18217.2773
221-322
18384.0685
220-321
7916.8722
514-606
8338.2170
515-606
8605.2257
111-212
8633.2821
101-202
8661.4051
110-211
9122.9767
202-111
9151.0664
202-110
12147.6632
615-707
12737.5449
616-707
12907.8177
212-313
12949.8399
202-303
12949.9731
221-322
12950.1063
220-321
12992.0868
211-312
16363.6487
716-808
17150.1551
717-808
17210.3849
313-414
17266.2978
303-404
17266.6049
322-423
3-Buten-1-ol (A)
Cont’d
3-Buten-1-ol
(B)19
C4H8O
72
19928.6325
2172.3737
2144.284
187
17266.9379
321-422
17270.0848
808-818
17322.7435
312-413
17380.3096
707-717
17477.1943
606-616
17560.5637
505-515
17630.2673
404-414
17686.1803
303-313
17728.2024
202-212
17756.2588
101-111
17784.3485
101-110
17812.4715
202-211
17854.7184
303-312
17911.1642
404-413
17981.9085
505-514
18067.0760
606-615
18166.8160
707-716
18281.3026
808-817
7771.8478
303-312
7839.4613
414-413
8176.6479
111-202
9593.0523
514-524
10000.5614
404-413
10776.5434
423-515
11061.6750
000-111
11290.2397
413-423
11643.0492
515-514
11850.3850
000-110
12098.6060
111-212
12463.8966
514-523
12603.4402
413-422
12680.4697
211-303
12732.7075
312-322
12794.6649
101-202
12991.6965
615-624
13053.0932
505-514
13187.7748
312-321
13676.0260
110-211
13854.0510
211-221
13946.7021
211-220
13957.0170
321-413
14412.0843
322-413
15046.5997
212-303
16220.1810
212-221
16312.8321
212-220
3-Buten-1-ol (B)
Cont’d
3-Buten-2-ol
(A)20
C4H8O
72
8234.201
3616.184
2827.474
188
16716.6230
101-212
16874.3209
606-615
17144.2304
312-404
17458.1082
313-322
17913.1754
313-321
18093.0468
212-313
18968.5578
202-303
7502.9598
404-313
8450.9579
404-312
9370.0000
111-212
9527.3072
101-202
9686.0000
110-211
12497.4642
303-212
12971.4642
303-211
14054.5679
212-313
14289.2287
202-303
14292.0000
221-322
14294.7713
220-321
14528.5660
211-312
17416.6928
202-111
17574.6928
202-110
18738.6200
313-414
8488.6232
111-202
9364.9599
413-423
9656.2674
404-413
10277.9690
000-111
10817.0146
312-322
10912.1764
413-422
11011.1431
514-523
11087.3420
000-110
11359.8120
312-321
11650.9662
413-422
11775.8290
514-523
11877.1029
000-110
11878.7926
312-321
11956.1040
423-515
12067.3148
111-212
12473.9912
615-624
12730.8916
211-221
12870.9051
211-220
13394.5750
101-202
14384.2230
211-303
14495.4338
505-514
15159.0106
110-211
15175.7450
212-221
3-Buten-2-ol (A)
Cont’d
3-Butynal
(Trans)21
C4H4O
68
29405
2461
2303
3-Methyl-1butene22
C5H10
70
7536.355
3550.987
2741.614
189
15663.5927
212-220
15718.5424
212-303
3-Methyl-1butene
15761.1970
321-413
Cont’d
16206.3901
313-321
16803.4849
606-615
16879.2896
312-404
17391.8019
414-423
17598.4333
212-313
17671.4288
202-221
18103.0154
303-322
18189.3160
101-211
18446.2164
202-303
18507.5575
624-633
18877.8030
221-322
18939.0183
414-422
9191.84
937-936
12328.44
111-212
13165.12
101-202
14319.54
110-211
17672.43
826-827
18400.14
212-313
9052.39
303-312
10804.93
111-212
11372.40
000-111
11528.80
404-413
11593.80
101-202
12415.00
212-303
12580.18
110-211
15084.60
413-422
15797.84
312-321
16149.15
212-313
16331.29
101-212
17152.28
202-303
17538.95
221-322
17925.17
220-321
18804.80
211-312
7983.7983
413-422
8032.4903
422-431
8814.1981
313-312
8883.1885
321-331
9049.2696
303-312
9080.2800
212-221
9111.2904
321-330
9199.4500
000-101
9776.3274
212-220
3-Methyl-1butyne23
C5H8
68
7969.48
3828.801
2833.216
3-Methylfuran24
C5H6O
82
8893.16
3366.91
2479.32
3,3Dimethylcyclop
ropene25
C5H8
68
6872.96
5353.25
3846.2
190
9788.9874
202-221
10719.1600
000-111
11709.9379
313-3223
11735.2579
322-331
11945.0094
303-322
11963.3598
322-330
12226.2100
000-110
13259.7579
423-432
13710.3891
414-413
13774.3376
404-413
13861.9836
431-440
14562.0073
313-321
14619.8689
423-431
14630.9977
312-331
14656.1777
413-432
14676.0926
110-202
14957.2932
220-303
15106.8088
414-423
15160.7912
432-441
15170.7574
404-423
15192.3647
220-313
15888.4121
221-313
16183.1426
111-202
16891.8500
111-212
17702.8526
101-202
18411.5600
101-212
18507.8170
505-514
7579.2970
202-313
7723.7280
413-423
7796.3610
413-422
7986.9930
312-322
8285.6370
414-515
8387.8780
202-312
8547.2280
404-505
8631.9590
423-524
8653.6500
441-542
8653.6500
440-541
8658.4170
432-533
8660.7170
431-532
8727.4880
422-523
8957.2400
413-514
9048.3030
303-414
9930.9900
515-616
10207.4520
505-606
10349.4320
524-625
3,3Dimethylcyclop
ropene
Cont’d
4-Ethylcyclohexanone26
C8H14O
126
3660.3669
931.6707
796.8889
191
10395.1420
533-634
10395.3550
303-413
4-Ethylcyclohexanone
10401.2730
532-624
Cont’d
10468.0460
404-515
10513.2090
523-624
10733.5470
514-615
11777.9650
110-221
12486.6990
404-514
13396.0520
211-321
13776.0800
212-322
8840.555
8-18-725
9110.013
211-303
9240.920
202-110
9443.908
6-25-716
9542.776
818-725
10648.419
414-413
10720.631
4-14-413
12014.990
212-303
12635.239
2-12-303
13145.960
615-5-24
14549.240
515-514
15164.478
615-524
15525.100
615-523
15968.452
515-514
15988.730
5-15-514
10762.69
312-321
11199.28
321-330
11286.13
202-211
15096.33
000-111
15827.74
212-221
16321.26
633-642
16689.83
431-440
16943.54
523-532
17606.04
743-752
18580.98
322-331
18667.58
413-422
10751.65
312-321
11257.72
321-330
11272.44
202-211
15074.08
000-111
15750.85
212-221
16308.24
633-642
16912.38
431-440
16937.69
523-532
17574.58
743-752
Acetaldehyde27
C2H4O
44
56507.105
10454.2525
9089.0386
Acetone (AA)28
C3H6O
58
10167.4780
7
8515.13359
4907.9621
1
Acetone (EE)28
192
18392.18
322-331
Acetone (EE)
18654.30
413-422
Cont’d
10730.96
312-321
Acetone (AE)28
11073.97
321-330
11252.52
202-211
15064.88
000-111
15736.24
212-221
16278.10
633-642
16475.02
431-440
16920.88
523-532
17489.68
743-752
18449.01
322-331
18630.98
413-422
10749.92
312-321
11468.30
321-330
11265.26
202-211
15038.56
000-111
15615.73
212-221
16312.39
633-642
16943.54
523-532
17281.52
431-440
17593.75
743-752
18037.07
322-331
18651.65
413-422
11143.089
000-101
15328.318
202-212
17791.189
212-303
17929.346
101-110
8902.197
000-101
17387.612
110-212
17801.32
101-202
18221.177
110-211
7763.867
212-313
8168.619
313-404
8217.876
202-303
8853.205
211-312
9083.646
615-624
9221.867
716-725
9224.728
514-523
9558.716
413-422
10322.950
313-414
10837.722
303-404
10886.979
202-313
11079.582
322-423
11218.708
414-505
Acetone (EA)28
Acrolein (cis)29
C3H4O
56
22831.6496
6241.0470
4902.2063
Acrolein
(trans)30
C3H4O
56
47640
4659.43
4242.71
Anisole31
C7H8O
104
5028.84414
1569.36430
1205.8256
193
11342.554
321-422
Anisole
11769.481
312-413
Cont’d
12860.811
414-515
13373.038
404-505
13742.136
515-524
14652.044
413-514
15831.915
505-606
17018.206
505-616
11334.3499
212-313
11789.5515
202-303
11990.1002
211-312
15104.4520
313-414
15712.2436
303-404
15980.7153
312-413
18867.5348
414-515
8481.5981
202-303
8698.3009
221-322
8918.4590
220-321
9375.7249
211-312
10550.2442
313-414
11080.1955
303-404
11553.2302
322-423
11691.3613
331-432
11716.2905
330-431
12073.9502
321-422
12423.8025
312-413
13106.8113
414-515
13554.0297
404-505
14370.5837
423-524
14640.2207
432-533
14725.7557
431-532
15314.1097
422-523
15393.3130
413-514
15626.8280
515-616
15953.0240
505-606
18251.9528
514-615
7752.2719
212-313
7950.5584
313-404
8207.0738
202-303
8306.8593
221-322
8406.6476
220-321
8810.1602
606-615
8829.9395
211-312
8848.4092
101-212
9618.3242
615-624
Ar-Acetylene
Complex32
C2H2-Ar
66
47550
2074.98
1856.99
Ar-Diacetylene
Complex33
C4H2-Ar
90
4435.9988
1688.16837
1212.7884
5
Benz-aldehyde31
C7H6O
106
5234.364
1564.274
1204.681
194
9688.4673
716-725
Benz-aldehyde
9807.9390
514-523
Cont’d
10087.7495
817-826
10171.4581
413-422
10309.4761
313-414
10612.1121
312-321
10795.6934
707-716
10831.4034
303-404
11016.1890
414-505
11035.4050
211-220
11056.2202
322-423
11087.9190
202-313
11122.9490
331-432
11129.5434
330-431
11300.5835
321-422
11741.2378
312-413
12089.0284
212-221
12643.6164
313-322
12846.7075
414-515
13190.3209
303-414
13375.1072
404-505
13390.3609
414-423
13788.8803
423-524
13900.5830
441-542
13900.9034
440-541
13920.1747
432-533
13943.0987
431-532
14012.6456
515-606
14258.2008
422-523
14332.5327
515-524
14621.7220
413-514
15205.6257
404-515
15362.4557
515-616
15470.9727
616-625
15843.1646
505-616
16500.8965
524-625
16698.7032
542-643
16700.1348
541-642
16723.1202
533-634
16783.4913
532-633
16802.7002
717-726
16905.5948
616-707
16907.7569
110-221
17192.9746
505-616
17269.9756
523-624
195
17292.4971
111-220
Benz-aldehyde
17459.5886
514-615
Cont’d
17518.4571
725-734
17856.9156
616-717
18168.1124
624-633
18255.4059
606-707
18319.9893
818-827
18654.5987
523-532
9878.05
000-101
10999.38
101-110
12387.99
202-211
14679.49
303-312
18080.54
404-413
18484.30
111-212
Butene-1 (cis)34
C4H8
56
15302.54
5574.92
4303.14
Butene-1
(skew)34
C4H8
56
22557.33
4156.28
4056.21
8212.45
000-101
16325.14
111-212
16424.43
101-202
16525.00
110-211
16721.29
808-818
17094.62
707-717
17425.56
606-616
17714.77
505-515
17957.80
404-414
18153.93
303-313
18302.10
202-212
18401.20
101-111
18501.28
111-110
18601.64
202-211
18753.00
303-312
9647.567
0-1
C2O 35
C2O
40
-
11545.597
-
9405.052
1-2
C4O 35
C4O
64
-
2351.2625
-
10152.516
2-3
12776.575
1-2
14107.563
2-3
14570.267
3-4
18062.594
2-3
C5H2 36
C5H2
62
34638.701
3424.8768
3113.6386
C5O 37
C5O
76
-
1366.84709
-
18810.072
3-4
10050.700
606-515
11298.472
212-303
12765.667
212-111
13074.705
202-101
13388.144
211-110
17412.085
707-616
18139.060
111-202
10394.76
3-4
196
13668.454
4-5
C5O
16402.136
5-6
Cont’d
8497.574
4-5
C6O 35
C6O
88
-
849.75709
-
9276.632
4-5
C7O 37
C7O
100
-
572.94105
-
Carbonic Acid38
CH2O3
62
11778.6808
11423.1345
5792.0741
Cyclopropaneca
rboxaldehyde
(cis)39
C4H6O
70
11417.29
3994.18
3849.92
Cyclopropaneca
rboxaldehyde
(trans)39
C4H6O
70
15885.40
3195.09
3040.95
Crotonaldehyde (transtrans)40
C4H6O
70
32636.58
2183.30
2073.32
9302.524
5-6
10197.083
5-6
10901.418
6-7
11091.621
5-6
11896.595
6-7
12514.009
7-8
12891.743
6-7
13596.102
7-8
14138.823
8-9
14678.164
7-8
15295.613
8-9
16452.366
8-9
9167.046
7-8
10312.924
8-9
16909.2223
202-211
17215.1941
000-101
17959.5836
212-221
7567.409
101-110
7844.081
000-101
7937.096
303-312
8242.14
404-413
8262.909
111-202
15267.253
000-111
15543.926
111-212
15686.081
101-202
15832.399
110-211
16242.904
212-303
12317.871
111-212
12470.645
101-202
12626.186
110-211
18475.945
212-313
18702.511
202-303
8403.32
111-212
8512.99
101-202
8623.14
110-211
12604.75
212-313
12769.03
202-303
12934.76
211-312
16806.04
313-414
17023.60
303-404
17246.15
312-413
197
7863.6921
505-515
8371.3801
404-414
8796.2468
303-313
9125.8412
202-212
9350.7780
101-111
9542.0079
414-322
9562.7900
414-321
9579.8730
101-110
9776.0030
111-212
9797.9265
312-404
9813.1262
202-211
10000.9398
101-202
10170.7697
303-312
10234.1930
110-211
10661.9998
404-413
11172.4493
313-404
11298.7967
505-514
12095.5236
606-615
12703.6674
312-221
12707.8256
312-220
13068.2545
707-716
14078.1902
313-221
14082.3485
313-220
14267.9926
413-505
14353.3270
000-111
14582.4220
000-110
14661.4288
212-313
14991.0232
202-303
15007.6470
221-322
15024.2708
220-321
15348.6666
211-312
16558.6124
414-505
18572.4522
514-606
8365.18
000-101
15482.10
111-212
16554.55
101-202
17978.55
110-211
8978.35
761-762
10799.79
652-651
12505.92
541-542
14013.86
432-431
15266.91
322-321
16229.25
212-211
18278.64
202-221
Crotyl Alcohol
(cis)41
C4H8O
72
11966.6
2615.822
2386.727
Cyclobutanone42
C4H7O
71
10785.2
4806.69
3558.47
Cyclobutene43
C4H6
54
12892.87
12226.03
6816.29
198
7766.4
111-212
8316.0
101-202
8532.4
422-432
9261.9
110-211
10381.3
515-514
11537.9
212-313
11702.9
101-211
12055.6
202-303
12771.7
221-322
13487.5
220-321
14539.0
110-220
15088.6
111-221
15217.1
313-414
16871.0
322-423
17135.5
202-312
17343.2
331-432
17537.4
330-431
18014.5
312-413
18340.3
321-422
18764.2
211-321
11064.03
111-212
11954.86
101-202
13251.61
110-211
16479.07
212-313
11125.99
111-212
12025.62
101-202
13349.21
110-211
16567.04
212-313
17554.91
202-303
18356.35
221-321
11565.9124
322-321
11603.0292
312-321
11862.0000
212-211
11869.4369
202-211
12463.6500
212-221
12471.0869
202-221
12497.0800
000-101
12697.6300
000-111
12769.0399
322-331
12806.1567
312-331
16651.6300
000-110
9629.565
422-431
9743.660
312-321
10280.616
432-441
11242.950
000-111
Cyclohexanone44
C6H10O
98
4195.30
2502.57
1754.49
Cyclopent-2-en1-one45
C5H6O
82
7410.56
3586.34
2492.55
Cyclopent-3-en1-one45
C5H6O
82
7378.47
3615.2
2503.59
Cyclopentadiene46
C5H6
66
8426.09
8225.54
4271.54
Cyclopentene47
C5H8
56
7293.6358
7226.9160
3949.382
3
199
16543.590
303-312
Cyclopentene
16574.050
413-422
Cont’d
16589.160
423-432
14945.53
101-202
16872.34
8029.987
15089.598
321-322
8372.78
726-725
9263.49
313-312
14105.74
000-101
15427.90
414-413
18343.14
101-110
10230.5000
414-321
10540.6400
505-514
11420.6455
312-221
11536.4474
000-111
11731.7598
606-615
12302.5023
414-505
13208.4570
707-716
14995.2940
808-817
15006.7695
101-212
16596.6445
515-606
18322.5566
202-313
11532.64
111-212
12369.58
101-202
13481.98
110-211
17218.19
212-313
18227.25
202-303
18760.73
221-322
12070.640
111-212
13026.162
101-202
14381.192
110-211
17990.248
212-313
11591.94
111-212
12011.68
101-202
12453.68
110-211
12878.93
101-110
13320.94
202-211
14004.44
303-312
14953.35
404-413
16198.53
505-514
17380.86
212-313
17775.35
606-615
17989.64
202-303
C5H6O
82
5709.38
4541.12
3248.97
110-211
Cyclopentene
Oxide48
110-111
Cyclo-propene49
C3H4
40
30061.00
21825.84
13795.83
Cyclopropenone50
C3H2O
54
32046.00
7824.95
6280.72
C3H2
38
-
-
-
Dihydroxyacetone52
C3H6O3
90
9801.294341
2051.52561
1735.165
Dimethylallene53
C5H8
56
8264.075
3614.154
2639.494
Dimethylketene54
C4H6O
70
8267.832
3884.101
2728.26
Divinyl Ether55
C4H6O
70
15669.73
3220.98
2790.20
Cyclopropenylidene51
200
18078.50
220-321
DVE Cont’d
8172.6410
110-202
Ethanol56
C2H6O
46
34891.5
9350.6
8135.1
Ethenol57
C2H4O
44
59660.2
10561.55
8965.82
Ethoxy Ethyne
(anti)58
C4H6O
70
29500
2502.178
2380.034
Ethoxy Ethyne
(guache)58
C4H6O
70
11799.84
3422.405
2887.691
Ethyl Acetate
(Trans A)59
C4H8O2
88
8452.754
2093.88122
1733.424
8448.6787
2094.03132
1733.525
9388.1410
111-202
17485.7000
000-101
9573.50
313-312
10081.92
202-111
10887.37
212-303
14463.30
212-313
14829.70
211-303
12595.35
101-202
13154.85
110-211
13318.52
505-514
15516.21
606-615
18113.07
212-313
18254.76
707-716
8606.360
404-413
9596.595
313-404
10252.843
000-111
10937.392
212-313
11199.529
606-615
11432.550
202-303
11492.364
221-322
11552.318
220-321
12028.748
211-312
13019.715
707-716
13719.739
101-212
14566.520
313-414
15174.526
303-404
15216.818
808-817
15311.440
322-423
15460.220
321-422
16019.842
312-413
16536.543
817-826
17010.480
202-313
17664.160
514-523
18182.484
414-515
18244.477
413-422
18860.951
404-505
8734.429
000-110
10306.385
505-515
11224.157
211-312
11371.928
202-303
11506.388
221-322
11518.656
220-321
11639.611
606-616
Ethyl Acetate
(Trans E)59
201
11734.432
212-313
12512.088
101-211
Ethyl Acetate
(Trans E)
13325.237
707-717
Cont’d
14364.854
413-505
14783.518
312-413
15111.373
303-404
15329.916
331-432
15341.698
330-431
15363.174
322-423
15374.878
808-818
16137.964
202-312
17788.625
909-919
17987.557
919-928
18165.309
818-827
18480.793
514-606
18601.234
717-726
18803.029
404-505
13478.27
111-212
14059.24
101-202
14731.48
110-211
10642.26
111-212
10962.44
101-202
11293.29
110-211
15498.26
202-211
15959.95
212-313
16004.65
303-312
16430.40
202-303
16451.67
221-322
16472.82
220-321
16697.93
404-413
16936.62
211-312
17593.71
505-514
8023.0000
111-110
11386.1000
101-110
15604.4650
211-220
11442.82
110-101
15790.68
220-211
14488.479
211-212
11592.09
111-212
12581.87
101-202
14004.66
110-211
17262.99
212-313
17439.78
514-515
18373.98
202-303
13431.2771
322-321
Ethyl Formate
(gauche)60
C3H6
42
9985.34
3839.48
3212.94
Ethyl Formate
(trans)60
C3H6
42
17746.6
2904.73
2579.14
Ethylene
Oxide61
C2H4O
44
25438.92
22120.82
14097.82
Ethylene Oxide
(v1)61
C2H4O
44
25453.45
22003.6
14010.75
CH2O
30
281970.54
38836.05
34002.2
C6H6
78
8186.134
3802.732
2596.436
C4H4O
68
9447.1210
9246.7419
4670.823
Formaldehyde62
Fulvene63
Furan64
202
13463.4215
312-321
Furan
13727.7555
212-211
Cont’d
13734.1932
202-211
13917.5653
000-101
14117.9444
000-111
14328.8928
212-221
14335.3305
202-221
14633.4223
322-331
14665.5668
312-331
18693.8629
000-110
7774.962
110-211
10424.1751
212-313
10970.7654
202-303
11049.3093
221-322
11127.8702
220-321
11639.0044
606-615
11649.7996
211-312
13103.3214
101-212
13484.8507
414-505
13877.2313
313-414
14537.9655
303-404
14717.0558
322-423
14770.0729
331-432
14773.6412
330-431
14911.4949
321-422
15508.6218
312-413
16180.9878
202-313
17313.4269
414-515
17338.161
413-422
17625.4318
515-606
17935.2879
312-321
18034.3380
404-505
18371.6700
423-524
18752.7120
422-523
10133.0130
322-414
10347.5918
101-202
10609.4420
414-423
10772.9277
110-211
11168.2440
414-422
11399.4910
432-524
11936.9400
211-303
12621.4700
431-523
12677.9800
101-212
12699.8170
624-634
12874.6560
515-523
Furfural65
C5H8O2
100
8191.7738
2045.929
1637.183
Glyceraldehyde (1)15
C3H6O3
90
5467.77597
2789.8577
2403.408
203
13096.2490
212-303
13437.0350
523-533
Glyceraldehyde (1)
13788.2200
321-413
Cont’d
13837.2910
101-211
13921.0210
422-432
13938.1267
422-431
14186.5479
321-331
14243.7500
423-515
14380.8320
322-330
14496.9395
423-515
14743.7969
524-532
14976.8135
212-313
15172.6143
616-624
15200.7988
625-633
15426.6387
202-303
15579.7363
221-322
15732.6846
220-321
15969.6758
726-734
16133.6064
211-312
16208.7891
312-404
17307.2031
202-313
18806.6060
110-221
18845.4840
110-220
10122.5576
000-111
10439.9355
606-615
11037.3428
313-404
11793.8828
212-313
12253.1943
202-303
12895.1748
211-312
13888.0625
101-212
14899.7080
101-211
15572.1191
414-505
16274.4248
303-404
16569.9990
514-523
17110.8408
413-422
17490.2754
202-313
17630.9609
312-321
9852.555
110-211
11691.408
101-212
12689.695
212-313
13387.862
202-303
13762.655
221-322
14137.345
220-321
14713.300
211-312
15302.808
202-313
Glyceraldehyde (2)15
C3H6O3
90
8239.8203
2219.9762
1882.753
Glyceraldehyde (3)15
C3H6O3
90
5826.323
2632.5216
1955.036
204
15566.700
313-404
16825.058
313-414
Glyceraldehyde (3)
17481.642
303-404
Cont’d
18274.440
322-423
18520.309
331-432
18572.074
330-431
12299.94
533-624
12804.11
633-726
15261.66
322-413
15614.97
844-937
15642.62
422-331
17716.34
423-330
17980.93
303-312
9138.6108
111-202
9285.3412
202-211
11949.4942
514-524
11959.2110
303-312
13497.7056
211-303
13514.7545
414-413
14208.9050
000-111
14332.2640
111-212
14739.7534
413-423
15499.4358
101-202
15540.8529
321-413
15572.8010
000-110
15997.3241
404-413
16498.7703
322-413
17060.0560
110-211
17169.7984
312-322
17458.1783
413-422
17589.3936
212-303
17671.2446
312-404
17756.3839
514-523
18127.7158
312-321
Glycolaldehyde66
C2H4O2
60
18446.410
6526.042
4969.274
Glyoxylic Acid67
C2H2O3
74
10966.813
4605.988
3242.092
000-101
H2O-HO2
Complex68
H3O3
51
32897
5656
4829
11202.058
202-313
Hexanol (1)69
C6H14O
102
6611.3383
1011.56061
926.5724
11366.296
515-616
11595.285
505-606
11624.393
524-625
11657.853
523-624
11711.966
202-313
11875.825
514-615
12972.144
303-414
13062.292
818-909
10374.6168
10380.3191
205
13257.225
616-717
Hexanol (1)
13513.496
606-707
Cont’d
13559.344
625-726
13569.953
652-753
13612.701
624-725
13821.953
303-413
13851.259
615-716
14703.257
404-515
15047.824
918-928
15146.65
717-818
15360.685
918-927
15418.249
817-827
15425.329
707-808
15493.173
726-827
15508.062
762-863
15509.290
753-854
15515.507
735-854
15517.208
734-835
15572.786
725-826
15618.234
817-826
15749.890
716-726
15824.815
716-817
15870.261
716-725
15977.852
404-514
16108.822
615-624
16293.240
514-524
16398.075
505-616
16503.587
413-423
16517.982
413-422
16672.387
312-322
16800.244
211-220
17034.421
818-919
17187.092
313-321
17330.132
808-909
17353.400
414-423
17425.715
827-928
17447.234
862-964
17448.943
854-955
17457.381
836-937
17460.504
835-936
17538.595
826-927
17601.388
515-523
17796.144
817-918
17825.930
616-625
17892.944
616-624
206
18060.011
606-717
Hexanol (1)
18128.048
717-725
Cont’d
18182.198
505-615
18248.421
717-725
18474.571
818-827
18674.553
818-826
18865.868
919-928
12746.768
303-414
13122.273
515-616
13300.186
505-606
13324.038
524-625
13520.417
514-615
14806.477
404-515
15306.444
616-717
15505.068
606-707
15542.647
625-726
15586.478
624-725
15770.572
615-716
17489.364
717-818
17704.715
707-808
18019.157
716-817
12647.508
303-414
13130.186
818-909
14348.268
404-515
15169.897
716-725
15414.338
615-624
15600.914
514-525
15639.256
514-523
15683.945
404-514
15837.587
413-422
16003.403
312-321
16011.592
505-616
16711.694
414-423
16936.594
515-524
17207.347
616-625
17641.373
606-717
17888.051
818-827
18298.805
919-928
12873.452
303-414
14599.939
404-515
15900.090
615-624
16290.693
505-616
16302.928
413-422
16459.638
312-321
17336.178
515-524
Hexanol (10)69
C6H14O
102
5302.463
1143.8617
1077.446
Hexanol (11)69
C6H14O
102
6380.0804
1003.1223
914.0581
Hexanol (12)69
C6H14O
102
6533.5516
1006.9431
923.3035
207
17887.499
717-726
Hexanol (12)
17949.058
606-717
Cont’d
18046.546
505-615
18228.511
818-827
18613.584
919-928
12357.240
515-616
12689.769
505-606
12798.341
524-625
12923.246
523-624
13213.847
514-615
14404.634
616-717
14753.815
606-707
16798.249
707-808
18484.614
818-919
18823.870
808-909
16524.150
717-818
16876.466
707-808
17107.191
726-827
17183.891
735-836
17379.540
725-826
17636.294
716-817
18572.115
818-919
18914.990
808-909
12925.019
303-414
13152.379
616-717
13410.503
606-707
13456.984
625-726
13511.064
624-725
13751.226
615-716
15077.027
918-928
15307.372
707-808
15376.147
726-827
15392.556
753-854
15394.122
918-927
15400.534
734-835
15450.380
817-827
15456.834
725-826
15653.073
817-826
15710.419
716-817
15784.646
716-726
15906.651
716-725
15924.811
404-514
16078.891
615-625
16318.181
505-616
16332.339
514-524
Hexanol (13)69
C6H14O
102
5296.26
1139.5507
996.2448
Hexanol (14)69
C6H14O
102
5394.22
1142.153
1001.502
Hexanol (2)69
C6H14O
102
6618.766
1004.6000
918.9220
208
16366.349
514-523
Hexanol (2)
16544.381
413-423
Cont’d
16558.965
413-422
16714.535
312-322
16899.490
818-919
17197.129
808-909
17233.450
313-321
17294.010
827-928
17326.120
836-937
17329.300
835-936
17408.405
826-927
17617.274
515-524
17651.285
515-524
17667.359
817-918
17945.416
616-624
17963.387
606-717
18116.777
505-615
18304.094
717-725
18531.467
818-827
13036.144
616-717
13297.966
606-707
13348.383
625-726
13407.055
624-725
13649.372
615-716
14355.422
404-515
14837.591
817-827
14893.569
717-818
15177.213
707-808
15179.384
716-726
15251.756
726-827
15276.296
735-836
15278.289
734-835
15339.240
725-826
15480.364
615-625
15593.538
716-817
15671.427
404-514
15739.710
514-524
15956.745
413-423
15972.588
413-422
16013.113
505-616
16749.197
818-919
16850.016
414-422
17048.818
808-909
17092.646
515-523
17153.716
827-928
Hexanol (3)69
C6H14O
102
6418.576
997.9263
910.1749
209
17177.498
862-964
Hexanol (3)
17179.345
853-954
Cont’d
17188.480
836-937
17192.140
835-936
17277.632
826-927
17535.241
817-918
17767.060
717-725
12678.088
303-414
13142.482
515-616
13172.910
303-413
13278.712
505-606
13292.425
524-625
13295.898
541-642
13308.224
523-624
13439.263
514-615
14772.661
404-515
15331.176
616-717
15484.971
606-707
15506.610
625-726
15512.270
642-743
15514.008
633-734
15514.864
404-514
15531.828
624-725
15677.280
615-716
16845.382
505-616
17519.118
717-818
17688.142
707-808
17720.234
726-827
17730.823
735-836
17731.476
734-835
17757.937
725-826
17884.368
505-615
17914.400
716-817
18759.403
211-321
18897.854
606-717
12172.808
716-726
12562.439
514-524
12713.212
413-423
12713.394
716-808
13202.454
313-321
13326.749
515-616
13331.240
414-422
13474.993
515-524
13491.990
505-606
13511.290
524-625
Hexanol (4)69
C6H14O
102
5169.271
1132.6212
1083.157
Hexanol (5)69
C6H14O
102
5465.031
1156.6301
1095.789
210
13515.945
541-642
Hexanol (5)
13533.612
523-624
Cont’d
13654.109
303-413
13659.536
616-625
13691.558
514-615
13704.236
616-624
14256.632
818-826
15149.544
404-515
15545.492
616-717
15731.002
606-707
15761.463
625-726
15797.074
624-725
15970.832
615-716
16062.100
404-514
17491.404
110-220
17763.202
717-818
17965.702
707-808
18010.856
726-827
18025.618
735-836
18064.024
725-826
18248.829
716-817
18504.460
505-616
14579.398
616-717
14699.913
616-624
14931.687
606-707
15098.085
625-726
15292.304
624-725
15578.081
615-716
15836.015
505-616
16647.044
717-818
17002.089
707-808
17168.120
110-221
17171.716
110-220
17242.981
726-827
17311.803
111-221
17315.401
111-220
17573.643
606-717
17782.389
716-817
12169.969
413-422
12475.714
313-322
12538.553
303-414
12553.548
414-423
12650.990
515-524
12768.152
616-625
12799.456
515-616
Hexanol (6)69
C6H14O
102
5386.535
1152.2818
1008.596
Hexanol (7)69
C6H14O
102
5196.2729
1095.8890
1057.121
211
12908.375
505-606
Hexanol (7)
12916.621
524-625
Cont’d
12926.185
523-624
13031.999
514-615
14596.139
404-515
14931.648
616-717
15055.598
606-707
15068.647
625-726
15083.930
624-725
15202.871
615-716
17063.372
717-818
17200.923
707-808
17243.212
725-826
17373.212
716-817
13002.570
303-414
13235.900
515-616
13404.324
505-606
13424.364
524-625
13447.519
523-624
13608.362
514-615
15439.424
616-717
15628.377
606-707
15659.998
625-726
15668.051
642-743
15696.926
624-725
15873.678
615-716
17641.877
717-818
17847.965
707-808
17894.821
726-827
17910.155
735-836
17911.266
734-835
17949.952
725-826
18137.664
716-817
12867.716
515-616
13030.820
505-606
13037.878
303-413
13051.484
524-625
13058.144
533-634
13058.404
532-633
13075.236
523-624
13230.511
514-615
15009.846
616-717
15192.405
606-707
15224.957
625-726
15235.594
634-735
Hexanol (8)69
C6H14O
102
5478.901
1150.0356
1087.909
Hexanol (9)69
C6H14O
102
5118.538
1118.1438
1057.639
212
15236.198
633-734
Hexanol (9)
15262.830
624-725
Cont’d
15368.674
404-514
15432.812
615-716
16413.851
110-220
16473.680
111-221
17150.887
717-818
17349.446
707-808
17454.128
725-826
18709.940
212-322
18707.6516
18709.0495
HOOO
Radical70
HO3
49
70778
9987
8750
Hydroxyacetone71
C3H6O2
74
10069.41
3810.412
2864.883
Isobutanol (A)1
C4H10O
74
7597.2434
3532.7089
2666.256
18711.3049
18787.5488
000-101
18792.0759
18795.7252
12405.0610
111-212
13251.3505
101-202
14296.1190
110-211
18548.5147
212-313
7618.5462
303-312
7618.8488
221-312
7657.8447
514-524
8209.0683
111-202
8586.4421
414-413
9433.1108
413-423
10180.8831
404-413
10263.4992
000-111
10369.3993
423-515
10978.0453
312-322
11129.9523
000-110
11152.4036
413-422
11332.8365
514-523
11531.4763
111-212
11583.5925
312-321
12193.6035
211-221
12193.9062
211-303
12273.6028
101-202
12317.9301
211-220
12357.8044
615-624
12691.7914
515-514
13264.3825
110-211
13642.7884
505-514
14660.0091
321-413
14792.9628
212-221
213
14793.2655
212-303
Isobutanol (A)
14917.2894
212-220
Cont’d
15265.5563
322-413
15596.0108
101-212
16062.7185
312-404
16165.4747
313-322
16771.0220
313-321
17224.3822
212-313
17846.2236
606-615
18019.5529
414-423
18115.3708
202-221
18115.6735
202-303
18195.3701
101-211
18596.5914
303-322
18596.8941
221-322
18768.1755
624-633
18851.5130
413-505
7711.0557
514-524
8126.4115
111-202
8347.8258
414-413
9446.7749
413-423
9967.6645
404-413
10190.3550
000-111
10405.1623
423-515
10952.8179
312-322
11032.4105
000-110
11087.4239
413-422
11227.1084
514-523
11447.9775
111-212
11529.5318
312-321
12128.8860
211-303
12136.0155
211-221
12171.7500
101-202
12176.4105
615-624
12254.2985
211-220
12346.4629
515-514
13132.0885
110-211
13322.5171
505-514
14464.1296
321-413
14655.0525
212-303
14662.1820
212-221
14780.4650
212-220
15040.8436
322-413
15493.3160
101-212
15994.6420
313-322
Isobutanol (B)1
C4H10O
74
7538.8745
3493.536
2651.481
214
16025.9970
312-404
Isobutanol (B)
16571.3559
313-321
Cont’d
17102.5895
212-313
17411.2393
606-615
17794.6007
414-423
17976.6186
202-303
17983.7480
202-221
18019.4825
101-211
18435.0495
221-322
18442.1790
303-322
18731.8586
624-633
18875.8830
413-505
18893.4804
220-321
7548.0487
221-312
7572.0245
514-524
7587.5550
303-312
8130.7008
111-202
8579.2177
414-413
9342.7714
413-423
10151.6325
404-413
10177.4886
000-111
10193.8527
423-515
10885.1588
312-322
11043.3121
000-110
11070.0581
413-422
11261.2276
514-523
11422.9815
111-212
11493.8812
312-321
12059.7520
211-303
12099.2583
211-221
12163.7869
101-202
12224.2764
211-220
12299.6770
615-624
12678.8196
515-514
13154.6285
110-211
13613.8324
505-514
14528.3777
321-413
14657.2225
212-303
14696.7288
212-221
14821.7469
212-220
15137.1001
322-413
15456.0676
101-212
15870.6264
312-404
16068.6746
313-322
16677.3971
313-321
Isobutanol (C)1
C4H10O
74
7538.8745
3505.113
2639.290
215
17061.2617
212-313
Isobutanol (C)
17812.8940
606-615
Cont’d
17921.9890
414-423
17949.5031
202-303
17989.0095
202-221
18053.5381
101-211
18433.2075
221-322
18472.7138
303-322
18601.6112
413-505
18607.5804
624-633
18916.9119
220-321
7537.2235
514-523
7859.7573
414-413
8360.9065
404-413
9105.4224
212-221
9286.3273
212-220
9427.7298
000-111
9936.6797
523-533
10231.5121
000-110
10413.8413
313-322
10605.1375
523-532
10713.9035
202-221
10994.3676
220-313
11175.2726
221-313
11176.6969
110-202
11265.1034
313-321
11384.2607
303-322
11425.0556
422-432
11603.1213
422-431
11619.3974
505-514
11980.4792
111-202
12175.4707
414-423
12417.7316
321-331
12443.9680
321-330
12676.6199
404-423
13268.9937
322-331
13295.2301
322-330
13588.9603
111-212
13708.0145
423-432
13886.0802
423-431
14211.8377
101-202
14355.8429
515-524
14458.4296
414-422
14505.9977
524-533
15174.4555
524-532
Isobutanol (D)1
C4H10O
74
6231.4353
4000.0768
3196.295
216
15196.5249
110-211
Isobutanol (D)
15347.7771
330-423
Cont’d
15374.0135
331-423
15616.2744
321-414
15790.8251
220-312
15820.3188
101-212
15971.7300
221-312
16467.5365
322-414
16898.9287
211-303
17509.9699
532-542
17537.6710
532-541
17630.7360
330-422
17656.9724
331-422
17997.8716
431-441
18001.0120
431-440
18175.9373
432-441
18178.4276
533-542
18179.0777
432-440
18206.1287
533-541
18231.6657
101-211
18886.3776
312-331
10238.3735
322-321
10734.9442
312-321
11297.2800
212-221
11398.9471
202-221
12997.7400
000-101
13551.9900
212-221
13653.6571
202-221
13749.3100
000-111
14739.1025
322-331
15235.6732
312-331
16667.8471
212-220
17515.0700
000-110
18362.2929
110-202
9072.1157
322-321
9408.7783
312-321
9886.9887
212-211
9955.5748
202-211
11608.2264
212-221
11676.8125
202-221
12509.4224
322-331
12834.9407
000-101
12846.0850
312-331
13408.6866
000-111
14398.7295
212-220
Isobutylene72
C4H8
56
9133.32
8381.75
4615.99
Isopropanol
(gauche)73
C3H8O
60
8639.0477
8065.3018
4769.639
217
16704.3495
000-110
17159.4802
423-422
Isopropanol
(gauche)
17195.3668
413-422
Cont’d
17616.5938
313-312
17621.7624
303-312
17953.2564
313-322
17958.4251
303-322
18129.8046
423-432
18165.6912
413-432
9187.6091
322-321
9398.5577
312-321
9830.1210
212-211
9872.8162
202-211
11171.3104
212-221
11214.0056
202-221
11867.4253
322-331
12078.3740
312-331
12807.1705
000-101
13254.2337
000-111
16530.9407
000-110
17022.0159
423-422
17039.8610
413-422
17315.7644
313-312
17318.3269
303-312
17526.7131
313-322
17529.2756
303-322
17638.2252
423-432
17656.0703
413-432
8287.7510
303-312
8638.4389
312-322
9213.8816
321-414
9425.2767
110-202
9763.4489
312-321
9827.1946
413-422
10160.8200
211-221
10338.8915
322-414
10398.2733
211-220
10475.1300
000-111
10551.9867
111-202
10977.7004
514-523
11048.1671
414-413
11601.8400
000-110
11887.2867
404-413
12388.6778
220-312
12626.1311
221-312
Isopropanol
(trans)73
C3H8O
60
8489.00189
8041.93877
4765.232
Isopropyl
Carboxaldehyde (g)74
C4H8O
72
7494.39
4107.45
2980.74
218
13049.6700
111-212
13540.9500
212-221
13778.4033
212-220
13938.9267
101-202
14032.6512
423-515
14499.2001
211-303
15303.0900
110-211
15364.2202
523-533
15366.5730
313-322
16038.6333
202-221
16071.4005
515-514
16193.1076
523-532
16436.6100
101-212
16475.7362
505-514
16491.5829
313-321
16926.1899
303-322
17419.4154
422-432
17638.3483
422-431
17827.1428
414-423
17879.3301
212-303
18138.2108
312-404
18666.2624
404-423
18762.0231
321-331
18794.1490
321-330
7925.18
615-616
16980.97
918-919
8583.68
313-312
11428.21
836-835
13298.87
111-212
13425.80
624-625
14085.25
414-413
14427.13
101-202
16174.57
110-211
17969.58
937-936
8582.07
313-312
11423.58
836-835
14082.80
414-413
17963.76
937-936
8738.8290
101-111
9384.5580
101-110
10064.7648
202-211
10113.7697
212-303
10764.6603
313-221
10799.1381
313-220
11148.4654
303-312
Isopropyl
Carboxaldehyde (g)
Cont’d
Ketene75
C2H2O
42
-
-
-
Methacrolein
(Trans A)76
C4H6O
70
8612.39
4403.08
2965.33
C4H6O
70
12124.795
3385.966
2740.237
Methacrolein
(Trans E)76
Methoxyallene77
219
11606.6770
111-212
12217.9282
101-202
12706.3998
404-413
12898.1350
110-211
13017.3410
312-404
14819.1314
505-514
14865.0320
000-111
15510.7610
000-110
15888.8435
524-615
16890.5625
313-404
17262.9214
413-505
17389.0137
212-313
17558.8460
606-615
18241.3475
202-303
18378.6090
221-322
18515.8705
220-321
16871.62
514-523
17005.60
413-422
17690.00
615-624
17782.70
312-321
18051.54
505-514
16866.13
514-523
17000.10
413-422
17685.21
615-624
17778.81
312-321
18049.14
505-514
14613.42
414-505
15187.77
707-716
16068.81
101-212
17580.12
808-817
14615.58
414-505
15185.48
707-716
16066.19
101-212
17577.83
808-817
8910.032
734-643
10718.577
533-624
11227.499
111-202
12219.209
000-101
14012.303
844-937
14681.231
101-110
14903.145
322-413
16431.284
202-211
13262.77
413-422
14429.81
514-523
17200.42
615-624
Methoxy-allene
Cont’d
C3H6O2
74
10406.81
4176.93
3077.18
C4H6O2
86
10301.98
2327.08
1922.29
Methyl
Formate79
C2H4O2
60
17522.369
9323.547
5312.699
Methyl Glyoxal
(trans)80
C3H4O2
72
9102.4332
4439.8832
3038.940
Methyl Acetate
(A)78
Methyl Acetate
(E)78
Methyl Acrylate
(A)78
Methyl Acrylate
(E)78
220
18219.28
101-212
MG (t) Cont’d
8432.98
101-111
C4H6O
70
11756.83
3323.73
2795.43
8747.44
211-303
8961.36
101-110
Methyl
Propargyl
Ether81
Methyl
Propiolate (A)78
C4H4O2
84
9615.89
2430.23
1962.71
Methyl Vinyl
Ketone (ap)82
C4H6O
70
8941.45
4274.48
2945.32
Methylallene83
C4H6
54
34021
4201.311
3928.122
Methylcyclopropane84
C4H8
56
15505.20
6363.39
5587
9513.82
202-211
10332.59
212-303
10386.97
303-312
11631.01
404-413
13305.68
505-514
13851.40
312-404
14552.11
000-111
15080.54
000-110
17020.82
313-404
13447.21
606-615
15504.10
101-212
15853.25
706-716
18572.06
817-826
18646.13
716-725
13445.67
606-615
15502.0
101-212
15851.63
706-716
18567.77
817-826
18641.58
716-725
10326.6
303-312
11554.4
625-624
11886.9
000-111
13110.4
111-212
13163.3
413-422
14194.1
101-202
14139.5
514-523
15769.0
110-211
16606.4
615-624
17144.4
212-303
17611.6
726-725
17777.5
101-212
8129.4330
000-101
9264.5043
413-505
11996.3384
414-505
13562.6915
202-111
13835.8805
202-110
15985.6770
111-212
16256.9975
101-202
16532.0550
110-211
11949.95
000-101
Methyl
Propiolate (E)78
221
8645.18
000-101
17164.41
111-212
17290.23
101-202
17422.24
110-211
8066.56
000-101
14989.22
111-212
15979.45
101-202
17270.27
110-211
8344.6848
221-312
8472.1452
312-322
8844.2100
000-111
8920.6322
414-413
9109.7073
413-422
9262.2520
312-321
9368.4999
423-515
9715.8000
211-221
9743.3707
514-523
9749.3800
000-110
9880.4417
211-220
9898.2342
404-413
10306.0500
111-212
10977.7832
211-303
11046.5783
101-202
11384.3756
615-624
12116.3900
110-211
12431.3100
212-221
12595.9517
212-220
13073.6317
515-514
13094.6011
431-523
13225.6689
432-523
13544.6500
101-212
13588.2524
505-514
13693.2932
212-303
13884.0711
313-322
13942.7760
312-404
14521.4517
624-633
14578.7582
321-413
14674.1780
313-321
14929.3817
202-221
15197.2926
523-533
15364.0689
212-313
15368.8650
322-413
15554.8468
303-322
15701.0523
523-532
15778.3480
413-505
Methylene
Ketene85
C3H2O
54
-
4387.07
4258.15
Methylenebutane42
C5H8
68
10373.9
4618.05
3459.87
Methylenecyclopentane86
C6H10
82
6493.99
3255.39
2350.22
222
15845.1010
414-423
16191.3648
202-303
Methylenecyclopentane
16260.1600
101-211
Cont’d
16699.2757
514-606
16774.5949
422-432
16816.8300
221-322
16822.7029
404-423
16905.6627
422-431
17442.2952
220-321
17600.4558
616-615
17760.8243
321-331
17779.9183
321-330
17851.9835
606-615
17862.1405
202-313
18030.3395
414-422
18060.4848
211-312
18294.7313
515-524
18550.9311
322-331
18570.0252
322-330
18809.3521
505-524
18959.8335
423-432
8585.98
313-312
12322.30
000-101
14278.70
414-413
16918.69
111-212
17286.02
101-202
17659.98
110-211
16955.50
111-212
17285.19
101-202
17621.40
110-211
8476.282
321-330
8524.940
422-431
10130.099
000-111
11548.031
212-221
13250.821
413-422
13680.005
322-331
14619.696
303-312
14754.000
111-202
16247.059
313-322
16410.832
101-212
16529.744
432-441
17124.973
423-432
9214.140
202-303
11334.277
211-312
11604.253
313-414
Methylenecyclopropane87
C4H6
54
19417.6
6877.11
5455.12
Methylketene
(A)88
C3H4O
56
38920
4507.349
4136.983
o-Benzyne89
C6H4
76
6989.7292
5706.8062
3140.371
o-Xylene90
C8H10
106
3163.930
2150.069
1300.835
Methylketene
(E)88
223
11738.564
303-404
o-Xylene
13515.960
322-423
Cont’d
14249.350
414-515
14292.463
404-505
14471.229
312-413
15556.680
321-422
16479.525
423-524
9484.334
21-10
9485.314
11-10
9485.736
01-10
9560.672
10-01
9562.038
10-11
9564.651
10-21
9569.527
11-01
9569.909
21-11
9570.893
11-01
9571.316
01-11
9572.523
21-21
9573.506
11-21
14951.418
21-32
14952.39
11-22
14952.779
01-12
14965.926
10-01
14965.946
10-11
14965.981
10-21
14973.802
21-11
14973.83
21-21
14974.757
11-01
14974.792
11-11
14974.826
11-21
14975.214
01-11
11073
313-404
8005.33
313-401
8354.16
606-615
8738.11
000-111
8965.74
000-110
9316.51
413-505
9418.53
212-313
9736.54
202-303
9764.22
221-322
9791.94
220-321
10101.29
211-322
11591.91
414-505
11765.23
101-212
11797.81
514-606
OH-OH2 91
H3O2
35
-
-
-
Ozone92
O3
48
106530.7
13348.3
11835.4
C5H12O
88
7224.557
1741.198
1513.558
Pentanol
(1)69
224
12448.13
101-211
Pentanol (1)
12550.22
313-414
Cont’d
12949.96
303-404
13013.53
322-423
13082.54
321-422
13459.95
312-413
13707.83
716-726
14059.80
615-707
14458.90
615-625
14553.32
716-725
14681.17
202-313
14935.49
615-624
15113.75
514-523
15208.61
515-606
15354.29
514-523
15666.51
413-423
15770.19
413-422
16046.84
202-312
16112.93
312-322
16136.81
404-505
16147.58
312-321
16296.07
432-533
16298.84
431-532
16395.14
422-523
16450.01
211-221
16456.94
211-220
16811.00
413-514
17132.90
212-221
17139.84
212-220
17478.60
313-322
17494.87
303-414
17513.26
313-321
17941.93
414-423
18045.59
414-422
18793.58
515-616
8513.94
000-111
8748.80
000-110
9263.74
212-313
9590.30
202-303
9620.68
221-322
9651.13
220-321
9968.17
211-312
11485.97
101-212
12190.57
101-211
12343.095
313-414
Pentanol (2)69
C5H12O
88
7027.907
1720.8889
1486.027
225
12751.87
303-404
Pentanol (2)
12821.65
322-423
Cont’d
12897.40
321-422
13281.62
312-413
14343.50
202-313
14544.46
514-524
15113.55
413-423
15415.58
414-515
15573.55
312-322
15752.54
202-312
15884.41
404-505
15921.02
211-221
16017.51
423-524
16058.98
432-533
16062.21
431-532
16167.59
422-523
16586.63
413-514
16633.22
212-220
17020.63
313-321
17574.93
414-422
18982.99
505-606
9336.05
212-313
9653.95
202-303
9681.44
221-322
10018.29
211-312
12420.65
101-211
12440.34
313-414
12840.09
303-404
12903.24
322-423
12971.84
321-422
13349.32
312-413
15198.83
514-524
15538.30
414-515
15991.51
202-312
15999.73
404-505
16120.39
423-524
16157.98
432-533
16160.71
431-532
16256.40
422-523
16672.80
413-514
17413.27
303-414
17596.41
313-321
7502.88
111-212
7667.77
101-202
7711.65
212-303
Pentanol (3)69
C5H12O
88
7238.730
1727.313
1499.861
Pentanol (4)69
C5H12O
88
5944.849
2003.4287
1833.164
226
7778.00
000-111
Pentanol (4)
7843.41
110-211
Cont’s
7948.26
000-110
10731.73
312-404
10825.27
514-524
11238.33
413-423
11250.95
212-313
11444.28
101-212
11488.15
202-303
11509.55
221-322
11531.11
220-321
11572.07
312-322
11599.04
312-321
11753.20
313-404
11761.63
211-312
11824.16
211-221
11955.07
101-211
12334.95
212-221
12340.35
212-220
12593.56
313-322
12620.51
313-321
13020.86
414-422
13563.21
515-523
14132.44
413-505
14995.14
313-414
15027.47
202-313
15292.53
303-404
15341.78
322-423
15395.47
321-422
15675.60
312-413
15834.32
414-505
16048.93
202-312
17384.72
514-606
18534.46
303-414
7705.7594
212-313
8131.1779
202-303
8204.0659
221-322
8267.9737
220-321
10254.4228
313-414
10758.9588
303-404
10924.4851
322-423
10973.4878
331-432
10977.3614
330-431
11104.2853
321-422
11549.6796
312-413
Phenylacetylene93
C8H6
102
5680.3275
1529.7419
1204.955
227
12787.8444
414-515
13323.4054
404-505
Phenylacetylene
13632.7221
423-524
Cont’d
13982.6267
422-523
14398.2504
413-514
15304.3511
515-616
15822.5672
505-606
16325.8391
524-625
16910.7345
523-624
17217.3048
514-615
17803.3972
616-717
18265.4226
606-707
10492.4660
000-101
12070.6348
101-110
13474.8990
202-211
15778.6700
303-312
8465.4380
202-212
8831.4524
110-202
9211.0721
101-111
9443.4874
000-101
9626.4958
111-202
10006.1147
101-110
10850.4745
202-211
12207.0284
303-312
12785.4596
322-413
14173.9682
404-413
16857.2836
505-514
17283.6446
211-303
18091.9350
111-212
18654.5578
000-111
18837.5722
101-202
18712.237
18728.071
Propanal (syn)94
C3H6O
58
16669.6
5893.50
4598.99
Propanol (Gt)95
C3H8O
60
14330.36118
5119.30633
4324.200
Propargyl
radical96
C3H3
39
288055
9523.6775
9206.881
Propene97
C3H6
42
46070
9305.28
8134.16
18728.355
18729.160
18729.340
18729.543
18731.034
000-101
18731.586
18732.163
18732.434
18747.167
18751.240
17439.49
000-101
228
8551.6
000-101
17007.9
111-212
17102.9
101-202
17198.5
110-211
11341.70
101-111
14047.52
303-312
15775.13
404-413
18104.06
505-514
9325.8121
000-101
18325.56
111-212
18650.33
101-202
18978.78
110-211
17091.718
0-1
9896.47
523-524
11062.41
101-202
11154.86
734-735
12955.95
110-211
15018.32
212-313
15651.65
202-303
7663.342
212-313
8111.083
202-303
8209.161
221-322
8307.234
220-321
10191.333
313-414
10705.327
303-404
10926.328
322-423
10991.894
331-432
10998.375
330-431
11166.486
321-422
11600.648
312-413
12699.715
414-515
13220.284
404-505
13627.049
423-524
13736.83
441-542
13737.149
440-541
13756.093
432-533
13778.604
431-532
14088.377
422-523
14446.934
413-514
15013.709
919-918
15186.969
515-616
15660.795
505-606
16307.476
524-625
16480.212
550-651
16480.212
551-652
Propenol98
C3H6O
58
25592.16
4323.30
4228.22
Propylene
Oxide99
C3H6O
58
18023.72
6682.12
5951.48
Propynal100
C3H2O
54
68035.2994
4826.3014
4499.511
Propyne101
C3H4
40
-
8545.877
-
C3H4O3
88
5536.0436
3583.7074
2204.857
C8H8
104
5163.385
1545.1699
1191.224
Pyruvic
Acid102
Styrene103
229
16501.900
542-643
Styrene
16503.300
541-642
Cont’d
16525.941
533-634
16585.175
532-633
17063.610
523-624
17251.339
514-615
17653.252
616-717
9377.2457
000-100
9377.4304
002-102
11921.50
970-9-80
11992.90
002-102
18755.04
102-202
18778.52
110-2-10
8216.65
542-541
8501.04
432-431
8730.68
322-321
8904.10
212-211
9268.68
202-221
9470.91
312-331
9757.71
422-441
10143.53
532-551
10645.53
642-661
10984.61
000-101
11281.42
752-771
12067.95
862-881
8410.499
101-202
9301.280
110-211
11574.937
212-313
12329.475
202-303
12798.978
221-322
13268.495
220-321
13868.467
211-312
15316.356
313-414
15993.274
303-404
16969.684
322-423
17276.994
331-432
17439.666
330-431
18040.691
321-422
18318.144
312-313
8082.421
8083.169
8784.056
8784.960
10736.576
10738.897
221-322
220-321
616-615
tert-Butanol104
C4H10O
74
4704.38
4674.41
4508.16
Tetrahydrofuran105
C4H8O
72
7096.87
6976.02
4008.01
Toluene106
C7H8
92
5729.47572
2517.47827
1748.856
Tropolone107
C7H6O2
122
2743.090
1659.893
1034.383
230
11344.035
11344.280
11423.560
11423.658
14409.046
14411.987
414-515
Tropolone
Cont’d
404-505
928-927
9074.74
000-101
17734.53
111-212
18146.58
101-202
18564.91
110-211
11457.80
111-212
11875.95
101-202
12310.00
110-211
13612.50
818-726
14167.85
414-505
17181.20
212-313
17633.85
101-110
17794.10
202-303
17825.90
221-322
17856.70
220-321
18068.10
202-211
18460.40
211-312
18734.10
303-312
11885.44
111-212
12004.64
101-202
12125.54
110-211
17827.57
212-313
18004.72
202-303
7985.7076
212-313
8053.7004
202-303
8122.3223
211-312
10647.5552
313-414
10738.0698
303-404
10739.4004
322-423
10739.7949
321-422
10829.7041
312-413
13309.3540
414-515
13422.2730
404-505
13424.1816
423-524
13424.9717
422-523
13537.0347
413-514
15971.0942
515-616
16106.2647
505-606
16108.9180
524-625
16110.3057
523-624
Vinyl
Acetylene108
C4H4
52
49900
4744.66
4329.68
Vinyl
Formate109
C3H4O2
72
20391.57
3184.15
2757.75
Vinylcyclopropane110
C5H8
68
15220
3061.41
2941.34
Vinyldiacetylene111
C6H4
76
40490
1365.0813
1319.542
231
16244.3018
514-615
18632.7598
616-717
Vinyldiacetylene
18790.0020
606-707
Cont’d
18793.5987
625-726
18795.8223
624-725
232
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VITA
238
VITA
Amanda Shirar grew up just south of West Lafayette in Indianapolis, Indiana. She
is an only child with two wonderful parents. Growing up, her siblings included two cats
and a dog. Now she lives with her own (very finicky) cat, who has enjoyed moving out of
her parents house. Her interests include reading, watching an eclectic array of movies and
listening to and playing music. In addition to singing, she enjoys playing the piano (solo
or in a church praise band) and has played in two different handbell choirs. She spent her
first twelve years of education at Heritage Christian School on the northeast side of
Indianapolis.
For her undergraduate degree, she went to a small liberal arts college in
Grantham, PA, Messiah College. As a freshman, she planned on obtaining a degree in
chemistry and secondary education but decided to focus solely on chemistry. After
finishing her bachelor’s degree at a small school, she decided to go to a large state school
for graduate school. Her parents appreciated her moving back to an in-state school and
enjoyed her frequent visits. After obtaining her doctoral degree, she plans on beginning a
career in environmental consulting. Her brief entries into this scientific field while doing
background research on her thesis simply built upon a passion that had already existed.
She looks forward to starting a career where she will be able to make such an important
impact in the world.
PUBLICATIONS
239
Chemical Physics Letters 508 (2011) 10–16
Contents lists available at ScienceDirect
Chemical Physics Letters
journal homepage: www.elsevier.com/locate/cplett
Additional conformer observed in the microwave spectrum of methyl vinyl ketone
David S. Wilcox, Amanda J. Shirar, Owen L. Williams, Brian C. Dian ⇑
Department of Chemistry, Purdue University, 560 Oval Drive, West Lafayette, IN 47907-2084, USA
a r t i c l e
i n f o
Article history:
Received 3 December 2010
In final form 1 April 2011
Available online 3 April 2011
a b s t r a c t
A chirped-pulse Fourier transform microwave spectrometer was used to record the rotational spectrum
of methyl vinyl ketone (MVK, 3-butene-2-one). Two stable conformations were identified: the previously
documented antiperiplanar (ap) conformer and synperiplanar (sp), which is reported for the first time in
this microwave study. Methyl torsional analysis resulted in V3 barrier heights of 433.8(1) and
376.6(2) cm1 for ap- and sp-MVK, respectively. Heavy atom isotopic species of both conformers were
detected in natural abundance allowing bond lengths and angles of the molecular frames to be calculated
through Kraitchman analysis. A comparison with ab initio calculations is included.
Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction
The simplest a,b-unsaturated ketone, methyl vinyl ketone
(MVK, 3-buten-2-one), is important in atmospheric chemistry as
a primary product of isoprene oxidation. Early laboratory studies
discovered significant yields of MVK from the reaction of isoprene
with the hydroxyl radical [1] or ozone [2]. MVK was later identified
in the atmosphere at concentrations strongly correlated with biogenic isoprene [3]. MVK reacts quickly with the hydroxyl radical
to generate other carbonyls including methylglyoxal and formaldehyde that contribute to the additional destruction of ozone [4].
Accurately determining the structure of MVK will improve analyses of its chemistry in the atmosphere.
The microwave spectrum of MVK was first measured in 1965 by
Foster et al. in the region of 7–33 GHz [5]. MVK was identified in
the antiperiplanar configuration (ap, Figure 1) and A–E frequency
doublets were observed arising from hindered internal rotation
of the methyl group. The authors were able to determine a V3 value
of 437(7) cm1 for the barrier to internal rotation from this splitting. The microwave spectrum was revisited at higher frequencies
(26.4–40 GHz) in 1987 where rotational transitions were determined from the ground and first excited torsional states of apMVK. A slightly smaller V3 value of 424(7) cm1 was reported for
internal methyl rotation [6]. No evidence for an additional conformation was reported in either study.
Infrared studies, however, have confirmed a second stable conformation, synperiplanar (sp, Figure 1), in the liquid and gas phases
[7]. From the measured enthalpies, sp-MVK was found to be
2.36 kJ/mol higher in energy than ap-MVK. Ab initio calculations
have predicted similar energies (0.12–2.36 kJ/mol) depending on
⇑ Corresponding author. Fax: +1 765 494 0329.
E-mail addresses: wilcoxds@purdue.edu (D.S. Wilcox), ashirar@purdue.edu (A.J.
Shirar), olwillia@purdue.edu (O.L. Williams), dianb@purdue.edu (B.C. Dian).
0009-2614/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.cplett.2011.04.001
the level of theory used [7,8]. This small energy difference suggests
that the sp conformer could be detected in the MVK rotational
spectrum with a chirped-pulse Fourier transform microwave (CPFTMW) spectrometer. Here we present the rotational spectrum of
both the ap- and sp-MVK conformers, as well as assignments for
the isotopic species (heavy atoms only) in natural abundance.
2. Experimental
The CP-FTMW spectrometer has previously been described in
detail [9] and will only be summarized here. Methyl vinyl ketone
was obtained from Sigma–Aldrich (99% pure) and used without
further purification. Approximately 1 mL of sample was introduced
into a tank using a freeze–pump-thaw method and balanced to
approximately 7 bar with a 30/70% He/Ne mixture. An arbitrary
waveform generator with a 10 GS/s sampling rate (Tektronix
AWG 7101) produced a 1 ls microwave chirped pulse (1.875–
4.625 GHz). The pulse was then quadrupled (Phase One) and the
power increased with a 200 W amplifier (Amplifier Research) to
create an 11 GHz bandwidth pulse (7.5–18.5 GHz). This polarizing
pulse was then broadcast with a microwave horn into a vacuum
chamber with a typical base pressure of 1 106 Torr. The sample
was introduced into the chamber with a Series 9 General Valve
(2 mm orifice) at a backing pressure of 1 bar resulting in a supersonic expansion that cooled the molecules to an estimated rotational temperature of 4 K. The microwave pulse induced a
macroscopic polarization and the resulting free inductive decay
was collected with a second microwave horn. The molecular signal
passed through a PIN diode limiter (Advanced Control Components) and a switch (single pole single throw, Advanced Technical
Materials) before entering a low noise amplifier (+45 dB gain, Miteq). The molecular signal was then mixed with an 18.9 GHz phase
locked dielectric resonator oscillator (Microwave Dynamics) and
the lower sideband was isolated and digitized at 40 GS/s for
240
11
D.S. Wilcox et al. / Chemical Physics Letters 508 (2011) 10–16
X
r ¼1
g¼a;b
qg ¼ kg
Figure 1. The two conformers of methyl vinyl ketone (MVK) designated as
antiperiplanar (ap) and synperiplanar (sp).
20 ls with a 12 GHz oscilloscope (Tektronix TDS6124C). In order to
signal average, the entire system was synchronized with a
100 MHz phase locked loop (Wenzel Associates) driven by a
10 MHz Rb-disciplined crystal oscillator (Stanford Research Systems) and the timing was controlled with a 20-output Masterclock
(Thales Laser). A Kaiser-Bessel digital filter was applied to the time
domain signal to suppress side lobes of the rotational transitions in
the frequency domain. The Fourier transform of the free induction
decay yielded a nominally 11 GHz bandwidth frequency spectrum
with 20 kHz resolution. The resolution was interpolated to 5 kHz
offline in the spectral fitting program JB95 [10]. A signal-to-noise
ratio of 10000:1 on the strongest transition was achieved with
100 000 time domain averages. The ground state microwave spectrum and background spectrum (to remove spurious microwave
resonances) were recorded in a combined 15 h.
3. Results and discussion
^2
^2
^2
Heff
A ¼ AA P a þ BA P b þ C A P c þ H cd ;
ð1Þ
where
ð2Þ
AA ¼ ARR þ FW 00 q2a
BA ¼ BRR þ
ð2Þ
FW 00
q2b
ð2Þ
C A ¼ C RR ;
and Hcd describes 4th order distortion. In the preceding equation,
ð2Þ
W 00 is the second order perturbation coefficient of the A-state in
the ground torsional level. ARR, etc. are the unperturbed rigid rotor
constants in the absence of internal rotation. F and qg (g = a, b)
are functions of the methyl rotor inertial moment, I/ ; the direction
cosines of the top with respect to the principal axis, kg ; and the principal moments of inertia Ig :
2
F¼
h
FR
¼
2rI/
r
ð3Þ
I/
Ig
ð4Þ
I/
:
Ig
ð5Þ
The (a b) plane of symmetry in both conformers simplifies
terms in the Hamiltonians since the c-principal axis is normal to
the methyl rotor axis (kc = 0). qc is therefore zero, indicating C RR
is unperturbed by internal rotation.
The E-state effective Hamiltonian includes linear operators that
describe the angular momentum of methyl hydrogen tunneling [14],
^2
^2
^2
Heff
E ¼ AE P a þ BE P b þ C E P c þ
X
Dg P^g þ Hcd
ð6Þ
g¼a;b
with the operator constants
ð2Þ
AE ¼ ARR þ FW 01 q2a
BE ¼ BRR þ
ð2Þ
FW 01
q2b
ð7Þ
C E ¼ C RR
ð1Þ
Dg ¼ FW 01 qg :
ð1Þ
W 01
ð8Þ
ð2Þ
and W 01 are the 1st and 2nd order perturbation coefficients, respectively, for the ground torsional level of the E-state.
The A- and E-state perturbation coefficients are tabulated elsewhere [11] as a function of the reduced barrier parameter, s, which
is in turn related to V3 by
s¼
An effective Hamiltonian describing the torsion-rotation interaction for each A- and E-state symmetry species is implemented
in the spectral fitting program JB95 [10]. The fitting parameters
in the least squares procedure are based on the perturbative torsion-rotation Hamiltonian. For high V3 barriers to internal rotation,
the torsion-rotation cross terms are treated as perturbations. The
Hamiltonian is factored through successive Van Vleck transformations into submatrices describing A- and E-internal rotation states
separately for each torsional level [11]. In the principal axis frame,
this method is straightforward and suitable for molecules in torsional states near the bottom of the potential well with V3 barriers
greater than approximately 100 cm1 [12]. Published microwave
[5,6] and IR/Raman [13] data have reported V3 barrier heights between 420–437 cm1 for ap-MVK. Furthermore, all recorded lines
originated from the ground torsional state (vt = 0 for both symmetry species) under the experimental conditions in this lab.
The effective Hamiltonian used to fit the A-states is of the form
[14]
k2g
4 V3
:
9 F
ð9Þ
The V3 barrier is thus obtained by determining s from the perturbation coefficients and fixing FR to the appropriate value (I/ is
assumed 3.18 lÅ2 from Ref. [11]). Perturbation coefficients, rigid
rotor constants, and direction cosines are obtained iteratively from
the best-fit parameters of the A- and E-state effective Hamiltonians
in a method described by Lavrich et al. [14]. This procedure, generalized for the case of multiple internal rotors, is implemented in a
program integrated with the JB95 GUI.
An additional torsional analysis was performed with the internal rotor program XIAM [15]. The precision of the V3 barrier becomes harder to determine in the principal axis frame (PAF) as
the barrier increases and the torsion-rotation cross terms rapidly
decrease to zero. Improved internal rotation parameters such as
kg and V3 may be obtained in a number of methods employing
Rho-axis frame (RAF) Hamiltonians [12]. A new coordinate axis
system is defined in the RAF that is related to the PAF by a rotation
of the z-axis parallel to the q vector. The contributions of qx and qy
are eliminated from the q vector (qx ¼ qy ¼ 0) in the RAF, permitting the separation of the Hamiltonian into purely torsional and
rotational components. In XIAM, the pseudorigid rotor Hamiltonian, HPAF
rot ; is expressed in the PAF
^2
^2
^2
HPAF
rot ¼ AP a þ BP b þ C P c þ H cd :
ð10Þ
Global rotational parameters A, B, C, and Hcd differ from those of
Eqs. (1) and (6) in that they simultaneously describe A- and E-state
transitions. The torsional Hamiltonian in the RAF contains internal
rotation terms that may be directly varied in the fitting procedure:
^ 2 1
^
HRAF
tor ¼ Fðp/ þ qP z Þ þ V 3 ð1 cosð3/ÞÞ:
2
ð11Þ
^/ is the conjugate
Here, / is the internal rotation angle and p
momentum operator. The final rotational parameters are expressed in the PAF by transforming Eq. (11) back into the PAF
and adding the contributions to the rotational constants of Eq.
(10) [16].
241
12
D.S. Wilcox et al. / Chemical Physics Letters 508 (2011) 10–16
The microwave spectrum of methyl vinyl ketone in the region of
6–18.9 GHz is presented in Figure 2. Using the reported rotational
constants of Fantoni et al. [6], the A-state lines of ap-MVK were
identified in our spectrum and the corresponding E-state transi-
Figure 2. The microwave spectrum of methyl vinyl ketone (MVK). The inset
demonstrates the signal-to-noise achieved with the CP-FTMW spectrometer. The
dashed line in the inset indicates the average peak height of isotopic species
measured in natural abundance for ap-MVK.
tions were fit to the operator constants of Eq. (6) in JB95. Several
new lines were included in the fit of the A- and E-states of apMVK, refining the assignments of Foster et al. [5] at higher resolution. Quartic distortion terms were required in both effective Hamiltonians to reduce the root-mean-square error. Improved values of
all fitted parameters were obtained by including the previously
measured high frequency transitions (19–40 GHz) with their
appropriate experimental weights into the analysis. The quantum
number assignments from JB95 were used as input for a global
analysis in XIAM. The fit parameters from JB95 and XIAM are reported in Tables 1 and 2, respectively [17].
Though no preceding microwave studies have identified a second MVK conformer, experimental IR data has provided evidence
for a stable conformation of MVK (synperiplanar of Figure 1) that
is 2.36 kJ/mol higher in energy than ap-MVK. Ab initio calculations
were performed to estimate conformational energies using the
Gaussian suite [18] and are reported in Table 3. Zero-point corrected energies predicted sp-MVK to be the most stable conformer
by 0.44 and 0.07 kJ/mol, respectively, at the Hartree–Fock (HF/6311++G(d,p)) and density functional (B3LYP/6-311++G(d,p)) levels
of theory in contradiction with experimental results [7]. Secondorder Møller–Plesset perturbation theory (MP2/6-311++G(d,p))
calculations verified the experimental data with ap-MVK as the
lower energy conformer by 1.16 kJ/mol. Calculated vibrational
mode frequencies of each optimized geometry were positive, indicating that each conformer was a minimum energy structure [17].
Regardless of the computational accuracy, both theory and IR data
offer strong evidence that the sp conformer should be observed in
the MVK microwave spectrum.
After assigning all possible lines to ap-MVK, unassigned doublets up to three orders of magnitude above the noise floor were
Table 1
List of JB95a parameters used to fit the methyl vinyl ketone conformers.
ap-MVKb
A (MHz)
B (MHz)
C (MHz)
DJ (MHz)
DJK (MHz)
DK (MHz)
dJ (MHz)
dK (MHz)
Da (MHz)
Db (MHz)
DI (lÅ2)d
j
ne
r (kHz)f
s
ð1Þ
W 01
ð2Þ
W 00
ð2Þ
W 01
g
q
F (cm1)h
ARR (MHz)
BRR (MHz)
CRR (MHz)
hA (°)
V3 (cm1)
a
b
c
d
e
f
g
h
ap-MVK combinedc
sp-MVK
A
E
A
E
A
E
8941.590(1)
4274.5443(6)
2945.3315(6)
8.1(1) 10–4
4.6(1) 103
5.4(7) 104
2.95(9) 104
2.6(1) 103
8941.525(1)
4274.1989(6)
2945.3364(6)
8.1(2) 104
4.56(9) 103
9.1(6) 104
2.62(9) 104
2.6(1) 103
3.107(1)
7.1(4)
3.17
0.56
29
7.08
8941.597(1)
4274.5458(4)
2945.3346(4)
8.77(8) 104
4.48(3) 103
8.5(4) 104
2.67(3) 104
2.99(5) 103
8941.528(1)
4274.1992(4)
2945.3384(5)
8.22(8) 104
4.57(3) 103
1.03(4) 103
2.53(3) 104
2.67(5) 103
3.109(1)
7.3(2)
3.17
0.56
66
123.94
10240.938(2)
3991.6351(7)
2925.648(1)
7.2(2) 104
2.2(2) 103
1.232(6) 102
1.8(1) 104
3.7(3) 103
10237.429(6)
3991.464(1)
2925.655(1)
9.1(6) 104
2.4(4) 103
4(2) 103
1.7(3) 104
3.3(6) 103
33.570(9)
9.6(9)
3.24
0.71
18
8.00
3.16
0.56
26
5.99
3.16
0.56
85
113.92
35.48702
0.001654
35.32942
0.001693
3.22
0.71
20
5.04
29.86706
0.003859
0.002000
0.002046
0.004661
0.001000
0.001023
0.002330
0.028731
5.453
8941.546(1)
4274.3149(6)
2945.3334(6)
78(1)
435(6)
0.028654
5.453
8941.552(1)
4274.3144(4)
2945.3365(5)
78.5(7)
433(4)
0.054057
5.583
10238.657(4)
3991.507(9)
2925.652(1)
36(5)
375(5)
Ref. [10].
Parameters obtained with transitions in our spectral bandwidth (6–18.9 GHz).
Includes transitions of Refs. [5] and [6] (6–40 GHz).
Inertial defect, DI = Ic Ib Ia.
Number of transitions in the fit.
Observed minus calculated root-mean-square deviation of the fit.
Modulus of the q-vector (unitless).
FR fixed to 5.3 cm1.
242
13
D.S. Wilcox et al. / Chemical Physics Letters 508 (2011) 10–16
Table 2
List of XIAMa parameters used in the global fit of the methyl vinyl ketone conformers.
ap-MVK
A (MHz)
B (MHz)
C (MHz)
DJ (MHz)
DJK (MHz)
DK (MHz)
dJ (MHz)
dK (MHz)
DI (lÅ2)c
nd
r (kHz)e
s
qf
F (cm-1)
FR (cm-1)
hA (°)
hRAM (°)h
V3 (cm-1)
a
c
d
e
f
g
h
sp-MVK
8941.547(1)
4274.3593(9)
2945.2903(9)
8.3(2) 10–4
4.45(6) 10–3
6.2(9) 10–4
2.47(5) 10–4
2.95(9) 10–3
3.17
0.56
150
101.47
35.35527
0.028736
5.453
5.300g
78.21(2)
66.41
433.8(1)
j
b
b
10238.610(1)
3991.6814(6)
2925.4885(1)
7.3(2) 10–4
1.9(2) 10–3
9.98(7) 10–3
1.8(1) 10–4
3.2(3) 10–3
3.22
0.71
38
5.38
30.31290
0.058276
5.523
5.214(3)
29.71(3)
12.54
376.6(2)
Ref. [15].
Includes transitions of Refs. [5] and [6] (6–40 GHz).
Inertial defect, DI = Ic Ib Ia.
Number of transitions in the fit.
Observed minus calculated root-mean-square deviation of the fit.
Modulus of the q-vector (unitless).
Fixed.
Angle of Rho-axis transformation.
attributed to sp-MVK. Based on the A–E splitting of each doublet,
the V3 barrier was expected to be similar but slightly lower than
that of ap-MVK. This was anticipated since steric hindrance to
internal rotation is increased when the methyl and methylene
groups are in close proximity as in the ap conformation. A relaxed
potential energy scan (RPES) about the methyl torsional coordinate
at the HF level of theory (6-311++G(d,p)) predicted V3 values of 375
and 398 cm1 for sp- and ap-MVK, respectively. DFT (B3LYP/6311++G(d,p)) underestimated the barrier of the known ap conformer by nearly 50% and predicted ap-MVK to be the higher barrier configuration (Table 3). Using the HF results, initial rotational
constants for the A- and E-internal rotation species of sp-MVK were
generated from Eqs. (2) and (7). The optimized geometry of spMVK was used to predict the unperturbed rigid rotor rotational
constants as well as F and q in a method described by Pitzer
[19]. The reduced barrier parameter, s, was determined from F
and V3 (Eq. (9)), and perturbation coefficients were estimated from
Herschbach’s tables [11]. By simulating the spectrum with the perturbed rotational constants, ambiguity in the relative frequency order of the A–E symmetry doublets was eliminated in the early
stages of the fit. The fit parameters of sp-MVK from JB95 and XIAM
are also reported in Tables 1 and 2, respectively [17].
The internal rotation analysis in JB95 gave a V3 barrier height of
435(6) cm1 for ap-MVK with our data set, and 433(4) cm1 when
high frequency transitions from Refs. [5,6] were included in the fit.
The most precise V3 barrier height, 433.8(1) cm1, was obtained
with the combined data in XIAM by fixing FR to 5.3 cm1 and floating V3 and the angle between the top axis and a-principal axis, ha. A
direct comparison with previously reported results (Table 3) is not
straightforward, since the assumptions regarding the reduced moment, F, are different in all three models. The V3 barrier of
424(7) cm1 reported by Fantoni et al. was obtained by fixing the
reduced moment, F [6]. In this study, only I/ was fixed, allowing
the structural variables of F to be varied in the fit. Similar assumptions were made by Foster et al. who obtained a more directly comparable V3 barrier of 437(7) cm1 with a slightly smaller fixed
value of I/ (3.164 lÅ2) [5].
The V3 barrier height of sp-MVK was determined to be
375(5) cm1 in JB95. Including 4th order corrections into the effective Hamiltonians to account for the lower barrier did not improve
the precision of V3. It is anticipated that incorporating data from
higher J transitions is necessary to reduce the uncertainty with this
method. A value of 376.6(2) cm1 was obtained with XIAM by
floating the internal rotation parameters FR, ha, and V3. In contrast
to ap-MVK, the quality of the fit increased 5-fold by varying FR. The
variables FR and V3 are highly correlated, particularly in high barrier cases. It is suspected that due to the lower barrier of spMVK, more information is carried in the FR parameter and thus it
was necessary to vary it in the fit. Tables 1–3 summarize internal
rotation parameters from this study, calculations, and literature.
The high signal-to-noise in the MVK rotational spectrum made
it possible to resolve isotopic species in natural abundance, particularly carbon-13 substituted isotopes (1.1% natural abundance).
The unperturbed rigid rotor constants of the isotopologues were
calculated with the same computational accuracy as the unsubstituted parent species, and rotational constants for the A- and Einternal rotation species were predicted with Eqs. (2) and (7)
Table 3
Comparison of rotational data, dipole moment, and relative energies for experimental, ab initio calculated, and published values for both conformers of methyl vinyl ketone.
ap-MVK
sp-MVK
Calculateda
A (MHz)
B (MHz)
C (MHz)
DI (lÅ2)d
V3 (cm1)
hA (°)
F (cm1)
la (D)
lb (D)
lc (D)
DE (kJ/mol)
DEZPEf (kJ/mol)
a
b
c
d
e
f
Calculateda
Published
Published
HFb
DFTc
MP2b
Ref. [5]
Ref. [6]
HFb
DFTc
MP2b
9056.92
4311.38
2974.36
3.11
397.5
80.72
5.53
2.82
2.30
0.00
0
0.44
8934.27
4258.51
2936.50
3.14
258.7
81.63
5.46
2.74
2.07
0.00
0
0.07
8912.33
4262.47
2936.26
3.15
8941.45(6)
4274.48(2)
2945.32(2)
3.17
437(7)
75(4)
5.50
6.39(14)e
3.64(16)e
8941.58(1)
4274.494(5)
2945.276(4)
3.16
424(7)
75.46(5)
5.38
10481.39
4012.61
2954.22
3.09
375.2
32.78
5.66
0.60
3.01
0.00
0.04
0
10229.70
3979.18
2916.42
3.12
290.3
32.71
5.59
0.57
2.88
0.00
0.59
0
10141.03
3982.27
2911.12
3.14
Ref. [18].
6-311++G(d,p) basis set.
B3LYP/6-311++G(d,p) basis set.
Inertial defect, DI = Ic Ib Ia.
l2 (D2).
Zero-point corrected energies.
83.22
5.42
3.13
2.35
0.00
0
0
32.72
5.56
0.66
3.19
0.00
2.00
1.16
Ref. [7]
2.36
243
14
D.S. Wilcox et al. / Chemical Physics Letters 508 (2011) 10–16
Table 4
Isotopic assignments for methyl vinyl ketone.
ap-MVK
A (MHz)
B (MHz)
C (MHz)
DJ (MHz)
DJK(MHz)
DK (MHz)
dJ (MHz)
dK (MHz)
Da (MHz)
DI (lÅ2)
j
n
r (kHz)
H313CAC(O)ACH@CH2
H3CA13C(O)ACH@CH2
H3CAC(O)A13CH@CH2
H3CAC(O)ACH@13CH2
13C1
13C2
13C3
13C4
H3CAC(18O)ACH@CH2
180
A
E
A
E
A
E
A
E
A
E
8645.609(8)
4255.802(3)
2903.729(2)
6(2) 104
5.4(3) 103
2(2) 103
1.7(4) 104
8645.569(8)
4255.467(3)
2903.731(2)
6(2) 104
5.4(6) 103
4(1) 103
2.4(4) 104
8941.059(4)
4265.567(2)
2941.013(2)
8(1) 104
3.7(4) 103
8878.795(2)
4247.300(2)
2925.623(2)
6(1) 104
6.4(2) 103
8939.992(4)
4137.874(3)
2879.631 (3)
9(2) 104
5.8(3) 103
8939.930(4)
4137.556(3)
2879.627(2)
7(2) 104
5.8(3) 103
8728.645(9)
4129.793(3)
2853.487(3)
3.8(2) 103
2.1(1) 102
N/A
N/A
N/A
1.8(3) 104
8878.739(6)
4246.962(2)
2925.621(2)
4(2) 104
5.4(2) 103
5(1) 103
2.7(4) 104
3.16
0.53
15
6.671
3.21(8)
3.17
0.53
16
7.277
3.16
0.56
15
3.76
8941.017(7)
4265.219(3)
2941.014(3)
4(2) 104
4.5(7) 103
5(1) 103
3.2(4) 104
1.3(7) 103
3.09(7)
3.17
0.56
16
5.838
3.17
0.56
16
4.927
3.17(5)
3.18
0.56
17
5.968
10240.006(5)
3985.490(2)
2922.289(2)
2.7(9) 104
4.8(2) 103
5.2(2) 103
10236.587(6)
3985.300(2)
2922.295(2)
5(1) 104
5.2(5) 103
6.0(2) 102
10145.439(5)
3970.554(5)
2906.569(3)
5(3) 104
3.6(4) 103
1.0(2) 102
2.2(6) 104
3.22
0.71
10
4.696
34.11(3)
3.24
0.71
11
4.614
2.1(2) 104
4.0(4) 103
1.9(5) 104
3.1(5) 104
3.16
0.58
15
8.196
3.01(7)
3.17
0.58
15
6.751
3.16
0.57
10
7.736
10236.318(5)
3869.542(2)
2859.530(2)
N/A
N/A
N/A
sp-MVK
A (MHz)
B (MHz)
C (MHz)
DJ (MHz)
DJK (MHz)
DK (MHz)
dJ (MHz)
dK (MHz)
Da (MHz)
DI (lÅ2)
j
n
r (kHz)
10140.193(3)
3899.994(3)
2868.105(2)
4(1) 104
3.8(2) 103
1.12(9) 102
1.6(3) 104
3.22
0.72
12
3.541
10136.738(3)
3899.832(1)
2868.102(1)
1.0(3)x103
3.3(1) 102
2.0(3) 104
33.71(1)
3.24
0.72
13
3.833
3.22
0.71
12
6.509
assuming the same barrier parameters as the parent conformers.
The perturbed rotational constants were then scaled by the difference between the calculated and experimental values of the parent
species, increasing the accuracy of the isotopically substituted predictions by 1–2 orders of magnitude. The a- and b-type lines of the
13
C isotopes of ap-MVK were appreciable in intensity (Table 3)
resulting in 15–17 line assignments per internal rotor state. Owing
to the large signal of the parent species, 10 A-state lines of 18O
substituted ap-MVK were also detected. While there were several
suspected E-state transitions, a reliable set of parameters was not
obtained.
The sp-MVK rotational spectrum, in contrast, consists primarily
of a b-type transition moment (Table 3). Consequently, the low
intensity a-type lines of the sp-MVK 13C isotopologues were difficult to detect; only 10–13 lines were assigned per internal rotor
state. We were unable to assign 18O substituted transitions due
to the lower signal intensity of the parent species and the relative
abundance of this isotope (0.2% natural abundance). The quantum
number assignments of the A–E symmetry doublets were confirmed by noting the similar magnitude of doublet splitting with
respect to the parent conformer. In general, it was only possible
to float combinations of several distortion constants in the fits of
the isotopologues. Rotational constants and 4th order distortion
terms resulting in the lowest residual error are given in Table 4.
With heavy atom substitution data, structural information on
the frame of ap-MVK and carbon backbone of sp-MVK was acquired through Kraitchman analysis [20]. Vibrational contributions
from the methyl torsion contaminate both A- and E-state effective
rotational constants. In order to obtain the most accurate substitution structures (rs), these effects may be removed by calculating
the unperturbed rigid rotor constants from Eqs. (2) and (7) in the
iterative process of Lavrich et al. [14]. Since the Db cross-terms
were not well-determined due to the low J transitions in the fits
of the isotopically substituted species, an approximate expression
for the rigid rotor constants was used instead [21]:
10142.05(2)
3970.25(3)
2906.58(3)
4.0(6) 103
3.7(7) 102
7(1) 102
2(1) 104
5(1) 102
31.97(5)
3.25
0.71
12
8.815
Aapprox
¼
RR
10239.778(6)
3869.713(4)
2859.546(3)
8(3) 104
4.5(7) 103
9(3) 103
3.22
0.73
11
7.605
1
ðAA þ 2AE Þ:
3
N/A
N/A
N/A
2.7(1) 102
3.1(5) 104
33.64(4)
3.24
0.73
11
5.494
ð12Þ
A similar relation holds for Bapprox
. To account for small differRR
ences on the order of 1 kHz between C A and C E , the average was taken to calculate C RR . Eq. (12) is an approximation which neglects
denominator corrections accounting for the small differences in
the rotational energy levels after Van Vleck transformations. However, a systematic analysis on the equilibrium structure of cismethyl formate (V3 = 372.67 cm1) found that inclusion of denominator corrections induced a negligible effect on the value of the
unperturbed rigid rotor constants [21].
The errors of the rs structures in Table 5 are propagated from the
rigid rotor constants, although the calculated precision is greater
than the zero-point fluctuations of the molecule. Small changes
in the pseudoinertial defect (the difference between the equilibrium and effective principal moments estimating the vibration–
rotation contribution to rs) upon isotopic substitution lead to systematic errors in the rs coordinates [22]. Corrections with Costain’s
method lead to more physically meaningful uncertainties that are
comparable with calculated equilibrium structures [23,24]. The adjusted error in Table 5 is given by das /Å = 0.0015/|as|, where as is the
Kraitchman coordinate. The larger Costain errors observed in the
substitution structures involving the C2 and C4 atoms of both conformers are a consequence of the close proximity of the atoms to
the a-principal axis (0.06–0.1 Å) [23]. Costain’s results give errors
less than 0.01 Å when coordinates are greater than 0.15 Å from
the principal axis; it may be possible to reduce this uncertainty
by double-isotopic substitution [25].
Calculated and observed bond lengths and angles generally
concurred at all levels of theory in this study with the differences
between calculated and experimental values being attributed to
the modest basis sets used (Table 5). Without E-state rotational
constants for the 18O substituted ap conformer, rigid rotor constants could not be determined. Because of the barrier height, rota-
244
15
D.S. Wilcox et al. / Chemical Physics Letters 508 (2011) 10–16
Table 5
Comparison of the molecular structure according to ab initio predictions and Kraitchmana analysis.
ap-MVK
sp-MVK
Ab initiob
rC1C2
rC2C3
rC3C4
rC2O
\C1C2O
\C1C2C3
\OC2C3
\C2C3C4
a
b
c
d
e
f
Ab initiob
Kraitchman
Kraitchman
HFc
DFTd
MP2c
rs e
Costainf
HFc
DFTd
MP2c
rs e
Costainf
1.512
1.494
1.321
1.191
121.27
119.57
119.16
125.27
1.517
1.489
1.335
1.218
121.20
119.48
119.32
125.40
1.516
1.489
1.345
1.225
121.64
118.92
119.45
124.58
1.4774(2)
1.49249(8)
1.33945(4)
1.2411(1)
122.56(2)
121.21(2)
116.23(1)
123.32(1)
1.48(2)
1.492(8)
1.340(6)
1.24(1)
123(2)
121(2)
116(1)
123(1)
1.510
1.498
1.321
1.190
122.02
116.07
121.91
121.66
1.514
1.497
1.333
1.215
121.92
116.12
121.96
121.81
1.513
1.499
1.343
1.222
122.31
115.80
121.90
121.32
1.5037(1)
1.4751(3)
1.3334(2)
1.504(9)
1.475(9)
1.333(7)
117.94(2)
118(1)
121.96(3)
122(1)
Ref. [20].
Ref. [18].
6-311++G(d,p) basis set.
B3LYP/6-311++G(d,p) basis set.
Error of substitution structure propagated from rotational constants.
Errors corrected with Costain’s method (Refs. [22] and [24]).
tional constants for the A-internal rotation species are expected to
be within 0.1% of the rigid rotor values. For these reasons, 18O
substituted coordinates were included in the Kraitchman analysis
and are in reasonable agreement with theory. One notable disagreement between calculated and experimental values is in the
rC1C2 methyl rotor axis bond of ap-MVK. At all levels of theory,
the predicted rC1C2 bond length is typical for a carbon–carbon single bond. Within Costain’s error, the experimental bond length is
shorter by nearly 2 pm. Ab initio calculations less accurately predicted the V3 barrier associated with the RPES about this bond, suggesting the deviations from experimental values are correlated.
It has been suggested that a non-planar species of MVK exists
[7], but a RPES conducted about the C2-C3 bond did not reveal
any additional minima. Though low in intensity, approximately
550 lines were unassigned in the spectrum with many appearing
to be A-E symmetry doublets. We attempted to find vibrationally
excited satellite states corresponding to skeletal and methyl torsional motion using the rotational constants of Fantoni et al. [6].
No such lines were found. Efficient vibrational cooling in the supersonic jet diminishes population in these low energy modes and
accordingly we do not generally observe vibrational satellites with
our spectrometer. From the suspected torsional splitting of the
unidentified peaks, dimerization or other clusters with the backing
gas in the supersonic jet may account for the remaining lines in the
spectrum.
On a final note, the high resolution, high signal-to-noise, and
broad bandwidth of the CP-FTMW spectrometer makes it an ideal
apparatus for obtaining substitution structures of carbon-based
gas phase molecules. By signal averaging, detection of isotopologues in natural abundance requires no additional searching for
these transitions in the measurement process, nor does it necessitate the synthesis of additional molecular samples. Though weaker
in intensity, the improved predictions for isotopic rotational constants significantly expedite the assignment process of low intensity transitions and hence the additional transitions are
efficiently identified in a single broadband spectrum.
Acknowledgements
The authors would like to thank David Plusquellic for discussions regarding methyl rotor analysis in JB95 and for providing
the program that was used to calculate barrier heights, rotor
angles, and rigid rotor constants. We are also indebted to Isabelle
Kleiner for helpful comments on the topic of internal rotation. This
material was supported by the Dr. Henry and Camille Dreyfus
Foundation New Faculty Award and Purdue University.
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.cplett.2011.04.001.
4. Conclusion
The microwave spectrum of methyl vinyl ketone was measured
using a CP-FTMW spectrometer. In addition to the previously observed ap-MVK conformer, a second conformer, sp-MVK, was assigned. A torsional analysis in JB95 gave V3 barriers of 433(4) and
375(5) cm1 for ap- and sp-MVK, respectively, with the former barrier including previously measured high frequency transitions. The
internal rotor program XIAM was used to determine more precise
barriers of 433.8(1) and 376.6(2) cm1 for ap- and sp-MVK, respectively. The observed differences in precision were found to be a
reflection of the models used to interpret the data rather than
the resolution of the measurements. The high signal-to-noise of
the CP-FTMW spectrometer resolved MVK isotopologues in natural
abundance. Kraitchman analysis determined the molecular structure using approximate rigid rotor rotational constants. Ab initio
calculations were also completed that resulted in several discrepancies with the experimental data concerning relative energies,
barrier heights, and structural parameters, particularly for the ap
conformer.
References
[1] E.C. Tuazon, R. Atkinson, Int. J. Chem. Kinet. 22 (1990) 1221.
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14 (1982) 955.
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[4] E.C. Tuazon, R. Atkinson, Int. J. Chem. Kinet. 21 (1989) 1141.
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362 (1996) 187.
[9] A.J. Shirar, D.S. Wilcox, K.M. Hotopp, G.L. Storck, I. Kleiner, B.C. Dian, J. Phys.
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[15] H. Hartwig, H. Dreizler, Z. Naturforsch 51a (1996) 923.
[16] J. Gillies, C. Gillies, J.-U. Grabow, H. Hartwig, E. Block, J. Phys. Chem. 100 (1996)
18708.
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[17] Assigned rotational transitions and calculated vibrational frequencies for apand sp-MVK are given in the Supplemental Information.
[18] M.J. Frisch et al., Gaussian 03, Revision C.02, Gaussian, Inc., Wallingford, CT,
2004.
[19] K.S. Pitzer, J. Chem. Phys. 14 (1946) 239.
[20] J. Kraitchman, Am. J. Phys. 21 (1953).
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246
pubs.acs.org/JPCL
Dissociation Pathways of 2,3-Dihydrofuran Measured
by Chirped-Pulse Fourier Transform Microwave
Spectroscopy
Chandana Karunatilaka, Amanda J. Shirar, Giana L. Storck, Kelly M. Hotopp, Erin B. Biddle,
Rickie Crawley, Jr., and Brian C. Dian*
Department of Chemistry, Purdue University, 560, Oval Drive, West Lafayette, Indiana, 47907-2084
ABSTRACT This experiment combines the use of an ultrabroadband chirpedpulse Fourier transform microwave (CP-FTMW) spectrometer with a pulsed discharge nozzle to study products formed during dissociation of 2,3-dihydrofuran
(2,3-DHF). Molecules identified in the spectrum include cyclopropanecarboxaldehyde (CPCA), acrolein, crotonaldehyde (CA), formaldehyde, propene, propyne and
cyclopropenylidene. Individual cis and trans isomers were detected for CPCA and
acrolein, but only the trans isomer of CA was observed. Although cis forms of CPCA
and CA would be the most likely structures produced from 2,3-DHF isomerization,
our discharge provides enough energy for the molecules to convert into multiple
conformers. The identification of formaldehyde in the spectrum supports a
proposed mechanism to form propyne directly from 2,3-DHF ring-opening, a
scheme that has been difficult to verify. Although acetylene cannot be detected
because of the lack of a permanent dipole moment, the existence of cyclopropenylidene (C3H2) is indirect evidence of its presence in the discharge.
SECTION Kinetics, Spectroscopy
E
with an argon bath. Hot discharge products were cooled to a
few Kelvin in the supersonic expansion, and the signal was
characterized by recording the 11 GHz broadband rotational
spectrum using a digital oscilloscope with a 40 Gs/s digitizer.
Unlike previously reported discharge experiments,1,11 our system has the capability of recording the rotational spectrum of
several molecules simultaneously.
According to a study by Lifshitz,9 21 products were proposed in the pyrolysis of 2,3-DHF. Altogether, seven molecules
have been positively identified in our discharge spectrum:
cyclopropanecarboxaldehyde (CPCA), acrolein (prop-2-enal),
crotonaldehyde (CA, but-2-enal), formaldehyde, propene,
propyne, and cyclopropenylidene. Of these products, three
molecules have cis and trans conformers. Since rotational
constants differ between conformers, our microwave spectrometer distinguishes between them. For CPCA and acrolein,
both cis and trans conformers were observed, but only the
trans form of CA was identified. Molecules containing sufficient rotational transitions within our spectral range were fit
using JB95,12 otherwise molecular transitions were simply
compared to published frequencies. As can be seen in Figure 1,
the present experiment not only identified molecular products created in the discharge process, but also distinguished
lectric discharge sources are commonly used to produce exotic chemical species such as radicals and ions
that are difficult, if not impossible, to prepare on a
laboratory benchtop. These species are important intermediaries in combustion processes and play significant roles
in interstellar chemistry.1,2 Despite the recent advances of
experimental techniques,3 the unambiguous identification of
important reaction intermediates has become increasingly
difficult as target compounds increase in size. Structural
isomers can have widely varying chemical reactivities,4 necessitating chemical identification to provide accurate comparison to kinetic models. The experiment of Taatjes et al.3
demonstrates the current experimental complexity in positively identifying multiple isomers of the same mass.
Ultrabroadband chirped-pulse Fourier transform microwave (CP-FTMW) spectroscopy has continued to find new
developments5-7 with recent advances to state-of-the-art
high speed digital electronics. 2,3-Dihydrofuran (2,3-DHF) is
used in the present experiment as a prototypical system to
demonstrate applying a CP-FTMW spectrometer as a shapesensitive detector to explore various dissociation pathways.
The cyclic ether 2,3-DHF was chosen as an appropriate
molecule to study since its dissociation products have been
studied extensively and are well documented.8-10 Our spectrometer employs a pulsed discharge source to generate an
electric field across two electrodes. The sample was introduced using a pulsed valve located before the electrodes, and
dissociation of 2,3-DHF was initiated via penning ionization
r XXXX American Chemical Society
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Accepted Date: April 22, 2010
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Figure 2. Relative zero point corrected energies for the four
conformers of CA. Conformers related to the carbon double bond
or the aldehyde group are identified by d- and s- respectively. The
lowest energy structure, the s-trans conformer of d-trans-CA, is set
at 0 kJ/mol.
Figure 1. Comparison of the discharge spectrum of 2,3-DHF
obtained from CP-FTMW spectrometer (top) and predicted spectrum using Pickett's SPCAT6 (bottom).
obtain the torsional potential for cis and trans isomers of
CPCA. The torsional barrier is 26.22 kJ/mol, and the trans
conformer is 0.68 kJ/mol more stable than the cis conformer.
Microwave work by Volltrauer and Schwendeman18 confirms
that stable forms of cis and trans CPCA exist with an energy
difference of 0.121 kJ/mol and a barrier height of 18.4 kJ/mol.
In regards to CA, the problem is more complicated. The
energies and vibrations of the four possible structures have
been calculated (B3LYP/6-31þG**), and the zero point corrected energies are presented in Figure 2. A recent quantumchemical study by Bokareva et al.19 calculated structures and
torsional barriers of acrolein derivatives, and their labeling
scheme of CA was adopted here (d- indicates isomers relating
to the carbon double bond). It can be seen in Figure 2 that
both the d-trans conformers are more stable than either d-cis
conformer. Dubnikova and Lifshitz10 predicted that 2,3-DHF
proceeds directly to the s-cis conformer of d-cis-CA, shown in
the left panel of Figure 2. In the discharge spectrum, only the
trans conformer of d-trans-CA was recorded. In order to
convert from the predicted theoretical structure to the experimentally observed structure, two separate torsions would
need to be applied: one to the aldehyde group and one around
the carbon double bond. The torsional barrier for the aldehyde group is fairly straightforward to calculate, and Bokareva
et al.19 published results of ∼25 kJ/mol for d-cis conformers
and ∼36 kJ/mol for d-trans conformers. Experimentally, only
the d-trans molecules have been studied, and vibrational
analysis20 determined this torsional barrier to be ∼68 kJ/
mol. Unfortunately, calculating the torsion around the double
bond is much more difficult and has not been studied in CA.
However, many studies involving the double bond in stilbene
have been conducted and the barrier to cis-trans isomerization in stilbene21 is ∼187 kJ/mol. This barrier height gives a
rough estimate of the possible barrier to convert from the cis
form of d-cis-CA to the trans form of d-trans-CA. Once CA has
been created in the discharge, it likely contains an excess of
internal energy that allows for torsion around a double bond
to relax into a lower energy structure. A purely trans assignment was confirmed by obtaining the pure rotational spectrum of cis and trans CA using our spectrometer system. In the
region of 14 GHz, a feature similar to cis CA was observed, but
between cis and trans conformers on the basis of their
unique rotational frequencies and transition patterns.
Two different pyrolysis experiments using 2,3-DHF have
been reported in the literature. In the first study,8 both CPCA
and CA molecules were identified by their melting points at
various pyrolysis temperatures (670-820 K). In the late
1980s, Lifshitz9 revisited this study of 2,3-DHF pyrolysis
behind reflected shocks in a single-pulse tube at increased
temperatures (up to 1300 K). Major reaction pathways and
rate constants for certain unimolecular reactions were determined, and additional pyrolytic products were detected.
Identification of products was derived from retention times
and gas chromatography/mass spectrometry (GC/MS), but
they could not distinguish between individual conformers.
Contrary to the experimental results obtained for tetrahydrofuran,13 furan14 and 2,5-dihydrofuran,15 Lifshitz discovered
that the major reaction path of 2,3-DHF was the unimolecular
isomerization process to yield CPCA. Another favorable unimolecular isomerization to form CA from CPCA at higher
temperatures was also proposed, but the reaction rate could
not be determined because of incomplete separation between GC signals. However, as can been seen in Figure 1,
our spectrometer has the capability to isolate both CPCA and
CA as well as the cis and trans conformers of CPCA since our
detector is especially sensitive to the shape of the molecule.
According to theory and experiment, the main thermal
reaction of 2,3-DHF is the furan ring-opening to form two
isomerization products, CPCA and CA. The original experimental studies9,16 focused on determining combustion products, but were unable to determine contributions from
conformers of any species. Later, density functional theory
calculations by Dubnikova and Lifshitz10 proposed that both
CPCA and CA are formed by unimolecular isomerization
processes and proceed via a concerted mechanism with a
single transition state. These calculations follow a specific
reaction coordinate that leads exclusively to the cis conformer
for each reactant. Both cis and trans conformers of CPCAwere
identified on our spectrum, but only the trans conformer of
CA. Density functional theory calculations (B3LYP/6-31þG**)
using the Gaussian 03 program suite17 were performed to
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be formed via the dissociative recombination27 seen in
reaction 3.
we were unable to reasonably fit these transitions. Splitting
on the K1 bands of these transitions does suggest that this
unassigned molecule has a methyl rotor. The presence of
these additional conformers suggests that the discharge
products contain enough internal energy to isomerize to
these conformers subsequent to dissociation. This excess
energy could be due to the discharge itself, or collisions
with other excited species. Although we are unable to determine the origin of this additional energy, the presence of
additional conformers implies the discharge environment
provides enough energy to initiate numerous isomerization
reactions.
In addition to the aldehydes, both propyne and propene
were observed. Several studies9,22 propose that propyne is
produced from either propene or 2,3-DHF ring-opening. To
form propyne directly from 2,3-DHF, a side product of formaldehyde is necessary (reaction 1).
Unfortunately acetylene, ethane, and allene cannot be detected by our CP-FTMW spectrometer because of their lack
of a permanent dipole. The strong signal of this carbene may
be from contributions from several sources and could be
indirect evidence for these molecules.
A few lines remain unidentified in our spectrum despite
extensive search for different isomerization and decomposition products. Small organic molecules characterized in the
previous pyrolysis study9 were not detected by our spectrometer because they either lack a permanent dipole moment
or their rotational frequencies are located outside the bandwidth of our spectrometer. The open and closed shell transition states proposed in previous DFT calculations10 and other
possible free radical species could also be responsible for
unassigned peaks.
We present here the use of 2,3-DHF as a prototypical system
to demonstrate CP-FTMW as a shape-sensitive detection device. In addition to identifying seven distinct discharge products, our spectrometer was able to discriminate between cis
and trans conformers of multiple species. Although previous
theory10 predicted isomerization mechanisms that ended with
only cis conformers of CPCA and CA, our results suggest that
product species have sufficient internal energy after dissociation to convert between molecular conformations. Formaldehyde was observed and helps support a ring-opening mechanism to form propyne. An unpredicted discharge product,
cyclopropenylidene, was detected and a mechanism derived
from 2,3-DHF was proposed. Further experiments are needed
to get rate information of the proposed reaction mechanisms
by using relative peak intensities from experiments with
varying concentrations of 2,3-DHF. Moreover, introducing
oxygen into the discharge with different molecules would more
accurately represent a combustion environment and should be
explored. We believe combining a CP-FTMW spectrometer
with a pulsed discharge source will continue to find new
applications that will help to discover the rich chemistry behind
these reaction mechanisms.
The authors were unable to verify this reaction, as no
formaldehyde was observed in the postshock samples.
Unlike those experiments, a strong formaldehyde signal
exists in our discharge spectrum (14488.495 MHz). We are
unable to distinguish between the two mechanisms, but
there is now evidence to support direct formation of
propyne from 2,3-DHF. Additionally, it has been proposed
that propene is formed by secondary thermal decomposition of the aldehyde products instead of a direct decomposition of 2,3-DHF.9 The kinetics derived in the
experiment did not agree with a single-step unimolecular
reaction. However, such conclusions cannot be drawn from
our data set without performing isotopic substituted experiments of 2,3-DHF.
A product not reported in the pyrolysis study, but tentatively assigned in our spectrum, is cyclopropenylidene (C3H2).
Possible precursors to this molecule could be propyne, allene,
acetylene, or ethane, which are all significant products of 2,3DHF pyrolysis.9 Since the presence of propyne can be confirmed in the discharge, a mechanism initiating from this
molecule is proposed as the primary source of cyclopropenylidene. With the excess of energy produced in the discharge, propyne can dissociate to form vinylidenecarbene23
(reaction 2).
EXPERIMENTAL SECTION
The spectrometer is divided into three distinct regions: (1)
pulse generation, (2) sample interaction, and (3) molecular
detection. A 1 μs, chirped microwave pulse was generated
using an arbitrary waveform generator (Tektronix, AWG7101)
covering the frequency ranges of 1.875-4.625 GHz. The
bandwidth of this pulse was then increased with an x4
frequency multiplier (Phase One PS06-0161) to create an
11 GHz bandwidth pulse (7.5-18.5 GHz). After increasing
the power of the signal with a 200 W amplifier (Amplifier
Research 200T8G18A), the pulse was broadcast and received
through a vacuum chamber using a set of microwave horns
Since the most stable conformer of C3H2 is cyclopropenylidene,24,25 vinylidenecarbene will rearrange to that configuration. Likewise allene, an isomer of propyne, can also
form vinylidenecarbene,23 and experiments have shown
that allene in a discharge26 generates cyclopropenylidene.
Another mechanism that could be occurring begins with a
stable cyclopropenyl cation, C3H3þ, that can be created from
acetylene and ethane.27-29 Cyclopropenylidene could then
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(Amplifier Research AT4004) where it interacted perpendicularly with the sample.
The sample was introduced into the chamber at 30 psi
through a pulsed nozzle located prior to the discharge. The
electric discharge nozzle (Amy Facility, Purdue University)
consisted of a set of aluminum electrodes separated by a
1.5 mm Delrin spacer and fit in a Delrin holder. Using a highvoltage pulse modulator (DEI model PVM-4210), an electric
pulse of þ300 V and -500 V was applied to the electrodes.
The high voltage pulse modulator was controlled using a
discharge timing control box with a pulse duration range
from 1 to 100 μs that was initiated after opening the
pulsed valve. The discharge initiated Penning ionization in Ar
to induce chemical dissociation. Discharge conditions were
optimized by monitoring the depletion of the 000-101 ground
state rotational transition of 2,3-DHF. After the discharge, the
sample entered a diffusion-pumped (Varian, VHS-10), low
pressure (∼10-5 Torr) vacuum chamber backed by a roots
blower and a two-stage mechanical pump. Molecules entering
the vacuum chamber expanded supersonically and as a
result, the molecules were at very low temperatures. We
estimated the rotational temperature to be 2.5 K by fitting
the intensities of the b-type Q-branch transitions of cis CPCA
using SPCAT.30 After interaction with the electric field in the
chamber, the molecular free induction decay was detected,
amplified, (Miteq, AMF-6F-06001800-15-10P) and mixed
down (Miteq, TB0440LW1) with an 18.9 GHz phase-locked
dielectric resonant oscillator (PDRO, Microwave Dynamics
PLO-2000-18.9) to allow collection and digitization with a
12 GHz oscilloscope (Tektronix TDS6124C 12 GHz, 40 Gs/s).
The low-noise amplifier was protected from the high power
polarization pulse by a PIN-diode limiter (Advanced Control
Components, ACLM 4619F-C36-1K) and a single-pole singlethrow microwave switch (Advanced Technical Materials,
S1517D) operating in series.
The rotational free inductive decay was digitized for
10 μs, yielding an experimental resolution of 40 kHz after
zero-padding of the time spectrum. Molecular emission
was signal averaged in the time domain before applying a
Kaiser-Bessel Digital filter and Fourier transforming to the
frequency domain. Molecules with a sufficient number of
rotational lines present in the spectrum, such as with CPCA,
were fit using published rotational constants. Molecules,
such as formaldehyde, that only contained a single transition were compared directly to frequencies from literature
values. Some of the difference between experimental and
published values could be due to unresolved A-E methyl
rotor splitting or the lack of centrifugal distortion terms
when fitting the data.
ACKNOWLEDGMENT This material was supported by the
ACS-PRF #47285-G6 and Purdue University. Special thanks to
Dr. Josh Newby and Dr. Tim Zwier for consultation regarding the
discharge nozzle.
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(2)
(3)
(4)
(5)
(6)
(7)
(8)
(9)
(10)
(11)
(12)
(13)
(14)
SUPPORTING INFORMATION AVAILABLE A full list of
molecules characterized in the discharge experiment and their
assigned frequencies. This material is available free of charge via
the Internet at http://pubs.acs.org.
(15)
AUTHOR INFORMATION
(16)
Corresponding Author:
(17)
*To whom correspondence should be addressed. E-mail: dianb@
purdue.edu.
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