McCunnMSS2012

Report
FTIR OF METHYL PYRUVATE
CONFORMERS IN AN ARGON MATRIX
Allison B. Combs, Jordan L. Preston, Sara E. Lilly,
Courtney D. Hatten, and Laura R. McCunn
Department of Chemistry, Marshall University
Huntington, West Virginia
Introduction
methyl pyruvate
pyruvic acid
• possible precursor to the methoxycarbonyl
radical, CH3-O-C=O
• similar to pyruvic acid, which has been
determined to have three conformers
(Reva et al. J. Phys. Chem. A 2001, 105, 4773-4780)
Introduction
methyl pyruvate
• identify conformers of methyl pyruvate and
determine their relative energies
• FTIR of methyl pyruvate isolated in an argon
matrix
• Gaussian 09 calculations
• evaluate ratio of conformers by heating methyl
pyruvate prior to matrix deposition
Conformers of Methyl Pyruvate
s-trans-
trans-cis-
s-cis-
cis-cis-
Relative Energies: B3LYP/6-311G++(d,p)
s-trans-
s-cis-
DG298 K = 0 kcal/mol
DG298 K = 1.03 kcal/mol
trans-cisDG298 K = 6.65 kcal/mol
cis-cis-
Relative Energies: B3LYP/6-311G++(d,p)
s-trans-
s-cis-
DG298 K = 0 kcal/mol
DG298 K = 1.03 kcal/mol
• severe steric hindrance
• structure could not be
optimized by HF, MP2, B3LYP
calculations
trans-cisDG298 K = 6.65 kcal/mol
cis-cis-
Relative Energies: B3LYP/6-311G++(d,p)
s-trans-
s-cis-
DG298 K = 0 kcal/mol
DG298 K = 1.03 kcal/mol
trans-cisDG298 K = 6.65 kcal/mol
Predicted and Experimental IR Spectra
Experimental spectra consist of 1:1000 methyl pyruvate in argon deposited on a
~20 K CsI window. FTIR spectra (1 cm-1 resolution, 120 scans) collected at 10 K.
Distinguishing Conformers
• trans-cis-methyl pyruvate is almost identical to the s-trans conformer
• the presence/absence of two peaks around 700 cm-1 can distinguish the
trans-cis conformer
trans-cis-methyl pyruvate
s-cis-methyl pyruvate
s-trans-methyl pyruvate
1400
1200
1000
800
-1
scaled frequency (cm )
600
Ruling out the highest-energy conformer
trans-cis-methyl pyruvate
0.05
Absorbance
s-trans-methyl
pyruvate
experimental
0.00
1400
1200
1000
800
600
scaled frequency (cm-1)
CO2 contamination
Distinguishing Conformers
•Initial
•The
transanalysis
andindicated
cis- conformers
8% s-cis
are
conformer,
expected but
to be
fluctuations
observableininthe
experimentalinFTIR
background
the spectra.
1200-1300 cm-1 region made integration unreliable.
trans-cis-methyl pyruvate
s-cis-methyl pyruvate
s-trans-methyl pyruvate
1400
1200
1000
800
-1
scaled frequency (cm )
600
Identification of s-cis-methyl pyruvate
Absorbance
s-cis
s-trans
To verify the FTIR bands
assigned to the s-cis conformer,
the mixture of methyl pyruvate
and argon was heated prior to
matrix deposition.
Gray sticks indicate (scaled)
B3LYP/6-311G++(d,p) predicted
frequencies for the s-cis and strans conformers
Identification of s-cis-methyl pyruvate
Absorbance
s-trans
s-cis
To verify the FTIR bands
assigned to the s-cis conformer,
the mixture of methyl pyruvate
and argon was heated prior to
matrix deposition.
Gray sticks indicate (scaled)
B3LYP/6-311G++(d,p) predicted
frequencies for the s-cis and strans conformers
Identification of s-cis-methyl pyruvate
Absorbance
s-cis
To verify the FTIR bands
assigned to the s-cis conformer,
s-trans the mixture of methyl pyruvate
and argon was heated prior to
matrix deposition.
Gray sticks indicate (scaled)
B3LYP/6-311G++(d,p) predicted
frequencies for the s-cis and strans conformers
Population of Conformers
cis
integrated signal(cis) predicted intensity(trans)


trans integrated signal(trans) predictedintensity(cis)
s-trans
~600 K
Absorbance
s-cis
cis
 0.051
trans
or 4.5% s-cis
~295 K
cis
 0.0093
trans
or 0.9% s-cis
Expected 15% s-cis at 295 K based
on DG298 = 1.03 kcal/mol
Conclusions
• The FTIR spectrum of methyl pyruvate
isolated in a 10 K argon matrix was
measured.
• DFT calculations indicate three possible
conformers of methyl pyruvate, but only the
s-trans and s-cis are observable.
• Heating the methyl pyruvate sample prior to
matrix isolation verified the presence of the
s-cis conformer.
Acknowledgements
Allison Combs
B.S. ‘12
Sara Lilly
B.S. ‘12
Courtney Hatten
Class of ‘13
• Jordan Preston
• Research Corporation for Science Advancement
• Camille and Henry Dreyfus Foundation
• MU-ADVANCE
• NASA-WV Space Grant Consortium

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