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, CH 3 -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- s-cis-

trans-cis cis-cis-

Relative Energies: B3LYP/6-311G++(d,p)

s-trans- D G 298 K = 0 kcal/mol s-cis- D G 298 K = 1.03 kcal/mol

trans-cis-

D G 298 K = 6.65 kcal/mol

cis-cis-

Relative Energies: B3LYP/6-311G++(d,p)

s-trans- D G • = 0 kcal/mol severe steric hindrance • structure could not be optimized by HF, MP2, B3LYP calculations s-cis- D G 298 K = 1.03 kcal/mol

trans-cis-

D G 298 K = 6.65 kcal/mol

cis-cis-

Relative Energies: B3LYP/6-311G++(d,p)

s-trans- D G 298 K = 0 kcal/mol s-cis- D G 298 K = 1.03 kcal/mol

trans-cis-

D G 298 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 scaled frequency (cm -1 ) 800 600

Ruling out the highest-energy conformer

trans

-

cis

-methyl pyruvate 0.05

s-

trans

experimental 0.00

1400 1200 1000 scaled frequency (cm -1 ) 800 600 CO 2 contamination

Distinguishing Conformers

• • The trans- and cis- conformers are expected to be observable in experimental FTIR spectra.

-1 region made integration unreliable.

trans

-

cis

-methyl pyruvate s-

cis

-methyl pyruvate s-

trans

-methyl pyruvate 1400 1200 1000 scaled frequency (cm -1 ) 800 600

Identification of s-cis-methyl pyruvate 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 s trans conformers

Identification of s-cis-methyl pyruvate 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 s trans conformers

Identification of s-cis-methyl pyruvate 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 s trans conformers

Population of Conformers

cis trans



integrated signal (cis) integrated signal (trans)



predicted intensity (trans) predicted intensity (cis)

s-trans ~600 K

cis trans

 0 .

051 or 4.5% s-cis s-cis ~295 K

cis trans

 0 .

0093 or 0.9% s-cis Expected 15% s-cis at 295 K based on D G 298 = 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