Praveenkumar Boopalachandran, 1 Jaan Laane 1 and Norman C. Craig 2 1 Department of Chemistry, Texas A&M University, College Station, Texas Department of Chemistry and Biochemistry, Oberlin College, Oberlin, Ohio Department of Chemistry and Biochemistry, Oberlin College, Oberlin, Ohio Gas Phase Raman Spectra of Butadiene and Butadiene-d 6 and the Internal Rotational Potential Energy Function

Outline Introduction Introduction Research Work Research Work  1,3-butadiene (-d 0 )  1,3-butadiene (-d 6 )  1,3-butadiene (-d 2 )  1,3-butadiene (-d 4 ) Summary Summary

1,3-butadiene It is a planar molecule with C 2h symmetry and possesses a trans geometry. Earlier studies to establish the structure of the higher energy conformer gave contradictory results. Our goal is to determine an experimental torsional potential. McKean, D. C.; Craig, N. C.; Panchenko, Y. N. J. Chem. Phys. 110 (2006) 8044 transgauchecis conformers of 1,3-butadiene CH 2 =CHCH=CH 2

Published Raman spectrum by Carreira Published Raman spectrum of Carreira shows trans and cis conformers Carreira’s potential shows higher energy conformer to be cis L.A. Carreira, J. Chem. Phys. 62 (1975) 3851 Low-frequency Raman spectrum of 1,3- butadiene limited data Potential for the torsional vibration in 1,3-butadiene trans cis trans cis

Published Raman spectrum by Engeln Low-frequency Raman spectrum of 1,3- butadiene Potential for the torsional vibration in 1,3-butadiene Engeln, R.; Consalvo, D.; Reuss, J. J. Chem. Phys. 160 (1992) 427 Published Raman spectrum of Engeln shows trans-gauche-gauche potential Published Raman spectrum of Engeln shows trans-gauche-gauche potential Engeln’s potential shows higher energy conformer to be gauche Engeln’s potential shows higher energy conformer to be gauche trans gauche cis

Published Calculated Potential trans gauche trans cis V (cm ) Torsional angle (degrees) V (cm -1 ) Feller, D.; Craig, N. C. J. Phys. Chem. 2009, 113, 1601 Karpfen, A.; Vudhichai, P. J. Mol. Phys. 102 (2004) 819 Theoretical calculations show the higher energy conformer to be gauche Theoretical calculations show the higher energy conformer to be gauche Plotted Karpfen’s potential Feller’s potential

Outline Introduction Introduction Research Work Research Work  1,3-butadiene (-d 0 )  1,3-butadiene (-d 6 )  1,3-butadiene (-d 2 )  1,3-butadiene (-d 4 ) Summary Summary

Raman instrument Raman spectrometer sample compartment heatable vapor cell

Vapor Raman spectrum of 1,3-butadiene (bottom) compared to the calculated DFT spectrum (top) cm -1 vapor ν4ν4 ν 12 ν 13 ν 16 ν ν4ν4 ν 12 ν 13 ν 16 ν 18 calculated   good agreement with each other

Low-frequency Raman spectrum of 1,3-butadiene (-d 0 ) The transition observed at cm -1 cannot be fitted with the Carreira potential The transition observed at cm -1 cannot be fitted with the Carreira potential With this observation, we could fit a better torsional potential which is expected to be similar to Engeln’s trans-gauche-gauche potential With this observation, we could fit a better torsional potential which is expected to be similar to Engeln’s trans-gauche-gauche potential cm -1

transition: cm -1 for d 0   Drop in the frequency   Mass Effect Low-frequency Raman spectrum of 1,3-butadiene (-d 6 ) cm -1

Low-frequency Raman spectrum of 1,3-butadiene (-d 2 ) cm

Low-frequency Raman spectrum of 1,3-butadiene (-d 4 ) cm

Potential functions for the torsional vibration in 1,3-butadiene Our new potential is similar to Engeln’s potential Our new potential is similar to Engeln’s potential Engeln, R.; Consalvo, D.; Reuss, J. J. Chem. Phys. 160 (1992) 427

Observed and Calculated Raman Transitions (cm -1 ) for the Internal Rotation of 1,3-Butadiene

ν(cm -1 ) Potential functions for the torsional vibration in 1,3-butadiene Feller, D.; Craig, N. C. J. Phys. Chem. 2009, 113, 1601 Karpfen, A.; Vudhichai, P. J. Mol. Phys. 102 (2004) 819 Feller This work Karpfen

Low-frequency Raman spectrum of 1,3-butadiene 260 o C) cm -1

Comparison of Low-frequency Raman spectrum of 1,3-butadiene (RT 260 o C) cm RT C

Intensity ratio of Raman transition at room temperature and C

Combination Bands (ν 12 + ν 13 ) of 1,3-butadiene cm ν 12 + ν 13

cm ν 10 + ν 13 Combination Bands (ν 10 + ν 13 ) of 1,3-butadiene

Energy level diagram of 1,3-butadiene

Outline Introduction Introduction Research Work Research Work  1,3-butadiene (-d 0 )  1,3-butadiene (-d 6 )  1,3-butadiene (-d 2 )  1,3-butadiene (-d 4 ) Summary Summary

Summary Raman spectra of 1,3-butadiene and 1,3-butadiene (-d 6 ) have been collected and the torsional vibration have been studied. Raman spectra of 1,3-butadiene and 1,3-butadiene (-d 6 ) have been collected and the torsional vibration have been studied. The torsional vibration confirms that the higher energy form has gauche conformation. The torsional vibration confirms that the higher energy form has gauche conformation. The experimental and theoretical potential energy functions are reasonably in good agreement with each other. The experimental and theoretical potential energy functions are reasonably in good agreement with each other. Future work would be carried out on other isotopic species and heated spectra. Future work would be carried out on other isotopic species and heated spectra.

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