1. Praveenkumar Boopalachandran, 1 Jaan Laane 1 and Norman C. Craig 2 1 Department of Chemistry, Texas A&M University, College Station, Texas 77840 2 Department of Chemistry and Biochemistry, Oberlin College, Oberlin, Ohio 44074 2 Department of Chemistry and Biochemistry, Oberlin College, Oberlin, Ohio 44074 Gas Phase Raman Spectra of Butadiene and Butadiene-d 6 and the Internal Rotational Potential Energy Function

2. 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

3. 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

4. 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 0-2 1-3 2-4 3-5 4-6 5-7 6-8 7-9 limited data Potential for the torsional vibration in 1,3-butadiene trans cis 1 0 2 3 4 6 7 5 trans cis

5. 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 0-2 1-3 4-6 5-7 6-8 3-5 2-4 7-9 0 - -2 - 0 + -2 + 1 - -3 - 1 + -3 + cis 1 0 2 3 4 6 7 5

6. Published Calculated Potential trans gauche trans cis 100200300 0 500 1000 1500 2000 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

7. 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

8. Raman instrument Raman spectrometer sample compartment heatable vapor cell

9. Vapor Raman spectrum of 1,3-butadiene (bottom) compared to the calculated DFT spectrum (top) cm -1 vapor 16001200800400 ν4ν4 ν 12 ν 13 ν 16 ν 18 16001200800400 ν4ν4 ν 12 ν 13 ν 16 ν 18 calculated   good agreement with each other

10. Low-frequency Raman spectrum of 1,3-butadiene (-d 0 ) 320 300 280 260 240 220 320 300 280 260 240 220 The transition observed at 214.9 cm -1 cannot be fitted with the Carreira potential The transition observed at 214.9 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 0-2 1-3 2-4 4-6 5-7 6-8 3-5 1 + -3 + 2-4 4-6 5-7 7-9 0 - -2 - 0 + -2 + 1 - -3 - cm -1

11. 280 260 240 220 200 180 0-2 4-6 1-3 2-4 6-8 3-5 5-7 7-9 0-2 transition: 322.4 cm -1 for d 0   Drop in the frequency   Mass Effect Low-frequency Raman spectrum of 1,3-butadiene (-d 6 ) cm -1

12. 320 310 300 290 280 270 260 250 320 310 300 290 280 270 260 250 Low-frequency Raman spectrum of 1,3-butadiene (-d 2 ) cm -1 0-2 1-3 6-8 3-52-4 4-6 5-7 7-9

13. Low-frequency Raman spectrum of 1,3-butadiene (-d 4 ) 300 280 260 240 220 200 300 280 260 240 220 200 cm -1 0-2 1-3 6-8 3-5 2-4 4-6 5-7 7-9

14. 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

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

16. 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

17. Low-frequency Raman spectrum of 1,3-butadiene (heated @ 260 o C) 340 320 300 280 260 240 220 0-2 1-3 2-4 3-5 4-6 5-7 6-8 7-9 0 - -2 - 0 + -2 + 1 - -3 - 1 + -3 + cm -1

18. Comparison of Low-frequency Raman spectrum of 1,3-butadiene (RT and @ 260 o C) 320 300 280 260 240 220 cm -1 0-2 1-3 2-4 3-5 4-6 5-7 6-8 7-9 0 - -2 - 0 + -2 + 1 - -3 - 1 + -3 + RT 260 0 C

19. Intensity ratio of Raman transition at room temperature and 260 0 C

20. Combination Bands (ν 12 + ν 13 ) of 1,3-butadiene 680 670 660 650 680 670 660 650 cm -1 0-1 1-2 2-3 3-4 4-5 5-6 6-7 ν 12 + ν 13

21. 1180 1170 1160 1150 cm -1 0-1 1-2 2-3 3-4 ν 10 + ν 13 Combination Bands (ν 10 + ν 13 ) of 1,3-butadiene

22. Energy level diagram of 1,3-butadiene

23. 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

24. 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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