Non-Planar Structure Of High Energy Rotamer Of 1,3-Butadiene

G. R. De Maré+ , Yu. N. Panchenko and J. Vander Auwera+

+Laboratoire de Chimie Physique Moléculaire,
Faculté des Sciences, CP 160/09,
Université Libre de Bruxelles,
50 av. F.-D. Roosevelt, B-1050 Bruxelles,
Belgium

Laboratory of Molecular Spectroscopy,
Department of Chemistry, Moscow State University,
Moscow 119899, Russian Federation, C. I.S

The experimental vibrational frequencies of s-trans-1,3-butadiene, for which the assignments are well-established, are used to determine the scale factors for its quantum mechanical force field obtained at the MP2/6-31G*// MP2/6-31G* level. The scale factors obtained are transferred to the MP2/6-31G*// MP2/6-31G* force fields of the s-cis and s- gauche rotamers and then their theoretical frequencies calculated. Comparison of the calculated vibrational frequencies of these three species indicates a special region of the IR spectrum of butadiene-1,3 (710-790 cm--1) in which only a band attributable to the s- gauche rotamer should be present. Investigation of the medium and high resolution IR spectra of 1,3-butadiene in the gas phase reveals the presence of a band at 749.220 (65) cm--1 possessing the typical B contour. Rotational analysis of the medium resolution spectrum of this band yields the rotational constants (25) cm--1 and (27) cm--1 , only about one-third of the experimental values for s-trans-1,3-butadiene. This clearly identifies the band as belonging to the high energy conformer of 1,3-butadiene. Moreover, the good agreement between the experimental and theoretical values of the band center (749 versus 735 cm--1 ), the clear B type contour, and the extremely complicated character of the high resolution spectrum of the band at 749.220 (65) cm--1 strongly suggest that the geometry of the high energy conformer of 1,3-butadiene in the gas phase is non-planar s-gauche and not planar s-cis [1].

Reference

1. G. R. De Maré, Yu. N. Panchenko and J. Vander Auwera, submitted.
Return to main WATOC Poster page