STRUCTURE OF THE HIGH-ENERGY CONFORMER OF 1,3-BUTADIENE

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 of theory, The scale factors are then tra nsferred to the MP2/6-31G//MP2/6-31G* force fields of the s-cis and s -gauche rotamers and their theoretical frequencies calculated. Compari son of the vibrational frequencies of these three species indicates a special region of the IR spectrum of 1,3-butadiene in the gas phase (7 20-790 cm(-1)) in which only a band attributable to the s-gauche rotam er should be present; i.e., it should be free both of the observed IR bands of the s-trans and of the calculated vibrational frequencies of the s-cis conformer. Investigation of the medium- and high-resolution IR spectra of 1,3-butadiene in the gas phase reveals the presence of a band at 749.22(20) cm(-1) possessing the typical B contour (consisten t with A symmetry, Ct group). Rotational analysis of the medium resolu tion spectrum of this band yields the rotational constants A '' - B '' = 0.4478(27) cm(-1) and A' - B' = 0.4455(25) cm(-1), only about one-t hird of the experimental values for s-trans-1,3-butadiene. This identi fies the band as belonging to the high-energy conformer of 1,3-butadie ne. The agreement between the experimental and theoretical values of t he band center (749 vs 735 cm(-1)), the clear B type contour, and the extremely complicated character of the high-resolution spectrum of the band at 749.22 cm(-1) strongly suggest that the geometry of the high- energy conformer of 1,3-butadiene in the gas phase is nonplanar s-gauc he and not planar s-cis.