10min:

YU. N. PANCHENKO, Department of Chemistry, Moscow State University, Moscow 119899, Russia.

A region of the IR spectrum of 1,3-butadiene containing only a band of the s-gauche rotamer was predicted by quantum mechanical calculations. Investigation of the IR spectrum using Fourier transform spectroscopy in the gas phase revealed the presence of a band at 749.22\,(20)\,cm^-1 possessing the typical b-type contour consistent with the predicted A species, (C₂ symmetry) for the ₁₂ band (=CH₂ twist) of the s-gauche rotamer. Rotational analysis of this band on the spectrum recorded for the first time at a resolution of 0.64\,cm^-1 yielded (A' - B') = 0.4455\,(25)\,cm^-1 and (A'' - B'') = 0.4478\,(27)\,cm^-1 . The extremely complicated character of the high resolution spectrum of this band suggests also non-planar s-gauche structure.