Vibrational Spectra and Structure of 3-Methyl-1,2-Butadiene

Abstract

The infrared and Raman spectra of gaseous and crystalline 3-methyl-1,2-butadiene have been recorded from 140 to 4000 cm−1. All spectra have been interpreted in detail, and the fundamentals have been assigned on the basis of depolarization ratios, vapor-phase band contours, relative band intensities, and group frequency correlations. The interesting a2 and b2 methyl torsional vibrations are assigned to bands at 185 and 236 cm−1, respectively, in the spectrum recorded at liquid nitrogen temperatures. From these frequencies the magnitude of the internal rotational barrier is estimated to be 2.7 kcal/mole. The barrier height and the observed top-top frequency splitting is discussed in conjunction with recent studies on similar molecules.