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 a 2 and b 2 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.