Torsional Barriers in the Methyl Substituted Ethylenes

The far-infrared spectra have been recorded from 350 to 33 cm−1 for propene, cis-2-butene, trans-2-butene, isobutene, 2-methyl-2-butene, and 2,3-dimethyl-2-butene, in the gaseous and solid states (−190°C). The Raman spectra of the liquids have also been recorded in the same frequency range. The bands corresponding to the torsional modes are assigned as appearing at 226; 195; 233 and 219; 183 and 144; 219 and 178; and 227, 201, 194, and 153 cm−1, in the infrared spectra of propene, trans-2-butene, isobutene, cis-2-butene, 2-methyl-2-butene, and 2,3-dimethyl-2-butene, respectively, in the solid state. The corresponding barriers to internal rotation are 2.68, 2.21, 3.24, 1.82, 2.45, and 2.25 kcal/mole.