Torsional Barriers in the Methyl Substituted Ethylenes

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Abstract
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.