Raman spectra of gases. XVII: Two‐quantum torsional transitions in some methyl‐substituted small ring compounds

Methyl torsional overtones (Δv = 2) have been observed for methylcyclopropane, propylene oxide, propylene sulfide, cis‐propylene imine, trans‐propylene imine, methylcyclobutane, and methylcyclopentane in the Raman effect. From the observed shifts, barriers to internal rotation have been calculated in the vapor state to be 2.95, 2.58, 3,26, 2.97, 2.51, ≥ 2.47, and 2.61–3.03 kcal mole−1, respectively. For this set of compounds the torsional overtones are difficult to identify because they fall very close to the methyl bending modes. Also, for the four‐ and five‐membered rings, interference was found from the low frequency ring motions. In general, the torsional overtones were not nearly as intense as those previously observed for the halogen substituted et hanes.