Torsional frequencies and barriers to methyl rotation in isobutylene, O -xylene, and durene

Abstract

The A₂ and B₂ torsional frequencies for liquid and solid isobutylene, solid O ‐xylene, and solid durene have been obtained at 173 206, 206 239, 146 182, and 144 180 cm⁻¹, respectively, by the ``small kappa'' method of inelastic neutron scattering. Using a truncated, two‐dimensional Fourier series to describe the potential energy interactions of the methyl groups, both the dominant threefold potential energy term and a methyl‐methyl coupling term have been calculated from the torsional frequencies. The splittings of the symmetric and asymmetric torsional modes are explained as due entirely to the potential coupling of the two methyl groups. Although not resolved, the scattering data on gaseous isobutylene yields an ``average'' torsional mode of 170 cm⁻¹. The total barrier heights for liquid and solid isobutylene of 2390 and 3070 cal/mole, which are much higher than gas phase barriers obtained by other methods, indicate rather large intermolecular forces affect the barriers in the condensed states. Due to large methyl‐methyl interactions, the total barrier of 1860 and 1820 cal/mole in O ‐xylene and durene is found to be lower than results obtained by other methods which neglect methyl‐methyl interactions.