Neutron-Scattering Study of Hindered Rotations in Methylbenzenes

Abstract

The low-frequency modes of a number of methylbenzenes in their solid and liquid phases have been investigated by the energy-gain scattering of cold neutrons. The compounds studied include benzene, o-xylene (1, 2), hemimellitene (1, 2, 3), mesitylene (1, 3, 5), durene (1, 2, 4, 5), prehnitene (1, 2, 3, 4), and hexamethylbenzene. The neutron spectra for molecules with adjacent methyl groups (e.g., o-xylene) exhibit bands peaked near 170 cm−1, which are assigned primarily to the 1→0 transitions of the torsional oscillations of the methyl groups. The hemimellitene and prehnitene spectra appear to show a second broad maximum in the region of 100 cm−1, which is attributed to torsional or hindered rotational modes due to methyl groups centered between two adjacent groups. Previous results on hexamethylbenzene show torsional bands peaked at 120 and 137 cm−1, respectively, above and below the λ transition near 115°K. The mesitylene spectra indicate a very low barrier to rotation of the methyl groups in this compound, with hindered-rotational levels below 100 cm−1. In addition, the results for all the compounds in their solid phases show broad bands below 90 cm−1, which are assigned to whole-molecule librations and translations. If simple threefold cosine potentials are assumed, an ``average'' barrier to rotation of about 2 kcal/mole is calculated from the average of the observed torsional peaks (170 cm−1) for methyl groups with one adjacent group. The lower-energy bands for the ``centered'' methyl groups suggest greater rotational freedom for these groups. A comparison of these results indicates that methyl groups surrounded by two adjacent groups on the benzene ring experience a considerably different potential (and possibly a significantly lower barrier) than that experienced by methyl groups with one neighboring group. The neutron results are compared in detail with NMR and thermodynamic data.