Neutron-Scattering Study of Hindered Rotational Motions and Phase Transitions in Hexamethylbenzene

The low-frequency motions of solid hexamethylbenzene (HMB) have been investigated above and below its λ transition near 110°K by the energy-gain scattering of cold neutrons. Measurements were also obtained above and below the high-temperature phase transition near 383°K. At temperatures below the λ point the neutron spectra exhibit a band peaked at 137±5 cm−1, which is assigned to the 1→0 transitions of the torsional oscillations of the methyl groups. Above the transition this band is broadened considerably and its maximum is shifted down to 120±5 cm−1. These results show that the λ transition is associated with a small change in the forces hindering rotation of the methyl groups. Assuming a simple threefold cosine potential for the methyl groups, the torsional peaks are used to calculate ``average'' barriers to rotation above and below the λ transition of 1.07±0.09 and 1.35±0.10 kcal/mole, respectively. These barriers are considerably lower than a recent estimate of 3–8 kcal/mole from thermodynamic results. Moreover, the torsional bands and calculated barriers are also significantly lower than those obtained recently by the neutron technique for the methyl groups in o-xylene and durene (vtors≃170 cm−1, V3≃2 kcal/mole). This indicates a considerable difference between the potential experienced by a methyl group surrounded by two other methyl groups on the benzene ring and that experienced by two adjacent groups. A second band with a maximum around 60 cm−1, which shows little change at the λ transition, is also observed in the low-temperature spectra and is attributed primarily to the over all librational motions of the HMB molecules. The spectra above and below the high-temperature transition show only small changes with the change in phase.