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

Abstract

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 (v_tors≃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.