Methyl group dynamics in glassy toluene:: A neutron scattering study

We present a neutron scattering study on methyl group dynamics in glassy toluene. The spectra in the whole temperature range, covering the transition from quantum rotational tunneling to classical hopping, have been successfully analyzed in terms of a potential barrier distribution model. The average barrier in the glass is found to be notably higher than the unique barrier of the crystalline beta -phase, whose short-range structure is known to be similar to that of the glass. Due to the mainly intermolecular origin of the interactions on the methyl groups in toluene, it is concluded that the rotational potentials are strongly affected by structural disorder. (C) 2001 American Institute of Physics.