Methyl rotational potentials and transferable pair potentials in toluene

The crystal structure of α-phase toluene at 5 K has been refined from high resolution neutron powder diffraction data and the charge distribution in the molecule calculated ab initio. From these data, methyl rotational potentials of the two inequivalent methyl groups have been calculated for various parametrizations of transferable pair potentials and compared with potentials derived from tunneling and librational transitions. The results demonstrate that the inclusion of Coulomb interactions is necessary to reproduce the equilibrium orientations of the methyl groups. However, none of the model pair potentials yield the very strong sixfold contribution of the rotational potential of methyl group 1. The best approximation to the experimentally determined values is obtained with the pair potential parameters of Kitaigorodskii [Acta Crystallogr. 18, 585 (1965)]. However, by the inclusion of existing results on p-xylene, the average agreement obtained with the parametrization of Claverie [Acta Crystallogr. Part A 31, 448 (1975)] is equally good. Overall, it was found that general improvements can be made if the repulsive parts of the model pair potentials are softened. The presence of two inequivalent methyl groups in toluene which are influenced in different ways by the various contributions to the interaction provides a rigorous test of the modeling techniques.