Methyl rotational excitations in p-xylene: A test of pair interaction potentials

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Abstract
The rotational potential of methyl groups in p‐xylene is calculated from transferable pair interaction potentials on the basis of the low temperature crystal structure. Various parametrizations of pair potentials are used. Rotational states in these potentials are calculated and compared to tunneling and librational transitions observed by inelastic neutron scattering. None of the pair interaction potentials can describe the rotational modes correctly. The best results are obtained with Williams IVb parameters. Improvement could be achieved by diminishing the hydrogen–hydrogen interaction strength at short distances.