Intermolecular dynamics of deuterated benzene

Of the 24 distinct intermolecular modes of deuterated benzene, with wave vectors corresponding to the four highest symmetry points R, U, S, and T at the surface of the Brillouin zone, 23 have been observed and identified by means of coherent neutron inelastic scattering experiments. These results complement the zero wave vector data (18 normal mode frequencies) obtained previously by far infrared and Raman scattering methods, and provide a more comprehensive test of any theoretical model of the intermolecular forces in this simplest of aromatic compounds. The results are interpreted in terms of a particular force model involving pairwise interatomic forces, specified by three Buckingham 6-exponential potential functions for C...C, C...D, and D...D pairs respectively, between atoms on different molecules (so-called nonbonding interactions). The agreement between theory and experiment, as regards both the normal mode frequencies and eigenvectors, is reasonably satisfactory, but the existence of more than one set of numerically suitable parameters casts considerable doubt on the uniqueness of the model.