The thermal diffuse scattering of X-rays and molecular dynamics of hexamethylenetetramine

The thermal diffuse scattering of X-rays from hexamethylenetetramine (HMT) has been measured in the (hk 1 / 2 ), (hk 3 / 2 ) interlayers by photographic techniques. The intensity distribution of the diffuse scattering in these interlayers is derived. A model for the lattice dynamics of molecular crystals composed of rigid molecules is presented, in which the intermolecular mode frequencies are expressed in terms of parameters describing specific interatomic bonds. A simple force system is assumed for HMT and the independent parameters of this 'interatomic' model are determined from the elastic constants of HMT and particular phonon frequencies of deuterated hexamethylenetetramine. The dispersion curves of intermolecular modes propagating along symmetry directions of HMT are calculated on the basis of this model. The one phonon X-ray thermal diffuse scattering in the (hk 1 / 2 ), (hk 3 / 2 ) interlayers is calculated, and a comparison between the observed and calculated distributions is made. The general agreement between the two indicates that the interatomic model can provide a realistic description of the intermolecular dynamics of HMT and possibly of other simple molecular solids.