Intermolecular dynamics of hexamethylenetetramine

Abstract

The technique of coherent inelastic neutron scattering has been used to measure the dis-persion curves of intermolecular modes propagating along various high symmetry directions in the organic molecular crystal deuterated hexamethylenetetramine, at 100 and 298 K. The results are analysed in terms of phenomenological models involving interactions between supposedly rigid molecules. Interactions with both first and second nearest-neighbour molecules are found to be necessary for an adequate description. A simplified five-parameter model, which assumes centrosymmetric molecules, provides almost as good a fit to the results as that given by a generalized twelve-parameter model. Group theory shows that the effects of assuming centrosymmetry may be significant in the [££0] or E direction. Assuming that the replacement of deuterium by hydrogen does not change the intermolecular forces, the frequency distribution of function and the heat capacity of hydrogenous hexamethylenetetramine are calculated. The agreement between the computed and measured heat capacities is good. The frequency distribution function shows many obvious critical points and other singular features. The existence of logarithmically infinite singularities in this function is investigated. The elastic constants computed from the force models show discrepancies from those measured ultrasonically, and these discrepancies are thought to be significant. The long wavelength optical librational frequency measured in the present work differs considerably from that deduced from optical measurements, and an explanation of this apparent discrepancy is suggested. The 2 % hydrogen content in the specimen results in the presence of hydrogenous 'defect’ molecules, and the effects of these on the normal vibrations of the host lattice are shown to be very small.