Lattice Dynamics of Hexamethylenetetramine

The frequencies of the normal modes of vibration of the organic molecular crystal deuterated hexamethylenetetramine (DHMT )have been measured using inelastic neutron scattering techniques. The measurements were made along the high symmetry directions [00δ], [δδδ], [δδ0] and [½½δ] at 296 °K, using the triple- axis spectrometer in its constant-Q mode of operation at the NRU reactor. Chalk River. DHMT is a simple molecular crystal which crystallizes in a body-centred-cubic structure. The molecules have cubic symmetry, and it is believed that the intramolecular forces are much stronger than the intermolecular ones. The molecules can thus be considered as rigid units with three translational and three librational degrees of freedom. For any general wave-vector there will thus be six normal modes, each comprising both translational and librational components. The effect of the librational components can be seen in the very different neutron group intensities observed along similar directions in reciprocal space, e. g. (200) → (221) and (330) → (331). The frequencies of certain triply degenerate modes are (units 1012 cps): H15 (1.96 ± 0.04); H'15 (0- 95 ± 0.02); Γ'15 (1.06 ±0.04). Cochran and Pawley have proposed a simple model for normal hexamethylenetetramine and calculated dispersion curves in the symmetry directions. The present results indicate that the model is a reasonable approximation and, by fitting the observed frequencies, the parameters of the model for DHMT can be determined. (author)