Hexamethylenetetramine: extinction and thermal vibrations from neutron diffraction at six temperatures.

Neutron diffraction data have been collected for hexamethylenetetramine (HMT) at 15, 50, 80, 120, 160 and 200K using a single crystal (mass 8.1 mg). The structure refinement at each temperature included two extinction parameters and third-order thermal parameters for the H nuclei. Extinction effects are very severe with extinction factors as small as 0.2Fkin2 for three reflections (800, 110 and 440). Application of the Sabine extinction theory indicates that the crystal domain size decreases from 115 microns at 200 K to 85 microns at 15 K. The half-width in the mosaic spread (7" of arc) is almost independent of temperature. An extinction model without phase correlations between mosaic blocks gives a slightly better fit to the diffraction data. The nuclear mean square thermal displacements have been analysed assuming no coupling between the external (rigid body) and internal vibrations. This gives mean square displacements for rigid-body vibration in which zero-point vibrational effects are apparent. The methylene H nuclei have internal vibrations approximately independent of temperature. At 200 K, the H nuclear vibrations have a small anharmonic component, but at temperatures below 160 K this becomes insignificant in terms of the experimental error.