## Abstract

On account of its cubic molecular and crystal symmetry, hexamethylenetetramine is a key substance in the study of the lattice vibrations of molecular crystals; its anisotropic atomicvibration amplitudes have been determined at 298, 100 and 34 $^\circ$K from three-dimensional single crystal X-ray data. Two independent sets of experiments were done with Cu and Mo radiation; these led to results in very close agreement with one another (and with results derived from previous X-ray and neutron data at room temperature). Typical final discrepancy factors obtained from full-matrix least-squares refinements are 3.4% for 95 Mo reflexions and 6.3% for 44 Cu reflexions at 34 $^\circ$K. The studies at the three temperatures provide direct support for the rotational-oscillation correction for atomic co-ordinates. The corrected weighted-mean dimensions from eight determinations are $\mathrm{C-N} = 1\cdot476 \pm 0\cdot002 \overset{\circ}{\mathrm A}, \angle N-C-N = 113\cdot6 \pm 0\cdot2,^\circ,$ $\angle \mathrm{C-N-C} = 107\cdot2 \pm 0\cdot1^\circ,$ and from the neutron study C-H = 1$\cdot$088 $\pm$ 0$\cdot$011$\overset{\circ}{mathrm A}$. The compact arrangement of molecules in the crystal is described.