HEXAMETHYLENETETRAMINE AT 298-K - NEW REFINEMENTS

New refinements of the crystal structure of hexamethylenetetramine (HM T, C6H12N4) have been carried out using previously reported neutron an d X-ray diffraction data collected at 298 K. A new feature in the stru cture model is the inclusion of third-order Gram-Charlier coefficients for the description of the anharmonic C-H bond stretching, which is f ound to be significant. The charge-density distribution is analyzed in terms of the pseudoatom model of Stewart [Acta Cryst. (1976). A32, 56 5-574]. Our experimental determination of the molecular octapole momen t gives [xyz] = + 1.0 (3) Absolute value of e angstrom3 . The refineme nt involving HMT provides a worst-case example of a general deficiency in the application of the multipole model to noncentrosymmetric struc tures. Strong least-squares correlations occur involving the electron population parameters of all atoms for certain of the multipole terms, namely those odd-order terms that are invariant under the symmetry op erations of the space group.