title: Hexamethylenetetramine at 298 K: new refinements.
abstract: New refinements of the crystal structure of hexamethylenetetramine (HMT, C6H12N4) have been carried out using previously reported neutron and X-ray diffraction data collected at 298 K. A new feature in the structure model is the inclusion of third-order Gram-Charlier coefficients for the description of the anharmonic C-H bond stretching, which is found to be significant. The charge-density distribution is analyzed in terms of the pseudoatom model of Stewart [Acta Cryst. (1976). A32, 565-574]. Our experimental determination of the molecular octapole moment gives <xyz> = +1.0 (3)/e/ A3. The refinement involving HMT provides a worst-case example of a general deficiency in the application of the multipole model to noncentrosymmetric structures. 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 operations of the space group.
authors.affiliation: Crystallography Department, University of Pittsburgh, PA 15260.
authors.forename: M
authors.initials: M
authors.lastname: Terpstra
authors.forename: B M
authors.initials: BM
authors.lastname: Craven
authors.forename: R F
authors.initials: RF
authors.lastname: Stewart
mesh_terms: D018360:Crystallography, X-Ray; D004583:Electrons; D005583:Fourier Analysis; D006358:Hot Temperature; D008709:Methenamine / Q000737:chemistry*; D015394:Molecular Structure; D009502:Neutrons
chemical_list: D008709:Methenamine