Single-Crystal Structure and Electron Density Distribution of Ammonia at 160 K on the Basis of X-ray Diffraction Data

Abstract

A single-crystal of ammonia, NH3, was grown in a thin-walled capillary at 178 K, and high-resolution X-ray diffraction data were obtained for this compound at 160 K in order to obtain information about electron density distribution. Conventional and multipole refinements and deformation electron density maps indicated small but significant N−H bond bending inside the NH3 tetrahedron that is in agreement with ab initio quantum-chemical calculations and the VSEPR model. Topological analysis of the experimental charge density distribution in the ammonia molecule has been performed, and the data are compared with high-level quantum-chemical calculations. Some features of the intermolecular hydrogen bonds in the crystal are discussed.

Wavefunctions derived from experiment. IV. Investigation of the crystal environment of ammonia

“…IV 245 research papers Figure 1 Schematic representations (Schaftenaar & Noordik, 2000) of the various NH 3 clusters used to generate theoretical structure-factor magnitudes. In all clusters, the X-ray diffraction values (Boese et al, 1997) …”

“…The experimental unit-cell parameters, bond lengths and angles for crystalline NH 3 obtained by Boese et al (1997) at 160 K were used throughout. The structure-factor magnitudes from this study (Boese, 1999) were used in both CHF calculations and in a least-squares multipole re®nement (see x2.2).…”

“…Secondary-extinction effects were accounted for by the Larson model (Larson, 1970) to minimize the 1 2 value. The Stewart model for basis function pairs was used to make the corrections for thermal motion (Stewart, 1969) during the CHF calculations in conjunction with the thermal parameters for model M of Boese et al (1997): for N, U 11 0X0372 and U 12 À0X0009 A Ê 2 ; for H, U 11 0X053 A Ê 2 .…”

“…The and H parameters, which control the radial extent of the spherical and aspherical density, respectively, were then re®ned while the multipole populations remained ®xed at the values obtained from the ®rst stage of re®nement. Both and H for the hydrogen atom were ®xed throughout at 1.2 (Boese et al, 1997;Koritsanszky et al, 2000). In the ®nal re®nement stage, the multipole functions (N) and H (N) were re®ned together, after which (N) and H (N) were 0.91 and 0.92, respectively.…”

“…In this work, the constrained Hartree±Fock procedure has been applied to experimental X-ray diffraction data for crystalline NH 3 (Boese et al, 1997) with particular emphasis placed upon the effect of choosing different sized NH 3 model clusters which are then propagated to obtain an electron density from which the crystal structure-factor magnitudes are calculated. The purpose of this was to study in detail the effect of the crystal lattice on the charge density of an NH 3 unit in the crystal, and to see whether or not the CHF procedure can reproducibly recover such effects in the electron density from the experimental data.…”