Electrostatic properties of ammonium fluoride and deuterated ice-Ih

Abstract

New single crystal X-ray diffraction data have been collected at reduced temperature (120 K) and high resolution (sinθ/λ ≤ 1.34 Å−1) for ammonium fluoride and for heavy ice-Ih, (D2O) in order to determine the detailed electron density distribution in these structures. The crystals are isostructural. Ammonium fluoride, NH4F, is hexagonal with space group P63mc, a = 4.4365(3), c = 7.1672(5) Å, Z = 2. Ice-Ih, D2O, is hexagonal with space group P63/mmc, a = 4.4950(5), c = 7.3170(10) Å, Z = 4. The X-ray data were collected with Nb-filtered MoKα radiation (λ = 0.7093 Å). Refinements with Stewart's rigid pseudoatom model gave R(F2) = 0.016 for 292 reflections for NH4F and R(F2) = 0.023 for 296 reflections for ice-Ih. Hydrogen nuclear positions and mean-square displacements were taken from previous neutron diffraction studies. The structures each contain two crystallographically distinct H(D) atoms, namely, those with N-H or O-D directed along the c-axis and those that are directed tetrahedrally with respect to c. For NH4F, maps of the total electron density, the total electrostatic potential, and the values of the Laplacian at the bond critical points show that the bonding interactions involving both kinds of H atom are the same within experimental error. Similar results are obtained for ice-Ih although there are complications and limitations owing to the disorder in the structure. Nevertheless, the pseudoatom model enables the electron density distribution to be determined for an ordered water molecule isolated from the crystal structure. The estimated molecular dipole moment for water in ice-Ih, is 2.1 debye. Key words: electron density, bond critical points, hydrogen bonding, water dipole moment.