The crystal structure of perdeuterated methanol monoammoniate (CD3OD•ND3) determined from neutron powder diffraction data at 4.2 and 180 K

The crystal structure of perdeuterated methanol monoammoniate, CD3OD center dot ND3, has been solved from neutron powder diffraction data collected at 4.2 and 180 K. The crystal structure is orthorhombic, space group Pbca (Z = 8), with unit-cell dimensions a = 11.02320 (7), b = 7.66074 (6), c = 7.59129 (6) angstrom, V = 641.053 (5) angstrom(3) [rho(calc) = 1162.782 (9) kg m(-3)] at 4.2 K, and a = 11.21169 (5), b = 7.74663 (4), c = 7.68077 (5) angstrom, V = 667.097 (4) angstrom(3) [rho(calc) = 1117.386 (7) kg m(-3)] at 180 K. The crystal structure was determined by ab initio methods from the powder data; atomic coordinates and anisotropic displacement parameters were subsequently refined by the Rietveld method to Rp < 3% at both temperatures. The crystal comprises a sheet-like structure in the bc crystallographic plane, consisting of strongly hydrogen bonded elements; these sheets are stacked along the a axis, and adjacent sheets are linked by what may be comparatively weak C-D center dot center dot center dot O hydrogen bonds. Within the strongly bonded sheet structure, ND3 molecules are tetrahedrally coordinated by the hydroxy moieties of the methanol molecule, accepting one hydrogen bond (O-D center dot center dot center dot N) of length similar to 1.75 angstrom, and donating three hydrogen bonds (N-D center dot center dot center dot O) of length 2.15-2.25 angstrom. Two of the methyl deuterons appear to participate in weak interlayer hydrogen bonds (C-D center dot center dot center dot O) of length 2.7-2.8 angstrom. The hydrogen bonds are ordered at both 4.2 and 180 K. The relative volume change on warming from 4.2 to 180 K, Delta V/V, is +4.06%, which is comparable to, but more nearly isotropic (as determined from the relative change in axial lengths, e. g. Delta a/a) than, that observed in deuterated methanol monohydrate.