Ammonia and deutero-ammonia have been studied by single-crystal and powder X-ray methods. There are four molecules in the cubic unit cell, and the space group is confirmed to be P213. The nitrogen parameter obtained by least-squares refinement is equal to 0-0401 for I~H 3 at -102 ° C. and the same for ND 3 at -160 ° C. Cell dimensions, determined by powder methods, are : a = 5.084 A for NH 3 at -196 °C., 5"073 A for ND~ at -196 °C., 5.091 /~ for ND 3 at -160 °C., and 5.138 A for NH 3 at -102 °C. Each ammonia molecule is involved in six hydrogen bonds of approximate length 3.4 A. A possible assignment of the hydrogen atoms to these bonds is given. Electron-density maps give evidence of hydrogen positions with coordinates (0.85, 0.14, 0.99), in agreement with this assignment, but with hydrogen off the bond axis, making the H-N-H angles more nearly tetrahedral.

The crystal structures of normal and deuterated potassium hydrogen carbonate, KHCO3 and KDCO3, have been studied at 298, 219 and 95K using three-dimensional single-crystal X-ray data collected on a semi-automatic Stoe-Philips Weissenberg diffractometer. Both unit cells are monoclinic, containing four formula units and having the centrosymmetric space group P2~/a (chosen to conform with previous work on KHCO3). The cell dimensions at 298K for KHCO3 are: a=15.1725 (11), b=5.6283 (5), c=3.7110 (4) A, fl=104.631 (6) °, V=306.62 A3; and for KDCOa: a=15.1948 (9), b=5.6307 (3), c= 3.7107 (3) A, fl= 104-567 (5) °, V= 307.27 A s. The KHCO3 structure, comprising (HCO3) 2-dimers and K + ions, is isomorphous with its deuterated counterpart. The heavy atoms of each HCOs ion within the centrosymmetric dimer are closely coplanar at all temperatures, the separation between these planes being ~0.22 A. The two symmetry-related O-H...O hydrogen bonds within the dimer have lengths 2"585 (2), 2.592 (2) and 2.591 (2) A at 298, 219 and 95K, respectively. The corresponding lengths in KDCO3 are: 2.607 (2), 2.613 (2) and 2.606 (2) A, implying length increases of 0.022, 0.021 and 0.015 A on deuteration at the three temperatures. The final R(F) values obtained for the refinements of KHCO3 were 0.037, 0.028 and 0.031, respectively. The corresponding values for KDCO3 were 0.034, 0.033 and 0.033.

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The crystal structure of HClO4·2H2O has been determined from single-crystal x-ray data obtained at −190°C. The crystals are orthorhombic, space group Pnma, with four formula units in a cell of dimensions: a = 5.819, b = 10.598, c = 7.369 Å. The water molecules are bonded to each other into pairs by a very short hydrogen bond (2.424 Å), forming H5O2+ ions. With the space group chosen there is a center of symmetry in the middle of this hydrogen bond; the H5O2+ ion is perfectly staggered. The H5O2+ ions are hydrogen bonded to the ClO4− ions into layers, which are held together by van der Waals forces. The average Cl–O distance in the perchlorate ion is 1.438 Å.

HNOa.H20, orthorhombic, P2xen (nonstandard setting of Pna2x, No. 33), Z= 4, a = 5.4647 (1), b=8.6439 (1), c=6.2308 (1) A, V=294.32 A a, Dx= 1-829 g cm -a at 85 K and a = 5.4759 (2), b = 8.7242 (2), c=6-3275 (2) A, V=302.28 A 3, Dx= 1.779 g cm -3 at 225K, Dm=l'816 g cm -a at 78K [Biltz, Hfilsmann, & Eickholz. Nachr. Ges. Wiss. Gb'ttingen (1935). No.3, 1, 95-102]. The crystal structure at both 85 and 225K consists of H30 + and NO~" ions each of which has pseudo-threefold symmetry. The oxonium ion is hydrogen bonded to three different nitrate ions to form infinite layers.

The crystal structure of trifluoromethanesulphonic acid monohydrate has been determined from threedimensional single-crystal X-ray diffraction data recorded on a Stoe-Philips two-circle Weissenberg diffractometer at 298 and 83°K. The crystals are monoclinic, space group P21/c, with four formula units in a unit cell of dimensions a=5.9634 (3), b=9.975 (3), c=9.708 (1).~, fl=98.661 (7) ° at 298°K and a=5"8416 (3), b=9.855 (3), c=9.548 (1)A and fl=99.711 (7) ° at 83°K. The structure comprises oxonium ions and trifluoromethanesulphonate ions hydrogen-bonded together to form layers. The oxonium ion is hydrogen-bonded to three sulphonate groups in a pyramidal bonding arrangement. The O-H...O distances obtained from a full-matrix least-squares refinement are, at 83 °K, 2.546 (3), 2-588 (3) and 2.672 (3) /~. The structure at room temperature is essentially the same as that found at 83 °K, with only those minor changes expected to accompany an increase in temperature.