X-ray study of solid ammonia

Abstract

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.