Polyhedral Clathrate Hydrates. XI. Structure of Tetramethylammonium Hydroxide Pentahydrate

Abstract

The crystal structure of tetramethylammonium hydroxide pentahydrate has been determined at 25°C by an analysis of the three‐dimensional x‐ray‐diffraction data. The unit cell is orthorhombic with a=12.57, b=10.96, c=7.91 Å and contains four formula units. The OH⁻ ions and H₂O molecules form a hydrogen‐bonded framework based on the space‐filling arrangement of truncated octahedra. Equivalent (CH₃)₄N⁺ ions occupy the four available polyhedral cages in the unit cell and distort the cubic symmetry of the idealized oxygen lattice formed from undistorted face‐sharing truncated octahedra. Three carbon atoms of each cation are disordered giving rise to a circular distribution of electron density about the fourth N–C bond axis, which suggests that the cations are behaving as slightly hindered axial rotors. In the framework structure, the number of hydrogen‐bonded edges equals the number of available protons, with ⅓ and ⅔ of the oxygen atoms forming 3 and 4 bonds, respectively. The framework of this hydrate resembles closely that found in the acid hydrate HPF₆·6H₂O. Although the clathration of the cationic guests is similar in principle to that in the tetra‐n‐butyl and tetraisoamyl ammonium salt hydrates, the structural geometry of the host lattice is quite different since it is derived from the truncated octahedron, instead of the pentagonal dodecahedron.