Polyhedral Clathrate Hydrates. V. Structure of the Tetra-n-butyl Ammonium Fluoride Hydrate

Abstract

The structure of (n‐C₄H₉)₄N⁺F^—·32.8 H₂O is tetragonal, of space group P4₂/m with the unit‐cell dimensions a=23.52 Å and c=12.30 Å. The clathrate host structure is similar to that of the $\mathrm{(n}{\mathrm{-C}}_4{\mathrm{H}}_9{)}_4{\mathrm{N}}^{+}{\mathrm{C}}_6{\mathrm{H}}_5\mathrm{-}{\mathrm{COO}}^{-}\text{·39.5}{\mathrm{H}}_2\mathrm{O}$ and consists of a tetragonal pseudo body‐centered arrangement of groups of five face‐sharing pentagonal dodecahedra interlinked by tetrakaidecahedra and pentakaidecahedra. Whereas in the benzoate there are four salt molecules per unit cell, in the fluoride there are five. This difference in the number of guests accommodated in essentially the same host structure is possible because the four tetrakaidecahedra occupied by the benzoate groups are available for the four alkyl groups of the additional cation. In the fluoride hydrate, four of the cations have the central N⁺ atom at the fourfold positions (j) and that of the fifth is disordered over the twofold position (f). Thus the structure has lower symmetry than the benzoate hydrate, which is P4₂/mnm by reason of the disorder of the four cations over eightfold positions. The fluoride ions replace water molecules in the host lattice, with F^—···O distances of 2.80 to 2.87 Å forming an anionic host lattice. As in the other clathrate hydrate structures of this type, some of the pentagonal dodecahedra which are distorted by the presence of the cations are occupied by a guest species presumed to be additional water molecules.