Hydrogen bonding. Part 74. Geometric and molecular orbital study of the role of halide ion in determining cluster or chain hydrate formation in quaternary ammonium ion halide monohydrates

We have used molecular modeling derived from crystallographic parameters an d ab initio (3-21G(*)) calculations of structure energies to determine the role played by halide anion in determining whether a quaternary ammonium ha lide monohydrate adopts the Type I (planar C-2h (H2O . X-)(2) cluster) or T ype II (extended ... HOH ... X ... HOH ... X-... chain) water-halide struct ure. Fluoride ion, which always forms Type I clusters, has the highest enth alpy for insertion of an X-. H2O into a growing chain, as the effect of str ong hydrogen bonds is counteracted by large inter-anion repulsions from the small F-... HOH ... F- units in the chain. In addition, the (H2O . F-)(2) cluster is extraordinarily stable relative to those of the other halides, a nd chain formation would come at large enthalpic cost. In contrast, iodide ion, which, with a single exception, always forms Type II chains, has the l owest enthalpy for insertion of an X-. H2O into a growing chain - even thou gh hydrogen bonds are weak, the large span of the chain unit reduces inter- anion repulsions even further - and a much less stable planar cluster. Brom ide and chloride monohydrates behave very similarly to each other, and are intermediate between the fluoride and iodide cases. Cation effects are para mount in the nearly even distribution of chloride and bromide monohydrates between Type I cluster and Type II chain structures. (C) 2000 Elsevier Scie nce B.V. All rights reserved.