HYDROGEN-BONDING - PART 67 - THERMODYNAMIC STUDY OF THE DISSOCIATION OF TRI-I-PENTYLAMMONIUM CHLORIDE AND BROMIDE MONOHYDRATES

Tri-i-pentylammonium bromide and chloride form very stable, highly cry stalline monohydrates which contain planar C-2h (H2O.X-)(2) clusters i n which two water molecules mutually bridge two halide ions. Each hali de ion in turn accepts an N-H...X- hydrogen bond from cation to give a dimeric unit in which each cluster is sandwiched between two large ca tions. The enthalpy of dissociation of these monohydrates consists of breaking HOH...X- hydrogen bonds (+Delta H), changing N-H...X- bonds I n hydrate to those in anhydrous salt (-Delta H), and change in lattice enthalpy (Delta H-1) between monohydrate and anhydrous salt (expected -Delta H). However, consideration of the thermodynamic values for the dissociation of these hydrates and comparision of these values with t hose for quaternary ammonium ion salts previously studied suggests tha t Delta H-L is positive for the dissociation of tri-i-pentylammonium b romide and chloride monohydrates; thus the presence of water has a sig nificant stabilizing effect on the crystal lattice which contributes t o the unusual stability of these monohydrates. (C) 1998 Elsevier Scien ce B.V. All rights reserved.