Hydrogen bonding: Part 24. Dichotomous hydration behavior of quaternary ammonium halides

On dehydration in vacuo quaternary ammonium halides show two entirely different types of behavior. Type A salts give first a liquid (4–6 H₂O) of very low vapor pressure, then a series of crystalline framework clathrates (2–4 H₂O), then very stable monohydrates with water—anion dimeric clusters. Type B salts give first a hypobarogenic clathrate (solid at reduced pressure, liquid at 760 torr), then crystalline monohydrates, which, when the pressure is returned to 760 torr, disproportionate into anhydrous salt and same hypobarogenic clathrate. Liquid—solid equilibria for type A at 760 torr is always between framework clathrate and saturated solution (or possibly liquid clathrate) and for type B is between anhydrous salt and hypobarogenic clathrate. Dissolution in water is exothermic for type A salts and endothermic for type B. Examples: type A, choline fluoride, tetramethylammonium fluoride, tetraethylammonium fluoride and chloride, tetrapropylammonium fluoride and chloride; type B, choline chloride, bromide, and iodide, tetramethylammonium chloride and bromide, tetraethylammonium bromide, tetrapropylammonium bromide. Type A behavior is favored by larger cation and smaller (more electronegative) anion, and type B by smaller cation and larger (less electronegative) anion.