Repulsive regularities of water structure in ices and crystalline hydrates

Abstract

Hydrogen-bonded structures display wide ranges of the various angles (such as O—H…O) and distances (such at H⋯O) used to describe their intermolecular geometry. For example, O⋯O distances are found to vary between ∼2.5 and 3.2 Å, and the hydrogen bond angles normally vary between 120 and 180°. Although correlations between distances and angles may be found (for example, between H⋯O distance and O—H⋯O angle), these geometrical characteristics are generally too ‘soft’ to be used as stereochemical restraints in, for example, refining the solvent structure in protein crystals. Moreover, although many potential functions exist for describing water–water interactions, none of them is totally satisfactory in reproducing experimental results, even for pure water¹. We present here the initial results of an analysis of water structures in high-resolution neutron crystal structures² which is much more successful in rationalizing the stereochemistry of water interactions. Rather than considering the structures in terms of weak, orientation-dependent attractions, a concentration on the repulsive interactions leads to a set of very much stronger stereochemical constraints, which not only rationalize the structures but appear largely to control the orientational correlations in aqueous systems. Looked at this way, water network structures in crystal hydrates and, we suspect, in the liquid itself, become for the first time comprehensible. The approach provides a much firmer base from which to build realistic potential functions to model and simulate solvent structures.