Evidence for two kinds of hydrogen bond in ice

Abstract

DESPITE its simplicity at the molecular level, water is a complex and poorly understood liquid¹. The reasons for this centre around the existence of a dynamic hydrogen-bonded network throughout the liquid. Attempts to describe the structure of liquid water have tended to invoke either continuum models such as the 'distortedbond' model², which assumes that the hydrogen-bonded structure relaxes on a timescale similar to that in other liquids, and mixture models, such as the 'flickering-cluster' model³, which postulate the coexistence of two or more long-lived structures in the liquid. Here we analyse inelastic neutron-scattering spectra for the ice I solid phase of water, which provide evidence for the existence of two different kinds of hydrogen-bond, of different strengths, in the solid. A model in which strong and weak hydrogen-bonds in the ratio of about 2:1 are randomly distributed throughout the network is able to reproduce the neutron spectra. If we can assume that the same kind of bimodal hydrogen-bonding exists in the liquid state, our model may be able to explain several of the anomalous properties of liquid water, such as the large specific heat and the unusual behaviour of water in thin films and clusters.