Dynamics of hydrogen bonding in an elementary model of water

Abstract

The dynamics of the hydrogen bond breaking and reforming are investigated using a model for water in which each water molecule is a hard sphere with four sticky spots located at the corners of a tetrahedron. H-bonding arises when a pair of particles attach at their sticky spots by means of the narrow, anisotropic square well potential. To escape the square well, and break the H-bond, a solvent molecule must collide with the bonded pair and communicate sufficient energy along the line of centers so as to exceed the threshold energy. The calculated time correlation function describes the fluctuation in the number of H-bonded pairs. Its correlation time, associated with the three-body direct rupture, is roughly 0.83 ps in water at 300 K and obeys an Arrhenius law. After bond rupture, the restituting solvent molecule can return (or backscatter) and in so doing, cause the initial dimer to reform its H-bond. As a result, the overall correlation time for bond breaking is roughly 14 ps. Employed here are aspects of the kinetic theory of square well fluids together with Wertheim’s theory for associating systems.