Hydrogen bonding and coordination in normal and supercritical water from x-ray inelastic scattering

A direct measure of hydrogen bonding in water under conditions ranging from the normal state to the supercritical regime is derived from first-principles calculations for the Compton scattering of inelastically scattered x rays. First, we show that a measure of the number of electrons n(e) involved in hydrogen bonding at varying thermodynamic conditions can be directly obtained from Compton profile differences. Then, we use first-principles simulations to provide a connection between n(e) and the number of hydrogen bonds n(HB). Our study shows that over the broad range studied, the relationship between n(e) and n(HB) is linear, allowing for a direct measure of bonding and coordination in water by coupling simulations with experiments. In particular, the transition to supercritical state is characterized by a sharp increase in the number of water monomers but also displays a significant number of residual dimers and trimers.