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

TL;DR

This study uses X-ray inelastic scattering to directly measure hydrogen bonding in water across normal and supercritical conditions, revealing a linear relationship between electron involvement and hydrogen bonds, and characterizing state transitions.

Contribution

It introduces a novel experimental approach linking Compton scattering data to hydrogen bonding, enabling direct measurement of water's bonding and coordination changes under various conditions.

Findings

Linear relationship between electron count and hydrogen bonds.

Sharp increase in monomers at supercritical transition.

Residual dimers and trimers persist in supercritical water.

Abstract

A direct measure of hydrogen bonding in water under conditions ranging from the normal state to the supercritical regime is derived from 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 experimental measure of bonding and coordination in water. 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.