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

Abstract

International audienceA 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

Oxygen disorder in ice probed by x-ray Compton scattering

“…Through the CP difference it is possible to measure the fraction of electrons directly involved in a phase transition of this kind, that is the electrons whose wave functions undergo change. The number n e of electrons involved in this change is defined as follow [17]:…”

ERKALE—A flexible program package for X‐ray properties of atoms and molecules

“…Difference profiles have been successfully used in studying, for example, temperature-dependent hydrogen bond networks in water, [99,100] configurational energetics in ice, [101] and solvation in water-ethanol mixtures. [102] Completeness-optimized basis sets…”

Microscopic structure of water at elevated pressures and temperatures

“…Likewise, NMR studies of water up to 600°C and 400 bar suggest a significant amount of H-bonding up to the highest temperatures and pressures studied (25). Sit et al (15) used Compton scattering to measure the bonding and coordination in water up to supercritical conditions and found a large increase in the number of water monomers on transition to the supercritical regime, but they also report a remaining number of higher coordinations, such as dimers and trimers. Based on X-ray Raman spectroscopy (XRS) measurements, Wernet et al (12) proposed that supercritical water consists of small H-bonded patches surrounded by less dense non-H-bonded regions in line with local density inhomogeneities revealed by small angle neutron scattering (26) but reported a relatively small proportion of non-H-bonded H 2 O species.…”

“…Due to experimental difficulties, water under supercritical conditions has only rarely been the subject of detailed in situ spectroscopic investigations to date (12,15). The pressure (p) and temperature (T) regime of supercritical water, however, is especially interesting for geoscientists because water plays a key role in heat and mass transfer as well as element fractionation processes in the Earth's lithosphere, such as volcanism and ore deposit formation (16,17).…”