Temperature dependence of the near-edge spectrum of water.

The interpretation of the oxygen near-edge spectrum of water has been debated intensively. We present new measurements of the temperature dependence of the spectrum and perform a van't Hoff analysis for the pre-edge intensity. Many microscopical and thermodynamic properties of liquid water have been described in the literature in terms of mixture models, which presume the existence of two distinct species with different local structures. Assuming such a two-component model here leads to a van't Hoff enthalpy change ΔH = 0.9 ± 0.2 kcal/mol for the conversion between the two presumed components contributing to the pre-edge intensity. The small value of ΔH compared to the average bond energy implies that the components are nearly equally bonded, suggesting that the pre-edge is sensitive to structural changes that leave hydrogen bonds intact. We further show that the pre-edge intensity in the vapor, liquid, and ice Ih spectra can be correlated with enthalpy changes. While the pre-edge intensity in water has often been interpreted to imply a large fraction of broken hydrogen bonds in the liquid, we propose that the current results indicate that those bonds would not be considered broken by energetical criteria.