Deciphering Charge Transfer and Hydrogen Bonding Characteristics from Liquid Water XAS Spectra

Abstract

X-ray absorption spectroscopy (XAS) provides critical insights into the molecular and electronic structures of complex condensed-phase systems. For example, the XAS spectrum of liquid water depends on the complex three-dimensional arrangements of water molecules and their effects on the core-to-valence electronic transitions. Consequently, a wealth of molecular and electronic information is encoded in the complex spectral patterns. To decode these structure–electronic property relationships, we simulated the XAS spectra of bulk water by combining advanced molecular dynamics (MD) simulations coupled with multiconfigurational wave function methods. Using three advanced MD approaches─ ab initio MD (AIMD), RexPoN, and MB-pol─we sampled the local solvation environments and hydrogen-bond (HB) networks, which were then used to predict the XAS spectra. Based on the theory–experiment comparisons of pair distribution functions and XAS spectra, we identified the MD method that most reliably predicts the local water structure. We further revealed that the charge-transfer (CT) character observed across the entire spectral range is strongly correlated to the hydrogen-bond (HB) network. Specifically, the extent of CT and the associated excitation energy are significantly influenced by the HB structure. These findings highlight the limitations of a purely local excitation model for interpreting bulk water XAS spectra. Instead, accurate sampling of HB structures and high-level wave function theory with dynamic correlation are essential for reliable spectral interpretation. This approach provides new insights into the relationship between the electronic structure and the molecular organization of water.