Relationship between Hydrogen-Bonding Motifs and the 1b₁ Splitting in the X‑ray Emission Spectrum of Liquid Water

The split of the 1b₁ peak observed in the X-ray emission (XE) spectrum of liquid water has been the focus of intense research. Although several hypotheses have been proposed to explain the origin of this split, a consensus has not yet been reached. Here, we introduce a novel theoretical/computation approach which, combining path-integral molecular dynamics simulations with the MB-pol model and time-dependent density functional theory calculations, predicts the 1b₁ splitting in liquid water and not in crystalline ice, in agreement with the experimental observations. A systematic analysis of the underlying local structure of liquid water at ambient conditions indicates that several different hydrogen-bonding motifs contribute to the overall XE line shape in the energy range corresponding to emissions from the 1b₁ orbitals. This suggests that it is not possible to unambiguously attribute the split of the 1b₁ peak to only two specific structural arrangements of the underlying hydrogen-bonding network.