On the 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 over the last two decades. Although several hypotheses have been proposed to explain the origin of the 1b₁ splitting, a general consensus has not yet been reached. In this study, we introduce a novel theoretical/computational approach which, combining path-integral molecular dynamics (PIMD) simulations carried out with the MB-pol potential energy function and time-dependent density functional theory (TD-DFT) calculations, correctly predicts the split of the 1b₁ peak in liquid water and not in crystalline ice. A systematic analysis in terms of the underlying local structure of liquid water at ambient conditions indicates that several different hydrogen-bonding motifs contribute to the overall XE lineshape in the energy range corresponding to emissions from the 1b₁ orbitals, which 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.