The interpretation of X-ray absorption spectra of water and ice

The changes in the X-ray Absorption Spectrum (XAS) of water upon formation of hydrogen bonds (H-bonds) are analyzed with the aid of Density Functional Theory (DFT) calculations. The tetrahedral symmetry of the ice structure removes the p-character from the 4a1 level, leading to very weak intensity in the pre-edge region. Breaking an accepting H-bond has very little effect on the absorption spectrum. A broken donating H-bond, however, is identified through a strong pre-edge feature in XAS. The asymmetry on the hydrogen side causes a1 and b2 orbital mixing and the orbitals localize along the internal O–H bonds.