Direct evidence for water antibonding orbital mixing in the hydrated electron from its X-ray absorption spectrum

The X-ray absorption spectrum (XAS) of the hydrated electron (e−(aq)) has been simulated using time- dependent density functional theory with a quantum mechanics/molecular mechanics description. A unique XAS peak at 533 eV is observed with an energy and intensity in quantitative agreement with recent time-resolved experiments, allowing its definitive assignment as arising from water O1s transitions to the singly occupied molecular orbital (SOMO) in which the excess electron resides. The transitions acquire oscillator strength due to the SOMO comprising an admixture of a cavity-localized orbital and water 4a1 and 2b2 antibonding orbitals. The mixing of antibonding orbitals has implications for the strength of couplings between e−(aq) and intramolecular modes of water.