Evidence for Water Antibonding Orbital Mixing in the Hydrated Electron from Its Oxygen 1s 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 assignment as arising from water O1 s 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 4 a 1 and 2 b 2 antibonding orbitals. The mixing of antibonding orbitals has implications for the strength of couplings between e (aq) – and intramolecular modes of water.