Computational studies of aqueous-phase photochemistry and the hydrated electron in finite-size clusters

A survey of recent ab initio calculations on excited electronic states of water clusters and various chromophore-water clusters is given. Electron and proton transfer processes in these systems have been characterized by the determination of electronic wave functions, minimum-energy reaction paths and potential-energy profiles. It is pointed out that the transfer of a neutral hydrogen atom (leading to biradicals) rather than the transfer of a proton (leading to ion pairs) is the generic excited-state reaction mechanism in these systems. The hydrated hydronium radical, (H₃O)_aq, plays a central role in this scenario. The electronic and vibrational spectra of H ₃O(H₂O)_n clusters and the decay mechanism of these metastable species have been investigated in some detail. The results suggest that (H₃O)_aq could be the carrier of the characteristic spectroscopic properties of the hydrated electron in liquid water.