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 H₃O(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.