Signature OH absorption spectrum from cluster models of solvation: a solvent-to-solute charge transfer state.

Ab initio electronic structure theory calculations on cluster models support the characterization of the signature absorption spectrum of a solvated hydroxyl OH radical as a solvent-to-solute charge transfer state modulated by the hydrogen-bonding environment. Vertical excited states in OH(H2O)n clusters (n = 0-7, 16) calculated at the TDDFT level of theory (with companion calculations at the EOM-CCSD level of theory for n </= 7) show an intense band in the region of approximately 250 nm. The calculations suggest that the intensity of the solvent-to-solute charge transfer transition depends strongly on a favorable alignment of the donor and acceptor molecular orbitals, as observed in one (n = 16) cluster. In the other (smaller) clusters, the transitions in this region were found to be weak as the clusters do not offer the necessary favorable alignment of orbitals. The present findings are consistent with the experimentally observed absorption at 230 nm that has been assigned to a solvent-to-solute charge transfer and provide insight into the electronic states and orbitals that give rise to the intensity of the band.