Electron-transfer reactions in water. Contributions from high-frequency librations?

From the solvent dependence of the absorption band energies for metal-to-metal charge-transfer bands in several symmetrical ligand-bridged mixed-valence dimers, water emerges as an anomalous solvent. Explanations based on specific ligand-solvent interactions or the involvement of high-frequency hydroxyl stretching modes are shown to be inconsistent with the available results. The experimental observations can be accomodated, however, by postulating the involvement of relatively high-frequency librational modes known to exist in water. The observation of anomalously high MMCT band energies in water has significant implications for related electron-transfer reactions such as thermally activated self-exchanges.