Electronic spectroscopy and photodissociation dynamics of hydrated Co2+ clusters: Co2+(H2O)(n) (n=4-7)

Solvated cluster ions Co2+ (H2O)(n) with n = 4-7 have been generated by ele ctrospray ionization and studied by laser photofragment spectroscopy. The s imilarity between the spectrum of gas-phase Co2+(H2O)(6) and the absorption spectrum of aqueous cobalt(II) suggests that Co2+(H2O)(6) ((aq)) is respon sible for the room-temperature solution absorption spectrum. The observed p hotodissociation spectrum of Co2+(H2O)(4) is similar to new bands which app ear in aqueous cobalt(II) at high temperatures and have been assigned to Co 2+ (H2O)(4 (aq)) by Swaddle and Fabes (Swaddle, T. W.; Fabes, L. Can. J. Ch em. 1980, 58, 1418-1426). The hexahydrate was found to dissociate by loss o f one or two water molecules, whereas the heptahydrate dissociates by loss of two or three water molecules. In both cases, loss of two water molecules is the preferred dissociation pathway. The tetrahydrate dissociates either by simple loss of water or by charge separation to form CoOH+(H2O)(2) and H3O+, with charge separation being the preferred dissociation channel. At 5 70 nm, photodissociation by charge separation leads to a kinetic energy rel ease of 110 +/- 20 kJ/mol, 48% of the available energy. This modest kinetic energy release is consistent with a "salt bridge" mechanism.