Stepwise solvatochromism of ketyl anions in the gas phase: photodetachment excitation spectroscopy of benzophenone and acetophenone radical anions microsolvated with methanol.

Electronic absorption spectra of bare and methanol-solvated radical anions of benzophenone ((C6H5)2CO) and acetophenone ((C6H5)CH3CO) were measured by monitoring the photodetachment efficiency in the gas phase. Strong absorption bands due to autodetachment after transitions to bound excited states were observed. Stepwise spectral shifts approaching the limit of the condensed phase spectra were found to occur as the cluster size increases. In the case of benzophenone radical anion, the solvation of two methanol molecules exhibits the near convergence to the limit, representing the full coordination with the solvent molecules around the carbonyl group. For the acetophenone case, the coordination number was not apparently determined because of their relatively small shifts. Relationships between hydrogen bonding and electronic structure are analyzed for the spectral shifts with the aid of calculations based on density functional theory. The calculational results show that the coordination angle of the solvent molecule is affected mostly by steric hindrance around the carbonyl group, and that there is no evidence for reorientation due to specific hydrogen bonding interaction with the singly occupied orbital, which has been formerly persisted for an interpretation of the transient absorption following pulse radiolysis in alcoholic solutions. An alternative possibility involving deformation with respect to intramolecular coordinates is discussed.