Photophysics of the excited uranyl ion in aqueous solutions. Part 3.—Effects of temperature and deuterated water: mechanisms of solvent exchange and hydrogen abstraction from water in excited states

Abstract

The effects of temperature and D₂O on excited uranyl decay in aqueous solution have been studied at several pH values and uranyl concentrations. The data are interpreted in terms of reversible crossing between two excited states of uranyl with different electronic distributions. The highest state, called U^*, has a π³_uϕ¹_u configuration whereas the lowest state, called X^*, is π³_uδ¹_u(Ω= 1). The reversible decay is interpreted in terms of a solvent-exchange process. The values of ΔH^† and ΔS^† for this process reveal that solvent exchange proceeds by different mechanisms for the U^* and X^* states. The activation parameters are pH dependent, but the U^* state always has more dissociative character for solvent exchange, whereas the X^* state has more associative character. A weak effect on the solvent-exchange rates was found in D₂O. The rates of the irreversible decays are attributed mainly to a hydrogen-abstraction reaction from water molecules by excited uranyl ions. The activation energies and the frequency factors are pH dependent, revealing that hydrogen abstraction from the coordinated equatorial H₂O and from the more loosely bound water in axial positions can occur. The temperature and the deuterium isotope effects on the decays were analysed in terms of the current theories of radiationless transitions for the involvement of the O—H and UO stretching modes in the abstraction process. The autoquenching processes and the effect of the exciting intensity on the decays are in agreement with the proposed mechanisms.