Photochemistry and exciplex of the uranyl ion in aqueous solution

Abstract

The effects of acidity, temperature, self-quenching and H-donor concentration on the luminescent state of the aqua-uranyl (VI) ion have been studied in aqueous acidic nitrate and perchlorate solution. The detailed results cannot be explained by any single simple mechanism such as radiative, nonradiative or spontaneous collisional quenching, or irreversible hydrogen abstraction from water. Quantitative analysis of the results shows a far more complex mechanism, involving the adiabatic formation of the species UO₂H²⁺ and U₂O₄H⁴⁺, as already proposed by the author. This mechanism is supported by state and m.o. correlations. The abstraction of hydrogen from water is shown to take place by H atom transfer in a uranyl–water complex intermediate, rather than by attack of H⁺ on the fully occupied π_u orbitals of uranium (V) in a well-defined uranyl water complex with strong charge transfer character. A qualitative description of the exciplex U₂O₄H⁴⁺ is shown to be possible in a v.b. formalism and the origin of its radiative properties is discussed on this basis.