Ab Initio Study of the Mechanism for Photoinduced Yl-Oxygen Exchange in Uranyl(VI) in Acidic Aqueous Solution

The mechanism for the photochemically induced isotope-exchange reaction U17/18O2 2+(aq) + H2 16O ⇄ U16O2 2+(aq) + H2 17/18O has been studied using quantum-chemical methods. There is a dense manifold of states between 22 000 and 54 000 cm−1 that results from excitations from the σu and πu bonding orbitals in the 1Σg + ground state to the nonbonding fδ and fϕ orbitals localized on uranium. On the basis of investigations of the reaction profile in the 1Σg + ground state and the excited states 3Δg (the lowest triplet state) and 3Γg (one of the several higher triplet states), the latter two of which have the electron configurations σufδ and πufϕ, respectively, we suggest that the isotope exchange takes place in one of the higher triplet states, of which the 3Γg state was used as a representative. The geometries of the luminescent 3Δg state, the lowest in the σufδ,ϕ manifold (the “σ” states), and the 1Σg + ground state are very similar, except that the bond distances are slightly longer in the former. This is presumably a result of transfer of a bonding electron to a nonbonding f orbital, which makes the excited state in some respects similar to uranyl(V). As is the case for all of the states of the πu f δ,ϕ manifold (the “π” states), the geometry of the 3Γg state is very different from that of the 3Δg “σ” state and has nonequivalent U−Oyl distances of 1.982 and 1.763 Å; in the 3Γg state, the yl-exchange takes place by transfer of a proton or hydrogen from water to the more distant yl-oxygen. The activation barriers for proton/hydrogen transfer in the ground state and the 3Δg and 3Γg states are 186, 219, and 84 kJ/mol, respectively. The relaxation energy for the 3Γg state in the solvent after photoexcitation is −86 kJ/mol, indicating that the energy barrier can be overcome; the “π” states are therefore the most probable route for proton/hydrogen transfer. They can be populated after UV irradiation but are too high in energy (∼36000−40000 cm−1) to be reached by a single-photon absorption at 436 nm (22 900 cm−1), where experimental data have demonstrated that exchange can take place. Okuyama et al. [ Bull. Res. Lab. Nucl. React. (Tokyo Inst. Technol.) 1978, 3, 39–50] have demonstrated that an intermediate is formed when an acidic solution of UO2 2+(aq) is flash-photolyzed in the UV range. The absorption spectrum of this short-lived intermediate (which has a maximum at 560 nm) indicates that this species arises from 436 nm excitation of the luminescent 3Δg state (w