Hydration of UO₂²⁺ and PuO₂²⁺

We study the hydration of the actinyl cations, uranyl UO22+ and plutonyl PuO22+, by performing Kohn−Sham Density Functional Theory calculations using a new quantum chemistry codeMAGIC. The calculations have been performed on the separate uranyl and plutonyl species, and on the complexes AcO22+·nH2O (Ac = U, Pu and n = 4, 5, and 6), in the gas and aqueous phases. The liquid-state environmental effects are included via a simple cavity model and by using the self-consistent reaction field method. The calculations find that the solvent effects are crucial. By this, we mean that a simple cavity model, alone, will be incapable of giving insight into the chemical behavior of such molecules. The short-ranged interactions between the actinyls and their closest water molecules are very strong and involve an appreciable amount of charge transfer, an effect that cannot be included in cavity models. The actinyls form strongly bound complexes with the surrounding water molecules, with n = 5 being the most stable. Thus, the short-range solvent effects are important. The binding energies of the complexes are very large, and in the gas phase they are about twice as large as in the aqueous phase. Thus, the bulk solvent effects are also important. Any reactivity of the actinyls with other species will thus be impeded by the existence of such strongly bound complexes, and the solvent will play an active role in such phenomena. Regarding the chemical behavior of the actinyls in aqueous solution, our studies provide preliminary evidence that there will be no qualitative and very little quantitative difference between the uranium and plutonium species.