Theoretical Studies of the Properties and Solution Chemistry of AnO₂²⁺ and AnO₂⁺ Aquo Complexes for An = U, Np, and Pu

The structures and vibrational frequencies of UO2(H2O)52+, NpO2(H2O)52+, and PuO2(H2O)52+ corresponding to An(VI) oxidation states and UO2(H2O)5+, Np(H2O)5+, and Pu(H2O)5+ corresponding to An(V) have been calculated using density functional theory (DFT) and relativistic effective core potentials (RECPs). The resulting structures are compared to EXAFS solution studies, and the Raman and IR vibrational frequencies of the actinyl unit are compared to experimental studies in solution. Free energies for reactions in solution are computed by combining thermodynamic free energies in the gas phase with a dielectric continuum model to treat solvent effects. The hydrolysis reaction of UO2(H2O)52+ to form UO2(H2O)4(OH)+ and the reactions for removing or adding a water to the first shell in UO2(H2O)52+ are examined using this approach. Multiplet and spin−orbit effects not included in a single-configuration DFT wave function are incorporated by model spin−orbit CI calculations. PuO2q+ is used as a model for the aquo c...