Theoretical studies of the properties and solution chemistry of AnO(2)(2+)and AnO(2)(+) aquo complexes for An = U, Np, and Pu

The structures and vibrational frequencies of UO2(H2O)(5)(2+), NPO2(H2O)(5) (2+), and PuO2(H2O)(5)(2+) corresponding to An(VI) oxidation states and UO2 (H2O)(5)(+), Np(H2O)(5)(+), and Pu(H2O)(5)(+) corresponding to An(V) have b een calculated using density functional theory (DFT) and relativistic effec tive core potentials (RECPs). The resulting structures are compared to EXAF S solution studies, and the Raman and IR vibrational frequencies of the act inyl unit are compared to experimental studies in solution. Free energies f or reactions in solution are computed by combining thermodynamic free energ ies in the gas phase with a dielectric continuum model to treat solvent eff ects. The hydrolysis reaction of UO2(H2O)(5)(2+) to form UO2(H2O)(4)(OH)(+) and the reactions for removing or adding a water to the first shell in UO2 (H2O)(5)(2+) are examined using this approach. Multiplet and spin-orbit eff ects not included in a single-configuration DFT wave function are incorpora ted by model spinorbit CI calculations. PuO2q+ is used as a model for the a quo complexes in a weak ligand field for the cases q = 3 (5f(1) configurati on), q = 2 (5f(2)) and q = 1 (5f(3)). The inclusion of these effects result s in a different ground state for NpO2(H2O)(5)(2+) and PuO2(H2O)(5)(2+) tha n that obtained in the original DFT calculations. The reduction potentials for all three AnO(2)(H2O)(5)(2+) complexes in solution is compared with ele ctrochemical experimental data. The trend for the reduction potentials NpO2 (H2O)(5)(2+) > PuO2(H2O)(5)(2+) > UO2(H2O)(5)(2+) is found in agreement wit h experiment, when multiplet and spin-orbit corrections are included, altho ugh the absolute reduction potentials are overestimated in all three cases. The possible reasons for this overestimate are examined using all-electron calculations using the ADF method.