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

Abstract

The structures and vibrational frequencies of UO 2 (H 2 O) 5 2+, NpO 2 (H 2 O) 5 2+, and PuO 2 (H 2 O) 5 2+ corresponding to An(VI) oxidation states and UO 2 (H 2 O) 5 +, Np(H 2 O) 5 +, and Pu(H 2 O) 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 UO 2 (H 2 O) 5 2+ to form UO 2 (H 2 O) 4 (OH) + and the reactions for removing or adding a water to the first shell in UO 2 (H 2 O) 5 2+ 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. PuO 2 q+ is used as a model for the aquo complexes in a weak ligand field for the cases q = 3 (5f 1 configuration), q = 2 (5f 2 ) and q = 1 (5f 3 ). The inclusion of these effects results in a different ground state for NpO 2 (H 2 O) 5 2+ and PuO 2 (H 2 O) 5 2+ than that obtained in the original DFT calculations. The reduction potentials for all three AnO 2 (H 2 O) 5 2+ complexes in solution is compared with electrochemical experimental data. The trend for the reduction potentials NpO 2 (H 2 O) 5 2+ > PuO 2 (H 2 O) 5 2+ > UO 2 (H 2 O) 5 2+ is found in agreement with experiment, when multiplet and spin−orbit corrections are included, although 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.