Quantum Chemical Calculations of Reduction Potentials of AnO22+/AnO2+ (An = U, Np, Pu, Am) and Fe3+/Fe2+ Couples

Abstract

The reduction potentials of the AnO(2)(H(2)O)(5)(2+)/AnO(2)(H(2)O)(5)(+) couple (An = U, Np, Pu, and Am) and Fe(H(2)O)(6)(3+) to Fe(H(2)O)(6)(2+) in aqueous solution were calculated at MP2, CASPT2, and CCSD(T) levels of theory. Spin-orbit effects for all species were estimated at the CASSCF level. Solvation of the hydrated metal cations was modeled both by polarizable conductor model (PCM) calculation and by solvating the solutes with over one thousand TIP3P water molecules in the QM/MM framework. The redox reaction energy calculated by QM/MM method agreed well with the PCM method after corrections using the classical Born formula for the contribution from the rest of the solvation sphere and correction for dynamic response of solvent polarization in the MM region. Calculated reduction potentials inclusive of spin-orbit effect, zero-point energy, thermal corrections, entropy effect, and PCM solvation energy were found to be comparable with experimental data. The difference between CASPT2 calculated and experimental reduction energies were less than 35 kJ/mol in all cases, which ensures that CASPT2 (and CCSD(T)) calculations provide reasonable estimates of the thermochemistry of these reactions.