Prediction of Redox\nPotentials for Ac, Th, and Pa\nin Aqueous Solution

Density functional theory in conjunction with small core\npseudopotentials\nand the associated basis sets was used to calculate potentials for\nmultiple redox couples, covering a range of oxidation states for Ac\n(0 to III), Th (0 to IV), and Pa (0 to V) in aqueous solution. Solvation\neffects were incorporated using a supermolecule-continuum approach,\nwith 30 water molecules representing two solvation shells, and the\nCOSMO and SMD implicit solvation models. The calculated geometries\nfor Ac(III), Th(IV), and Pa(V) were in reasonable agreement with the\navailable experimental data. Using the COSMO model with the B3LYP\nfunctional, the calculated redox potentials were within ±0.2\nV from experiment for most redox couples. Several pathways were explored\nfor the Pa(V/IV) redox couple for different forms of Pa(V) and Pa(IV).\nMost Pa(V/IV) redox couples have very similar potentials, ranging\nfrom 0 to −0.4 V up to a pH of 1.4. At pH = 1.4, the potentials\nshift to values that are more negative than −0.7 V, reflecting\nthe growing unfavorable nature of the redox process at higher pH levels.\nThe calculated values for An(III/II) potentials were consistent with\nprior estimates and the available experimental data. The predicted\nredox potentials for An(II/I) were highly negative, as expected. For\nAn(I/0) potentials, Th and Pa exhibited positive values, contrasting\nwith the negative values calculated for Ac. The An<sup>+m</sup>/An(0)\npotentials agreed better with the experimental data when using the\nCOSMO solvation model as compared to the SMD model.