Prediction of RedoxPotentials for Different OxidationStates of U, Np, Pu, and Am in Alkaline Aqueous Solution

The redox potentials for U, Np, Pu, and Am for oxidation states +III up to +VIII in alkaline aqueous solutions were predicted using density functional theory (DFT) and small-core pseudopotentials and their basis sets, with a hybrid explicit/implicit solvent model using SHE = 4.28 V. For each oxidation state, various oxo/hydroxo complexes were evaluated, resulting in a variety of one-electron redox pathways. For An(VIII/VII) couples, the predicted redox potentials for the [An(VIII)O₅(OH)]⁻³/[An(VII)O₄(OH)₂]⁻³ or [An(VIII)O₄(OH)₂]⁻²/[An(VII)O₄(OH)₂]⁻³ couples are in good agreement with existing estimates. For An(VII/VI) redox couples, all couples, particularly [An(VII)O₄(OH)₂]⁻³/[An(VI)O₂(OH)₄]⁻², were in agreement with experimental values for U, Np, and Pu, but the results for Am showed larger differences from the estimated potentials. The An(VI/V) couples were consistent with experiments for dioxo/tetrahydroxo couples, and the An(V/IV) couples showed acceptable agreement based on actinide-specific couples, with neutral hydroxides often favored in the +IV state. The An(IV/III) couples were consistent with the literature values when modeled as soluble neutral hydroxides. The use of our approach yielded calculated redox potentials that were within ±0.2 V of experimental or estimated values consistent with our prior calculations on redox potentials of actinides from Ac to Am in acidic aqueous solutions. This supports the robustness of our DFT-based methodology for predicting actinide redox potentials, offering valuable insights into actinide chemistry in aqueous solutions.