Probing the protonation and reduction of heptavalent neptunium with computational guidance

Abstract

Influence of pH on the speciation and stability of heptavalent neptunium is poorly understood although it is frequently invoked in the literature to explain experimental observations. The present study employs Density Functional Theory (DFT) methodology to assess the thermodynamic feasibility of protonation reactions for the Np(VII) anion complex and the impact on its reduction to Np(VI). This theoretical framework is then explored experimentally through the titration and systematic protonation of Np(VII) in solution and solid-state samples while monitoring them spectroscopically. Computational results reveal that protonation reactions with the axial OH⁻ ligands of the Np(VII) anionic complex, [NpO₄(OH)₂]³⁻, are more thermodynamically favorable than the equatorial oxo ligands. In addition, DFT studies indicated that up to four sequential protonation reactions may be feasible before reduction becomes thermodynamically favorable. Experimental results also uncover that protonation leads to distinct changes in the observable vibrational signals and UV-Vis absorption features. Overall, we observed that the protonation of [NpO₄(OH)₂]³⁻ in solution and in the solid-state occurs before reduction to the Np(VI)O₂²⁺ species.