Molecular characterization of actinide oxocations from protactinium to plutonium

For industrial, environmental and public health purposes, actinide chemistry has been the subject of considerable efforts since the 50’s. Aqueous redox chemistry, ionic selective recognition, uptake by specific biomolecules or compartments of the geosphere are some of the major fields of investigation. The physical-chemical properties of the actinide elements strongly depend on the 5f/6d electronic configuration. Some of them (U, Np, Pu and Am) can form AnO2n+ (n= 1, 2) oxocations, so-called actinyls, with two strong An-O covalent bonds. In any case, the cation polyhedron is characterized by large, flexible coordination spheres with various stable and metastable metal oxidation states. Protactinium, as the first actinide with 5f-electrons involved in bonding, occupies a key position in the actinide series. At formal oxidation state V (its most stable oxidation state in solution as well as in the solid state) Pa(V) corresponds to the formal 5f0 electronic configuration. U(VI) also corresponds to the formal oxidation state 5f0 and is most often encountered as the stable oxocationic form UO22+. The first stable form of U, Np or Pu at formal oxidation state (V) under atmospheric conditions is NpO2+ with formal 5f2 electronic configuration. U(V) is highly unstable under atmospheric conditions and Pu(V) dismutates into Pu(IV) and Pu(VI). On the other hand, the existence of the PaO2+ form in solution and in solid state is highly improbable and has never been reported. In solution, XAFS at the actinide LIII edge is an ideal structural probe of the cation coordination sphere. Furthermore, coupling the XAFS data with molecular dynamics calculations leads to a better description of the cation-solvent interactions. In addition, disorder can explicitly be taken into account using time spaced snapshots of the molecule. This is particularly important when large polyhedra are composed of ligands of similar types as water molecules. This presentation addresses the structural characterization of actinide cations at oxidation states (V) and (VI) as one walks across the periodic table from Z = 91 (protactinium) to Z = 94 (plutonium). For the fist time to our knowledge, the occurrence of the oxocation form of Pa(V) in H2SO4 solution has been inferred from EXAFS and XANES data at the Pa LIII edge. A structural comparison between Pa, U, Np and Pu oxocations in aqueous solution at formal oxidation states (V) and (VI) is carried out. These results are corroborated by quantum chemical and molecular dynamics calculations.