Abstract

New stability constants for NpV and PuV carbonates have been established by coupling capillary electrophoresis (CE) and ICP‐MS at the ionic strength of 0.202 mol/kgwater in NaClO4 and at various temperatures. The thermodynamic data [variation of the molar enthalpy, ΔrHm(T°) and entropy, ΔrSm(T) of reaction] associated with the formation of the AnO2CO3– (An = Np, Pu), AnO2(CO3)23–, and AnO2(CO4)35– complexes were determined. A drastic decrease in molar entropy was observed between the second and third complexes, suggesting important structural changes. Ab initio molecular dynamics showed that the 1:3 complex is formed with two carbonate ligands located at a shorter distance and one at a longer distance, suggesting that the closer ligands are coordinated in a bidentate fashion to the plutonyl(V) moiety whereas the third ligand is monodentate. The variation of the electrophoretic mobilities as function of charge/size ratio confirms unambiguously that the triscarbonate complex of pentavalent plutonium structurally differs from that of hexavalent plutonium where all carbonates anions are bidentate.

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