On the Structure of Np(VI) and Np(VII) Species in Alkaline Solution Studied by EXAFS and Quantum Chemical Methods

The bond distances and coordination numbers of the predominant Np(VII) complex in strongly alkaline solution have been determined using EXAFS transmission measurements. The stoichiometry and structure of NpO4(OH)2 3- has been deduced by combining these data with different structure models, mostly determined by using DFT based methods. The experimental and theory based distance Np(VII)−Ooxo is 1.894 and 1.90 Å, respectively, whereas the Np(VII)−OH- distance is 2.326 and 2.33 Å, respectively. Theory based geometry and bond distances have been obtained also for other Np(VII) and Np(VI) complexes, NpO2(OH)4 -, NpO4 -, NpO4(OH)2 4-, and NpO2(OH)4 2-, NpO4 2-. The “NpO6” − unit has a square bipyramidal geometry both in NpO4(OH)2 3-/4- and in NpO2(OH)4 1-/2-, albeit with some difference in bond distances. The close similarity in structure indicates that no major rearrangements are necessary on electron transfer between Np(VI) and Np(VII), a possible explanation for the stable and reproducible Np(VII)/Np(VI) redox potential observed in alkaline solution. The structure data indicate that new Np(VII) species may be identified by oxidation of Np(VI) solutions at lower hydroxide concentrations.