| 19441797 |
Hennig C, Ikeda-Ohno A, Tsushima S, Scheinost AC: The sulfate coordination of Np (IV), Np (V), and Np (VI) in aqueous solution. Inorg Chem. 2009 Jun 15;48(12):5350-60.
The coordination and redox behavior of Np (IV), Np (V), and Np (VI) sulfate in aqueous solution were investigated by Np L (3)-edge extended X-ray absorption fine structure (EXAFS) spectroscopy, cyclic voltammetry, and density functional theory (DFT) calculations. The sulfate coordination mode, that is, monodentate versus bidentate, was determined by using neptunium-sulfur distances R (Np-S) and coordination numbers N (S) obtained by EXAFS spectroscopy. Np (VI) is coordinated by sulfate in the bidentate (R (Np-S) = 3.12 +/- 0.02 A) and monodentate (R (Np-S) = 3.61 +/- 0.02 A) modes at a low sulfate concentration of [SO (4)(2-)]/[NpO (2)(2+)] = 1. At higher [SO (4)(2-)]/[NpO (2)(2+)] ratios, bidentate coordination prevails. Approximately two bidentate sulfate groups are coordinated to Np (VI) with 2.0 M SO (4)(2-) and at pH 1.1. Np (V) is coordinated by sulfate in the bidentate (R (Np-S) = 3.16 +/- 0.02 A) and monodentate (R (Np-S) = 3.67 +/- 0.02 A) modes. However, sulfate coordination is less pronounced and does not exceed one SO (4)(2-) per Np (V) with 2.0 M SO (4)(2-). The redox reaction between the Np (VI)/Np (V) couple can be basically categorized as quasi-reversible. It becomes a more irreversible character at high sulfate concentrations due to structural rearrangement of the sulfate ligands. Finally, Np (IV) also shows bidentate (R (Np-S) = 3.06 +/- 0.02 A) and monodentate (R (Np-S) = 3.78 +/- 0.02 A) coordination modes. The sulfate coordination increases with an increasing [SO (4)(2-)]/[Np (4+)] ratio. A comparison of other tetravalent actinides shows that the monodentate sulfate coordination decreases whereas the bidentate coordination increases along the series Th (IV)-U (IV)-Np (IV). This trend was studied by DFT calculations and is discussed in terms of solvation energy and increasing number of unpaired electrons. |
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