1. Chemical Speciation of Neptunium(VI) under Strongly Alkaline Conditions. Structure, Composition, and Oxo Ligand Exchange.
- Author
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Clark, David L., Conradson, Steven D., Donohoe, Robert J., Gordon, Pamela L., Keogh, D. Webster, Palmer, Phillip D., Scott, Brian L., and Tait, C. Drew
- Subjects
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NEPTUNIUM , *ALKALINE solutions , *OXO compounds , *AQUEOUS solutions , *SINGLE crystals , *X-ray diffraction , *CHEMICAL bonds - Abstract
Hexavalent neptunium can be solubilized in 0.5-3.5 M aqueous MOH (M = Li+, Na+, NMe4+ = TMA+) solutions. Single crystals were obtained from cooling of a dilute solution of Co(NH3)6Cl3 and NpO22+ in 3.5 M [N(Me)4]OH to 5 °C. A single-crystal X-ray diffraction study revealed the molecular formula of [Co(NH3)6]2[NpO2(OH)4]3·H2O, isostructural with the uranium analogue. The asymmetric unit contains three distinct NpO2(OH)42- ions, each with pseudooctahedral coordination geometry with trans-oxo ligands. The average Np=O and Np-OH distances were determined to be 1.80(1) and 2.24(1) Å, respectively. EXAFS data and fits at the Np LIII-edge on solid [Co(NH3)6]2[NpO2(OH)4]3·H2O and aqueous solutions of NpO22+ in 2.5 and 3.5 M (TMA)OH revealed bond lengths nearly identical with those determined by X-ray diffraction but with an increase in the number of equatorial ligands with increasing (TMA)OH concentration. Raman spectra of single crystals of [Co(NH3)6]2[NpO2(OH)4]3·H2O reveal a v1(O=Np=O) symmetric stretch at 741 cm-1. Raman spectra of NpO22+ recorded in a 0.6-2.2 M LiOH solution reveal a single v1 frequency of 769 cm-1. Facile exchange of the neptunyl oxo ligands with the water solvent was also observed with Raman spectroscopy performed with 16O- and 18O-enriched water solvent. The combination of EXAFS and Raman data suggests that NpO2(OH)42- is the dominant solution species under the conditions of study and that a small amount of a second species, NpO2(OH)53-, may also be present at higher alkalinity. Crystal data for [Co(NH3)6]2[NpO2(OH)4]3·H2O: monoclinic, space group C2/c, a = 17.344(4) Å, b = 12.177(3) Å, c = 15.273 Å, ß = 120.17(2)°, Z = 4, R1 = 0.0359, wR2 = 0.0729. [ABSTRACT FROM AUTHOR]
- Published
- 2013
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