Structures of Plutonium(IV) and Uranium(VI) with N,N-Dialkyl Amides from Crystallography, X-ray Absorption Spectra, and Theoretical Calculations

Abstract

The structures of plutonium(IV) and uranium(VI) ions with a series of N,N-dialkyl amides ligands with linear and branched alkyl chains were elucidated from single-crystal X-ray diffraction (XRD), extended X-ray absorption fine structure (EXAFS), and theoretical calculations. In the field of nuclear fuel reprocessing, N,N-dialkyl amides are alternative organic ligands to achieve the separation of uranium(VI) and plutonium(IV) from highly concentrated nitric acid solution. EXAFS analysis combined with XRD shows that the coordination structure of U(VI) is identical in the solution and in the solid state and is independent of the alkyl chain: two amide ligands and four bidentate nitrate ions coordinate the uranyl ion. With linear alkyl chain amides, Pu(IV) also adopt identical structures in the solid state and in solution with two amides and four bidentate nitrate ions. With branched alkyl chain amides, the coordination structure of Pu(IV) was more difficult to establish unambiguously from EXAFS. Density functional theory (DFT) calculations were consequently performed on a series of structures with different coordination modes. Structural parameters and Debye-Waller factors derived from the DFT calculations were used to compute EXAFS spectra without using fitting parameters. By using this methodology, it was possible to show that the branched alkyl chain amides form partly outer-sphere complexes with protonated ligands hydrogen bonded to nitrate ions.

Probing the existence of uranyl trisulfate structures in the AMEX solvent extraction process

“…The experimental spectra clearly differ from the typical uranyl spectra previously recorded from solvent extraction experiments: [10][11][12] in addition to the first U-Oax contribution from the linear transdioxo bonds (R + = 1.2 Å) and to the equatorial oxygen shell (R +  = 1.8 to 2.4 Å) the spectra exhibit unusual intense contributions in the second coordination sphere (R + > 2.5 Å). Considering the extraction equilibrium (eqn (1)), this feature could be due to the sulfur atoms from the sulfate anions coordinated with UO2 2+ .…”

Hexavalent Actinide Extraction Using N,N-Dialkyl Amides

“…Recent studies have suggested high nitric acid concentration increases the amount of p rotonated monoamide ligands (HL) in the organic phase, bringing about an outer-sphere coordination environment. 15,16 Similarly, elevated nitric acid conditions give rise to anionic trinitrate uranyl species, [UO 2 (NO 3 ) 3 ] -. 17,18 The mixed 1:1 and 1:2 stoichiometry could be due a protonated monoamide complexing with anionic uranyl (UO 2 (NO 3 ) 3 HL) and neptunyl species while the 1:2 ratio is synonymous with two ligands extracting the neutral hexavalent actinide dinitrate species (UO 2 (NO 3 ) 2 L 2 ).…”

Insights from quantum chemical calculations into inner and outer-sphere complexation of plutonium(iv) by monoamide and carbamide extractants

“…Siddall et al 6 have demonstrated that a simple modification of the acyl group can suppress Pu(IV) extraction. [18][19][20] In the series R = CH 3 , C 2 H 5 , CH(CH 3 ) 2 , C(CH 3 ) 3 , Pu(IV) distribution ratio D Pu continuously drops from 21 (R = CH 3 ) to 0.001 (R = C(CH 3 ) 3 ), while U(VI) distribution coefficient decreases much less significantly from 9.9 to 0.6 (upon extraction from a 3 M nitric acid aqueous solution to a toluene organic phase with 0.5 M of amide extractant). 6 Accordingly, U/Pu separation factor increases from 0.5 to ~600.…”

“…However, recent studies have revealed that plutonium extraction with amide derivatives is more complex than with TBP. 17,19 By coupling UV-Vis spectroscopy, EXAFS and quantum chemical calculations, it has been shown that the plutonium hexanitrato complex Pu(NO 3 ) 6…”