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.

with dipicolinic acid derivatives were synthesized and characterized by powder XRD, SQUID magnetometrya nd NMR spectroscopy.I na ddition, Np VI and Pu VI complexes were described by first principles CAS based and two-component spin-restricted DFT methods. The analysis of the 1 Hp aramagnetic NMR chemical shifts for all protons of the ligands according to the X-rays structures shows that the Fermi contact contribution is negligible in agreement with spin density determined by unrestrictedDFT.The magnetic susceptibility tensor is determined by combiningS QUID, pNMRs hifts and Evans' method. The SO-RASPT2 results fit well the ex-perimental magnetic susceptibility and pNMR chemical shifts.T he role of the counterions in the solid phaseisp ointed out;t heir presence impacts the magnetic properties of the Np VI complex.T he temperature dependence of the pNMR chemical shifts has as trong 1=T contribution, contrarily to Bleaney'st heory for lanthanide complexes. The fitting of the temperature dependence of the pNMR chemical shifts and SQUID magnetic susceptibility by at wo-Kramers-doublet model for the Np VI complex and an on-Kramers-doublet modelf or the Pu VI complex allows for the experimental evaluation of energy gaps and magnetic moments of the paramagnetic center.

N,N-Dialkylamides are extensively studied as alternative organic ligands to achieve the extraction and separation of uranium(vi) and plutonium(iv). We report here the coordination structures of the plutonium(iv) ion with N,N-di(2-ethylhexyl)-n-butanamide as a function of nitric acid concentration in the aqueous phase. The coordination structure of Pu(iv) evolves gradually with increasing nitric acid concentration from an inner-sphere with two coordinated amide ligands toward an outer-sphere hexanitrate complex with only nitrate ions in the first coordination sphere and protonated amide ligands in the outer shell.

Carbamide and monoamide derivatives are very promising molecules to achieve U­(VI) and Pu­(IV) extraction and separation from spent nuclear fuels through solvent extraction. Herein, coordination structures of U­(VI) and Pu­(IV) complexes with carbamide derivatives were characterized using X-ray crystallography as well as infrared, UV–visible, and EXAFS spectroscopies. Coordination structures are compared to those obtained for monoamide derivatives in order to better understand the role of coordination chemistry in extraction properties. Single crystals were first synthesized with a short alkyl chain carbamide analog. Carbamide complexation in the solid state is found analogous to that in the monoamide. In organic solution, upon solvent extraction from nitric acid aqueous solution, it is shown that both amide derivatives can bind in the inner and outer coordination spheres of uranium­(VI) and plutonium­(IV). The amount of outer sphere coordination complexes increases with the amount of nitric acid. With uranium­(VI), at a nitric acid concentration up to 5 mol·L–1, amide derivatives operate predominantly in the inner coordination sphere. In contrast, Pu­(IV) coordination geometry is much more sensitive toward acid concentration or ligand structure than U­(VI). Pu­(IV) changes from inner sphere complexation at 0.5 mol·L–1 HNO3 to mostly outer sphere complexation at 4 mol·L–1. The proportion of outer-sphere complexes is strongly influenced by the ligand structure. Higher Pu­(IV) extraction is found to be correlated with the amount of Pu­(IV) outer sphere species. Secondary interactions in the outer sphere coordination shell appear to be of primary importance for plutonium extraction.