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The first structural studies of protactinium(V) in sulfuric and hydrofluoric acid media using X-ray absorption spectroscopy was presented. The results show that the absence of the trans-dioxo bond that characterizes the other early actinide element such as U dnd Np.

The UV-visible absorption spectroscopy and NMR studies are conducted to explain the characterization of Me[Bu.sub.3][N.sup.+], Bu[Me.sub.2][Im.sup.+], and BuMe[Im.sup.+] cations, which are shown to help in the solvation of U[Cl.sub.6.sup.2-] anionic complexes. Different cations are shown to exhibit different affinities towards the anions.

The dissolution of Th[.sub.2] powdered samples in various conditions of pH and concentrations of anions in the leachate and the effect of pH on the dissolution of Th[O.sub.2] in both nitric and hydrochloric media are examined. The redox properties of plutonium in acidic media, like disproportionation of Pu(IV) and Pu(V) and reduction of Pu(VI) and Pu(IV) in Pu(III) by [H.sub.2][O.sub.2], has increased the dissolution of plutonium.

A UV/vis/near-IR spectroscopic study shows that in [BuMelm][(CF3SO2)2N] hydrophobic room-temperature ionic liquid solutions, [BuMelm]2[AnCl6] complexes, where BuMelm+ is 1-n-butyl-3-methylimidazolium and An(IV) is Np(IV) or Pu(IV), have an octahedral An(IV) environment similar to that observed in solid complexes. Water has no influence on the absorption spectra of AnCl62- complexes, indicating their stability to hydrolysis in ionic liquid. Adding [BuMelm]Cl modifies the UV/vis/near-IR absorption spectra of An(IV) in the ionic liquid and causes solids to precipitate. The solid-state reflectance spectra of the precipitates reveal considerable differences from the corresponding An(IV) hexachloro complexes. A voltammetric study indicates that AnCl62- complexes are electrochemically inert in [BuMelm][(CF3SO2)2N] at the glassy carbon working electrode. By contrast, quasi-reversible electrochemical reduction An(IV)/An(III) and An(IV) oxidation are observed in ionic liquids in the presence of [BuMelm]Cl. The oxidation wave of noncoordinated chloride ions interferes with the An(IV) oxidation waves. The spectroscopic and voltammetric data clearly indicate the formation of nonoctahedral actinide(IV) chloride complexes with a Cl-/An(IV) ratio exceeding 6/1 in [BuMelm][(CF3SO2)2N] in excess chloride ions. [ABSTRACT FROM AUTHOR]

A new mixed valent neptunium compound [BuMeIm]5[Np(NpO2)3(H2O)6Cl12] was synthesized by evaporation of an ethyl acetate-acetone mixture where the salt [BuMeIm]2[NpCl6] was dissolved. The crystal structure was determined using single crystal X-ray diffraction; it consists of four crystallographically unique Np centers with two different coordination environments in two different oxidation states. In addition, five crystallographically independent dialkylimidazolium cations stabilize the crystal through C-H⋯Cl hydrogen bonds. [Np(NpO2)3(H2O)6Cl12] unit presents a highly symmetric structure with the formation of three Np(IV)-Np(V)O2+ bonds. According to the crystal structure and density functional theory (DFT) calculations, the stability of the three Np4+-NpO2+ interactions is due to the presence of chloride ions around NpO2+ which brings electronic charge into the system and also to the presence of water molecules which create hydrogen bond with chloride ions. [ABSTRACT FROM AUTHOR]

This study concerns the formation of complexes with actinides at oxidation state +IV, from Th to Am, through a lacunary heteropolyanion ligand P2W17O6110-. The preparation of original single crystals of complexes with 1:2 stoichiometry provided structural data through single-crystal X-ray diffraction for the entire An(IV) series. An(lV) atoms in these complexes have a coordination number of 8, and their coordination polyhedron is a distorted square antiprism. The data were used to compute a mean (An-O) interatomic distance for each actinide. When the interatomic distance is plotted versus the inverse ionic radius of these tetravalent actinides, it appears that the simple electrostatic model is not suitable for Am(IV) and Pu(IV), although this trend must be confirmed by further investigation with other examples from the An(IV) series. [ABSTRACT FROM AUTHOR]

Actinide +IV complexes (An IV = Th IV , U IV , Np IV , and Pu IV ) with two dipicolinic acid derivatives (DPA and Et-DPA) have been studied by 1 H and 13 C NMR spectroscopies and first-principles calculations. The Fermi contact and dipolar contributions to the actinide-induced shifts (AIS) are evaluated from a temperature dependence analysis, combined with ab initio results. It allows an experimental estimation of the axial anisotropy of the magnetic susceptibility Δχ ax and of the hyperfine coupling constants of the NMR-active nuclei. Due to the compactness of the coordination sphere, the magnetic anisotropy of the paramagnetic center is small, and this makes the contact contribution to be the dominant one, even on the remote atoms. The sign of the hyperfine coupling constants and related spin densities is alternating on the nuclei of the ligand cycle, denoting a preponderant spin polarization mechanism. This is well reproduced by unrestricted density functional theory (DFT) calculations. Those values are furthermore slightly decreasing in the actinide series, which indicates a small decrease of the covalency from U IV to Pu IV .

Multiparametric study of the hydrothermal synthesis of thorite, ThSiO 4 , was performed with the aim to determine the most efficient conditions to form single phase thorite samples. Among the experimental parameters investigated, temperature of the hydrothermal process, concentration of carbonate ions, thorium and silicon reactants, and pH of the reactive media significantly affect the composition of the final system obtained. Single phase samples of ThSiO 4 were prepared in weakly basic reactive media and at temperatures over 150 °C, for thorium and silicate concentrations higher than 8 × 10 -3 mol L -1 and carbonate concentrations of at least 8 × 10 -2 mol L -1 . Although the synthesis of thorite in carbonate media was already described in the literature, this study gives new insights to explain the key role of carbonate ions in the preparation of thorite. Especially, beyond their simple role of pH buffer, carbonate ions are involved in the formation of thorium-carbonate complexes at high pH, increasing the apparent solubility of thorium in weakly basic media. The presence of carbonate ions has an important impact not only on the domain of formation of thorite but also on the morphology of the silicate phase.

A multiparametric study of the hydrothermal synthesis of ThSiO 4 , thorite, was performed with the aim of determining the most efficient conditions to form single-phase samples. Among the experimental parameters examined, significant effects were found for the concentration of reactants in the starting mixture, pH of the reactive media, and temperature of the hydrothermal process. Such parameters affected both the rate of formation of thorite and the morphology of the final products synthesized. Precipitation of pure ThSiO 4 was obtained over a wide range of pH on going from C HNO3 = 0.3 mol L -1 to pH 9.1 with a yield of over 95%. Temperatures higher than 160 °C favor the formation of thorite. Finally, thorium and silicon concentrations above 2.1 × 10 -3 mol L -1 are required to obtain pure thorium silicate.