Spectroscopic properties of tetravalent actinide ions in solids

Determination of Uranium Central-Field Covalency with 3d4f Resonant Inelastic X-ray Scattering

Understanding the nature of metal-ligand bonding is a major challenge in actinide chemistry. We present a new experimental strategy for addressing this challenge using actinide 3d4f resonant inelastic X-ray scattering (RIXS). Through a systematic study of uranium(IV) halide complexes, [UX6]2-, where X = F, Cl, or Br, we identify RIXS spectral satellites with relative energies and intensities that relate to the extent of uranium-ligand bond covalency. By analyzing the spectra in combination with ligand field density functional theory we find that the sensitivity of the satellites to the nature of metal-ligand bonding is due to the reduction of 5f interelectron repulsion and 4f-5f spin-exchange, caused by metal-ligand orbital mixing and the degree of 5f radial expansion, known as central-field covalency. Thus, this study furthers electronic structure quantification that can be obtained from 3d4f RIXS, demonstrating it as a technique for estimating actinide-ligand covalency.

Neptunium Alkoxide Chemistry: Expanding Alkoxides to the Transuranium Elements

Two oxo-containing neptunium(IV) tert-butoxides, [Np₃O(O^tBu)₁₀] (1) and [K₄Np₂O(O^tBu)₁₀] (2), were synthesized using the ligand substitution between neptunium(IV) silylamides and HO^tBu, whereas the salt metathesis between [NpCl₄(DME)₂] (DME = dimethoxyethane) and various amounts of LiO^tBu resulted in the formation of oxo-free alkoxides [Np(O^tBu)₄(py)₂] (3; py = pyridine) and [Li(THF)]₂[Np(O^tBu)₆] (4; THF = tetrahydrofuran). These complexes are the first structurally characterized neptunium(IV) alkoxides using single-crystal X-ray diffraction and solid-state absorption spectroscopy, which provide data for the development of anhydrous metal-organic neptunium chemistry.

Sequestration of tetravalent neptunium from acidic feeds using diglycolamide-functionalized dendrimers in a room temperature ionic liquid: extraction, spectroscopic and electrochemical studies

Extraction of Np(iv) was studied using RTIL containing two poly(propyleneimine) based dendrimer ligands from HNO3 solutions. Studies of the extracted complexes were also carried out by Vis-NIR and cyclic voltammetry.

Perrhenate and Pertechnetate Complexes of U(IV), Np(IV), and Pu(IV) with Dimethyl Sulfoxide as an O-Donor Ligand

A series of new dimethyl-sulfoxide-containing pertechnetates and perrhenates of tetravalent U, Np, and Pu were synthesized and structurally characterized by the X-ray diffractometry. In all the synthesized compounds, the actinide atoms were coordinated by eight DMSO molecules with or without an extra XO₄^- anion in the coordination sphere. This resulted in the square antiprismatic or capped square antiprismatic coordination of An atoms. Three or four XO₄^- anions play the role of outer-sphere anions. The electron and IR spectra of the compounds correlated with their crystal structure.

Crystallographic structure and crystal field parameters in the [AnIV(DPA)3]2− series, An = Th, U, Np, Pu

The [An^IV(DPA)₃]²⁻ series with An = Th, U, Np, Pu has been synthesized and characterized using SC-XRD, vibrational spectroscopy, and first principles calculations.

5f3 → 5f 26d Absorption Spectrum Analysis of U3+−SrCl2

The 5f3--> 5f26d1 absorption spectra of the U3+ ions incorporated in SrCl2 single crystals were recorded at 4.2 K in the 15,000-50,000 cm(-1) spectral range. From an analysis of the vibronic structure, 32 zero-phonon lines corresponding to transitions from the 4I9/2 ground multiplet of the 5f3 configuration to the 5f26d(eg)1 excited levels were assigned. A theoretical model proposed by Reid et al. (Reid, H. F.; van Pieterson, L.; Wegh, R. T.; Meijerink, A. Phys. Rev. B 2000, 62, 14744) that extends the established model for energy-level calculations of nf N states has been applied for analysis of the spectrum. The Fk(ff) (k = 2, 4), zeta(5f)(ff), B0(4)(ff), B0(6)(ff), Fk(fd) (k = 2, 4), and Gj(fd) (j = 1, 3) Hamiltonian parameters were determined by a least-squares fitting of the calculated energies to the experimental data. A good overall agreement between the calculated and experimentally observed energy levels has been achieved, with the root-mean-square (rms) deviation equal to 95 cm(-1) for 32 fitted levels and 9 varied parameters. Adjusted values of Fk(ff) and zeta(5f)(ff) parameters for the 5f2 core electrons are closer to the values characteristic of the 5f2 (U4+) configuration than to those of the 5f3 (U3+) configuration. For the U3+ ion, the f-d Coulomb interaction parameters are significantly more reduced from the values calculated using Cowan's computer code than they are for lanthanide ions. Moreover, because of weaker f-d Coulomb interactions for the U3+ ion than for the isoelectronic Nd3+ lanthanide ion, the very simple model assuming the coupling of crystal-field levels of the 6d1 electron with the lattice and the multiplet structure of the 5f2 configuration may be employed for the qualitative description of the general structure of the U3+ ion f-d spectrum.

Effects of Precursor and Support Variation in the Genesis of Uranium Oxide Catalysts for CO Oxidation and Selective Reduction of NO: Synthesis and Characterization

A range of uranium oxide-based catalysts, derived from UO2(NO3)2.6H2O and UCl4 precursors, and supported on gamma-Al2O3, SiO2 and mesoporous H1SiO2, have been synthesized and then characterized using the following methods: isothermal nitrogen adsorption/desorption measurements, diffuse reflectance infrared spectroscopy (DRIFTS), gas titration of surface hydroxyl groups using Grignard reagents, U L(III) extended X-ray absorption fine structure (EXAFS), powder X-ray diffraction (PXRD), and thermogravimetric and differential thermal analysis. Brij76-templated H1SiO2 mesoporous silicas are found to be essentially stable under flowing oxygen after 16 h at 1073 K. At temperatures above this, however, extensive structural collapse, together with extensive dehydroxylation, ensues. Titration of the accessible hydroxyl group concentrations shows that in these materials the density of OH groups is considerably lower than in their amorphous counterparts. The adsorption of uranyl nitrate onto these dispersants results in a supported, and partially dehydrated, phase of the parent molecule with little obvious structural distortion; however, the adsorption of UCl4 leads to a complex adstructure which may best be described as U(O)2Cl2. The subsequent formation of the uranium oxide phase, nominally active for the oxidation of CO and selective reduction of NO (generally accepted to be U3O8), is found to be a considerable function of both the precursor and support system employed. Calcination of such systems to 1073 K results in extensive extrusion of the supported uranium phase from mesoporous supports, resulting in the formation of very large orthorhombic U3O8 domains. PXRD, however, shows that on amorphous SiO2 and gamma-Al2O3 similar treatment results in the formation of a hexagonal phase of U3O8. The formation of U3O8 is found to be promoted in mesoporous systems and by the presence of Cl in the catalyst make up. Some evidence is also found that suggests that a persistence of Cl limits the growth of U3O8 domains.