Satoru Tsushima

Speciation and Structural Study of U(IV) and -(VI) in Perchloric and Nitric Acid Solutions

In order to elucidate the uranium solution chemistry at the high HNO(3) concentrations typically employed for the reprocessing of spent nuclear fuels, speciation and complex structures of U(IV) and U(VI) are studied in aqueous HNO(3) solutions, as well as in HClO(4) solutions, by means of UV-visible-near-infrared and X-ray absorption spectroscopies and density functional theory calculations. In 1.0 M HClO(4), U(IV) exists as a spherical cation of U(4+), which is surrounded by 9-10 water molecules in the primary coordination sphere, while it forms a colloidal hydrous oxide, U(IV)O(2) x nH(2)O, at a lower acidic concentration of 0.1 M HClO(4). U(VI) exists as a transdioxo uranyl cation, UO(2)(2+), and forms a 5-fold pure hydrate complex of [U(VI)O(2)(H(2)O)(5)](2+) in 1.0 M HClO(4). With increasing HNO(3) concentration, the water molecules of the U(IV) and U(VI) hydrate complexes are successively replaced by planar bidentate coordinating nitrate ions (NO(3)(-)), forming dominant species of [U(IV)(H(2)O)(x)(NO(3))(5)](-) in 9.0 M HNO(3) and [U(VI)O(2)(NO(3))(3)](-) in 14.5 M HNO(3), respectively. The present multitechnique approach also suggests the formation of two intermediate U(VI) species, a 5-fold mononitrato complex ([U(VI)O(2)(H(2)O)(3)(eta(2)-NO(3))](+)) and a 6-fold dinitrato complex ([U(VI)O(2)(H(2)O)(2)(eta(2)-NO(3))(2)](0)), involving an increase in the total coordination number on the uranyl(VI) equatorial plane from 5 to 6 with increasing HNO(3) concentration. The presence of unidentate coordinate nitrato complexes or tetranitrato U(VI) complexes is less probable in the present HNO(3) system.

Hydration numbers of pentavalent and hexavalent uranyl, neptunyl, and plutonyl

Hydration numbers of the pentavalent and hexavalent actinyls (U, Np, and Pu) have been studied using the ab initio Hartree‐ Fock method including the effective core potentials. The calculations were carried out inclusive of the primary and the secondary hydration spheres and the dissociation energy was used to discuss the most stable structure. The results suggest that the hydration number na 5 is the most stable for the actinyls we have studied. The result for neptunyl(V) was in conflict with the previous Hartree‐Fock calculation, which only included the primary hydration sphere. It was concluded that the secondary hydration sphere is quite important in studying the hydration numbers of the actinyls. The atomic bond lengths of hydrated uranyl(VI) and neptunyl(V) obtained by the MP2 level calculation gave good agreement with experimental results. q 2000 Elsevier Science B.V. All rights reserved.

Theoretical Gibbs free energy study on UO2(H2O)n2+ and its hydrolysis products

Abstract Hydration of uranyl ion (UO 2 2+ ) in aqueous system was investigated using hybrid density functional theory B3LYP method. Gibbs free energies and solvation energies for different UO 2 (H 2 O) n 2+ ( n =4, 5, and 6) (including the first and the second solvation shell) clusters were calculated. Polarized continuum model (PCM) was used to calculate the solvation energy. The results show that the hydration number of UO 2 2+ in liquid phase is 5. The hydrolysis reactions of UO 2 (H 2 O) 5 2+ were investigated. The temperature and pressure effects on the reaction energies were studied. It was found that temperature and pressure significantly changed the equilibrium constants of hydrolysis reactions.

Aqueous uranium(VI) complexes with acetic and succinic acid: speciation and structure revisited.

We employed density functional theory (DFT) calculations, and ultraviolet-visible (UV-vis), extended X-ray absorption fine-structure (EXAFS), and attenuated total reflection Fourier-transform infrared (IR) spectroscopy analyzed with iterative transformation factor analysis (ITFA) to determine the structures and the pH-speciation of aqueous acetate (ac) and succinate (suc) U(VI) complexes. In the acetate system, all spectroscopies confirm the thermodynamically predicted pH-speciation by Ahrland (1951), with the hydrated uranyl ion and a 1:1, a 1:2 and a 1:3 U(VI)-ac complex. In the succinate system, we identified a new 1:3 U(VI)-suc complex, in addition to the previously known 1:1 and 1:2 U(VI)-suc complexes, and determined the pH-speciation for all complexes. The IR spectra show absorption bands of the antisymmetric stretching mode of the uranyl mojety (υ3(UO2)) at 949, 939, 924 cm(-1) and at 950, 938, 925 cm(-1) for the 1:1, 1:2 and 1:3 U(VI)-ac and U(VI)-suc complexes, respectively. IR absorption bands at 1535 and 1534 cm(-1) and at 1465 and 1462 cm(-1) are assigned to the antisymmetric υ3,as(COO) and symmetric υ3,s(COO) stretching mode of bidentately coordinated carboxylic groups in the U(VI)-ac and U(VI)-suc complexes. The assignment of the three IR bands (υ3(UO2), υ3,as(COO), υ3,s(COO)) and the stoichiometry of the complexes is supported by DFT calculations. The UV-vis spectra of the equivalent U(VI)-ac and U(VI)-suc complexes are similar suggesting common structural features. Consistent with IR spectroscopy and DFT calculations, EXAFS showed a bidentate coordination of the carboxylic groups to the equatorial plane of the uranyl moiety for all uranyl ligand complexes except for the newly detected 1:3 U(VI)-suc complex, where two carboxylic groups coordinate bidentately and one carboxylic group coordinates monodentately. All 1:1 and 1:2 complexes have a U-Owater distance of ∼2.36 Å, which is shorter than the U-Owater distance of ∼2.40 Å of the hydrated uranyl ion. For all complexes the U-Ocarboxyl distance of the bidentately coordinated carboxylic group is ∼2.47 Å, while the monodentately coordinated carboxylic group of the 1:3 U(VI)-suc complex has a U-Ocarboxyl distance of ∼2.36 Å, that is, similar to the short U-Owater distance in the 1:1 and 1:2 complexes.

The Sulfate Coordination of Np(IV), Np(V), and Np(VI) in Aqueous Solution

The coordination and redox behavior of Np(IV), Np(V), and Np(VI) sulfate in aqueous solution were investigated by Np L(3)-edge extended X-ray absorption fine structure (EXAFS) spectroscopy, cyclic voltammetry, and density functional theory (DFT) calculations. The sulfate coordination mode, that is, monodentate versus bidentate, was determined by using neptunium-sulfur distances R(Np-S) and coordination numbers N(S) obtained by EXAFS spectroscopy. Np(VI) is coordinated by sulfate in the bidentate (R(Np-S) = 3.12 +/- 0.02 A) and monodentate (R(Np-S) = 3.61 +/- 0.02 A) modes at a low sulfate concentration of [SO(4)(2-)]/[NpO(2)(2+)] = 1. At higher [SO(4)(2-)]/[NpO(2)(2+)] ratios, bidentate coordination prevails. Approximately two bidentate sulfate groups are coordinated to Np(VI) with 2.0 M SO(4)(2-) and at pH 1.1. Np(V) is coordinated by sulfate in the bidentate (R(Np-S) = 3.16 +/- 0.02 A) and monodentate (R(Np-S) = 3.67 +/- 0.02 A) modes. However, sulfate coordination is less pronounced and does not exceed one SO(4)(2-) per Np(V) with 2.0 M SO(4)(2-). The redox reaction between the Np(VI)/Np(V) couple can be basically categorized as quasi-reversible. It becomes a more irreversible character at high sulfate concentrations due to structural rearrangement of the sulfate ligands. Finally, Np(IV) also shows bidentate (R(Np-S) = 3.06 +/- 0.02 A) and monodentate (R(Np-S) = 3.78 +/- 0.02 A) coordination modes. The sulfate coordination increases with an increasing [SO(4)(2-)]/[Np(4+)] ratio. A comparison of other tetravalent actinides shows that the monodentate sulfate coordination decreases whereas the bidentate coordination increases along the series Th(IV)-U(IV)-Np(IV). This trend was studied by DFT calculations and is discussed in terms of solvation energy and increasing number of unpaired electrons.

Direct Spectroscopic Characterization of Aqueous Actinyl(VI) Species: A Comparative Study of Np and U

The hydrolysis reactions of Np(VI) were investigated under an ambient atmosphere by attenuated total reflection Fourier transform infrared (ATR FT-IR) spectroscopy, NIR absorption spectroscopy, and speciation modeling applying the updated NEA thermodynamic database. For the first time, spectroscopic results of Np(VI) hydrolysis reactions are provided in the submillimolar concentration range and at pH values up to 5.3. The calculated speciation pattern and the results from FT-IR spectroscopy are comparatively discussed with results obtained from the U(VI) system under identical conditions. For both actinides, the formation of similar species can be derived from infrared spectroscopic results at pH values < or = 4, namely, the free cation AnO(2)(2+) (An = U, Np) and monomeric hydrolysis products. At higher pH, the infrared spectra evidence structurally different species contributing to the speciation of both actinides. At pH 5, the formation of a carbonate-containing dimeric complex, that is, (NpO(2))(2)CO(3)(OH)(3)(-), probably occurs during the hydrolysis reactions of neptunium, which is supported by the calculated speciation and results from NIR spectroscopy. For uranium, the presence of additional hydroxo complexes is assumed in this pH range. However, an unequivocal assignment of the spectral features to distinct species remains difficult. In particular, in the concentration range (0.5 mM) that constitutes the lower limit for the spectroscopic investigations of Np(VI) in the present work, monomeric and polymeric species obviously contribute to the U(VI) speciation considerably increasing the complexity of the spectral data.

Ab initio study on the structures of Th(IV) hydrate and its hydrolysis products in aqueous solution

Ab initio calculations have been performed to study the structures of thorium(IV) hydrate and its hydrolysis products in aqueous solution. The conductor-like polarizable continuum model (CPCM) has been used to perform geometry optimization calculations in aqueous solution. The calculated results demonstrate that the molecule geometries obtained in solvent are generally consistent with the experiments. The coordination number of thorium(IV) hydrolysis products has been investigated. The effect of the relativistic effective core potential (RECP) on the structures is also discussed.

The relationship of monodentate and bidentate coordinated uranium(VI) sulfate in aqueous solution

Abstract The coordination of U(VI) sulfate complexes has been investigated by uranium LIII-edge EXAFS and HEXS measurements with the aim to distinguish monodentate and bidentate coordinated sulfate in aqueous solution. UV-vis absorption spectroscopy has been used to differentiate the species and to determine the species distribution as a function of the [SO42−]/[UO22+] ratio. A monodentate coordination prevails in solutions with [SO42−]/[UO22+] ratio of 1, where UO2SO4 is the dominant species. Besides the dominating monodentate sulfate a small amount of bidentate sulfate could be observed, indicating that two isomers may exist for UO2SO4. With increasing [SO42−]/[UO22+] ratio the UO2(SO4)22− species becomes the main species. The uranium atom of this species is coordinated by two bidentate sulfate groups.

The Role of Water H-Bond Imbalances in B-DNA Substate Transitions and Peptide Recognition Revealed by Time-Resolved FTIR Spectroscopy

The conformational substates B(I) and B(II) of the phosphodiester backbone in B-DNA are thought to contribute to DNA flexibility and protein recognition. We have studied by rapid scan FTIR spectroscopy the isothermal B(I)-B(II) transition on its intrinsic time scale. Correlation analysis of IR absorption changes occurring within seconds after a reversible incremental growth of the DNA hydration shell identifies water populations w(1) (PO(2)(-)-bound) and w(2) (non-PO(2)(-)-bound) exhibiting weaker and stronger H-bonds, respectively, than those dominating in bulk water. The B(II) substate is stabilized by w(2). The water H-bond imbalance of 3-4 kJ mol(-1) is equalized at little enthalpic cost upon formation of a contiguous water network (at 12-14 H(2)O molecules per DNA phosphate) of reduced ν(OH) bandwidth. In this state, hydration water cooperatively stabilizes the B(I) conformer via the entropically favored replacement of w(2)-DNA interactions by additional w(2)-water contacts, rather than binding to B(I)-specific hydration sites. Such water rearrangements contribute to the recognition of DNA by indolicidin, an antimicrobial 13-mer peptide from bovine neutrophils which, despite little intrinsic structure, preferentially binds to the B(I) conformer in a water-mediated induced fit. The FTIR spectra resolve sequential steps leading from PO(2)(-)-solvation to substate transition and eventually to base stacking changes in the complex. In combination with CD-spectral titrations, the data indicate that, in the absence of a bulk aqueous phase, as in molecular crowded environments, water relocation within the DNA hydration shell allows for entropic contributions similar to those assigned to water upon DNA ligand recognition in solution.

X-ray absorption fine structures of uranyl(V) complexes in a nonaqueous solution.

The structures of three different U(V) complexes, [U(V)O(2)(salophen)DMSO](-), [U(V)O(2)(dbm)(2)DMSO](-), and [U(V)O(2)(saldien)](-), in a dimethyl sulfoxide (DMSO) solution were determined by X-ray absorption fine structure for the first time.