| 1. |
- Moll, H., et al.
(författare)
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The hydrolysis of dioxouranium(VI) investigated using EXAFS and O-17-NMR
- 2000
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Ingår i: Radiochimica Acta. - : Walter de Gruyter GmbH. - 0033-8230 .- 2193-3405. ; 88:7, s. 411-415
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Tidskriftsartikel (refereegranskat)abstract
- The structure of dioxouranium(VI) as a function of pH at different (CH3)(4)N-OH concentrations has been investigated with the aid of U L-III-edge EXAFS. Polynuclear hydroxo species were identified by an U-U interaction at 3.80(8) Angstrom at pH = 4.1. The precipitate formed at pH = 7 has a schoepite like structure. In solution at high pH [0.5 M (CH3)(4)N-OH], the EXAFS data are consistent with the formation of a monomeric four coordinated uranium(VI) hydroxide complex UO2(OH)(4)(2-) of octahedral geometry. The first shell contains two O atoms with a U=O distance of 1.83(o) Angstrom, and four O atoms were identified at a U-O distance of 2.26(5) Angstrom. In strong alkaline solutions [>1 M (CH3)(4)N)-OH],O-17-NMR spectra indicate the presence of two species, presumably UO2(OH)(4)(2-) and UO2(OH)(5)(3-), the latter in low concentration, which are in rapid equilibrium with one another at 268 K in aqueous solution.
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| 2. |
- Szabo, Zoltan, et al.
(författare)
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Structure and dynamics in the complex ion (UO2)(2)(CO3)(OH)(3)(-)
- 2000
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Ingår i: Journal of the Chemical Society. Dalton Transactions. - : Royal Society of Chemistry (RSC). - 1472-7773 .- 1364-5447 .- 1470-479X. ; :18, s. 3158-3161
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Tidskriftsartikel (refereegranskat)abstract
- The structure and ligand exchange dynamics of the ternary complex (UO2)(2)(CO3)(OH)(3)(-) have been investigated by EXAFS and NMR spectroscopy. Very broad signals can be observed in both the C-13 and the O-17 NMR spectra. The EXAFS data show the presence of 1.3 +/- 0.3 short uranium-oxygen distances at 2.26 Angstrom, consistent with single bonded hydroxide and 3.9 +/- 0.6 distances at 2.47 Angstrom for the other ligands in the first co-ordination shell. There is also evidence for a U ... U interaction at 3.90 Angstrom. Based on the EXAFS and NMR data we suggest the presence of three isomers with different bridge arrangements, the dominant one, C, contains 80% of the uranium and the minor ones A and B, 5 and 15%, respectively. The ligand exchange reactions between these isomers are slow. The NMR data indicate that the main reactions involve intramolecular exchanges between isomers with different positions of the non-bridging ligands in A, B and C. We suggest that these take place through water exchange as discussed earlier for other ternary uranium(VI) complexes.
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| 3. |
- Vallet, V., et al.
(författare)
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Solvent effects on uranium(VI) fluoride and hydroxide complexes studied by EXAFS and quantum chemistry
- 2001
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Ingår i: Inorganic Chemistry. - : American Chemical Society (ACS). - 0020-1669 .- 1520-510X. ; 40:14, s. 3516-3525
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Tidskriftsartikel (refereegranskat)abstract
- The structures of the complexes UO2Fn(H2O)(5-n)(2-n), n = 3-5, have been studied by EXAFS. All have pentagonal bipyramid geometry with U-F of and U-H2O distances equal to 2.26 and 2.48 Angstrom, respectively. On the other hand the complex UO2(OH)(4)(2-) has a square bipyramid geometry both in the solid state and in solution. The structures of hydroxide and fluoride complexes have also been investigated with wave function based and DFT methods in order to explore the possible reasons for the observed structural differences. These studies include models that describe the solvent by using a discrete second coordination sphere, a model with a spherical, or shape-adapted cavity in a conductor-like polarizable continuum medium (CPCM), or a combination of the two. Solvent effects were shown to give the main contribution to the observed structure variations between the uranium(VI) tetrahydroxide and the tetrafluoride complexes. Without a solvent model both UO2(OH)(4)(H2O)(2-) and UO2F4(H2O)(2-) have the same square bipyramid geometry, with the water molecule located at a distance of more than 4 Angstrom from uranium and with a charge distribution that is very near identical in the two complexes. Of the models tested, only the CPCM ones are able to describe the experimentally observed square and pentagonal bipyramid geometry in the tetrahydroxide and tetrafluoride complexes. The geometry and the relative energy of different isomers of UO2F3(H2O)(2)(-) are very similar, indicating that they are present in comparable amounts in solution. All calculated bond distances are in good agreement with the experimental observations, provided that a proper model of the solvent is used.
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| 4. |
- Vallet, V., et al.
(författare)
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Structure and bonding in solution of dioxouranium(VI) oxalate complexes : Isomers and intramolecular ligand exchange
- 2003
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Ingår i: Inorganic Chemistry. - : American Chemical Society (ACS). - 0020-1669 .- 1520-510X. ; 42:6, s. 1982-1993
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Tidskriftsartikel (refereegranskat)abstract
- Structural isomers of [UO2(oxalate)(3)](4-), [UO2(oxalate)F-3](3-), [UO2(oxalate)(2)F](3-), and [UO2(oxalate)(2)(H2O)](2-) have been studied by using EXAFS and quantum chemical ab initio methods. Theoretical structures and their relative energies were determined in the gas phase and in water using the CPCM model. The most stable isomers according to the quantum chemical calculations have geometries consistent with the EXAFS data, and the difference between measured and calculated bond distances is generally less than 0.05 Angstrom. The complex [UO2(oxalate)(3)](4-) contains two oxalate ligands forming five-membered chelate rings, while the third is bonded end-on to a single carboxylate oxygen. The most stable isomer of the other two complexes also contains the same type of chelate-bonded oxalate ligands. The activation energy for ring opening in [UO2(oxalate)F-3](3-), DeltaU(double dagger) = 63 kJ/mol, is in fair agreement with the experimental activation enthalpy, DeltaH(double dagger) = 45 +/- 5 kJ/mol, for different [UO2(PiCOlinate)F-3](2-) complexes, indicating similar ring-opening mechanisms. No direct experimental information is available on intramolecular exchange in [UO3(oxalate)(3)](4-). The theoretical results indicate that it takes place via the tris-chelated intermediate with an activation energy of AV = 38 kJ/mol; the other pathways involve multiple steps and have much higher activation energies. The geometries and energies of dioxouranium(VI) complexes in the gas phase and solvent models differ slightly, with differences in bond distance and energy of typically less than 0.06 Angstrom and 10 kJ/mol, respectively. However, there might be a significant difference in the distance between uranium and the leaving/entering group in the transition state, resulting in a systematic error when the gas-phase geometry is used to estimate the activation energy in solution. This systematic error is about 10 kJ/mol and tends to cancel when comparing different pathways.
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