Solution and solid-state structural chemistry of actinide hydrates and their hydrolysis and condensation products.

The title compounds, [AnO(2)(H(2)O)(5)](n)(+), n = 1 or 2 and An = U, Np, and Pu, are studied using relativistic density functional theory (DFT). Three rather different relativistic methods are used, small-core effective core potentials (SC-ECP), a scalar four-component all-electron relativistic method, and the zeroeth-order regular approximation. The methods provide similar results for a variety of properties, giving confidence in their accuracy. Spin-orbit and multiplet corrections to the An(VI)/An(V) reduction potential are added in an approximate fashion but are found to be essential. Bulk solvation effects are modeled with continuum solvation models (CPCM, COSMO). These models are tested by comparing explicit (cluster), continuum, and mixed cluster/continuum solvation models as applied to various properties. The continuum solvation models are shown to accurately account for the effects of the solvent, provided that at least the first coordination sphere is included. Reoptimizing the structures in the presence of the bulk solvent is seen to be important for the equatorial bond lengths but less relevant for energetics. Explicit inclusion of waters in the second coordination sphere has a modest influence on the energetics. For the first time, free energies of solvation are calculated for all six [AnO(2)(H(2)O)(5)](n)(+) species. The calculated numbers are within the experimental error margins, and the experimental trend is reproduced correctly. By comparison of different relativistic methods, it is shown that an accurate relativistic description leads to marked improvements over the older large-core ECP (LC-ECP) method for bond lengths, vibrational frequencies, and, in particular, the An(VI)/An(V) reduction potential. Two approximate DFT methods are compared, B3LYP, a hybrid DFT method, and PBE, a generalized gradient approximation. Either method yields An(VI)/An(V) reduction potentials of comparable quality. Overall, the experimental reduction potentials are accurately reproduced by the calculations.

https://pubs.acs.org/doi/pdf/10.1021/jp0662855

Density functional studies of actinyl aquo complexes studied using small-core effective core potentials and a scalar four-component relativistic method.

Previous spectroscopic research suggested that uranium(VI) adsorption to iron oxides is dominated by ternary uranyl−carbonato surface complexes across an unexpectedly wide pH range. Formation of such complexes would have a significant impact on the sorption behavior and mobility of uranium in aqueous environments. We therefore reinvestigated the identity and structural coordination of uranyl sorption complexes using a combination of U LIII-edge extended X-ray absorption fine structure (EXAFS) spectroscopy and iterative transformation factor analysis, which enhances the resolution in comparison to conventional EXAFS analysis. A range of conditions (pH, CO2 partial pressure, ionic strength) made it possible to quantify the variations in surface speciation. In the resulting set of spectral data (N = 11) the variance is explained by only two components, which represent two structurally different types of surface complexes: (1) a binary uranyl surface complex with a bidentate coordination to edges of Fe(O,OH)6...

Identification of uranyl surface complexes on ferrihydrite: Advanced EXAFS data analysis and CD-MUSIC modeling.

Molecular Simulation of the Diffusion of Uranyl Carbonate Species in Aqueous Solution

Solvent- and catalyst-free synthesis of an azine-linked covalent organic framework and the induced tautomerization in the adsorption of U(VI) and Hg(II)

Structural properties of uranyl cations in acidic aqueous perchlorate and triflate solutions were investigated using uranium L 11 -edge extended X-ray absorption fine-structure spectroscopy (EXAFS) in conjunction with quantum mechancial calculations of gas-phase model complexes. EXAFS spectra were measured in aqueous solutions of up to 10 M triflic and 11.5 M perchloric acid, as well as mixtures of perchloric acid and sodium perchlorate. In no case is the perchlorate anion coordinated to UO 2 2+ . The number of equatorial water molecules bound to UO 2 2+ is always about five In the case of the 10 M CF 3 SO 3 H solution, an inner-sphere complexation of the triflate is observed with a U-S radial distance of 3.62 A These results are in qualitative agreement with quantum mechanical calculations of model uranyl complexes, according to which the interaction energies of anions follow the order perchlorate < triflate << nitrate.

André Roßberg

Papers

Do perchlorate and triflate anions bind to the uranyl cation in an acidic aqueous medium? A combined EXAFS and quantum mechanical investigation.