Structures and Stoichiometry of Actinide Complexes: Challenges of Combining EXAFS and Quantum Chemistry


Structures and Stoichiometry of Actinide Complexes: Challenges of Combining EXAFS and Quantum Chemistry

Tsushima, S.

EXAFS spectroscopy has been proved to be a very powerful tool for studying the structures of actinide complexes in aqueous and non-aqueous systems. The application of EXAFS spectroscopy has been extended to the systems with mixed species and / or species with very dilute concentration. Using EXAFS alone, however, has inherent limitations such as a lack of spatial information of the molecules and rather poor radial resolutions which make it difficult to differentiate two or more bonds with similar distances. Factor analysis by Rossberg et al., for example, was proven to be an effective method for separating individual species from the mixed ones [1]. Such method becomes even more powerful if corresponding 3D structural information can be provided by quantum chemical calculations.
Here, I show two examples in which density functional theory (DFT) calculations are proved to be very helpful for actinide speciation studies. In first case, Th(IV) hydroxo and sulfato complexes were studied by DFT calculations. A significant decrease in the coordination number (CN) was observed as a stepwise hydrolysis reaction of Th4+ ion proceeds. The fourth hydrolysis product, Th(OH)4 0, has a reduced CN of 6 and readily forms a dimeric complex via a Th-OH-Th bridging. The oligomerization makes Th(OH)4 0 to be an unstable aqueous species. In case of sulfate, the CN and the average Th-O distance of Th(SO4)2 0 remains essentially as same as Th4+ aquo ion. This result is in agreement with the EXAFS study by Hennig et al. [2].
In the second example, the reduction potential of Pu(VIII) / Pu(VII) couple was studied as the same way as in a recent study [3]. The geometries and the energies of the complexes were calculated at the B3LYP level. The spin-orbit effect for Pu(VII) species was corrected at the CASSCF level. The redox potential of the PuVIIIO4(OH)2 2-/ PuVIIO4(OH)2 3- couple was found to be as high as ~1.7V in alkaline solution which indicates that it is difficult to obtain Pu(VIII) in aqueous solution.
REFERENCES
1. A. Roßberg, T. Reich, G. Bernhard, Anal. Bioanal. Chem. 376, 631-638, (2003).
2. C. Hennig, K. Schmeide, V. Brendler, H. Moll, S. Tsushima, A. C. Scheinost, Inorg. Chem. 46, 5882-5892 (2007).
3. S. Tsushima, U. Wahlgren, I. Grenthe, J. Phys. Chem. A 110, 9175-9182 (2006).

  • Invited lecture (Conferences)
    Actinide XAS 2008, 15.-17.07.2008, Soleil, France

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