DFT and EXAFS, DFT and EXAFS, mutually enhancing tools for structure elucidation of Tc complexes

The redox-sensitive fission product technetium-99 (Tc) is of great interest in nuclear waste disposal studies because of its potential for contaminating the geosphere due to its very long half-life (2.13105 year) and high mobility under oxidising conditions, where technetium forms pertechnetate (TcO4-). Under suitable reducing conditions, e.g. in the presence of an iron(II) containing solid phase which can act as an electrondonor, the solubility can be limited by the reduction of pertechnetate followed by the formation of a surface precipitate [1]. However, by association with mobile humic substances (HS) or other associating/complexing species, the solubility of reduced Tc species may be drastically enhanced [2]. The elucidation of the identity and geometrical structure of the species causing this enhanced solubility often remains a difficult issue. EXAFS/XANES analysis can be very helpful to determine the molecular surroundings of reduced Tc organic complexes of Tc(IV) colloid-HS-associations. The interpretation of the EXAFS spectra is however not a straightforward process due to the possibly strong influence of multiple scattering paths on the spectra.. Contrary to single scattering analysis which can be easily performed on the experimental EXAFS spectra, multiple scattering analysis can only be done, based on good approximations of the possible geometrical structures of the species under consideration. It has been shown that DFT can be used to perform geometry optimizations of technetium species and therefore can be utilized to predict the structure of technetium complexes [3]. The success and computational effort needed for such structure predictions are however highly dependent on the quality of the initial structure from which the geometry optimization is started. Single scattering analysis of experimental Tc EXAFS/XANES spectra of the Tc-pyrogallol complexes under consideration have yielded relatively good estimates of the coordination number, surrounding atom types and bond lengths of the chemical environment of the Tc atom in the species under investigation. This information was used to construct a first guess for an initial geometry from which a DFT geometry optimization was started. The DFT-optimized geometry was in its turn used to create high quality theoretical standards with ab initio multiple scattering software like FEFF [4] which were used to perform high quality fits of the experimental EXAFS/XANES spectra being studied.status: publishe