Structural characterization of thermodynamically predicted U(VI)-tartrate complexes: Combing Factor Analysis with Landweber inversion method

An intrinsic problem of EXAFS shell fitting is that the radial pair distribution function (PDF) is approximated by Gaussians functions imitating the coordination shells. Different combinations of shells can yield different structures with similar fit quality, thereby making the structural solution non-unique. Even the so-called F-test often does not yield a unique solution. The shell fit analysis of complicated spectra, as in the case of thermodynamically predicted aqueous complexes of U(VI) with tartaric acid, is much more eased if the PDF for the first shells are computed precisely and independently. Solely based on the FEFF scattering theory, the Landweber inverse method [1] yields the PDF for the aqueous mono-, bi- and tri-nuclear U(VI)-tartaric acid complexes without predefined assumptions about the form of the PDF. With this information and density functional theory (DFT) calculations, the spatial structures of the complexes were determined. Depending on the pH the U(VI)-tartaric acid complexes coexist, hence the EXAFS signal is a mixture of the spectral contributions of the complexes. In order to accomplish this special structural analysis we measured an EXAFS pH-series and isolated the spectra of the single complexes from their mixtures by using factor analysis [2].

[1] A. Rossberg, et al., Journal of Synchrotron Radiation, 17, 280-288 (2010).
[2] A. Rossberg, et al., Analytical and Bioanalytical Chemistry, 376, 631-638 (2003).