A combined EXAFS spectroscopic and quantum chemical study on the complex formation of Americium(III) with formate

The pH-dependent (pH 2 - 4) formation of aqueous Am(III) complexes with formate (Form) is studied by EXAFS, iterative target transformation factor analysis (ITFA) and by quantum chemical calculations [1]. According to thermodynamical calculations three complexes (aq. Am3+, AmForm2+, AmForm2+) coexist and change their fractions, so that EXAFS spectral mixtures occur. A maximal spectral change of only 9% is observed between the different Am LIII-edge EXAFS spectra, which rules out the direct use of conventional shell fit analysis for structural investigation prior to the decomposition of the spectra into the single spectral components by ITFA. By combining pH-speciation calculations with quantum chemistry, the pH-dependent number of coordinated water and formate molecules is calculated and used as constraint for a modified ITFA-approach. The decomposition results in the separate spectral contributions of the exchanged molecules, hence the signal of a coordinated water and a monodentate coordinated formate molecule for which the structural parameters are determined by shell-fitting and are compared with twelve complex structures gained by quantum chemical calculations. Methodologically consistent, the prevailing coordination numbers are 9, 9 and 8 for the aq. Am3+, AmForm2+ and AmForm2+ complexes, respectively [2]. Low concentrations of species with other coordination numbers and modes cannot be excluded, as energy differences obtained by our quantum chemical calculations are small [2].

This work shows the power of the proposed ITFA-framework in obtaining structural information for weak ligand systems like formate, where conventional EXAFS data analysis fails due to the lower spectroscopic resolution in analysing mixtures of metal species.

References
[1] Rossberg et al., Anal. Bioanal. Chem. 376, 631-638 (2003).
[2] Fröhlich et al., Inorg. Chem. submitted (2017).