Speaker
Dr
Adela Muñoz Páez
(University of Seville)
Description
X-ray absorption spectroscopies (XAS) provide accurate information of local order around atoms in solid or liquid states. In the EXAFS region of a XAS spectrum, the high kinetic energy of the electron renders the analysis rather simple, since single scattering contributions are dominant. The adjustable parameters are coordination distances, R, coordination numbers, N, and Debye-Waller (DW) factors, which take into account static and dynamic disorder. The structure around the absorbing atom within a radius of 4 A˚, can be determined with uncertainties of 0.01 °A in coordination distances, while that for coordination numbers is much higher (10–15%), because they are highly correlated with DW factors. To overcome this correlation, DW factors can be eliminated by summing over a set of Molecular dynamics MD structural arrangements (snapshots) large enough to be representative of the whole system. The information provided by the analysis of the XANES region is complementary to that supplied by EXAFS because, due to the low kinetic energy of the electron, it is sensible to coordination polyhedron, although this renders data analysis more complex.1
The first example selected to show the capabilities of the technique are the highly stable and symmetric aqua ions with octahedral configuration [Cr(H2O)6]3+, [Rh(H2O)6]3+ and [Ir(H2O)6]3+, for which we determined the first and second hydration sphere by analyzing the EXAFS region taking into account single and multiple scattering contributions.2-4 The second example, the analysis of the asymmetric and lest stable aqua ions [Y(H2O)8]3+ and [Cf(H2O)6]3+, required the quantitative analysis of the XANES region and the use of MD simulations. It was found that the first hydration shell of Y(III) and Cf(III) cations was formed by eight water molecules. The value for Cf(III)–O coordination distance, 2.43 A˚ together with those of other three valent lighter actinides, pointed to the existence of an actinide contraction similar to that found for the lanthanide cations.5 Additional examples will be those of Cu(II) complexes containing N-coordinating ligands of relevance to understand the interactions of this cation with biomolecules.
