Structural investigation of lanthanoid coordination: A combined XANES and molecular dynamics study

This is the first systematic study exploring the potentiality of the X-ray absorption near edge structure (XANES) technique as a structural tool for systems containing lanthanoid(III) Ions. A quantitative analysis of the XANES spectra at the K- and L3-edges has been carried out for three hydrated lanthanoid(III) ions, namely, Yb, Nd, and Gd, in aqueous solution and in the isostructural trifluoromethanesulfonate salts. The structural and dynamic properties of the hydrated lanthanoid(III) ions in aqueous solution have been investigated by a combined experimental-theoretical approach employing X-ray absorption spectroscopy and molecular dynamics (MD) simulations. This method allows one to perform a quantitative analysis of the XANES spectra of ionic solutions using a proper description of the thermal and structural fluctuations. XANES spectra have been computed starting from the MD trajectory, without carrying out any minimization in the structural parameter space. A comparative K- and L3-edge XANES data analysis is presented, demonstrating the clear advantages of the L3-edge XANES analysis over the K-edge studies for structural investigations of lanthanold compounds. The second hydration shells provide a detectable contribution to the L3-edge spectra while the K-edge data are insensitive to the more distant coordination spheres because of the strong damping and broadening of the signal caused by the extremely large core hole widths. The XANES technique has been found to be a new valuable tool for the structural characterization of metal complexes both in the solid and in the liquid state, especially in the presence of low symmetry. © 2009 American Chemical Society.