Detection of second hydration shells in ionic solutions by XANES: Computed spectra for Ni2+ in water based on molecular dynamics

A general procedure to compute X-ray absorption near-edge structure (XANES) spectra within multiple-scattering theory starting from molecular dynamics (MD) structural data has been developed and applied to the study of a Ni2+ aqueous solution. This method allows one to perform a quantitative analysis of the XANES spectra of disordered systems using a proper description of the thermal and structural fluctuations. The XANES spectrum of Ni2+ in aqueous solution has been calculated using the structural information obtained from the MD simulations without carrying out any minimization in the structural parameter space. A very good reproduction of the experimental data was obtained including the second-shell water molecules in the calculation, thus showing that the second hydration shell provides a detectable contribution to the XANES spectra of ionic solutions. The analysis including the first-shell cluster only permitted us to quantitatively determine the effect of disorder on the amplitude of the XANES spectra for molecular complexes. These results simultaneously confirm the reliability of the procedure and the structural results obtained from MD simulations. The combination of MD and XANES is found to be very helpful to get important new insights into the quantitative estimation of structural properties of disordered systems.