Influence of the Second Coordination Shell on the XANES Spectra of the Zn2+ Ion in Water and Methanol

A quantitative analysis of the X-ray absorption near-edge structure (XANES) spectra of the Zn2+ ion in methanol and aqueous solution has been carried out starting from the microscopic description of the systems derived from molecular dynamics (MD) simulations. The different sensitivity of XANES towards the second-shell solvent molecules in the two solvents has been highlighted: in methanol solution, the Zn2+ second solvation shell was found to provide a negligible contribution to the XANES spectrum, and it has been possible to reproduce the experimental data using a single coordination sphere in the calculation. Conversely, a reliable reproduction of the experimental spectrum of Zn2+ in aqueous solution could be obtained only by including the second-shell water molecules in the calculation. The combination of MD and XANES was found to be essential to evaluate the contribution of the ion second solvation shell to the XANES spectra of a metal ion in organic solvents.