X-ray absorption spectroscopy and molecular dynamics studies of Zn2+ hydration in aqueous solutions

The hydration of Zn2+ ions in aqueous solutions was studied at room temperature by x-ray absorption spectroscopy and molecular dynamics (MD) simulation. The extended x-ray absorption fine structure (EXAFS) above the Zn K-edge was interpreted using the multiple-scattering approach by taking into account only one coordination Shell composed of 6 +/- 0.2 water molecules at R(Zn-O) = 2.06 +/- 0.02 Angstrom with a mean square relative displacement (MSRD) sigma(2) = 0.009 +/- 0.002 Angstrom(2). NO evidence-of significant contributions from the second hydration shell to the EXAFS signal was found in the solutions. This is explained by the cancellation interference effect between double-scattering and single-scattering EXAFS signals in the second shell due to large thermal/static disorder (sigma(2) similar to 0.1 Angstrom(2)) as predicted by our MD simulation and by known results of diffraction techniques.