A coupled molecular dynamics and XANES data analysis investigation of aqueous cadmium(II)

The flexible nature of the first hydration shell of the cadmium(II) ion has been definitively assessed through an extensive study, combining X-ray absorption near-edge structure (XANES) spectroscopy and molecular dynamics (MD) simulations. The structural and dynamic properties of the cadmium(II) hydration shell have been determined from long-time MD simulations, and the influence of water-water interactions has been evaluated using the SPC/E and TIP5P water models. Comparison of the theoretical results with EXAFS data suggests that the TIP5P simulation provides a better description of the cadmium(II) hydration properties. XANES spectra have been computed starting from MD trajectories, without carrying out any minimization in the structural parameter space. The octahedral solvation of cadmium(II) in aqueous solution cannot be reconciled with the XANES results, while a flexible hydration shell is fully consistent with the experimental data, which unambiguously show the presence of a dominant percentage of heptahydrated species.