Structure of the hydrated Ca2+ and Cl−: Combined X-ray absorption measurements and QM/MM MD simulations study

Abstract

A combination of X-ray absorption spectroscopy (XAS) measurements and quantum mechanical/molecular mechanical (QM/MM) molecular dynamics (MD) simulations has been applied to elucidate detailed information on the hydration structures of Ca²⁺ and Cl⁻. The XAS spectra (extended X-ray absorption fine structure, EXAFS, and X-ray absorption near-edge structure, XANES) measured from aqueous CaCl₂ solution were analyzed and compared to those generated from snapshots of QM/MM MD simulations of Ca²⁺ and Cl⁻ in water. With regard to this scheme, the simulated QM/MM–EXAFS and QM/MM–XANES spectra, which correspond to the local structure and geometrical arrangement of the hydrated Ca²⁺ and Cl⁻ at molecular level show good agreement with the experimentally observed EXAFS and XANES spectra. From the analyses of the simulated QM/MM–EXAFS spectra, the hydration numbers for Ca²⁺ and Cl⁻ were found to be 7.1 ± 0.7 and 5.1 ± 1.3, respectively, compared to the corresponding values of 6.9 ± 0.7 and 6.0 ± 1.7 derived from the measured EXAFS data. In particular for XANES results, it is found that ensemble averages derived from the QM/MM MD simulations can provide reliable QM/MM–XANES spectra, which are strongly related to the shape of the experimental XANES spectra. Since there is no direct way to convert the measured XANES spectrum into details relating to geometrical arrangement of the hydrated ions, it is demonstrated that such a combined technique of XAS experiments and QM/MM MD simulations is well-suited for the structural verification of aqueous ionic solutions.