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 Ca(2+) and Cl(-). The XAS spectra (extended X-ray absorption fine structure, EXAFS, and X-ray absorption near-edge structure, XANES) measured from aqueous CaCl(2) solution were analyzed and compared to those generated from snapshots of QM/MM MD simulations of Ca(2+) 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 Ca(2+) 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 Ca(2+) 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.

Bicarbonate and chloride anion transport in anion exchange membranes

“…These results suggest that the dominant factors that dictate the water absorbance are hydration number, dissociation factor and the coulombic interaction between the counter ion and the cationic group. Indeed, the approximate coordination number of Cl À in water was found in experimental reports to vary from 4 to 9 and in theoretical studies, from 5.1 to 8.4 [23,24]. However, HCO 3 À has a hydration shell of 7-8 water molecules, higher than Cl À form, with 3-4 hydrogen bonds formed between the oxygen atoms of the bicarbonate ion and the hydrogen atoms from the solvating water molecules [25].…”

The Aqueous Ca2+ System, in Comparison with Zn2+, Fe3 +, and Al3 +: An Ab Initio Molecular Dynamics Study

“…This MD‐XAFS approach has previously been demonstrated by ourselves25 to provide a reliable benchmarking of AIMD data against highly accurate XAFS experiments. We also note that previous MD‐EXAFS studies on the aqueous Ca 2+ system have been limited to CMD26 and QM/MM27 approaches.…”

Communication: Electronic structure of the solvated chloride anion from first principles molecular dynamics

“…37 position of the first maximum (r max ) and minimum (r min ) of the Cl−O and Cl−D pair correlation functions and the corresponding coordination numbers are reported in Table I, together with experimental values. The latter ones differ depending on the technique used (i.e., neutron diffraction or X-ray measurements) and the type of counterions present in the solution, 5,6,[36][37][38][39][40][41] whose effect however appears to be minor. The dependence of experimental data on concentration is instead more pronounced (at least for the Cl−D coordination).…”