Structure of the hydrated Ca(2+) and Cl(-): Combined X-ray absorption measurements and QM/MM MD simulations study.
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.
Tongraar
Anan
A
School of Chemistry, Institute of Science, Suranaree University of Technology, Nakhon Ratchasima 30000, Thailand.
T-Thienprasert
Jiraroj
J
Rujirawat
Saroj
S
Limpijumnong
Sukit
S
D002118:Calcium / Q000737:chemistry*; D002712:Chlorides / Q000737:chemistry*; D056004:Molecular Dynamics Simulation; D011789:Quantum Theory; D014867:Water / Q000737:chemistry*; D056928:X-Ray Absorption Spectroscopy
D002712:Chlorides; D014867:Water; D002118:Calcium