Hydration of the Calcium Ion. An EXAFS, Large-Angle X-ray Scattering, and Molecular Dynamics Simulation Study

Abstract

The structure of the hydrated calcium(II) ion in aqueous solution has been studied by means of extended X-ray absorption fine structure spectroscopy (EXAFS), large-angle X-ray scattering (LAXS), and molecular dynamics (MD) methods. The EXAFS data displayed a broad and asymmetric distribution of the Ca-O bond distances with the centroid at 2.46(2) A. LAXS studies on four aqueous calcium halide solutions (1.5-2 mol dm(-)(3)) gave a mean Ca-O bond distance of 2.46(1) A. This is consistent with a hydration number of 8 determined from correlations between mean distances and coordination numbers from crystal structures. The LAXS studies showed a second coordination sphere with a mean Ca.O(II) distance of 4.58(5) A, and for the hydrated halide ions the distances Cl.O 3.25(1) A, Br.O 3.36(1) A, and I.O 3.61(1) A were obtained. Molecular dynamics simulations of CaCl(2)(aq) were performed using three different Ca(2+)-OH(2) pair potentials. The potential from the GROMOS program gave results in agreement with experiments, i.e., a coordination number of 8 and an average Ca-O distance of 2.46 A, and was used for further comparisons. Theoretical EXAFS oscillations were computed for individual MD snapshots and showed very large variations, though the simulated average spectrum from 2000 snapshots gave satisfactory agreement with the experimental EXAFS spectra. The effect of thermal motions of the coordinated atoms is inherent in the MD simulation method. Thermal disorder parameters evaluated from simulated spatial atom distribution functions of the oxygen atoms coordinated to the calcium ion were in close agreement with those from the current LAXS and EXAFS analyses. The combined results are consistent with a root-mean-square displacement from the mean Ca-O distance of 0.09(2) A in aqueous solution at 300 K.

Model for kinetic effects on calcium isotope fractionation (δ44Ca) in inorganic aragonite and cultured planktonic foraminifera

“…An atomic mass around 520 amu (O. universa) is compatible with the maximum value for a hydration number of ϳ25 which has recently been reported by Spångberg et al (2000), Jalilehvand et al (2001), and Koneshan et al (1998). Values of more than 600 amu (inorganic aragonite) exceed the empirically determined and accepted hydration number for a Ca 2ϩ -aquocomplex.…”

“…Molecules of the second shell are bound by hydrogen bondings to the water molecules of the first hydration shell. The number of waters that may occupy the second hydration sphere varies depending on the model selected from 11 to 18 (Jalilehvand et al, 2001). The concept of strongly bound first shell and weakly bound second shell molecules is also supported by the findings that the residence time of a water molecule in the first shell (Ca[H 2 O] 8 2ϩ ) of a Ca 2ϩ -aquocomplex is ϳ40 times longer than in the second shell.…”

“…(Jalilehvand et al, 2001;Koneshan et al, 1998), although in earlier publications between 6 and 10 water molecules are reported (Kaufman Katz et al, 1996;Langmuir, 1997;Spångberg et al, 2000). Modeling and experimental results indicate a strongly bound, clearly defined inner hydration shell and a more weakly bound second shell (Jalilehvand et al, 2001;Koneshan et al, 1998). Molecules of the second shell are bound by hydrogen bondings to the water molecules of the first hydration shell.…”

The solvation of bromide anion in acetonitrile: a structural study based on the combination of theoretical calculations and X-ray absorption spectroscopy

“…The interplay of XAS spectroscopy with quantum mechanics and numerical simulations has shown to be as an appropriate strategy to get insight into structural information on ionic aqueous solutions, [18][19][20][21][22][23][24][25][26][27] and will be explored for the case of a non-aqueous solution.…”

Analysis and simulation of the structure of nanoparticles that undergo a surface-driven structural transformation

“…In an alternative approach, MD simulations have been used to combine a statistically significant number of individual thermal configurations of periodic bulk materials [45] or hydrated ions [46]. However, this is impractical for a 3 nm nanoparticle containing > 700 atoms.…”