Solvation of Calcium Ion in Polar Solvents:  An X-ray Diffraction and ab Initio Study

Abstract

Ab initio calculations and X-ray diffraction experiments were carried out to study the structure of solutions of calcium chloride in water and methanol. Ab initio calculations were performed at MP2 level and density functional calculations at B3LYP level on calcium-water and calcium-methanol clusters yielding the formation of stable calcium-water clusters with up to eight water molecules and calcium-methanol clusters with up to seven methanol molecules. The experiments were performed in a wide concentration range both in water and in methanol (1-6 M and 1-2 M, respectively). The coordination number of the cation in low-concentration (1 M) aqueous and methanol solutions could only be determined with great uncertainty due to the low weights of cation-solvent contributions to the X-ray scattering intensity for both series of solutions. It was found that in 1 M solutions the Ca 2+ ion is surrounded by eight (five to ten) water and six (four to seven) methanol molecules, respectively. The coordination numbers decrease with an increase in concentration. The accuracy of the coordination numbers determined increases with increasing concentration. The solvation shell of Clion is composed of six solvent molecules in each solution. We have found evidence of both contact and solvent-separated Ca-Cl ion pair formation at higher concentrations. On the basis of the stoichiometry of the solution and structural parameters obtained, different models are suggested to explain the liquid structure of the solutions.

MD Simulation of the Ion Solvation in Methanol-Water Mixtures

“…A comparison of the radial distribution functions for the cations in aqueous and methanolic solutions shows that the average distance to the methanol's oxygen is almost the same as to the water's oxygen. Such feature is in good agreement with the experimental results (Megyes et al, 2004;Neilsen & Enderby, 1979). The first peaks of all ion-oxygen functions in methanolic solutions are higher that those in aqueous solutions.…”

“…The first shell of Ca 2+ is larger; it contains eight or more water molecules and does not show any regularity (Owczarek et al, 2007). X-ray diffraction studies (Tamura et al, 1992, Megyes et al, 2004 have suggested that all these cations are sixcoordinated in methanolic solutions thus their shells are octahedral.…”

“…Therefore both a preferential hydration (Convington & Dunn, 1989;Hawlicka, 1995), as well as a lack of preferences (Holtz et al, 1977) have been postulated for alkali and halide ions in methanol-water mixtures. Though X-ray and neutron scattering measurements should provide a direct insight into the ion coordination shell, their results cannot be decisive for methanol-water mixture, because distances between these ions and the oxygens of either water (Neilson & Enderby, 1979;Licheri et al, 1975) or methanol (Megyes, 2004) are almost the same. Moreover a direct correlation between the alkali earth cations and the methyl group is lacking (Radnai et al, 1995).…”

Multiple active zones in hybrid QM/MM molecular dynamics simulations for large biomolecular systems

“…The CN values and distances from calcium ion to water oxygen atom are in agreement with the experimental data that show a high CN (6 -10) with reported distances of 2.39 -2.46 Å. 56 In addition to examining the coordination of copper ions in the central QM1 active zone we have further investigated the reduced electron density gradient (Fig. 8) using the NCIPlot program.…”

Hydration of Metal and (CH3)4N+ Ions in Solutions of Mixed Salts

“…Its position coincides with the distance 0.41 nm extracted from X-ray scattering [26]. The first and second maxima of g MgO are well separated, thus the number of water molecules in the second shell can be determined unambiguously.…”