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

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 Ca2+ 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 Cl- ion 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.