Molecular dynamics simulation, <i>ab initio</i> calculation, and size-selected anion photoelectron spectroscopy study of initial hydration processes of calcium chloride

Abstract

To understand the initial hydration processes of CaCl₂, we performed molecular simulations employing the force field based on the theory of electronic continuum correction with rescaling. Integrated tempering sampling molecular dynamics were combined with ab initio calculations to overcome the sampling challenge in cluster structure search and refinement. The calculated vertical detachment energies of CaCl₂(H₂O)_n^- (n = 0-8) were compared with the values obtained from photoelectron spectra, and consistency was found between the experiment and computation. Separation of the Cl-Ca ion pair is investigated in CaCl₂(H₂O)_n^- anions, where the first Ca-Cl ionic bond required 4 water molecules, and both Ca-Cl bonds are broken when the number of water molecules is larger than 7. For neutral CaCl₂(H₂O)_n clusters, breaking of the first Ca-Cl bond starts at n = 5, and 8 water molecules are not enough to separate the two ion pairs. Comparing with the observations on magnesium chloride, it shows that separating one ion pair in CaCl₂(H₂O)_n requires fewer water molecules than those for MgCl₂(H₂O)_n. Coincidentally, the solubility of calcium chloride is higher than that of magnesium chloride in bulk solutions.