Ab initio calculations and reduced density gradient analyses of the structure and energetics of hydrated calcium fluoride and calcium carbonate

Abstract

We studied microhydrated calcium fluoride, calcium carbonate and their ions at the MP2/6-311++G** level of theory due to the lack of basic thermodynamic information about solvation of these salts and of systematic analyses of hydrogen bonding in their solvated species. Low-lying configurations were obtained through a molecular dynamics search involving stepwise hydration of the species of interest. The molecular dynamics employed a semiempirical Hamiltonian. The resulting configurations were then geometry-optimized at the MP2/6-311++G** level of theory and characterized as energy minima through vibrational analysis. We report a first estimate of the enthalpies of hydration at infinite dilution for calcium fluoride, calcium carbonate and their constituent anions. We also find that the dissociation processes of both hydrated calcium fluoride and calcium carbonate are endothermic processes. We analyze the interrelation of hydrogen bonding and van der Waals interactions in defining the structure of the first solvation shells of calcium fluoride, fluoride ion, calcium carbonate and carbonate ion by invoking geometric criteria, by calculations yielding critical points obtained from quantum theory of atoms in molecules (QTAIM) and by examination of reduced density gradient (RDG) surfaces. RDG surfaces reveal that water–water non-covalent interactions tend to destabilize the solvation shell, and are compensated for by cooperative hydrogen bonds.