Local Aqueous Solvation Structure Around Ca2+ During Ca2+•••Cl- Pair Formation

The molecular details of single ion solvation around Ca2+ and ion-pairing of Ca2+center dot center dot center dot Cl- are investigated using ab initio molecular dynamics. The use of empirical dispersion corrections to the BLYP functional are investigated by comparison to experimentally available extended X-ray absorption fine structure (EXAFS) measurements that probe the first solvation shell in great detail. Besides finding differences in the free-energy for both ion-pairing and the coordination number of ion solvation between the quantum and classical descriptions of interaction, there were important differences found between dispersion corrected and uncorrected density functional theory (DFT). Specifically, we show significantly different free-energy landscapes for both coordination number of Ca2+ and its ion-pairing with Cl- depending on the DFT simulation protocol. Our findings produce a self-consistent treatment of short-range solvent response to the ion and the intermediate to long-range collective response of the electrostatics of the ion-ion interaction to produce a detailed picture of ion-pairing that is consistent with experiment.