Solvent electronic polarization effects on Na(+)-Na(+) and Cl(-)-Cl(-) pair associations in aqueous solution

The formation of like-ion pairs, Na(+)-Na(+) and Cl(-)-Cl(-), in aqueous solution was studied by high-level ab initio methods, classical molecular dynamics (MD), QM/TIP5P, and QM/EFP MD (quantum mechanics/effective fragment potential molecular dynamics). Ab initio calculations on model clusters revealed that the Na(+)(H2O)nNa(+) (n = 2-4) clusters were significantly more stabilized by bridged waters than the corresponding Cl(-)(H2O)nCl(-) clusters. QM/EFP MD simulations in solution also predicted a clear local minimum near 3.6 Å only for the Na(+)-Na(+) pair, suggesting that Na(+)-Na(+) pairs may be more likely to form than Cl(-)-Cl(-) pairs in solution. Analysis of the hydration structures further showed that two-water bridged Na(+)-Na(+) pairs were dominant at the local minimum. The preferred formation of Na(+) like-ion pairs in solution appeared to come from significant short-range effects, in particular, charge delocalization (polarization) between the bridged oxygen p and the vacant valence Na(+) orbitals.