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⁺(H ₂O)_nNa⁺ (n = 2-4) clusters were significantly more stabilized by bridged waters than the corresponding Cl^-(H ₂O)_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.