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.