Solvent Electronic Polarization Effects on Na<sup>+</sup>–Na<sup>+</sup> and Cl<sup>–</sup>–Cl<sup>–</sup> Pair Associations in Aqueous Solution

The formation of like-ion pairs, Na<sup>+</sup>–Na<sup>+</sup> and Cl<sup>–</sup>–Cl<sup>–</sup>, 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<sup>+</sup>(H<sub>2</sub>O)<sub><i>n</i></sub>Na<sup>+</sup> (<i>n</i> = 2–4) clusters were significantly more stabilized by bridged waters than the corresponding Cl<sup>–</sup>(H<sub>2</sub>O)<sub><i>n</i></sub>Cl<sup>–</sup> clusters. QM/EFP MD simulations in solution also predicted a clear local minimum near 3.6 Å only for the Na<sup>+</sup>–Na<sup>+</sup> pair, suggesting that Na<sup>+</sup>–Na<sup>+</sup> pairs may be more likely to form than Cl<sup>–</sup>–Cl<sup>–</sup> pairs in solution. Analysis of the hydration structures further showed that two-water bridged Na<sup>+</sup>–Na<sup>+</sup> pairs were dominant at the local minimum. The preferred formation of Na<sup>+</sup> 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<sup>+</sup> orbitals.