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.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry
- Surfaces, Coatings and Films
- Materials Chemistry