title: Solvent electronic polarization effects on Na(+)-Na(+) and Cl(-)-Cl(-) pair associations in aqueous solution.
abstract: 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.
authors.affiliation: Department of Chemistry and Green-Nano Materials Research Center, College of Natural Sciences, Kyungpook National University, Taegu 702-701, Korea. cchoi@knu.ac.kr
authors.forename: Cheol Ho
authors.initials: CH
authors.lastname: Choi
authors.forename: Suyong
authors.initials: S
authors.lastname: Re
authors.forename: Mohammad H O
authors.initials: MH
authors.lastname: Rashid
authors.forename: Hui
authors.initials: H
authors.lastname: Li
authors.forename: Michael
authors.initials: M
authors.lastname: Feig
authors.forename: Yuji
authors.initials: Y
authors.lastname: Sugita
mesh_terms: D002712:Chlorides / Q000737:chemistry*; D007477:Ions / Q000737:chemistry; D056004:Molecular Dynamics Simulation; D011789:Quantum Theory; D012964:Sodium / Q000737:chemistry*; D012997:Solvents / Q000737:chemistry*; D013816:Thermodynamics; D014867:Water / Q000737:chemistry
chemical_list: D002712:Chlorides; D007477:Ions; D012997:Solvents; D014867:Water; D012964:Sodium