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Solvent Electronic Polarization Effects on Na+–Na+ and Cl––Cl– Pair Associations in Aqueous Solution
journal contribution
posted on 2013-08-08, 00:00 authored by Cheol Ho Choi, Suyong Re, Mohammad H. O. Rashid, Hui Li, Michael Feig, Yuji SugitaThe
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.