Presentation Critical solvent structure for the rate constant of ion-pair dissociation

米谷, 佳晃

Different ion pairs exhibit different dissociation kinetics; however, the underlying mechanism remains unclear. In this study, to examine the origin of different kinetic rate constants for this process, molecular dynamics simulations were conducted for LiCl, NaCl, KCl, and CsCl in water. The results showed substantial differences in dissociation rate constant, following the trend kLiCl < kNaCl < kKCl < kCsCl. Analysis of the free-energy landscape with a solvent reaction coordinate showed that the differences arose predominantly from the variation in solvent-state distribution between the ion pairs. The formation of a water-bridging configuration was identified as a key step in this process: water-bridge formation lowers the related dissociation free-energy barrier, thereby increasing the probability of ion-pair dissociation. Consequently, a higher probability of water-bridge formation leads to a higher ion-pair dissociation rate.

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