Origin of distinct ion-pair dissociation kinetics revealed by the solvent-coordinate free-energy landscape analysis

溶媒座標自由エネルギー地形計算による溶液中イオン対の解離カイネティクス解析

米谷 佳晃

Yonetani, Yoshiteru

Different ion pairs exhibit different dissociation kinetics. Though such kinetic event is essential for understanding various molecular events, the underlying mechanism remains unclear. In this study, to examine the mechanism of how different kinetics appears, we carried out molecular dynamics simulations for LiCl, NaCl, KCl and CsCl in water. The results showed that the dissociation rate constant is significantly different. The difference in the rate constant mainly arises from the different solvent-state distribution of each ion pair. Analysis of the free-energy landscape with a solvent reaction coordinate and the related rate component analysis clearly showed this tendency. The key is the occurrence of a specific water configuration, where the water takes a bridge configuration by bonding with both ions at the same time. Such water-bridge formation lowers the related dissociation free-energy barrier, thereby making the ion-pair dissociation more likely. Consequently, higher probability of water-bridge formation leads to a higher ion-pair dissociation rate.