Distinct dissociation kinetics among ion pairs; Solvent-coordinate free-energy landscape analysis

溶液中会合イオンの解離カイネティクスの多様性; 溶媒座標自由エネルギー地形解析

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 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, different probability of water-bridge formation leads to the different ion-pair dissociation kinetics.