Liu Xu, Xie Jing, Zhang Jiaxu, Yang Li, Hase William L
MIIT Key Laboratory of Critical Materials Technology for New Energy Conversion and Storage, School of Chemistry and Chemical Engineering, Harbin Institute of Technology , Harbin 150001, People's Republic of China.
Department of Chemistry, University of Minnesota , Minneapolis, Minnesota 55455, United States.
J Phys Chem Lett. 2017 Apr 20;8(8):1885-1892. doi: 10.1021/acs.jpclett.7b00577. Epub 2017 Apr 13.
Influences of solvent molecules on S2 reaction dynamics of microsolvated F(HO) with CHI, for n = 0-3, are uncovered by direct chemical dynamics simulations. The direct substitution mechanism, which is important without microsolvation, is quenched dramatically upon increasing hydration. The water molecules tend to force reactive encounters to proceed through the prereaction collision complex leading to indirect reaction. In contrast to F(HO), reaction with higher hydrated ions shows a strong propensity for ion desolvation in the entrance channel, diminishing steric hindrance for nucleophilic attack. Thus, nucleophilic substitution avoids the potential energy barrier with all of the solvent molecules intact and instead occurs through the less solvated barrier, which is energetically unexpected because the former barrier has a lower energy. The work presented here reveals a trade-off between reaction energetics and steric effects, with the latter found to be crucial in understanding how hydration influences microsolvated S2 dynamics.
通过直接化学动力学模拟，揭示了溶剂分子对n = 0 - 3时微溶剂化F(HO)与CHI的S2反应动力学的影响。在没有微溶剂化时很重要的直接取代机制，在水合作用增加时会急剧淬灭。水分子倾向于迫使反应性碰撞通过预反应碰撞复合物进行，从而导致间接反应。与F(HO)相反，与水合程度更高的离子反应在入口通道中表现出强烈的离子去溶剂化倾向，减少了亲核攻击的空间位阻。因此，亲核取代避免了所有溶剂分子保持完整时的势能垒，而是通过溶剂化程度较低的势垒发生，这在能量上是出乎意料的，因为前一个势垒能量更低。本文展示的工作揭示了反应能量学和空间效应之间的权衡，发现后者对于理解水合作用如何影响微溶剂化S2动力学至关重要。
