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Ion Pairing Kinetics Does not Necessarily Follow the Eigen-Tamm Mechanism
Qiang Zhang1,2, Bing-bing Zhang2,3, Ling Jiang*2, Wei Zhuang*2
1.Department of Chemistry, Bohai University, Jinzhou 121000, China;2.State Key Laboratory of Molecular Reaction Dynamics, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China;3.College of Chemistry and Molecular Engineering, Zhengzhou University, Zhengzhou 450001, China
Abstract:
The most recognized and employed model of the solvation equilibration in the ionic solutions was proposed by Eigen and Tamm, in which there are four major states for an ion pair in the solution: the completely solvated state, 2SIP (double solvent separate ion pair), SIP (single solvent separate ion pair), and CIP (contact ion pair). Eigen and Tamm suggested that the transition from SIP to CIP is always the slowest step during the whole pairing process, due to a high free energy barrier between these two states. We carried out a series of potential of mean force calculations to study the pairing free energy profiles of two sets of model mono-atomic 1:1 ion pairs 2.0:x and x:2.0. For 2.0:x pairs the free energy barrier between the SIP and CIP states is largely reduced due to the salvation shell water structure. For these pairs the SIP to CIP transition is thus not the slowest step in the ion pair formation course. This is a deviation from the Eigen-Tamm model
Key words:  Ion pairing, Eigen-Tamm model, Potential of mean force
FundProject:
离子配对动力学不一定遵循Eigen-Tamm机理
张强1,2, 张冰冰2,3, 江凌*2, 庄巍*2
1.渤海大学化学化工与食品安全学院,锦州121000;2.中国科学院大连化学物理研究所分子反应动力学国家重点实验室,大连116023;3.郑州大学化学与分子工程学院,郑州450001
摘要:
Eigen-Tamm模型认为水溶液中的离子对从单水分子桥链离子对到紧密离子对是离子对溶剂化平衡的动力学控制性步骤,两态之间自由能垒最高,相互转化速率最慢. 设计了两类带有单位电荷的离子对模型(2.0:x和x:2.0,固定离子对中的阳离子或阴离子的半径为2 ?,另一个为不同半径的阴阳离子),通过计算离子对不同距离的平均力势来获取自由能曲线,发现2.0:x系列离子对由于溶剂水的作用,从单水分子桥链离子对到紧密离子对转化的自由能垒有明显减低,并不是离子成对动力学平衡中的最慢步骤,与Eigen-Tamm 模型描述存在偏差.
关键词:  离子对,Eigen-Tamm模型,平均力势
DOI:10.1063/1674-0068/26/06/694-700
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