MD study of SN1 reactivity of 2-chloro-2-methylpropane in the room-temperature ionic liquid 1-ethyl-3-methylimidazolium hexafluorophosphate.
MD study of SN1 reactivity of 2-chloro-2-methylpropane in the room-temperature ionic liquid 1-ethyl-3-methylimidazolium hexafluorophosphate.
复制标题
2-氯-2-甲基丙烷在室温离子液体1-乙基-3-甲基咪唑六氟磷酸盐中SN1反应性的MD研究。
作者：
Youngseon Shim;H.
通过分子动力学计算机模拟研究了离子液体1-乙基-3-甲基咪唑鎓六氟磷酸盐([emim](+)P(-)(6))中2-氯-2-甲基丙烷的S(N)1电离反应RX→R(+) + X(-)。采用二态价键描述反应配合物的电子结构变化，通过热力学积分方法计算了[emim](+)P(-)(6))中与SN1电离相关的自由能曲线，并与水和乙腈中的自由能曲线进行了比较。发现三种溶剂的详细反应机理有所不同。具体来说，2-氯-2-甲基丙烷在[emim](+)P(-)(6))中的解离是一个逐步过程，包括溶剂分离的离子对的形成和随后的解离，而在乙腈中的解离似乎在没有任何稳定的反应中间体的情况下进行。另一方面，水中的 S(N)1 途径的特点是形成接触离子对，然后解离为自由离子。水中的活化自由能远低于[emim](+)P(-)(6))和乙腈中的活化自由能。在后两种溶剂之间，[emim](+)P(-)(6)) 中的势垒高度低于乙腈中的势垒高度，表明 2-氯-2-甲基丙烷的 S(N)1 反应性在 [emin](+)P(-)(6)) 中高于乙腈中。简要讨论了它对这些溶剂中溶剂分解的影响。
The S(N)1 ionization reaction RX --> R(+) + X(-) for 2-chloro-2-methylpropane in ionic liquid 1-ethyl-3-methylimidazolium hexafluorophosphate ([emim](+)P(-)(6)) is studied via molecular dynamics computer simulations. By employing a two-state valence-bond description for electronic structure variations of the reaction complex, the free energy curve relevant to its SN1 ionization in [emim](+)P(-)(6)) is computed via the thermodynamic integration method and compared with those in water and in acetonitrile. It is found that the detailed reaction mechanism differs among the three solvents. To be specific, the dissociation of 2-chloro-2-methylpropane in [emim](+)P(-)(6)) is a stepwise process consisting of the formation of a solvent-separated ion pair and ensuing dissociation, while that in acetonitrile appears to proceed without any stable reaction intermediates. The S(N)1 pathway in water on the other hand is characterized by the formation of a contact ion pair, followed by dissociation to free ions. The activation free energy in water is much lower than those in [emim](+)P(-)(6)) and acetonitrile. Between the two latter solvents, the barrier height is lower in [emim](+)P(-)(6)) than in acetonitrile, indicating that the S(N)1 reactivity of 2-chloro-2-methylpropane would be higher in [emin](+)P(-)(6)) than in acetonitrile. Its implication for solvolysis in these solvents is briefly discussed.