TRAJECTORY STUDIES OF SN2 NUCLEOPHILIC-SUBSTITUTION .2. NONSTATISTICAL CENTRAL BARRIER RECROSSING IN THE CL(-)+CH3CL SYSTEM
TRAJECTORY STUDIES OF SN2 NUCLEOPHILIC-SUBSTITUTION .2. NONSTATISTICAL CENTRAL BARRIER RECROSSING IN THE CL(-)+CH3CL SYSTEM
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DOI：
10.1063/1.462331
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发表时间：
1992-06-01
影响因子：
4.4
通讯作者：
HASE, WL
中科院分区：
文献类型：
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作者：
CHO, YJ;VANDELINDE, SR;HASE, WL
对于 Cl- + CH3Cl S(N)2 亲核取代反应，过渡态理论预测穿过势能表面的中心势垒区域是速率控制步骤。 在这项工作中，经典轨迹在中央障碍处初始化。 势能面考虑了四种不同的模型。 在轨迹中观察到大量的中心势垒重新穿过，这表明过渡态理论是计算 Cl- + CH3Cl S(N)2 速率常数的不完整模型。 在轨迹中观察到两种类型的重交叉：中间重交叉，其中轨迹在中心势垒附近徘徊；复杂重交叉，其中捕获在 Cl-...CH3Cl 复合物中的轨迹返回到中心势垒区域。 如果在轨迹初始条件下，将零点能量添加到与反应坐标正交的振动模式，则中间重交叉非常重要。 Rice-Ramsperger-Kassel-Marcus (RRKM) 理论预测 Cl- ...CH3Cl 复合物广泛解离为 Cl- + CH3Cl，并且在轨迹计算中复合物重新交叉可忽略不计。 与此预测相反，观察到可忽略不计的 Cl- + CH3Cl 形成，并且连续发生复合物重新交叉，其时间尺度比复合物的 RRKM 寿命长。 这些结果表明遍历假设对于 Cl- ...CH3Cl 络合物无效。 考虑了引起中间和复杂重新交叉的相空间瓶颈。
For the Cl- + CH3Cl S(N)2 nucleophilic substitution reaction transition-state theory predicts that crossing the central barrier region of the potential-energy surface is the rate-controlling step. In this work classical trajectories are initialized at the central barrier. Four different models are considered for the potential-energy surface. A significant amount of central barrier recrossing is observed in the trajectories, which suggests that transition-state theory is an incomplete model for calculating the Cl- + CH3Cl S(N)2 rate constant. Two types of recrossings are observed in the trajectories: intermediate recrossings in which trajectories linger near the central barrier and complex recrossings in which trajectories trapped in the Cl- ...CH3Cl complex return to the central barrier region. Intermediate recrossings are important if, in the trajectory initial conditions, zero-point energy is added to the vibrational modes orthogonal to the reaction coordinate. Rice-Ramsperger-Kassel-Marcus (RRKM) theory predicts extensive dissociation of the Cl- ...CH3Cl complex to Cl- + CH3Cl and negligible complex recrossings in the trajectory calculations. In contrast to this prediction, negligible Cl- + CH3Cl formation is observed and continual complex recrossings occur, on a time scale longer than the complex's RRKM lifetime. These results indicate the ergodic assumption is invalid for the Cl- ...CH3Cl complex. Phase-space bottlenecks which give rise to the intermediate and complex recrossings are considered.