Departamento de Química Física, Facultad de Ciencias Químicas, Universidad de Salamanca, 37008 Salamanca, Spain.
J Chem Phys. 2012 Sep 7;137(9):094303. doi: 10.1063/1.4747548.
In this work we present a dynamically biased statistical model to describe the evolution of the title reaction from statistical to a more direct mechanism, using quasi-classical trajectories (QCT). The method is based on the one previously proposed by Park and Light [J. Chem. Phys. 126, 044305 (2007)]. A recent global potential energy surface is used here to calculate the capture probabilities, instead of the long-range ion-induced dipole interactions. The dynamical constraints are introduced by considering a scrambling matrix which depends on energy and determine the probability of the identity/hop/exchange mechanisms. These probabilities are calculated using QCT. It is found that the high zero-point energy of the fragments is transferred to the rest of the degrees of freedom, what shortens the lifetime of H(5)(+) complexes and, as a consequence, the exchange mechanism is produced with lower proportion. The zero-point energy (ZPE) is not properly described in quasi-classical trajectory calculations and an approximation is done in which the initial ZPE of the reactants is reduced in QCT calculations to obtain a new ZPE-biased scrambling matrix. This reduction of the ZPE is explained by the need of correcting the pure classical level number of the H(5)(+) complex, as done in classical simulations of unimolecular processes and to get equivalent quantum and classical rate constants using Rice-Ramsperger-Kassel-Marcus theory. This matrix allows to obtain a ratio of hop/exchange mechanisms, α(T), in rather good agreement with recent experimental results by Crabtree et al. [J. Chem. Phys. 134, 194311 (2011)] at room temperature. At lower temperatures, however, the present simulations predict too high ratios because the biased scrambling matrix is not statistical enough. This demonstrates the importance of applying quantum methods to simulate this reaction at the low temperatures of astrophysical interest.
在这项工作中，我们提出了一个动态偏置统计模型，用于描述标题反应从统计机制到更直接机制的演化，使用准经典轨迹（QCT）。该方法基于 Park 和 Light 之前提出的方法[J. Chem. Phys. 126, 044305 (2007)]。这里使用最近的全局势能面来计算捕获概率，而不是长程离子诱导偶极相互作用。通过考虑依赖于能量的 scrambling 矩阵来引入动力学约束，该矩阵确定了身份/跳跃/交换机制的概率。这些概率使用 QCT 计算。结果表明，碎片的高零点能转移到其余自由度，从而缩短了 H(5)(+)复合物的寿命，因此交换机制的产生比例较低。零点能（ZPE）在准经典轨迹计算中没有得到很好的描述，因此进行了近似，即在 QCT 计算中降低反应物的初始 ZPE，以获得新的 ZPE 偏置 scrambling 矩阵。这种 ZPE 的降低是通过需要校正 H(5)(+)复合物的纯经典能级数来解释的，正如在单分子过程的经典模拟中所做的那样，以及使用 Rice-Ramsperger-Kassel-Marcus 理论获得等效的量子和经典速率常数。该矩阵允许获得跳跃/交换机制的比率α(T)，与 Crabtree 等人最近的实验结果[J. Chem. Phys. 134, 194311 (2011)]在室温下相当吻合。然而，在较低的温度下，目前的模拟预测的比率过高，因为有偏 scrambling 矩阵不够统计。这证明了在天体物理感兴趣的低温下应用量子方法模拟该反应的重要性。
