Collision Frequency for Energy Transfer in Unimolecular Reactions.
Collision Frequency for Energy Transfer in Unimolecular Reactions.
复制标题
单分子反应中能量转移的碰撞频率。
DOI：
10.1021/acs.jpca.8b00444
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发表时间：
2018
期刊：
影响因子：
--
通讯作者：
A. Matsugi
中科院分区：
文献类型：
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作者：
A. Matsugi
单分子反应速率的压力依赖性由反应物与浴气体分子碰撞中的能量转移决定。压力依赖的速率常数可以通过求解单分子反应的主方程从理论上确定。通常，主方程公式使用碰撞频率和每次碰撞传递的能量的概率分布模型来描述能量传递过程。本研究提出了一种新的方法来确定碰撞频率从经典的轨迹计算的结果。用三阶密度泛函紧束缚方法结合简单的两两相互作用势计算了几种多原子分子（乙烷、甲烷、四氟甲烷和环己烷）与单原子碰撞机（Ar、Kr和H2O）碰撞的经典轨道.从弹道中提取了失活碰撞能量传递的低阶（包括非整数阶）矩，并与一些概率分布模型的结果进行了比较。比较表明，传统的Lennard-Jones碰撞模型表示的碰撞频率的不足，并建议一个强大的方法来评估碰撞频率，这是符合一个给定的概率分布模型，如指数下降模型。指数下降模型的碰撞频率大大高于Lennard-Jones碰撞频率，并接近（假设的）色散相互作用的捕获速率常数。还简要讨论了指数下降模型的实用性。
Pressure dependence of unimolecular reaction rates is governed by the energy transfer in collisions of reactants with bath gas molecules. Pressure-dependent rate constants can be theoretically determined by solving master equations for unimolecular reactions. In general, master equation formulations describe energy transfer processes using a collision frequency and a probability distribution model of the energy transferred per collision. The present study proposes a novel method for determining the collision frequency from the results of classical trajectory calculations. Classical trajectories for collisions of several polyatomic molecules (ethane, methane, tetrafluoromethane, and cyclohexane) with monatomic colliders (Ar, Kr, and Xe) were calculated on potential energy surfaces described by the third-order density-functional tight-binding method in combination with simple pairwise interaction potentials. Low-order (including non-integer-order) moments of the energy transferred in deactivating collisions were extracted from the trajectories and compared with those derived using some probability distribution models. The comparison demonstrates the inadequacy of the conventional Lennard-Jones collision model for representing the collision frequency and suggests a robust method for evaluating the collision frequency that is consistent with a given probability distribution model, such as the exponential-down model. The resulting collision frequencies for the exponential-down model are substantially higher than the Lennard-Jones collision frequencies and are close to the (hypothetical) capture rate constants for dispersion interactions. The practical adequacy of the exponential-down model is also briefly discussed.