Zhang Hui, Zhang Xin, Truhlar Donald G, Xu Xuefei
State Key Laboratory of Chemical Resource Engineering, Institute of Materia Medica, College of Science, Beijing University of Chemical Technology , Beijing 100029, P. R. China.
Center for Combustion Energy and Department of Thermal Engineering, Tsinghua University , Beijing 100084, P. R. China.
J Phys Chem A. 2017 Nov 30;121(47):9033-9044. doi: 10.1021/acs.jpca.7b09374. Epub 2017 Nov 16.
The reaction between H and benzene is a prototype for reactions of radicals with aromatic hydrocarbons. Here we report calculations of the reaction rate constants and the branching ratios of the two channels of the reaction (H addition and H abstraction) over a wide temperature and pressure range. Our calculations, obtained with an accurate potential energy surface, are based on variational transition-state theory for the high-pressure limit of the addition reaction and for the abstraction reaction and on system-specific quantum Rice-Ramsperger-Kassel theory calibrated by variational transition-state theory for pressure effects on the addition reaction. The latter is a very convenient way to include variational effects, corner-cutting tunneling, and anharmonicity in falloff calculations. Our results are in very good agreement with the limited experimental data and show the importance of including pressure effects in the temperature interval where the mechanism changes from addition to abstraction. We found a negative temperature effect of the total reaction rate constants at 1 atm pressure in the temperature region where experimental data are missing and accurate theoretical data were previously missing as well. We also calculated the H + CH/CD and D + CH/CD kinetic isotope effects, and we compared our H + CH results to previous theoretical data for H + toluene. We report a very novel nonmonotonic dependence of the kinetic isotope effect on temperature. A particularly striking effect is the prediction of a negative temperature dependence of the total rate constant over 300-500 K wide temperature ranges, depending on the pressure but generally in the range from 600 to 1700 K, which includes the temperature range of ignition in gasoline engines, which is important because aromatics are important components of common fuels.
氢与苯之间的反应是自由基与芳烃反应的一个典型例子。在此，我们报告了在很宽的温度和压力范围内该反应的两个通道（氢加成和氢提取）的反应速率常数及分支比的计算结果。我们基于精确的势能面进行计算，对于加成反应和提取反应的高压极限，采用变分过渡态理论；对于加成反应中压力效应的计算，采用由变分过渡态理论校准的特定系统量子 Rice-Ramsperger-Kassel 理论。后者是在衰减计算中纳入变分效应、斜切隧道效应和谐波性的一种非常便捷的方法。我们的结果与有限的实验数据非常吻合，并且表明在反应机理从加成转变为提取的温度区间内纳入压力效应的重要性。我们发现在 1 个大气压下，在尚无实验数据且之前也没有精确理论数据的温度区域，总反应速率常数呈现负温度效应。我们还计算了 H + CH/CD 和 D + CH/CD 的动力学同位素效应，并将我们关于 H + CH 的结果与之前关于 H + 甲苯的理论数据进行了比较。我们报告了动力学同位素效应随温度呈现出一种非常新颖的非单调依赖性。一个特别显著的效应是预测在 300 - 500 K 的宽温度范围内，总速率常数呈现负温度依赖性，这取决于压力，但一般在 600 至 1700 K 的范围内，该温度范围涵盖了汽油发动机的着火温度范围，这一点很重要，因为芳烃是常见燃料的重要成分。
