OH + CH4 -> H2O + CH3反应动力学研究中的再交叉和隧道效应
Recrossing and tunnelling in the kinetics study of the OH + CH4 -> H2O + CH3 reaction
摘要：
基于精细全维解析势能面PES-2014，在200-2000 K温度范围内，采用两种动力学方法，环状聚合物分子动力学（RPMD）和多维隧道变分过渡态理论（VTST/MT），对OH + CH 4夺氢反应的热速率常数和几种动力学同位素效应进行了评估.对于OH + CH 4反应，在低温下，T = 200 K，其中量子隧道效应更重要，RPMD高估了实验速率常数，这是由于PES-2014在深隧道机制中的问题以及已知的该方法在不对称反应中的高估，而VTST/MT呈现出更好的一致性，差异约为10%，由于多种因素的补偿，PES-2014的不准确性和忽略非谐性。在相反的极端，T = 1000 K，再交叉效应起主要作用，两种方法之间的差异现在较小，为1.5倍。考虑到RPMD结果在高温极限下是准确的，差异是由于VTST/MT方法中使用的方法，例如忽略沿反应路径的最低振动频率的非谐性，这导致反应物和产物之间的分界面位置不正确。通过对几种动力学同位素效应（OH + CD_4、OD + CH_4和OH +12 CH_4/13 CH_4）的分析，揭示了这些问题，并证实了以前的结论。在一般情况下，与现有的实验数据的协议是合理的，虽然差异持续存在，他们已被分析为影响理论计算的许多因素的功能：动力学方法和势能面的局限性，以及在实验测量的不确定性。
英文摘要：
Thermal rate constants and several kinetic isotope effects were evaluated for the OH + CH4 hydrogen abstraction reaction using two kinetics approaches, ring polymer molecular dynamics (RPMD), and variational transition state theory with multidimensional tunnelling(VTST/MT), based on a refined full-dimensional analytical potential energy surface, PES-2014, in the temperature range 200-2000 K. For the OH + CH4 reaction, at low temperatures, T = 200 K, where the quantum tunnelling effect is more important, RPMD overestimates the experimental rate constants due to problems associated with PES-2014 in the deep tunnelling regime and to the known overestimation of this method in asymmetric reactions, while VTST/MT presents a better agreement, differences about 10%, due to compensation of several factors, inaccuracy of PES-2014 and ignoring anharmonicity. In the opposite extreme, T = 1000 K, recrossing effects play the main role, and the difference between both methods is now smaller, by a factor of 1.5. Given that RPMD results are exact in the high-temperature limit, the discrepancy is due to the approaches used in the VTST/MT method, such as ignoring the anharmonicity of the lowest vibrational frequencies along the reaction path which leads to an incorrect location of the dividing surface between reactants and products. The analysis of several kinetic isotope effects, OH + CD4, OD + CH4, and OH + 12CH4/13CH4, sheds light on these problems and confirms the previous conclusions. In general, the agreement with the available experimental data is reasonable, although discrepancies persist, and they have been analysed as a function of the many factors affecting the theoretical calculations: limitations of the kinetics methods and of the potential energy surface, and uncertainties in the experimental measurements.
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