Tan Ting, Yang Xueliang, Krauter Caroline M, Ju Yiguang, Carter Emily A
†Department of Chemistry, ‡Department of Mechanical and Aerospace Engineering, §Program in Applied and Computational Mathematics, and ∥Andlinger Center for Energy and the Environment, Princeton University, Princeton, New Jersey 08544, United States.
J Phys Chem A. 2015 Jun 18;119(24):6377-90. doi: 10.1021/acs.jpca.5b03506. Epub 2015 May 28.
The kinetics of hydrogen abstraction by five radicals (H, O((3)P), OH, CH3, and HO2) from methyl acetate (MA) is investigated theoretically in order to gain further understanding of certain aspects of the combustion chemistry of biodiesels, such as the effect of the ester moiety. We employ ab initio quantum chemistry methods, coupled cluster singles and doubles with perturbative triples correction (CCSD(T)) and multireference averaged coupled pair functional theory (MRACPF2), to predict chemically accurate reaction energetics. Overall, MRACPF2 predicts slightly higher barrier heights than CCSD(T) for MA + H/CH3/O/OH, but slightly lower barrier heights for hydrogen abstraction by HO2. Based on the obtained reaction energies, we also report high-pressure-limit rate constants using transition state theory (TST) in conjunction with the separable-hindered-rotor approximation, the variable reaction coordinate TST, and the multi-structure all-structure approach. The fitted modified Arrhenius expressions are provided over a temperature range of 250 to 2000 K. The predictions are in good agreement with available experimental results. Abstractions from both of the methyl groups in MA are expected to contribute to consumption of the fuel as they exhibit similar rate coefficients. The reactions involving the OH radical are predicted to have the highest rates among the five abstracting radicals, while those initiated by HO2 are expected to be the lowest.
为了进一步了解生物柴油燃烧化学的某些方面，如酯基部分的影响，理论上研究了五种自由基（H、O((3)P)、OH、CH3和HO2）从乙酸甲酯（MA）中夺取氢的动力学。我们采用从头算量子化学方法，即耦合簇单双激发并包含微扰三重激发校正（CCSD(T)）和多参考平均耦合对函数理论（MRACPF2），来预测化学上精确的反应能量。总体而言，对于MA + H/CH3/O/OH反应，MRACPF2预测的势垒高度比CCSD(T)略高，但对于HO2夺取氢的反应，其预测的势垒高度略低。基于获得的反应能量，我们还使用过渡态理论（TST）结合可分离受阻转子近似、可变反应坐标TST和多结构全结构方法报告了高压极限速率常数。在250至2000 K的温度范围内给出了拟合的改进阿伦尼乌斯表达式。这些预测与现有的实验结果吻合良好。预计从MA的两个甲基夺取氢都会对燃料消耗有贡献，因为它们表现出相似的速率系数。预计在这五种夺取自由基中，涉及OH自由基的反应速率最高，而由HO2引发的反应速率预计最低。
