Tan Ting, Yang Xueliang, 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 B. 2016 Mar 3;120(8):1590-600. doi: 10.1021/acs.jpcb.5b07959. Epub 2015 Oct 14.
The dissociation and isomerization kinetics of the methyl ester combustion intermediates methoxycarbonyl radical (CH3OĊ(═O)) and (formyloxy)methyl radical (ĊH2OC(═O)H) are investigated theoretically using high-level ab initio methods and Rice-Ramsperger-Kassel-Marcus (RRKM)/master equation (ME) theory. Geometries obtained at the hybrid density functional theory (DFT) and coupled cluster singles and doubles with perturbative triples correction (CCSD(T)) levels of theory are found to be similar. We employ high-level ab initio wave function methods to refine the potential energy surface: CCSD(T), multireference singles and doubles configuration interaction (MRSDCI) with the Davidson-Silver (DS) correction, and multireference averaged coupled-pair functional (MRACPF2) theory. MRSDCI+DS and MRACPF2 capture the multiconfigurational character of transition states (TSs) and predict lower barrier heights than CCSD(T). The temperature- and pressure-dependent rate coefficients are computed using RRKM/ME theory in the temperature range 300-2500 K and a pressure range of 0.01 atm to the high-pressure limit, which are then fitted to modified Arrhenius expressions. Dissociation of CH3OĊ(═O) to ĊH3 and CO2 is predicted to be much faster than dissociating to CH3Ȯ and CO, consistent with its greater exothermicity. Isomerization between CH3OĊ(═O) and ĊH2OC(═O)H is predicted to be the slowest among the studied reactions and rarely happens even at high temperature and high pressure, suggesting the decomposition pathways of the two radicals are not strongly coupled. The predicted rate coefficients and branching fractions at finite pressures differ significantly from the corresponding high-pressure-limit results, especially at relatively high temperatures. Finally, because it is one of the most important CH3Ȯ removal mechanisms under atmospheric conditions, the reaction kinetics of CH3Ȯ + CO was also studied along the PES of CH3OĊ(═O); the resulting kinetics predictions are in remarkable agreement with experiments.
采用高水平从头算方法和 Rice-Ramsperger-Kassel-Marcus（RRKM）/主方程（ME）理论，对甲酯燃烧中间体甲氧基羰基自由基（CH3OĊ(═O)）和（甲酰氧基）甲基自由基（ĊH2OC(═O)H）的离解和异构化动力学进行了理论研究。发现在杂化密度泛函理论（DFT）和耦合簇单双激发并包含微扰三重激发校正（CCSD(T)）理论水平下获得的几何结构相似。我们采用高水平从头算波函数方法来优化势能面：CCSD(T)、带有戴维森 - 西尔弗（DS）校正的多参考单双激发组态相互作用（MRSDCI）以及多参考平均耦合对函数（MRACPF2）理论。MRSDCI + DS 和 MRACPF2 捕捉到了过渡态（TSs）的多组态特征，并且预测的势垒高度比 CCSD(T) 更低。使用 RRKM/ME 理论在 300 - 2500 K 的温度范围和 0.01 atm 至高压极限的压力范围内计算了温度和压力依赖的速率系数，然后将其拟合到修正的阿伦尼乌斯表达式中。预测 CH3OĊ(═O) 分解为 ĊH3 和 CO2 的速度比分解为 CH3Ȯ 和 CO 快得多，这与其更大的放热性一致。预测 CH3OĊ(═O) 和 ĊH2OC(═O)H 之间的异构化在所研究的反应中是最慢的，即使在高温高压下也很少发生，这表明这两个自由基的分解途径没有强烈耦合。有限压力下预测的速率系数和分支比与相应的高压极限结果有显著差异，特别是在相对较高的温度下。最后，由于 CH3Ȯ + CO 反应动力学是大气条件下最重要的 CH3Ȯ 去除机制之一，因此还沿着 CH3OĊ(═O) 的势能面研究了该反应动力学；所得的动力学预测与实验结果非常吻合。
