Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139, USA.
Phys Chem Chem Phys. 2019 Oct 28;21(40):22248-22258. doi: 10.1039/c9cp04554f. Epub 2019 Oct 4.
Hydrogen-abstraction-CH-addition (HACA) is one of the most important pathways leading to the formation of naphthalene, the simplest two-ring polycyclic aromatic hydrocarbon (PAH). The major reaction channels for naphthalene formation have previously been calculated by Mebel et al., but few experiments exist to validate the theoretical predictions. In this work, time-resolved molecular beam mass spectrometry (MBMS) was used to investigate the time-dependent product formation in the reaction of a phenyl radical with CH for the first time, at temperatures of 600 and 700 K and pressures of 10 and 50 Torr. A pressure-dependent model was developed with rate parameters derived from Mebel et al.'s calculations and from newly calculated pathways on the CH PES at the G3(MP2,CC)//B3LYP/6-311G** level of theory. The model prediction is consistent with the MBMS product profiles at a mass-to-charge ratio (m/z) of 102 (corresponding to the H-loss product from CH, phenylacetylene), 103 (the initial CH adduct and its isomers plus the C isotopologue of phenylacetylene), 128 (naphthalene), and 129 (CH isomers plus the C isotopologue of naphthalene). An additional CH isomer, bicyclo[4.2.0]octa-1,3,5-trien-7-yl, not considered by Mebel et al.'s calculations, contributes significantly to the signal at m/z 103 due to its stable energy and low reactivity. At high CH concentrations, bimolecular reactions dominated the observed chemistry, and the m/z 128 and m/z 102 MBMS signal ratio was measured to directly determine the product branching ratio. At 600 K and 10 Torr, branching to the H-loss product (phenylacetylene) on the CH PES accounted for 7.9% of phenyl radical consumption, increasing to 15.9% at 700 K and 10 Torr. At 50 Torr, the branching was measured to be 2.8% at 600 K and 6.2% at 700 K. Adduct stabilization is favored at higher pressure and lower temperature, which hinders the formation of the H-loss product. The pressure-dependent model predicted the observed branching ratios within the experimental uncertainty, indicating that the rate parameters reported here can be used in combustion mechanisms to provide insights into phenyl HACA reactions and PAH formation.
氢提取-CH 加成（HACA）是导致萘形成的最重要途径之一，萘是最简单的两个环稠环芳香烃（PAH）。此前，Mebel 等人已经计算了萘形成的主要反应通道，但很少有实验来验证理论预测。在这项工作中，首次使用时间分辨分子束质谱（MBMS）在 600 和 700 K 的温度和 10 和 50 Torr 的压力下研究了苯基自由基与 CH 反应中产物形成的时间依赖性。开发了一个压力依赖模型，其速率参数来自 Mebel 等人的计算和在 G3(MP2,CC)//B3LYP/6-311G**理论水平上 CH PES 上新计算的途径。模型预测与在质荷比（m/z）为 102（对应于 CH 的 H 损失产物，苯乙炔）、103（初始 CH 加合物及其异构体加苯乙炔的 C 同位素）、128（萘）和 129（CH 异构体加萘的 C 同位素）的 MBMS 产物谱一致。Mebel 等人的计算中没有考虑的另一种 CH 异构体，双环[4.2.0]辛-1,3,5-三烯-7-基，由于其稳定的能量和低反应性，对 m/z 103 的信号有很大贡献。在高 CH 浓度下，双分子反应主导了观察到的化学，并且 m/z 128 和 m/z 102 MBMS 信号比直接测量以直接确定产物分支比。在 600 K 和 10 Torr 下，CH PES 上的 H 损失产物（苯乙炔）的分支占苯基自由基消耗的 7.9%，在 700 K 和 10 Torr 下增加到 15.9%。在 50 Torr 下，在 600 K 下测量的分支比为 2.8%，在 700 K 下为 6.2%。在较高压力和较低温度下有利于加合物稳定，这阻碍了 H 损失产物的形成。压力依赖模型在实验不确定度内预测了观察到的分支比，表明这里报道的速率参数可用于燃烧机制，以提供对苯基 HACA 反应和 PAH 形成的深入了解。
