Selby Talitha M, Goulay Fabien, Soorkia Satchin, Ray Amelia, Jasper Ahren W, Klippenstein Stephen J, Morozov Alexander N, Mebel Alexander M, Savee John D, Taatjes Craig A, Osborn David L
Department of Mathematics and Natural Sciences, University of Wisconsin-Milwaukee, West Bend, Wisconsin 53095, United States.
C. Eugene Bennett Department of Chemistry, West Virginia University, Morgantown, West Virginia 26506, United States.
J Phys Chem A. 2023 Mar 23;127(11):2577-2590. doi: 10.1021/acs.jpca.2c08121. Epub 2023 Mar 11.
The mechanism for hydrocarbon ring growth in sooting environments is still the subject of considerable debate. The reaction of phenyl radical (CH) with propargyl radical (HCCCH) provides an important prototype for radical-radical ring-growth pathways. We studied this reaction experimentally over the temperature range of 300-1000 K and pressure range of 4-10 Torr using time-resolved multiplexed photoionization mass spectrometry. We detect both the CH and CH + H product channels and report experimental isomer-resolved product branching fractions for the CH product. We compare these experiments to theoretical kinetics predictions from a recently published study augmented by new calculations. These transition state theory-based master equation calculations employ high-quality potential energy surfaces, conventional transition state theory for the tight transition states, and direct CASPT2-based variable reaction coordinate transition state theory (VRC-TST) for the barrierless channels. At 300 K only the direct adducts from radical-radical addition are observed, with good agreement between experimental and theoretical branching fractions, supporting the VRC-TST calculations of the barrierless entrance channel. As the temperature is increased to 1000 K we observe two additional isomers, including indene, a two-ring polycyclic aromatic hydrocarbon, and a small amount of bimolecular products CH + H. Our calculated branching fractions for the phenyl + propargyl reaction predict significantly less indene than observed experimentally. We present further calculations and experimental evidence that the most likely cause of this discrepancy is the contribution of H atom reactions, both H + indenyl (CH) recombination to indene and H-assisted isomerization that converts less stable CH isomers into indene. Especially at low pressures typical of laboratory investigations, H-atom-assisted isomerization needs to be considered. Regardless, the experimental observation of indene demonstrates that the title reaction leads, either directly or indirectly, to the formation of the second ring in polycyclic aromatic hydrocarbons.
在富烟环境中碳氢化合物环生长的机制仍然是一个备受争议的话题。苯基自由基（CH）与炔丙基自由基（HCCCH）的反应为自由基 - 自由基环生长途径提供了一个重要的原型。我们使用时间分辨多路复用光电离质谱法，在300 - 1000 K的温度范围和4 - 10 Torr的压力范围内对该反应进行了实验研究。我们检测到了CH和CH + H产物通道，并报告了CH产物的实验异构体分辨产物分支比。我们将这些实验与最近发表的一项研究中的理论动力学预测进行了比较，并通过新的计算进行了补充。这些基于过渡态理论的主方程计算采用了高质量的势能面，对于紧密过渡态采用传统过渡态理论，对于无障碍通道采用基于直接CASPT2的可变反应坐标过渡态理论（VRC - TST）。在300 K时，仅观察到自由基 - 自由基加成的直接加合物，实验和理论分支比之间具有良好的一致性，支持了无障碍入口通道的VRC - TST计算。随着温度升高到1000 K，我们观察到另外两种异构体，包括茚（一种二环多环芳烃）和少量双分子产物CH + H。我们计算的苯基 + 炔丙基反应的分支比预测的茚比实验观察到的要少得多。我们提供了进一步的计算和实验证据，表明这种差异最可能的原因是H原子反应的贡献，即H + 茚基（CH）重组为茚以及H辅助异构化将较不稳定的CH异构体转化为茚。特别是在实验室研究典型的低压条件下，需要考虑H原子辅助异构化。无论如何，茚的实验观察表明，该标题反应直接或间接地导致了多环芳烃中第二个环的形成。
