Hendrix Josie, Hait Diptarka, Michelsen Hope A, Head-Gordon Martin
Department of Chemistry, University of California, Berkeley, CA 94720.
Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720.
Proc Natl Acad Sci U S A. 2024 Dec 17;121(51):e2202744121. doi: 10.1073/pnas.2202744121. Epub 2024 Dec 9.
Polycyclic aromatic hydrocarbons (PAHs) play a major role in the chemistry of combustion, pyrolysis, and the interstellar medium. Production (or activation) of radical PAHs and propagation of their resulting reactions require efficient dehydrogenation, but the preferred method of hydrogen loss is not well understood. Unimolecular hydrogen ejection (i.e., direct C─H bond fission) and bimolecular radical abstraction are two main candidate pathways. We performed a computational study to characterize the role of H ejection, particularly as a driver for radical-centric hydrocarbon-growth mechanisms and particle formation. Electronic structure calculations establish that C─H bond strengths span a broad range of energies, which can be weaker than 30 kcal/mol in some C and C PAH radicals. At > 1200 K, calculated thermal rates for hydrogen ejection from weak C─H bonds at zigzag sites on PAH radicals are significantly larger than typical H-abstraction rates. These results are highly relevant in the context of chain reactions of radical species and soot inception under fuel-rich combustion conditions. Furthermore, calculated microcanonical rates that include the additional internal energy released by bond formation (e.g., ring closure to yield CH) yield significantly higher rates than those associated with full thermalization. These microcanonical considerations are relevant to the astrochemical processes associated with hydrocarbon growth and processing in the low-density interstellar environment.
多环芳烃（PAHs）在燃烧、热解和星际介质化学中起着重要作用。自由基PAHs的产生（或活化）及其后续反应的传播需要高效的脱氢作用，但氢损失的首选方式尚未得到很好的理解。单分子氢逸出（即直接C─H键断裂）和双分子自由基夺取是两个主要的候选途径。我们进行了一项计算研究，以表征氢逸出的作用，特别是作为以自由基为中心的碳氢化合物生长机制和颗粒形成的驱动因素。电子结构计算表明，C─H键强度跨越了广泛的能量范围，在某些C和C PAH自由基中，其强度可能低于30 kcal/mol。在>1200 K时，计算得出PAH自由基锯齿形位点上弱C─H键的氢逸出热速率明显高于典型的氢夺取速率。这些结果在富燃料燃烧条件下自由基物种的链反应和烟灰起始的背景下具有高度相关性。此外，计算得出的微正则速率包括键形成（例如环化生成CH）释放的额外内能，其速率明显高于与完全热化相关的速率。这些微正则考虑与低密度星际环境中与碳氢化合物生长和加工相关的天体化学过程有关。
