Kaiser Ralf I, Zhao Long, Lu Wenchao, Ahmed Musahid, Krasnoukhov Vladislav S, Azyazov Valeriy N, Mebel Alexander M
Department of Chemistry, University of Hawaii at Manoa, Honolulu, HI, 96822, USA.
Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA, 94720, USA.
Nat Commun. 2022 Feb 10;13(1):786. doi: 10.1038/s41467-022-28466-7.
Polycyclic aromatic hydrocarbons (PAHs) are prevalent in deep space and on Earth as products in combustion processes bearing direct relevance to energy efficiency and environmental remediation. Reactions between hydrocarbon radicals in particular have been invoked as critical molecular mass growth processes toward cyclization leading to these PAHs. However, the mechanism of the formation of PAHs through radical - radical reactions are largely elusive. Here, we report on a combined computational and experimental study of the benzyl (CH) radical self-reaction to phenanthrene and anthracene (CH) through unconventional, isomer-selective excited state dynamics. Whereas phenanthrene formation is initiated via a barrierless recombination of two benzyl radicals on the singlet ground state surface, formation of anthracene commences through an exotic transition state on the excited state triplet surface through cycloaddition. Our findings challenge conventional wisdom that PAH formation via radical-radical reactions solely operates on electronic ground state surfaces and open up a previously overlooked avenue for a more "rapid" synthesis of aromatic, multi-ringed structures via excited state dynamics in the gas phase.
多环芳烃（PAHs）作为燃烧过程的产物，在深太空和地球上普遍存在，这与能源效率和环境修复直接相关。特别是烃基之间的反应，被认为是导致这些多环芳烃环化的关键分子质量增长过程。然而，通过自由基 - 自由基反应形成多环芳烃的机制在很大程度上仍然难以捉摸。在这里，我们报告了一项关于苄基（CH）自由基通过非常规的、异构体选择性激发态动力学自反应生成菲和蒽（CH）的计算与实验相结合的研究。菲的形成是通过两个苄基自由基在单重基态表面上无障碍重组引发的，而蒽的形成则是通过激发态三重态表面上的一个奇特过渡态经环加成开始的。我们的研究结果挑战了传统观念——即通过自由基 - 自由基反应形成多环芳烃仅在电子基态表面上进行，并且开辟了一条以前被忽视的途径，即通过气相中的激发态动力学更“快速”地合成芳香族多环结构。
