Combining synchrotron vacuum-ultraviolet photoionization mass spectrometry and gas chromatography-mass spectrometry for isomer-specific mechanistic analysis with application to the benzyl self-reaction.

Abstract

Elucidating the formation mechanism of polycyclic aromatic hydrocarbons (PAHs) is crucial to understand processes in the contexts of combustion, environmental science, astrochemistry, and nanomaterials synthesis. An excited electronic-state pathway has been proposed to account for the formation of 14π aromatic anthracene in the benzyl (b-C₇H₇) self-reaction. Here, to improve our understanding of anthracene formation, we investigate C₇H₇ bimolecular reactions in a tubular SiC microreactor through an isomer-resolved method that combines in situ synchrotron-radiation VUV photoionization mass spectrometry and ex-situ gas chromatography-mass spectrometry. We observe the formation of o-tolyl (o-C₇H₇) radical isomer, and identify several C₁₄H₁₀ products (diphenylacetylene, phenanthrene and anthracene) and key C₁₄H₁₄ and C₁₄H₁₂ intermediates. These isomer-specific results support the occurrence of reactions on the electronic ground-state potential energy surface, with no evidence for key intermediates of the proposed excited-state pathway as the key pathway. Furthermore, theoretical calculations unveil new facile reaction pathways that may contribute to the enhanced production of anthracene, and these mechanistic findings are further substantiated by pyrolysis experiments. The results add insight into the molecular formation of PAHs in C₇H₇ bimolecular reaction, and contribute to establishing accurate models to predict PAH chemistry in diverse laboratory, environmental, and extraterrestrial contexts.