Hydrogen ejection from hydrocarbons: Characterization and relevance in soot formation and interstellar chemistry

• 成果类型: Article
• 署名作者: Hendrix, Josie; Hait, Diptarka; Michelsen, Hope A.; Head, Martin
• 署名单位: University of California System; University of California Berkeley; United States Department of Energy (DOE); Lawrence Berkeley National Laboratory; Stanford University; Stanford University; University of Colorado System; University of Colorado Boulder
• 刊物名称: PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA
• ISSN/ISSBN: 0027-11099
• DOI: 10.1073/pnas.2202744121
• 发表日期: 2024-12-09
• 关键词: polycyclic aromatic-hydrocarbons; bond-dissociation energies; radical-chain-reactions; gas-phase formation; c-h; nascent soot; combustion; kinetics; flames; indene

摘要

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-9 and C-13 PAH radicals. At T > 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 C9H9) 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.