Abstract
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 (C7H7) radical self-reaction to phenanthrene and anthracene (C14H10) 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.
Highlights
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
Since the discovery of the 14π-aromatic anthracene molecule (C14H10) as a product of coal-tar distillation by Dumas and Laurent in 18321, polycyclic aromatic hydrocarbons (PAHs)—hydrocarbons composed of multiple fused benzenoid rings2,3—have been contemplated in molecular mass growth processes in combustion systems along with cold molecular clouds, hydrocarbon-rich atmospheres of planets and their moons[4], and circumstellar envelopes of carbon-rich Asymptotic Giant Branch stars like intrinsic reaction coordinate (IRC) + 10216 covering temperatures from 10 K to a few 1000 K5–8
PAHs can transverse unconventional reaction pathways, and can prove a rich tapestry to map out excited state chemical dynamics as is seen here in the self-reaction of the benzyl (C7H7) radical leading eventually to the formation of phenanthrene and anthracene
Summary
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. We report on a combined computational and experimental study of the benzyl (C7H7) radical self-reaction leading eventually to the formation of phenanthrene (C14H10, p1) and anthracene (C14H10, p2) as prototype 14π aromatic systems carrying three fused benzene rings.
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