Unconventional pathway for the gas-phase formation of 14π-PAHs via self-reaction of the resonantly stabilized radical fulvenallenyl (C₇H₅˙)

Resonantly stabilized free radicals (RSFRs) are contemplated to be the reactive intermediates in molecular mass-growth processes leading to polycyclic aromatic hydrocarbons (PAHs), which are prevalent in deep space and on Earth. The self-reaction routes of two RSFRs have been recognized as fundamental but more-efficient pathways to form fused benzenoid rings. The present experiment, which exploits a chemical microreactor in combination with an isomer-selective identification technique through fragment-free photoionization utilizing a tunable vacuum ultraviolet (VUV) light in tandem with the detection of the ionized molecules by a high-resolution reflection time-of-flight mass spectrometer (Re-TOF-MS), provides compelling evidence for the formation of phenanthrene and a minor amount of anthracene in the presence of fulvenallenyl (C₇H₅˙). Further theoretical calculations of the potential energy surfaces of C₁₄H₁₀ and C₁₄H₁₁ reveal that phenanthrene and anthracene can be efficiently produced via a hydrogen-assisted multi-step mechanism [C₇H₅˙ + C₇H₅˙ → i3, i3 = (3,4-di(cyclopenta-2,4-dien-1-ylidene)cyclobut-1-ene); i3 + H → phenanthrene + H/anthracene + H or i3 + H → i8 + H → phenanthrene + H/anthracene + H, i8 = (1-(cyclopenta-2,4-dien-1-ylidene)indene)] at low pressures, rather than through the one-step recombination-isomerization of fulvenallenyl radicals. This study provides a novel growth mechanism for tricyclic PAHs, especially in hydrogen-rich environments such as combustion and interstellar environments, which advances the knowledge of PAH propagation and even the formation mechanisms of carbonaceous nanoparticles in our universe.