A Free‐Radical Pathway to Hydrogenated Phenanthrene in Molecular Clouds—Low Temperature Growth of Polycyclic Aromatic Hydrocarbons

Abstract

The hydrogen-abstraction/acetylene-addition mechanism has been fundamental to unravelling the synthesis of polycyclic aromatic hydrocarbons (PAHs) detected in combustion flames and carbonaceous meteorites like Orgueil and Murchison. However, the fundamental reaction pathways accounting for the synthesis of complex PAHs, such as the tricyclic anthracene and phenanthrene along with their dihydrogenated counterparts, remain elusive to date. By investigating the hitherto unknown chemistry of the 1-naphthyl radical with 1,3-butadiene, we reveal a facile barrierless synthesis of dihydrophenanthrene adaptable to low temperatures. These aryl-type radical additions to conjugated hydrocarbons via resonantly stabilized free-radical intermediates defy conventional wisdom that PAH growth is predominantly a high-temperature phenomenon and thus may represent an overlooked path to PAHs as complex as coronene and corannulene in cold regions of the interstellar medium like in the Taurus Molecular Cloud.

On the formation and the isomer specific detection of methylacetylene (CH3CCH), propene (CH3CHCH2), cyclopropane (c-C3H6), vinylacetylene (CH2CHCCH), and 1,3-butadiene (CH2CHCHCH2) from interstellar methane ice analogues

“…23 Meanwhile, 1,3-butadiene's reaction with the phenyl radical generates 1,4-dihydronaphthalene (C 10 H 10 ), 41,136 the reaction with the tolyl radical forms 5-and 6-methyl-1,4-dihydronaphthalene (C 11 H 12 ), 42 and its reaction with the 1-napthyl radical synthesizes dihydrophenanthrene (C 14 H 12 ). 137 Furthermore, 1,3-butadiene can also react with the cyano radical to form pyridine (C 9 H 5 N), 3,138 and its reaction with pyridyl radicals (C 5 H 4 N) can produce 1,4-dihydro (iso)quinolone (C 9 H 9 N). 3 The other C 4 H 6 isomers detected here, 1,2-butadiene, 1-butyne, and 2-butyne, have also been shown to react with dicarbon to make RSFRs.…”

An Aromatic Universe–A Physical Chemistry Perspective

“…If the reacting aromatic doublet radical moiety carries an alkyl group such as methyl, HAVA can also lead to the formation of, for example, methyl-substituted PAHs such as 1- and 2-methylnaphthalene as demonstrated via crossed molecular beam experiments of tolyl (C 7 H 7 ) radicals with vinylacetylene in which the methyl group acts as a spectator . Modified HAVA routes involving hydrogenated reactants, for example, 1,3-butadiene along with their methyl-substituted counterparts, lead to (methyl-substituted) dihydrogenated naphthalenes …”

“…48 Modified HAVA routes involving hydrogenated reactants, for example, 1,3-butadiene along with their methyl-substituted counterparts, lead to (methylsubstituted) dihydrogenated naphthalenes. 28 To sum up, HAVA has the capability to expand PAHs by "adding" a benzene ring to an existing PAH moiety via a single collision event involving two neutral reactants. The barrierless nature of this mechanism is driven by the initial formation of a van der Waals complex, a submerged barrier to addition, and formation of an RSFR intermediate that successively isomerizes via hydrogen shifts and cyclization prior to the atomic hydrogen loss and PAH formation.…”

“…Therefore, this complexity requires a systematic elucidation of the fundamental, elementary reactions involved in the formation of PAHs through well-defined molecular beam studies. These experiments are conducted either under single-collision conditions as provided in crossed molecular beam experiments , or in pyrolytic microreactors under very dilute conditions . Furthermore, flame-sampling molecular-beam studies can provide immediate insights into PAH formation kinetics.…”

“…We bring together emerging concepts of low- and high-temperature aromatization reactions via neutral–neutral collisions from monocyclic systems to PAHs. It compiles developing trends on the formation and molecular growth processes of PAHs via five elementary mechanisms made available by new molecular beam studies combined with high-level electronic structure calculations (for details of these calculations, we refer the reader to the original literature): H ydrogen A bstraction– C 2 H 2 (acetylene) A ddition (HACA), − H ydrogen A bstraction– V inylacetylene A ddition (HAVA), ,,,,,− P henyl A ddition–Dehydro C yclization (PAC), R adical- R adical R eactions (RRR), and M ethylidyne A ddition- C yclization- A romatization (MACA) . The experimental and computational verification of these reaction networks provides a constrained framework of PAH formation chemistry in extreme environments from low temperatures in molecular clouds (10 K) and hydrocarbon-rich atmospheres of planets and their moons (35–150 K) to high-temperature environments like circumstellar envelopes of carbon-rich AGB stars and combustion systems at a few 1000 K.…”