Gas-phase product detection studies of o-hydroxyphenyl radical and O2 are reported at 373, 500, and 600 K, at 4 Torr (533.3 Pa), using VUV time-resolved synchrotron photoionisation mass spectrometry. The dominant products are assigned as o-benzoquinone (C6H4O2, m/z 108) and cyclopentadienone (C5H4O, m/z 80). It is concluded that cyclopentadienone forms as a secondary product from prompt decomposition of o-benzoquinone (and dissociative ionization of o-benzoquinone may contribute to the m/z 80 signal at photon energies ≳9.8 eV). Ion-trap reactions of the distonic o-hydroxyphenyl analogue, the 5-ammonium-2-hydroxyphenyl radical cation, with O2 are also reported and concur with the assignment of o-benzoquinone as the dominant product. The ion-trap study also provides support for a mechanism where cyclopentadienone is produced by decarbonylation of o-benzoquinone. Kinetic studies compare oxidation of the ammonium-tagged o-hydroxyphenyl and o-methylphenyl radical cations along with trimethylammonium-tagged analogues. Reaction efficiencies are found to be ca. 5% for both charge-tagged o-hydroxyphenyl and o-methylphenyl radicals irrespective of the charged substituent. G3X-K quantum chemical calculations are deployed to rationalise experimental results for o-hydroxyphenyl + O2 and its charge-tagged counterpart. The prevailing reaction mechanism, after O2 addition, involves a facile 1,5-H shift in the peroxyl radical and subsequent elimination of OH to yield o-benzoquinone that is reminiscent of the Waddington mechanism for β-hydroxyperoxyl radicals. These results suggest o-hydroxyphenyl + O2 and decarbonylation of o-benzoquinone serve as plausible OH and CO sources in combustion.

The reactions of the indenyl radicals with acetylene (C₂ H₂ ) and vinylacetylene (C₄ H₄ ) is studied in a hot chemical reactor coupled to synchrotron based vacuum ultraviolet ionization mass spectrometry. These experimental results are combined with theory to reveal that the resonantly stabilized and thermodynamically most stable 1-indenyl radical (C₉ H₇^. ) is always formed in the pyrolysis of 1-, 2-, 6-, and 7-bromoindenes at 1500 K. The 1-indenyl radical reacts with acetylene yielding 1-ethynylindene plus atomic hydrogen, rather than adding a second acetylene molecule and leading to ring closure and formation of fluorene as observed in other reaction mechanisms such as the hydrogen abstraction acetylene addition or hydrogen abstraction vinylacetylene addition pathways. While this reaction mechanism is analogous to the bimolecular reaction between the phenyl radical (C₆ H₅^. ) and acetylene forming phenylacetylene (C₆ H₅ CCH), the 1-indenyl+acetylene→1-ethynylindene+hydrogen reaction is highly endoergic (114 kJ mol^-1 ) and slow, contrary to the exoergic (-38 kJ mol^-1 ) and faster phenyl+acetylene→phenylacetylene+hydrogen reaction. In a similar manner, no ring closure leading to fluorene formation was observed in the reaction of 1-indenyl radical with vinylacetylene. These experimental results are explained through rate constant calculations based on theoretically derived potential energy surfaces.

Polycyclic aromatic hydrocarbons (PAHs) represent the link between resonance-stabilized free radicals and carbonaceous nanoparticles generated in incomplete combustion processes and in circumstellar envelopes of carbon rich asymptotic giant branch (AGB) stars. Although these PAHs resemble building blocks of complex carbonaceous nanostructures, their fundamental formation mechanisms have remained elusive. By exploring these reaction mechanisms of the phenyl radical with biphenyl/naphthalene theoretically and experimentally, we provide compelling evidence on a novel phenyl-addition/dehydrocyclization (PAC) pathway leading to prototype PAHs: triphenylene and fluoranthene. PAC operates efficiently at high temperatures leading through rapid molecular mass growth processes to complex aromatic structures, which are difficult to synthesize by traditional pathways such as hydrogen-abstraction/acetylene-addition. The elucidation of the fundamental reactions leading to PAHs is necessary to facilitate an understanding of the origin and evolution of the molecular universe and of carbon in our galaxy.

The three-ring polycyclic aromatic hydrocarbons (PAHs) 3H-benz[e]indene (C₁₃H₁₀) and 1H-benz[f]indene (C₁₃H₁₀) along with their naphthalene-based isomers 2-(prop-2-yn-1-yl)naphthalene (C₁₃H₁₀), 2-(prop-1-yn-1-yl)naphthalene (C₁₃H₁₀), and 2-(propa-1,2-dien-1-yl)naphthalene (C₁₃H₁₀) were formed through a "directed synthesis"via a high temperature chemical micro reactor under combustion-like conditions (1300 ± 35 K) through the reactions of the 2-naphthyl isomer (C₁₀H₇˙) with allene (C₃H₄) and methylacetylene (C₃H₄). The isomer distributions were probed utilizing tunable vacuum ultraviolet radiation from the Advanced Light Source (ALS) by recording the photoionization efficiency curves at mass-to-charge of m/z = 166 (C₁₃H₁₀) and 167 (¹³CC₁₂H₁₀) of the products in a supersonic molecular beam. Complemented by electronic structure calculations, our study reveals critical mass growth processes via annulation of a five-membered ring from the reaction between aryl radicals and distinct C₃H₄ isomers at elevated temperatures as present in combustion processes and in circumstellar envelopes of carbon stars. The underlying reaction mechanisms proceed through the initial addition of the 2-naphthyl radical with its radical center to the π-electron density of the allene and methylacetylene reactants via entrance barriers between 8 and 14 kJ mol^-1, followed by isomerization (hydrogen shifts, ring closure), and termination via atomic hydrogen losses accompanied by aromatization. The reaction mechanisms reflect the formation of indene - the prototype PAH carrying a single five- and a single six-membered ring - synthesized through the reaction of the phenyl radical (C₆H₅˙) with allene and methylacetylene. This leads us to predict that aryl radicals - upon reaction with allene/methylacetylene - may undergo molecular mass growth processes via ring annulation and de facto addition of a five-membered ring to form molecular building blocks essential to transit planar PAHs out of the plane.

Abstract: Polycyclic aromatic hydrocarbons (PAHs) represent the link between resonance‐stabilized free radicals and carbonaceous nanoparticles generated in incomplete combustion processes and in circumstellar envelopes of carbon rich asymptotic giant branch (AGB) stars. Although these PAHs resemble building blocks of complex carbonaceous nanostructures, their fundamental formation mechanisms have remained elusive. By exploring these reaction mechanisms of the phenyl radical with biphenyl/naphthalene theoretically and experimentally, we provide compelling evidence on a novel phenyl‐addition/dehydrocyclization (PAC) pathway leading to prototype PAHs: triphenylene and fluoranthene. PAC operates efficiently at high temperatures leading through rapid molecular mass growth processes to complex aromatic structures, which are difficult to synthesize by traditional pathways such as hydrogen‐abstraction/acetylene‐addition. The elucidation of the fundamental reactions leading to PAHs is necessary to facilitate an understanding of the origin and evolution of the molecular universe and of carbon in our galaxy.