Unconventional excited-state dynamics in the concerted benzyl (C7H7) radical self-reaction to anthracene (C14H10)

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.

Exotic Reaction Dynamics in the Gas-Phase Preparation of Anthracene (C14H10) via Spiroaromatic Radical Transients in the Indenyl–Cyclopentadienyl Radical–Radical Reaction

“…The onset of the ion counts at 7.40 ± 0.05 eV correlates nicely with the NIST-evaluated adiabatic ionization energy (IE) of anthracene ( p2 ) of 7.439 ± 0.006 eV (Figure a–d). Additional ion counts from phenanthrene ( p1 ), which has an adiabatic ionization energy (IE) of 7.891 ± 0.001 eV, are also critical; nevertheless, ion counts from phenanthrene (C 14 H 10 , p1 ) and anthracene (C 14 H 10 , p2 ) alone cannot replicate the experimental PIE curve. This discrepancy can be resolved by including a third isomer: benzofulvalene (C 14 H 10 , p3 ).…”

“…Validated through electronic structure calculations, discrete spiroaromatic hydrocarbon radical transients1a′,6a′-dihydro-5′ H -spiro­[cyclopentane-1,1′-cyclopropa­[ a ]­indene]-2,4-dienyl ( i3 ), 4a′ H -spiro­[cyclopentane-1,2′-naphthalene]-2,4-dienyl ( i4 ), 9b,9c-dihydro-3 H -cyclopentadiene­[2,3]­cyclopropa­[1,2- a ]­naphthalenyl ( i5 ), spiro­[bicyclo[3.1.0]­hexane-6,2′-inden]-2-enyl ( i12 ), and spiro­[cyclohexane-1,2′-indene]-2,5-dienyl ( i13 ) (C 14 H 11 • )prepared through carbon–carbon coupling of the five-membered rings from the doublet radical reactants were revealed to be essential in the gas-phase preparation of phenanthrene (C 14 H 10 , p1 ) and anthracene (C 14 H 10 , p2 ). Cemented through the isomer-selective identification of phenanthrene (C 14 H 10 , p1 ), anthracene (C 14 H 10 , p2 ), and benzofulvalene (C 14 H 10 , p3 ) in a molecular beam via fragment-free, soft photoionization by tunable synchrotron vacuum ultraviolet (VUV) light coupled with a reflectron time-of-flight mass spectrometer (Re-TOF-MS; − Supporting Information), these findings afford persuasive testimony on a hitherto elusive class of aromatic, reactive intermediates involved in molecular mass growth processes of aromatics in our universe: spiroaromatics.…”

The Identity and Chemistry of C7H7 Radicals Observed during Soot Formation

“…Colket and Seery 68 demonstrated that benzyl self-reactions can lead to the formation of PAHs, such as anthracene, phenanthrene, fluorene, acenaphthylene, and pyrene. The mechanism for the production of phenanthrene was later investigated by Hirsch et al, 77 and mechanisms for the production of phenanthrene and anthracene were studied by Kaiser et al 78 In low-pressure pyrolysis experiments, Buckingham et al 64 demonstrated that benzyl loses a hydrogen atom and rearranges to form fulvenallene (C 7 H 6 ). Sivaramakrishnan et al 63 performed shock tube measurements on isotopically labeled benzyl and came to a similar conclusion.…”

“…Colket and Seery demonstrated that benzyl self-reactions can lead to the formation of PAHs, such as anthracene, phenanthrene, fluorene, acenaphthylene, and pyrene. The mechanism for the production of phenanthrene was later investigated by Hirsch et al, and mechanisms for the production of phenanthrene and anthracene were studied by Kaiser et al…”

Probing combustion and catalysis intermediates by synchrotron vacuum ultraviolet photoionization molecular-beam mass spectrometry: recent progress and future opportunities

“…11 This method is able to record the intensity of species, not only as a function of the mass-to-charge ratio (mass spectrum), but also as a function of photon energy (photoionization efficiency (PIE) spectrum), providing impressive isomer-specific measurement capability. 7,12 In recent decades, the application of SVUV-PIMS method has promoted the development of various fields including reaction kinetics, combustion chemistry, [13][14][15] catalysis, 16,17 aerosol chemistry, 18 planetary and interstellar chemistry, [19][20][21] etc.…”