Bay capping via acetylene addition to polycyclic aromatic hydrocarbons: Mechanism and kinetics

Tuli, Lotefa Binta; Mebel, Alexander M.; Frenklach, Michael

doi:10.1016/j.proci.2022.08.018

Cited by 5 publications

(6 citation statements)

References 42 publications

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“…Furthermore, as the PAH size increases to three rings and beyond, the increasing number of vibrational modes enhances collisional stabilization at lower pressures . Also, because a second acetylene addition is not required for larger PAH growth, HACA becomes more efficient from three rings on, producing new five-membered rings on zigzag edges , and new six-membered rings on armchair edges via bay capping . However, HACA mechanisms may be insufficient to fully describe the rate of PAH growth in combustion, and other reactions classes need to be invoked.…”

Section: Discussionmentioning

confidence: 99%

Radical–Radical Reactions in Molecular Weight Growth: The Phenyl + Propargyl Reaction

et al. 2023

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The mechanism for hydrocarbon ring growth in sooting environments is still the subject of considerable debate. The reaction of phenyl radical (C6H5) with propargyl radical (H2CCCH) provides an important prototype for radical–radical ring-growth pathways. We studied this reaction experimentally over the temperature range of 300–1000 K and pressure range of 4–10 Torr using time-resolved multiplexed photoionization mass spectrometry. We detect both the C9H8 and C9H7 + H product channels and report experimental isomer-resolved product branching fractions for the C9H8 product. We compare these experiments to theoretical kinetics predictions from a recently published study augmented by new calculations. These ab initio transition state theory-based master equation calculations employ high-quality potential energy surfaces, conventional transition state theory for the tight transition states, and direct CASPT2-based variable reaction coordinate transition state theory (VRC-TST) for the barrierless channels. At 300 K only the direct adducts from radical–radical addition are observed, with good agreement between experimental and theoretical branching fractions, supporting the VRC-TST calculations of the barrierless entrance channel. As the temperature is increased to 1000 K we observe two additional isomers, including indene, a two-ring polycyclic aromatic hydrocarbon, and a small amount of bimolecular products C9H7 + H. Our calculated branching fractions for the phenyl + propargyl reaction predict significantly less indene than observed experimentally. We present further calculations and experimental evidence that the most likely cause of this discrepancy is the contribution of H atom reactions, both H + indenyl (C9H7) recombination to indene and H-assisted isomerization that converts less stable C9H8 isomers into indene. Especially at low pressures typical of laboratory investigations, H-atom-assisted isomerization needs to be considered. Regardless, the experimental observation of indene demonstrates that the title reaction leads, either directly or indirectly, to the formation of the second ring in polycyclic aromatic hydrocarbons.

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Section: Discussionmentioning

confidence: 99%

Radical–Radical Reactions in Molecular Weight Growth: The Phenyl + Propargyl Reaction

et al. 2023

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“…54 But it could also form from a pentalink that is followed by acetylene addition to the bay site (see Supporting Information section S4). 55 In addition to the partially embedded five-membered rings, we also resolved a fully embedded five-membered ring and a fully embedded seven-membered ring; see A9 and A19 in Figure 4g, respectively. The A9 compound has a central moiety made of a corannulene-like motif with the difference of presenting a seven-membered ring instead of a six-membered ring fused on one edge of the central five-membered ring.…”

Section: Resultsmentioning

confidence: 99%

Portraits of Soot Molecules Reveal Pathways to Large Aromatics, Five-/Seven-Membered Rings, and Inception through π-Radical Localization

Lieske

Commodo²,

Martin

et al. 2023

ACS Nano

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Incipient soot early in the flame was studied by high-resolution atomic force microscopy and scanning tunneling microscopy to resolve the atomic structure and orbital densities of single soot molecules prepared on bilayer NaCl on Cu(111). We resolved extended catacondensed and pentagonalring linked (pentalinked) species indicating how small aromatics cross-link and cyclodehydrogenate to form moderately sized aromatics. In addition, we resolved embedded pentagonal and heptagonal rings in flame aromatics. These nonhexagonal rings suggest simultaneous growth through aromatic cross-linking/cyclodehydrogenation and hydrogen abstraction acetylene addition. Moreover, we observed three classes of open-shell π-radical species. First, radicals with an unpaired π-electron delocalized along the molecule's perimeter. Second, molecules with partially localized π-electrons at zigzag edges of a π-radical. Third, molecules with strong localization of a π-electron at pentagonal-and methylene-type sites. The third class consists of π-radicals localized enough to enable thermally stable bonds, as well as multiradical species such as diradicals in the open-shell triplet state. These π-diradicals can rapidly cluster through barrierless chain reactions enhanced by van der Waals interactions. These results improve our understanding of soot formation and the products formed by combustion and could provide insights for cleaner combustion and the production of hydrogen without CO 2 emissions.

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“…At 1 atm, a falloff of the total rate constant from the HP limit can be seen above 1200 K reaching a factor of ∼2.9 at 2500 K. At lower temperatures, the reaction is dominated by collisional stabilization of the intermediate i1 and then i4 , but at T > 1250 K, the well-skipping channels leading to the bimolecular products become predominant; i4 becomes unstable above 1500 K. In the 1300–2500 K temperature range, the bay-capped product p1 has the highest yield with the rate constant for its formation reaching a maximum at 2000 K. The rate constant for the formation of the ethynyl-substituted product p2 steadily grows with temperature and nearly equalizes with that for the formation of p1 at 2500 K. At 1500 K, the bay-capping rate constant involving the reaction of a seven-membered ring closure next to the E-bridge, 3.8 × 10 –12 cm 3 molecule –1 s –1 , is similar to those for bay-capping processes with a six-membered ring formation, which were found to vary in the 8.9 × 10 –13 –7.5 × 10 –12 cm 3 molecule –1 s –1 range. 38 …”

Section: Resultsmentioning

confidence: 99%

“…At 1 atm, a falloff of the total rate constant from the HP limit can be seen above 1200 K reaching a factor of ∼2.9 at 2500 K. At lower temperatures, the reaction is dominated by collisional stabilization of the intermediate i1 and then i4, but at T > 1250 K, the well-skipping channels leading to the bimolecular products become predominant; i4 becomes unstable above 1500 K. In the 1300−2500 K temperature range, the baycapped product p1 has the highest yield with the rate constant for its formation reaching a maximum at 2000 K. The rate constant for the formation of the ethynyl-substituted product p2 steadily grows with temperature and nearly equalizes with that for the formation of p1 at 2500 K. At 1500 K, the baycapping rate constant involving the reaction of a sevenmembered ring closure next to the E-bridge, 3.8 × 10 −12 cm 3 molecule −1 s −1 , is similar to those for bay-capping processes with a six-membered ring formation, which were found to vary in the 8.9 × 10 −13 −7.5 × 10 −12 cm 3 molecule −1 s −1 range. 38 As mentioned above, the second seven-membered closure in the C 36 H 15 + C 2 H 2 reaction is less favorable than the closure of the first bay due to the lower exothermicity and higher barriers along the reaction path. This is indeed reflected in the calculated rate constants, displayed in Figure 3.…”

Section: Calculation Methodsmentioning

confidence: 94%

On the Mechanism of Soot Nucleation. IV. Molecular Growth of the Flattened E-Bridge

Frenklach

Mebel

2022

J. Phys. Chem. A

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Rotationally excited dimerization of aromatic moieties, a mechanism proposed recently to explain the initial steps of soot particle inception in combustion and pyrolysis of hydrocarbons, produces a molecular structure, termed E-bridge, combining the two aromatics via five-membered aromatic rings sharing a common bond. The present study investigates a hydrogen-mediated addition of acetylene to the fused fivemembered ring part of the E-bridge forming a seven-membered ring. The carried out quantum-mechanical and rate theoretical calculations indicate the plausibility of such capping reactions, and kinetic Monte Carlo simulations demonstrate their frequent occurrence. The capping frequency, however, is limited by "splitting" the fused five-membered bridge due to five-membered ring migration. A similar migration of edge seven-membered rings is shown to be also rapid but short, as their encounter with five-membered rings converts them both into six-membered rings.

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Bay capping via acetylene addition to polycyclic aromatic hydrocarbons: Mechanism and kinetics

Cited by 5 publications

References 42 publications

Radical–Radical Reactions in Molecular Weight Growth: The Phenyl + Propargyl Reaction

Radical–Radical Reactions in Molecular Weight Growth: The Phenyl + Propargyl Reaction

Portraits of Soot Molecules Reveal Pathways to Large Aromatics, Five-/Seven-Membered Rings, and Inception through π-Radical Localization

On the Mechanism of Soot Nucleation. IV. Molecular Growth of the Flattened E-Bridge

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