Inception of Carbonaceous Nanostructures via Hydrogen-Abstraction Phenylacetylene-Addition Mechanism

Less-Dominant Resonance Configuration of Propargyl Radical Leads to a Growth Mechanism for Polycyclic Aromatic Hydrocarbons that Preserves the Cyclopenta Ring

Understanding the growth of polycyclic aromatic hydrocarbons (PAHs) is essential for combustion, astrochemistry, and carbon-based nanomaterial synthesis. This study presents theory-guided experiments on radical-radical combination reactions of propargyl (•C3H3). The addition of •C3H3 to three cyclopenta-fused PAH radicals─1-indenyl (•1-C9H7), acenaphthenyl (•C12H9), and 4H-cyclopenta[def]phenanthrenyl (•C15H9)─revealed that the reaction between the dominant propyne-3-yl resonance configuration of •C3H3 and the three radicals consistently produces PAHs with all hexagonal rings, while the reaction between the less dominant allene-1-yl resonance configuration of •C3H3 and the three radicals selectively preserves the cyclopenta ring and forms a new hexagonal ring. Elusive intermediates and isomeric products were observed and identified by combining molecular beam-sampling synchrotron photoionization mass spectrometry with gas chromatography-mass spectrometry. The complementary results suggest a high selectivity of the allene-1-yl addition pathway, which is thermodynamically controlled. The findings presented here are based on a combination of experimental capabilities, and they provide new mechanisms and insights into the selective formation of bowl-shaped PAHs, serving as templates for fullerene and nanotube structures. The high selectivity of the allene-1-yl pathway provides a rational synthetic strategy for cyclopenta-fused PAHs, bearing barrierless and facile radical-radical reaction pathways in various environments, including high-temperature combustion, circumstellar envelopes, and cold molecular clouds.

Don't forget the trans: double bond isomerism radical-acetylene growth reactions affect the primary stages of PAH and soot formation

In combustion, acetylene is a key species in molecular-weight growth reactions that form polycyclic aromatic hydrocarbons (PAHs) and ultimately soot particles. Radical addition to acetylene generates a vinyl radical intermediate, which has both trans and cis isomers. This isomerism can lead to profound changes in product distributions that are as yet insufficiently investigated. Herein, we explore acetylene addition to substituted trans-vinyl radicals, including potential rearrangement to cis structures, for eight combustion-related hydrocarbon radicals calculated using a composite method (G3X-K). Of these eight systems, the phenyl trans- and cis-Bittner-Howard HACA (hydrogen abstraction, C2H2 Addition) process, where acetylene successively adds to a phenyl radical via a β-styryl intermediate, is simulated using a unified Master Equation model. Including the trans-Bittner-Howard pathway changes the products significantly at all simulated temperatures (550-1800 K) and pressures (5.33 × 102-107 Pa), relative to a cis-only model. Typically, naphthalene remains the dominant product, but its abundance decreases at higher temperatures and pressures. For example, at 1200 K and 105 Pa, its branching ratio decreases from 78.5% to 62.9% when the trans pathway is included. At higher temperatures this decrease corresponds to the formation of alternative C10H8 isomers, including the cis product benzofulvene with 8% maximum abundance at 1200 K and 5.33 × 102 Pa, and E-2-ethynyl-1-phenylethylene, a trans product with 26% maximum abundance at 1800 K, with little pressure dependence. At higher pressures, our model predicts a range of C10H9 radicals, including resonance-stabilised radicals (RSRs). The impact of trans-vinyl radical chemistry in reactive environments means that they are essential to accurately describe combustion reactions and inhibit soot formation.

Probing the Reaction of Propargyl Radical with Molecular Oxygen by Synchrotron VUV Photoionization Mass Spectrometry

Self-reaction of propargyl (C3H3) radical is the main pathway to benzene, the formation of which is the rate-controlling step toward the formation of polycyclic aromatic hydrocarbons (PAHs) and soot. Oxidation of C3H3 is a promising strategy to inhibit the formation of hazardous PAHs and soot. In the present study, we studied the C3H3 + O2 reaction from 650 to 1100 K in a laminar flow reactor and identified the intermediates and products by synchrotron VUV photoionization mass spectrometry. 2-Propynal, ethenone, formaldehyde, CO, CO2, C2H2, C2H4, and C3O2 were identified. Among them, 2-propynal, ethenone, and formaldehyde provided direct evidence for the branching reaction of C3H3 + O2 → HCCCHO + OH, C3H3 + O2 → H2CCO + CHO, and C3H3 + O2 → H2CO + CHCO, respectively. Potential energy surface calculation and mechanistic analysis of the C3O2 formations implied that C3H3 + O2 → CCCHO + H2O and C3H3 + O2 → HCCCO + H2O could occur, despite lacking direct observations of CCCHO and HCCCO radicals. The formation of ethenone and CO suggested the occurrence of the two CO elimination channels. We incorporated these validated reactions and the corresponding rate coefficients in the kinetic model of NUIGMech1.3, and the simulation showed obvious improvements toward the measured mole fractions of C3H3 and H2CCO, suggesting that the new C3H3 + O2 reaction channels were crucial in the overall combustion modeling of the important intermediate propyne (C3H4).

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

Soft photoionization molecular-beam mass spectrometry (PI MBMS) with synchrotron vacuum ultraviolet light (SVUV) has has a significant development and broad applications in recent decades. Particularly, the tunability of SVUV enables...