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

Abstract

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.

Gas-phase formation of the resonantly stabilized 1-indenyl (C 9 H 7 • ) radical in the interstellar medium

“…It is possible that 3-phenyl-1-propyne and phenylallene can undergo unimolecular decomposition via H loss to form the corresponding C 9 H 7 radicals ( m / z = 115), namely, 1-phenylpropargyl (1PPR; C 6 H 5 CHCCH) and 3-phenylpropargyl (3PPR; C 6 H 5 CCCH 2 ) radicals. From our fitting results, the match of the reference indenyl PIE with our experimental PIE curve of m / z = 115 demonstrates that indenyl should be the dominant product for m / z = 115 at our high-temperature experimental condition of 1373 ± 10 K. These results are supported by the recent kinetic calculations for the branching ratios for the C 6 H 5 plus C 3 H 3 reaction at different pressure and temperatures, which reveal that 1PPR and 3PPR are relatively minor products compared to 1-indenyl ( 41 ). Until now, there is very limited experimental PIE data for 1PPR and 3PPR ( 37 ).…”

“…However, the electron impact ionization could neither reveal the source of C 9 H 7 + , i.e., nascent reaction product or dissociative electron impact ionization of C 9 H 8 , nor elucidate the nature of the structural isomers of C 9 H 8 . Selby et al (41) studied this reaction over 300 to 1000 K and 4 to 10 torr using time-resolved multiplexed photoionization mass spectrometry. The isomer-resolved branching ratios for the C 9 H 8 products are reported from 300 to 1000 K. On the basis of their photoionization spectra, at 300 K and 4 torr, only the direct C 9 H 8 adducts 3-phenyl-1-propargyl and phenylallene are observed with the branching ratios of 77 ± 5% and 23 ± 3%, respectively.…”

Combustion in a Sustainable World: From Molecules to Processes

“…The parallel and joint exploration of multiple influencing factors through theory, experiment, and numerical modeling is increasingly contributing to a more comprehensive picture of the process. Individual reactions in the decisive range of molecular precursors toward ring formation are the subject of several studies. − The phenyl + propargyl reaction, which is a prototype for a radical–radical ring-growth pathway, was investigated experimentally by time-resolved multiplexed photoionization mass spectrometry in a quartz flow reactor tube . Experimental results were complemented by theoretical calculations for a better understanding of isomer formation and branching ratios, including the formation of the two ring aromatic indene .…”

“…Individual reactions in the decisive range of molecular precursors toward ring formation are the subject of several studies. − The phenyl + propargyl reaction, which is a prototype for a radical–radical ring-growth pathway, was investigated experimentally by time-resolved multiplexed photoionization mass spectrometry in a quartz flow reactor tube . Experimental results were complemented by theoretical calculations for a better understanding of isomer formation and branching ratios, including the formation of the two ring aromatic indene . The chemistry of the C 7 H 7 radical was explored in terms of isomer and pathway identification for its formation and decomposition reactions in two contributions involving automated theoretical kinetics calculations and aerosol mass spectrometry experiments both in premixed flames and under pyrolysis conditions. , …”

“…26 Experimental results were complemented by theoretical calculations for a better understanding of isomer formation and branching ratios, including the formation of the two ring aromatic indene. 26 The chemistry of the C 7 H 7 radical was explored in terms of isomer and pathway identification for its formation and decomposition reactions in two contributions involving automated theoretical kinetics calculations and aerosol mass spectrometry experiments both in premixed flames and under pyrolysis conditions. 27,28 The efficiency of carbon cluster growth from small polycyclic aromatic compounds can be enhanced by trace metals, such as iron, present in the combustion environment or in targeted combustion synthesis processes, as reported by Zhou et al for binary coronene collisions.…”