A VUV photoionization study of the multichannel reaction of phenyl radicals with 1,3-butadiene under combustion relevant conditions

We studied the reaction of phenyl radicals (C₆H₅) with 1,3-butadiene (H₂CCHCHCH₂) exploiting a high temperature chemical reactor under combustion-like conditions (300 Torr, 873 K). The reaction products were probed in a supersonic beam by utilizing VUV radiation from the Advanced Light Source and by recording the experimental PIE curves at mass-to-charge ratios of m/z = 130 (C₁₀H₁₀ ⁺), 116 (C₉H₈⁺), and 104 (C ₈H₈⁺). Our data suggest that the atomic hydrogen (H), methyl (CH₃), and vinyl (C₂H₃) losses are open with estimated branching ratios of about 86 ± 4%, 8 ± 2%, and 6 ± 2%, respectively. The isomer distributions were probed further by fitting the experimentally recorded PIE curves with a linear combination of the PIE curves of individual C₁₀H₁₀, C ₉H₈, and C₈H₈ isomers. These fits indicate the formation of three C₁₀H₁₀ isomers (trans-1,3-butadienylbenzene, 1,4-dihydronaphthalene, 1-methylindene), three C₉H₈ isomers (indene, phenylallene, 1-phenyl-1- methylacetylene), and a C₈H₈ isomer (styrene). A comparison with results from recent crossed molecular beam studies of the 1,3-butadiene-phenyl radical reaction and electronic structure calculations suggests that trans-1,3-butadienylbenzene (130 amu), 1,4-dihydronaphthalene (130 amu), and styrene (104 amu) are reaction products formed as a consequence of a bimolecular reaction between the phenyl radical and 1,3-butadiene. 1-Methylindene (130 amu), indene (116 amu), phenylallene (116 amu), and 1-phenyl-1-methylacetylene (116 amu) are synthesized upon reaction of the phenyl radical with three C₄H₆ isomers: 1,2-butadiene (H ₂CCCH(CH₃)), 1-butyne (HCCC₂H₅), and 2-butyne (CH₃CCCH₃); these C₄H₆ isomers can be formed from 1,3-butadiene via hydrogen atom assisted isomerization reactions or via thermal rearrangements of 1,3-butadiene involving hydrogen shifts in the high temperature chemical reactor.