Five vs. six membered-ring PAH products from reaction of o-methylphenyl radical and two C3H4 isomers

Abstract

Gas-phase reactions of the o-methylphenyl (o-CH₃C₆H₄) radical with the C₃H₄ isomers allene (H₂CCCH₂) and propyne (HCC–CH₃) are studied at 600 K and 4 Torr (533 Pa) using VUV synchrotron photoionisation mass spectrometry, quantum chemical calculations and RRKM modelling. Two major dissociation product ions arise following C₃H₄ addition: m/z 116 (CH₃ loss) and 130 (H loss). These products correspond to small polycyclic aromatic hydrocarbons (PAHs). The m/z 116 signal for both reactions is conclusively assigned to indene (C₉H₈) and is the dominant product for the propyne reaction. Signal at m/z 130 for the propyne case is attributed to isomers of bicyclic methylindene (C₁₀H₁₀) + H, which contains a newly-formed methylated five-membered ring. The m/z 130 signal for allene, however, is dominated by the 1,2-dihydronaphthalene isomer arising from a newly created six-membered ring. Our results show that new ring formation from C₃H₄ addition to the methylphenyl radical requires an ortho-CH₃ group – similar to o-methylphenyl radical oxidation. These reactions characteristically lead to bicyclic aromatic products, but the structure of the C₃H₄ co-reactant dictates the structure of the PAH product, with allene preferentially leading to the formation of two six-membered ring bicyclics and propyne resulting in the formation of six and five-membered bicyclic structures.