Fivevs.six membered-ring PAH products from reaction ofo-methylphenyl radical and two C3H4isomers

Gas-phase reactions of theo-methylphenyl (o-CH3C6H4) radical with the C3H4isomers allene (H2C-C-CH2) 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 C3H4addition:m/z116 (CH3loss) and 130 (H loss). These products correspond to small polycyclic aromatic hydrocarbons (PAHs). Them/z116 signal for both reactions is conclusively assigned to indene (C9H8) and is the dominant product for the propyne reaction. Signal atm/z130 for the propyne case is attributed to isomers of bicyclic methylindene (C10H10) + H, which contains a newly-formed methylated five-membered ring. Them/z130 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 C3H4addition to the methylphenyl radical requires anortho-CH3group - similar too-methylphenyl radical oxidation. These reactions characteristically lead to bicyclic aromatic products, but the structure of the C3H4co-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.