Five six membered-ring PAH products from reaction of -methylphenyl radical and two CH isomers

Gas-phase reactions of the o -methylphenyl ( o -CH 3 C 6 H 4 ) radical with the C 3 H 4 isomers allene (H 2 C&z.dbd;C&z.dbd;CH 2 ) and propyne (HC&z.tbd;C-CH 3 ) 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 C 3 H 4 addition: m / z 116 (CH 3 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 (C 9 H 8 ) 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 (C 10 H 10 ) + 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 C 3 H 4 addition to the methylphenyl radical requires an ortho -CH 3 group - similar to o -methylphenyl radical oxidation. These reactions characteristically lead to bicyclic aromatic products, but the structure of the C 3 H 4 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. Gas-phase o -methylphenyl reactions with propyne and allene form PAHs: with preferences for either six-six or five-six bicyclic products.