Role of methyl radicals in the growth of PAHs.

The role of methyl radicals in the networking of sp(2) carbons has been explored through kinetic analysis of mass spectra of the gas-phase products of the pyrolysis of toluene and toluene/acetone mixtures. Pyrolytic reactions were performed in a flow tube reactor at temperatures of 1140-1320 K and a constant total pressure of 10.38 Torr with a residence time of 0.585 s. On addition of acetone, methyl substituted products and their derivatives were enhanced. Mass peaks were observed in several sequences at an interval of 14 mass units; these ions correspond to methyl substituted products formed as a result of hydrogen abstraction (-H) followed by methyl radical addition (+CH(3)). Each major peak was usually preceded by a peak at two mass units lower, which was likely produced through dehydrogenation/dehydrocyclization (-H(2)) of methyl substituted products. Detected species include a large number of alkyl, cyclotetrafused (CT), cyclopentafused (CP) mono-, di-, and polycyclic aromatic hydrocarbons (PAHs) along with primary PAHs. The analysis showed that MAC (methyl addition/cyclization) has a unique capacity to induce the sequential growth of hexagonal networks of sp(2) carbons from all fusing sites of a PAH. Moreover, MAC was found capable of answering an important question in PAH growth, which is expansion of the CT --> CP --> hexagonal network for which other reported mechanisms are inefficient.