The Role of Methylaryl Radicals in the Growth of Polycyclic Aromatic Hydrocarbons: The Formation of Five-Membered Rings

The regions of the C13H11 potential energy surface (PES) related to the unimolecular isomerization and decomposition of the 1-methylbiphenylyl radical and accessed by the 1-/2-methylnaphthyl + C2H2 reactions have been explored by ab initio G3(MP2,CC)//B3LYP/6-311G(d,p) calculations. The kinetics of these reactions relevant to the growth of polycyclic aromatic hydrocarbons (PAH) under high-temperature conditions in circumstellar envelopes and in combustion flames has been studied employing the RRKM-Master Equation approach. The unimolecular reaction of 1-methylbiphenylyl proceeding via a five-membered ring closure followed by H elimination is predicted to be very fast, on a submicrosecond scale above 1000 K and to result in the formation of an embedded five-membered ring in the 9H-fluorene product. The 1-/2-methylnaphthyl + C2H2 reaction mechanism involves acetylene addition to the radical on the methylene group followed by a six- or five-membered ring closure and aromatization via an H atom loss. Despite of the complexity of the C13H11 PES, these straightforward pathways are dominant in the high-temperature regime (above similar to 1000 K), with the prevailing products being phenalene, with a significant contribution of 1H-cyclopenta(a)naphthalene, for 1-methylnaphthyl + C2H2, and 1H-cyclopenta(b)naphthalene and 3H-cyclopenta(a)naphthalene, for 2-methylnaphthyl + C2H2. The methylnaphthyl reactions with acetylene represent a clean source of the three-ring PAHs, but they are relatively slow owing to the high entrance barriers of similar to 10 kcal/mol, with the rate constants of about an order of magnitude lower as compared to those for naphthyl + allene and sigma-aryl + C2H2. The 1-methylnaphthyl + C2H2 and 2-methylnaphthyl + C2H2 reactions represent prototypes for PAH growth by an extra six- and five-membered ring on a zigzag edge or a corner of PAH and the generated modified Arrhenius expressions are recommended for kinetic modeling of PAH expansion by the mechanism of acetylene addition to methylaryl radicals.