On the kinetics of the C5H5 + C5H5 reaction

The possibility that the reaction between two cyclopentadienyl radicals (cC5H5) may lead to the production of naphthalene has been the subject of considerable theoretical and experimental studies. Though it has been proposed that this reaction may be the main channel for the formation of naphthalene in many combustion environments, the elementary mechanism leading from the initial adduct (C5H5_C5H5) to naphthalene is still not clear. In this study the portion of the C10H10 PES accessible to C5H5_C5H5 has been theoretically re-examined using density functional theory to locate stationary points and the CBS-QB3 computational protocol to determine energies. A new reaction pathway leading to the formation of a set of azulyl radicals was identified. Since it is known that azulyl radicals can easily decompose to naphthalene and atomic H, the proposed pathway provides an effective route for the formation of naphthalene. Channel specific kinetic constants were determined between 1100 K and 2000 K integrating the master equation for a PES comprising both this reaction pathway and the literature reaction pathway, which main product is the fulvalenyl radical. It was found that the main reaction channel is decomposition to reactants in the whole temperature range investigated and that the azulyl reaction channel is dominant over the fulvalenyl pathway up to 1450 K. The rate constants calculated at 1 bar for the azulyl and fulvalenyl reaction channels are 1014.72T(K)−0.853 exp(−3650/T(K)) and 1010.30T(K)0.951 exp(−7948/T(K)) cm3/mol/s, respectively. The rate constant for the formation of naphthalene through the azulyl channel is consistent with recent estimates based on the kinetic simulation of the pyrolysis and oxidation of cyclopentadiene.