Theoretical kinetics of HO2 + C5H5: A missing piece in cyclopentadienyl radical oxidation reactions.

The resonantly-stabilized cyclopentadienyl radical (C 5 H 5) is a key species in the combustion and molecular growth kinetics of mono and poly-aromatic hydrocarbons (M/PAHs). At intermediate-to-low temperatures, the C 5 H 5 reaction with the hydroperoxyl radical (HO 2) strongly impacts the competition between oxidation to smaller products and growth to PAHs, precursors of soot. However, literature estimates for the HO 2 + C 5 H 5 reaction rate are inaccurate and inconsistent with recent theoretical calculations, thus generating discrepancies in global combustion kinetic models. In this work, we perform state-of-the-art theoretical calculations for the HO 2 + C 5 H 5 reaction including variable reaction coordinate transition state theory for barrierless channels, accurate thermochemistry, and multi-well master equation (ME) simulations. Contrary to previous studies, we predict that OH + 1,3-C 5 H 5 O is the main reaction channel. The new rate constants are introduced in two literature kinetic models exploiting our recently developed ME based lumping methodology and used to perform kinetic simulations of experimental data of MAHs oxidation. It is found that the resonantly-stabilized 1,3-C 5 H 5 O radical is the main C 5 H 5 O isomer, accumulating in relevant concentration in the system, and that the adopted lumping procedure is fully consistent with results obtained with detailed kinetics. The reactivity of C 5 H 5 O with OH and O 2 radicals is included in the kinetic mechanisms based on analogy rules. As a result, C 5 H 5 O mostly reacts with O 2 producing smaller C 3 /C 4 species and large amounts of C 5 H 4 O, suggesting that further investigations of the reactivity of both C 5 H 5 O and C 5 H 4 O with oxygenated radicals is necessary. Overall, this work presents new reliable rate constants for the HO 2 + C 5 H 5 reaction and provides indications for future investigations of relevant reactions in the sub-mechanisms of cyclopentadiene and MAH oxidation. [ABSTRACT FROM AUTHOR]