Structural effects on the growth of large polycyclic aromatic hydrocarbons by C2H2

Soot particles are composed of planar and curved polycyclic aromatic hydrocarbons with different types of reactive sites, where their growth and oxidation reactions occur. This study presents the effect of curvatures in PAHs present in soot on their growth in the flame environment. For this, six planar and curved model PAH molecules having five to nine aromatic rings with armchair and zigzag sites are selected. Density functional theory (B3LYP functional and 6-311G (d,p) basis set) is used to study the reaction energetics for their growth through hydrogen-abstraction-C2H2-addition mechanism that is primarily used in all soot models for their chemical growth. The rate constants evaluated using transition state theory for the reactions involved in the mechanism are provided. Through energetics and kinetics comparison, the differences in the reactivity of planar and curved PAHs at different site types are observed. The growth at armchair site was found to be sensitive to the PAH structure, with the curved one having higher growth rate at all temperatures between 1000 and 2500K studied in this work. The PAH curvature had less impact on the growth at zigzag sites of the model PAHs. The fast conversion of planar PAH to a curved one through ring addition and the high growth rate of curved PAHs at low temperatures suggests that tortuous PAHs can also be present in nascent soot. (C) 2019 The Combustion Institute. Published by Elsevier Inc. All rights reserved.