Theoretical Investigation of PAH Polymerization Reactions Involving Indenyl Radicals or Ethynyl-Substituted PAHs

Soot is an important topic in health, environment, materials, astronomy, and combustion. However, the formation of soot from its precursor polycyclic aromatic hydrocarbons (PAHs) is not well understood. The chemical-based soot inception pathway can survive at the flame temperature and is regarded as a promising solution for explaining soot formation. In this work, chemical-based polymerization of various connection pathways was comprehensively investigated via density functional theory calculations, transition state theory, and RRKM-ME simulations (1000-2500 K, 1-100 atm). The PAH polymerization reactions were investigated individually by considering the site effects for the reactions between the indenyl radical (C₉H₇) and phenylacetylene (A₁C₂H), reactions between the 3-ethynylphenyl radical and different aromatic molecules, as well as the reactions between 2-ethynyllnaphthalene and the 2-ethynyllnaphthyl radical. The energy well for the adduct with radical characters ranged from 26.8 to 61.8 kcal/mol, and the adduct became more stable if it formed from PAH with C₂H or C₄H₃ substitution. The results showed that the addition reaction on the indenyl radical is unfavorable compared to the bridging reaction involving PAH with substitution, in terms of the reaction rate coefficients and product yield. For the 2-A₂C₂H dimerization reaction in this work, the kinetic results highlighted its priority in comparison with those of other polymerization pathways. Furthermore, reactivity degeneration was observed for the adduct with the radical located on chain substitution.