On the radical behavior of large polycyclic aromatic hydrocarbons in soot formation and oxidation

The mechanism of evolution of polycyclic aromatic hydrocarbons (PAHs) into carbonaceous particles in combustion, atmosphere, and interstellar space has been the subject of intense debate. Recently, there has been emerging evidence supporting resonantly-stabilized radicals as key players in PAH growth. In this work, we build on this hypothesis and propose that, beyond a critical size, PAH reactivity can be assimilated to that of radicals. We found that odd-C-numbered PAHs embedding 5-membered rings rapidly lose a hydrogen atom to form resonantly-stabilized radicals in combustion conditions, while even-C-numbered PAHs react as open-shell rather than closed-shell molecules independently of temperature, as usually assumed. Acenes were used as molecular models of large even-C-numbered PAHs. The construction of a kinetic model including these findings allows to interpret experimental soot oxidation data otherwise irreconcilable with existing chemical kinetic mechanisms.