Locating extrema on the potential energy surface of a reacting system methyl and fluorenyl via quantum chemical methods

The study of aromatic and resonance-stabilized radicals reaction mechanisms has a key importance for understanding the processes of molecular mass growth of polycyclic aromatic hydrocarbons (PAHs) and carbon nanoparticles in various astrophysical conditions, such as molecular clouds, circumstellar envelopes, and combustion systems. In this paper, we present a study of the gas-phase high-temperature reaction between methyl $CH_{3}^{\bullet}$ and fluorenyl $C_{13}H_{9}^{\bullet}$ radicals using high-level quantum chemical computational methods. Based on the local extrema found on the potential energy surface of the reacting system $CH_{3}^{\bullet}$ + $C_{13}H_{9}^{\bullet}$, reaction paths leading to the formation of three-ring PAHs: anthracene and phenanthrene were found. Two different mechanisms of their formation were identified, based on the recombination of radicals and the expansion of five-membered rings, which is of particular importance for understanding the processes of PAH growth and the formation of carbon structures under extreme conditions.