The reactions of N(2D) with monocyclic and polycyclic aromatic hydrocarbons and their role in the atmospheric chemistry of Titan

We present a quantum mechanical study showing that the reaction mechanism established for N( 2 D) and monocyclic aromatic compounds (MACs) also applies to small polycyclic aromatic hydrocarbons (PAHs). Our results indicate that π bond conjugation in fused rings does not interfere with the reaction mechanisms, even though several strained rings are formed along the minimum energy path. In all cases investigated (N( 2 D) + benzene, toluene, pyridine, naphthalene, and phenanthrene), the main reaction channels occur via ring-contraction mechanisms that involve the elimination of HCN and the formation of cyclopentadienyl, substituted cyclopentadienyl, indenyl, and fluorenyl radicals. At first sight, these reactions contrast the growth of larger aromatic structures, since there is a loss of aromaticity in the ring attacked by N( 2 D). However, the products formed in ring-contraction mechanisms are resonance-stabilized radicals known to contribute to PAH formation in various media. N( 2 D) is produced in Titan’s upper atmosphere, where benzene peaks and aerosol formation starts. Therefore, these reactions, along with others leading to the formation of the first rings, can contribute to the formation of PAHs. Minor channels lead to the formation of 7-membered cyclic compounds incorporating N, the evolution of which remains to be determined. Another general conclusion is that aromatic rings are less resistant to chemical attack by energetic reactants, such as electronically excited species, than previously believed.