Formation of Nitrogen-Containing Aromatic Compounds in the Envelopes of Asymptotic Giant Branch Stars

The formation of the bicyclic N‑heterocyclic aromatic compound indole (C8H7N) from aniline (C6H5NH2) has been shown to proceed via the widely recognized “hydrogen abstraction–acetylene addition” (HACA) mechanism that is commonly employed to describe molecular mass growth in polycyclic aromatic hydrocarbons. Hydrogen abstraction from the NH2 functional group produces the N‑phenylamino radical (C6H5NH), whereas abstraction from the benzene ring produces the o‑aminophenyl radical (C6H4NH2). The mechanisms and kinetics of reactions between acetylene (C2H2) and the C6H5NH and C6H4NH2 radicals under the conditions of carbon‑rich circumstellar envelopes of asymptotic giant branch (AGB) stars have been elucidated using high‑level quantum chemical methods combined with advanced transition state theory. Based on the constructed potential energy surface diagrams and calculated dependencies of the kinetic constants of the processes, the reaction paths and their relative contributions to the composition of the final products have been identified. Notably, under AGB stellar envelope conditions, indole formation dominates over all other reaction products at temperatures of T < 2000 K.