2016ApJ...830..128P

Small hydrocarbons such as acetylene is present in circumstellar envelopes of carbon-rich stars, but the processes that yield larger molecules, and eventually polycyclic aromatic hydrocarbons (PAHs), remain poorly understood. To gain additional insight into the early steps of such processes, electronic structure calculations were performed on the potential energy surfaces of C₄H₃⁺, C₆H₃⁺ and C₆H₅⁺. The results establish reactive pathways from acetylene and its ion to formation of the first aromatic ring. We characterize the stable isomers, their spectroscopic properties, and many of the transition structures that represent barriers to isomerization. The pathways to stabilized C₄H₃⁺ and C₆H₃⁺ are most likely to arise from unimolecular decomposition of hot C₄H₄⁺ and C₆H₄⁺ by H atom elimination. By contrast, we found an ion-molecule pathway to C₆H₅⁺ to be very stable to fragmentation and elimination reactions even without collisional stabilization. This aromatic species is a good nucleation center for the growth of larger PAHs in interstellar conditions.