INSIGHTS INTO HYDROCARBON CHAIN AND AROMATIC RING FORMATION IN THE INTERSTELLAR MEDIUM: COMPUTATIONAL STUDY OF THE ISOMERS OF AND AND THEIR FORMATION PATHWAYS

Abstract

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 , and . 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 and are most likely to arise from unimolecular decomposition of hot and by H atom elimination. By contrast, we found an ion-molecule pathway to 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.