Bottom-Up Formation of Aromatic Molecules via Sequential Ion-Molecule Reactions Starting at C3H5

Aromatic molecules are important in many fields of chemistry due to their chemical stability and structural rigidity. Recently, multiple polycyclic aromatic hydrocarbon (PAH) species have been observed in the interstellar medium (ISM). While it is well known that PAHs are present in the ISM, how these molecules are formed in these environments is not fully understood. In order to better understand interstellar PAH formation, two classes of mechanisms have been considered, top-down and bottom-up. Top-down mechanisms explore PAH formation through fragmentation processes, and bottom-up mechanisms explore PAH formation through small-molecule aggregation. In bottom-up formation mechanisms, it has long been theorized that the formation of the first aromatic ring via ion-molecule reactions is the rate-limiting step. In this study, we experimentally demonstrate the bottom-up formation of at least three aromatic ions through sequential ion-molecule reactions under interstellar conditions. The assignment of the structures of these molecules is supported through a combination of experimental chemical reactivity studies and quantum chemical calculations. These results reveal that aromatic rings can form with high efficiency in the gas-phase through barrierless reactions and introduce a new potential formation pathway for polycyclic aromatic hydrocarbons throughout the ISM.