Formation of Monocyclic and Polycyclic Hydrocarbons from Sequential Toluene Reactions in Coulomb Crystals

Understanding the formation and reactivity of aromatic molecules is important for understanding chemistry in a wide range of environments, including the interstellar medium. The clear observation of polycyclic aromatic hydrocarbons (PAHs) in space has intensified efforts to identify efficient formation pathways capable of producing such complex species under cold, dilute conditions. In astrochemical environments, ion-molecule reactions are typically the dominant type of reactions due to their high efficiency compared to neutral-neutral reactions. However, current models of PAH formation largely neglect ion-molecule reactions beyond the initial formation of benzene. Here, we present results from a series of ion-molecule reactions carried out in Coulomb Crystals beginning with the protonation of toluene to identify possible reaction pathways that could lead to interstellar PAH growth. In this work, we observe the formation of monocyclic aromatic species, C₆H₅⁺ and C₇H₇⁺, via the initial dissociative proton transfer to toluene. We also observe an associative proton transfer product, resulting in C₇H₉⁺. We find the sequential reaction between C₇H₉⁺ and C₇H₈ results in the formation of C₇H₅⁺, C₁₂H₅⁺, and C₁₂H₈⁺, which provides the first experimental evidence of a possible interstellar formation mechanisms of polycyclic cations beginning with monocyclic precursors.