Barrierless Gas-Phase Formation of the Phenyl Radical (C6H5, X2A1) via Atomic Carbon Induced Ring Expansion–A Versatile Framework to Huckel Aromatic Aryl Radicals in Molecular Clouds

Abstract The low-temperature formation of aromatic molecules in the interstellar medium remains a major challenge in astrochemistry. Here, crossed molecular beam experiments combined with high-level electronic structure and Rice–Ramsperger–Kassel–Marcus (RRKM) calculations reveal the barrierless formation of the simplest aryl radical, the 6π Hückel aromatic phenyl radical (C6H5•), through the reaction of ground-state atomic carbon with 1,3-cyclopentadiene (c-C5H6). The reaction proceeds via carbon addition, ring expansion, hydrogen atom elimination, and aromatization, with all transition states lying below the energy of the separated reactants. The measured reaction exoergicity agrees with theory, rendering the reaction operative even in the coldest interstellar environments. More broadly, carbon-atom-induced ring-expansion can transform cyclopentadiene motifs embedded within larger polycyclic aromatic hydrocarbons into aryl radical centers, providing a previously unrecognized route to aromatic molecular growth. These findings establish carbon-atom-mediated skeletal editing as a viable low-temperature mechanism for building increasingly complex aromatic carbonaceous molecules in space.