The Hidden Path to the Resonance-Stabilized Fulvenallenyl Radical (C₇H₅) via the Bimolecular Reaction of Tricarbon (C₃, X¹Σ_g⁺) with 1,3-Butadiene (C₄H₆; X¹A_g)

We report a novel bimolecular reaction pathway forming the resonance-stabilized fulvenallenyl radical (C₇H₅) via the gas-phase reaction of tricarbon (C₃, X¹Σg⁺) with 1,3-butadiene (C₄H₆, X¹A_g). Crossed molecular beam experiments combined with high-level electronic structures and statistical calculations reveal a rich potential energy surface. The reaction proceeds via tricarbon addition to a double bond of 1,3-butadiene, overcoming a 30 kJ mol^–1 barrier, followed by ring closure, isomerizations (ring opening/closures, hydrogen shifts), and eventual hydrogen atom loss, yielding the fulvenallenyl radical (p1) almost exclusively via two dominant pathways. Considering the entrance barrier, this reaction may occur in high-temperature environments, like circumstellar envelopes of carbon stars, but is suppressed in colder regions such as molecular clouds and Titan’s atmosphere. These findings highlight the unexpected reactivity of small carbon clusters in shaping the molecular complexity of our universe, from the flicker of a flame to the death of a star.