2022A&A...663L...9C

We report the detection of fulvenallene (c-C₅H₄CCH₂) in the direction of TMC-1 with the QUIJOTE¹ line survey. Thirty rotational transitions with K_a = 0,1,2,3 and J = 9-15 were detected. The best rotational temperature fitting of the data is 9 K and a derived column density is (2.7 ± 0.3) x 10¹² cm^–2, which is only a factor of 4.4 below that of its potential precursor cyclopentadiene (c-C₅H₆), and 1.4-1.9 times higher than that of the ethynyl derivatives of cyclopentadiene. We searched for fulvene (c-C₅H₄CH₂), a CH₂ derivative of cyclopentadiene, for which we derive a 3σ upper limit to its column density of (3.5 ± 0.5) x 10¹² cm^–2. Upper limits were also obtained for toluene (C₆H₅CH₃) and styrene (C₆H₅C₂H₃), the methyl and vinyl derivatives of benzene. Fulvenallene and ethynyl cyclopentadiene are likely formed in the reaction between cyclopentadiene (c-C₅H₆) and the ehtynyl radical (CCH). However, the bottom-up gas-phase synthesis of cycles in TMC-1 underestimates the abundance of cyclopentadiene by two orders of magnitude, which strengthens the need to study all possible chemical pathways to cyclisation in cold dark cloud environments, such as TMC-1. However, the inclusion of the reaction between C₃H₃ and C₂H₄ produces a good agreement between model and observed abundances.