2021A&A...652L..12S

Context. The chemical pathways linking the small organic molecules commonly observed in molecular clouds to the large, complex, polycyclic species long suspected of being carriers of the ubiquitous unidentified infrared emission bands remain unclear.
Aims. To investigate whether the formation of mono- and polycyclic molecules observed in cold cores could form via the bottom-up reaction of ubiquitous carbon-chain species with, for example, atomic hydrogen, a search is made for possible intermediates in data taken as part of the GOTHAM (GBT Observations of TMC-1: Hunting for Aromatic Molecules) project.
Methods. Markov chain Monte Carlo (MCMC) source models were run to obtain column densities and excitation temperatures. Astrochemical models were run to examine possible formation routes, including (a) a novel grain-surface pathway involving the hydrogenation of C₆N and HC₆N, (b) purely gas-phase reactions between C₃N and both propyne (CH₃CCH) and allene (CH₂CCH₂), and (c) via the reaction CN + H₂CCCHCCH.
Results. We report the first detection of cyanoacetyleneallene (H₂CCCHC₃N) in space toward the TMC-1 cold cloud using the Robert C. Byrd 100 m Green Bank Telescope. Cyanoacetyleneallene may represent an intermediate between less-saturated carbon chains, such as the cyanopolyynes, that are characteristic of cold cores and the more recently discovered cyclic species, such as cyanocyclopentadiene. Results from our models show that the gas-phase allene-based formation route in particular produces abundances of H₂CCCHC₃N that match the column density of 2x10¹¹cm^–2 obtained from the MCMC source model, and that the grain-surface route yields large abundances on ices that could potentially be important as precursors for cyclic molecules.