Detection of interstellar HCCCHCN: A possible link between chains and rings in cold cores

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 CN and HCN, (b) purely gas-phase reactions between CN and both propyne (CHCCH) and allene (CHCCH), and (c) via the reaction CN + HCCCHCCH. Results. We report the first detection of cyanoacetyleneallene (HCCCHCN) 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 HCCCHCN that match the column density of 2 × 1011 cm 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.