Identification of the interstellar 1-cyano propargyl radical (HCCCHCN) in TMC-1

We report the first detection in interstellar medium of the 1-cyano propargyl radical, HC₃HCN. This species is an isomer of the 3-cyano propargyl radical (CH₂C₃N), which was recently discovered in TMC-1. The 1-cyano propargyl radical was observed in the cold dark cloud TMC-1 using data from the ongoing QUIJOTE line survey, which is being carried out with the Yebes 40m telescope. A total of seven rotational transitions with multiple hyperfine components were detected in the 31.0–50.4 GHz range. We derived a column density of (2.2 ± 0.2) × 10¹¹ cm⁻² and a rotational temperature of 7±1 K. The abundance ratio between HC₃HCN and CH₂C₃N is 1.4. The almost equal abundance of these isomers indicates that the two species may be produced in the same reaction with a similar efficiency, probably in the reaction C + CH₂CHCN and perhaps also in the reaction C₂ + CH₃CN and the dissociative recombination with electrons of CH₂C₃NH⁺.