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, HC${}_3$HCN. This species is an isomer of the 3-cyano propargyl radical (CH${}_2$C${}_3$N), 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$\pm$0.2)$\times$10${}^{11}$ cm${}^{-2}$ and a rotational temperature of 7$\pm$1,K. The abundance ratio between HC${}_3$HCN and CH${}_2$C${}_3$N is 1.4. The almost equal abundance of these isomers indicates that the two species may be produced in the same reaction with a similar efficiency, probably in the reaction C + CH${}_2$CHCN and perhaps also in the reaction C${}_2$ + CH${}_3$CN and the dissociative recombination with electrons of CH${}_2$C${}_3$NH${}^{+}$