2020A&A...642L...8C

We present a study of the isocyano isomers of the cyanopolyynes HC₃N, HC₅N, and HC₇N in TMC-1 and IRC+10216 carried out with the Yebes 40m radio telescope. This study has enabled us to report the detection, for the first time in space, of HCCCCNC in TMC-1 and to give upper limits for HC₆NC in the same source. In addition, the deuterated isotopologues of HCCNC and HNCCC were detected, along with all ¹³C substitutions of HCCNC, also for the first time in space. The abundance ratios of HC₃N and HC₅N, with their isomers, are very different in TMC-1 and IRC+10216, namely, N(HC₅N)/N(HC₄NC) is ∼300 and ≥2100, respectively. We discuss the chemistry of the metastable isomers of cyanopolyynes in terms of the most likely formation pathways and by comparing observational abundance ratios between different sources.