Discovery of HC 4 NC in TMC-1: A study of the isomers of HC 3 N, HC 5 N, and HC 7 N

We present a study of the isocyano isomers of the cyanopolyynes HC 3 N, HC 5 N, and HC 7 N 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 HC 6 NC in the same source. In addition, the deuterated isotopologues of HCCNC and HNCCC were detected, along with all 13 C substitutions of HCCNC, also for the first time in space. The abundance ratios of HC 3 N and HC 5 N, with their isomers, are very different in TMC-1 and IRC+10216, namely, N (HC 5 N)/ N (HC 4 NC) 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.