Abstract
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.