Discovery of HC4NC in TMC-1: A study of the isomers of HC3N, HC5N, and HC7N

TL;DR

This paper reports the first detection of HCCCCNC in space within TMC-1, along with other isomers and isotopologues of cyanopolyynes, providing new insights into their chemistry and formation pathways in interstellar environments.

Contribution

First-time detection of HCCCCNC and various isotopologues of cyanopolyynes in space, with analysis of their abundance ratios and formation chemistry.

Findings

HCCCCNC detected in TMC-1 for the first time

Abundance ratios of cyanopolyynes vary significantly between sources

Upper limits established for HC6NC in TMC-1

Abstract

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.