2020ApJ...900L...9X

We report an astronomical detection of HC₄NC for the first time in the interstellar medium with the Green Bank Telescope toward the TMC-1 molecular cloud with a minimum significance of 10.5σ. The total column density and excitation temperature of HC₄NC are determined to be 3.29_–1.20^+8.60×10¹¹ cm^–2 and 6.7_–0.3^+0.3 K, respectively, using Markov chain Monte Carlo analysis. In addition to HC₄NC, HCCNC is distinctly detected whereas no clear detection of HC₆NC is made. We propose that the dissociative recombination of the protonated cyanopolyyne, HC₅NH⁺, and the protonated isocyanopolyyne, HC₄NCH⁺, are the main formation mechanisms for HC₄NC while its destruction is dominated by reactions with simple ions and atomic carbon. With the proposed chemical networks, the observed abundances of HC₄NC and HCCNC are reproduced satisfactorily.