Detection of Interstellar HC4NC and an Investigation of Isocyanopolyyne Chemistry under TMC-1 Conditions

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 ${3.29}_{-1.20}^{+8.60}\times {10}^{11}$ cm^-2 and ${6.7}_{-0.3}^{+0.3}\ {\rm{K}}$ , 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.