2024A&A...688L..29F

We report the discovery of HNC₅ in TMC-1. Six lines have been found in harmonic relation, with quantum numbers J = 12-11 up to J = 17-16. The lines can be reproduced with the standard frequency relation for linear molecules with B = 1361.75034 ± 0.00033 MHz and D = 32.2 ± 0.7 Hz. The assignment of the carrier to iminopentadienylidene was achieved through examining the possible candidates at a high level of theoretical ab initio calculations. Motivated by the good agreement between the observed B and the calculated value for HNC₅, we searched for it in the laboratory and observed the transitions J = 5-4 to 7-6. The derived rotational and distortion constants are 1361.74998 ± 0.00040 MHz and 26.5 ± 5.5 Hz, respectively. Hence, we solidly conclude that the carrier of the lines found in TMC-1 is HNC₅. The calculated dipole moment for this species is 7.7 D and the derived column density is (1.3 ± 0.2) × 10¹⁰ cm^–2. We used the new QUIJOTE data to improve previous observations of HC₄NC and found that the abundance ratio HC₄NC/HNC₅ is 10 ± 2. The abundance ratio of HC₅N and its two isomers HC₄NC and HNC₅ is 500 ± 80 and 5100 ± 800, respectively. These abundance ratios are higher by a factor of ∼10 than those of the equivalent isomers of HC₃N. Chemical models reproduce the observed abundances reasonably well when a chemistry similar to that of the smaller species C₃HN isomers is adopted. The formation of HNC₅ and HC₄NC arises from the dissociative recombination with electrons of the cations HC₅NH⁺ and HC₄NCH⁺.