Space and laboratory discovery of iminopentadienylidene, HNC5

Abstract

We report the discovery of HNC$_5$ 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 $B$=1361.75034pm 0.00033 MHz and $D$=32.2pm 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 HNC$_5$, we searched for it in the laboratory and observed the transitions $J$=5-4 to 7-6. The derived rotational and distortion constants are 1361.74998pm 0.00040 MHz and 26.5pm 5.5 Hz, respectively. Hence, we solidly conclude that the carrier of the lines found in TMC-1 is HNC$_5$. The calculated dipole moment for this species is 7.7\,D and the derived column density is (1.3pm 0.2)times 1010 $. We used the new QUIJOTE data to improve previous observations of HC$_4$NC and found that the abundance ratio HC$_4$NC/HNC$_5$ is 10pm 2. The abundance ratio of HC$_5$N and its two isomers HC$_4$NC and HNC$_5$ is 500pm 80 and 5100pm 800, respectively. These abundance ratios are higher by a factor of sim 10 than those of the equivalent isomers of HC$_3$N. Chemical models reproduce the observed abundances reasonably well when a chemistry similar to that of the smaller species C$_3$HN isomers is adopted. The formation of HNC$_5$ and HC$_4$NC arises from the dissociative recombination with electrons of the cations HC$_5$NH$^+$ and HC$_4$NCH$^+$.