Discovery of a new molecular ion, HC₇NH⁺, in TMC-1

We report the detection of the protonated form of HC 7 N in TMC-1. The discovery of the cation HC 7 NH + was carried out via the observation of nine harmonically related lines in the Q -band using the Yebes 40 m radiotelescope. The observed frequencies allowed us to obtain the rotational constants B 0 = 553.938802 (160) MHz and D 0 = 3.6292 (705) Hz. The identification of HC 7 NH + is further supported by accurate ab initio calculations. We derived a column density of (5.5 ± 0.7) × 10 10 cm −2 , which constitutes another piece of evidence for the identification of the carrier. In addition, we revised the HC 7 N column density and we derived a new value of (2.1 ± 0.2) × 10 13 cm −2 . Hence, the abundance ratio HC 7 N/HC 7 NH + is ∼380, while those for HC 3 N/HC 3 NH + and HC 5 N/HC 5 NH + are ∼230 and ∼240, respectively. Here, we discuss these results within the framework of a chemical model for protonated molecules in cold dense clouds.