Observed transitions of the isomers of HC₅N in TMC-1. The lines of HNC₅ are shown in the left panels, and the lines of HC₄NC are shown in the right panels. The abscissa corresponds to the rest frequency, and the ordinate is the antenna temperature, corrected for atmospheric and telescope losses, in milli Kelvin. The blank channels correspond to negative features produced when folding the frequency-switched data. The red lines correspond to the synthetic spectra calculated for a column density of 1.3 × 10¹⁰ cm⁻² for HNC₅ (T_rot = 6 K), and of 1.3 × 10¹¹ cm⁻² for HC₄NC (T_rot = 7.5 K).