J = 4 − 3 and J = 5 − 4 transitions of the single substituted isotopologs of HCCNC. The abscissa corresponds to the velocity with respect to the local standard of rest. The derived line parameters are given in Table A.1. The ordinate is the antenna temperature, corrected for atmospheric and telescope losses, in mK. Blanked channels correspond to negative features produced when folding the frequency-switched data. The red line shows the computed synthetic spectra for these lines (see Sect. 3.3). The second row panels correspond to a zoom in intensity of the lines of the main isotopolog (first row) to show the emission from its weak hyperfine components. The J = 4 − 3 line of DCCNC (marked with a cyan star) is affected by the negative features produced by the hyperfine structure of CH₃CN which is ∼8 MHz above in frequency.