Observed lines for the six isotopologues of CH₃CCCCH in TMC-1 in the 31.0−50.4 GHz range. Frequencies and line parameters are given in Table A.1. Quantum numbers for the observed transitions are indicated in each panel. The red line shows the synthetic spectrum computed for a rotational temperature of 9 K and the column densities from Table 1. Blank channels correspond to negative features created during the folding of the frequency-switching data.