Observed lines of 1-ECP in the 31-50 GHz frequency range towards TMC-1. Line parameters for the complete list of detected lines of 1-ECP are given in Table A.2. The abscissa corresponds to the rest frequency assuming a local standard of rest velocity of 5.83 km s⁻¹. The ordinate is the antenna temperature corrected for atmospheric and telescope losses in mK. The red line shows the synthetic spectrum obtained for an assumed T_r of 9 K and N(1-ECP) = (2.0 ± 0.4) × 10¹² cm⁻². The rotational quantum numbers are indicated in each panel. Blanked channels correspond to negative features produced in the folding of the frequency switching data.