Observed lines of HC₄NC found in the 31–50 GHz frequency range towards TMC-1. The abscissa corresponds to the local standard of rest velocity in km s⁻¹. Frequencies and intensities for the observed lines are given in Table 2. The ordinate is the antenna temperature corrected for atmospheric and telescope losses in mK. The J = 17 − 16 line is not detected within a 3σ level of 5.7 mK. Spectral resolution is 38.1 kHz.