Observed lines of CH₃CH₂CCH in the Q-band toward TMC-1. 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 using the parameters derived from a rotational diagram of the observed lines (T_r = 5 K and N(CH₃CH₂CCH) = 9.0 × 10¹¹ cm⁻²). The rotational quantum numbers are provided in the upper right corners of each panel.