Spectrum of TMC-1 taken with the Yebes 40 m telescope around 37.46 GHz (black histogram). The frequencies of the six most intense hyperfine components of the 2_0, 2–1_0, 1 transition of ortho CH₂CCH are indicated by red dotted vertical lines. Transition quantum numbers, frequencies, and derived line parameters are given in Table 1. The synthetic spectrum (red line) was computed for a column density of ortho CH₂CCH of 6.5 × 10¹³ cm⁻², a rotational temperature of 10 K, an emission size of 40″ of radius, and a linewidth of 0.72 km s⁻¹ (see main text).