(a) Observed group of lines assigned to the hyperfine structure of the 2_0, 2–1_0, 1 transition of HDCCN (black histogram). (b) The observed hyperfine structure of the same transition of o-H₂CCN. (c) Selected hyperfine components of the 2₁₂–1₁₁ transition of p-H₂CCN. (d) Selected hyperfine components of the 2₁₁–1₁₀ transition of p-H₂CCN. In all panels, the abscissa represents the rest frequency (in MHz) assuming a local standard of rest velocity for the source of 5.83 km s⁻¹ (Cernicharo et al. 2020c). The ordinate is the antenna temperature in mK. The red curve in each panel represents the best model fit to the emission of this species in TMC-1 for a rotational temperature of 10 K. A similar fit is obtained for other rotational temperatures (see Sect. 4.1 and Table 2). Negative features are due to the folding of the frequency-switching data.