Spectra of the nondetected (2–1) transitions of PO, PN, HCP, and CP. The upper x-axis shows the rest frequency (in MHz) and the lower one is a velocity axis (in km s⁻¹). The red dashed line indicates the 3σ level and the blue dashed line shows the transition frequency of the corresponding molecule. In the case of PO, we show as an example one of the observed transitions at 108.998 GHz.