Notes. Parameters of the transitions obtained from the molecular databases CDMS (Müller et al. 2005; Endres et al. 2016) and JPL (Pickett et al. 1998). In more detail, the CH₃CN and ¹³CH₃ CN entries are based on Müller et al. (2015, 2009), respectively, with important additional data especially from Cazzoli & Puzzarini (2006) and Pearson & Müller (1996). The dipole moment of CH₃ CN used to derive the Einstein A_ul values is taken from Gadhi et al. (1995); the dipole moment of ¹³CH₃CN is assumed to agree with that of the main isotopologue, as is frequently done.