Rotational transitions in the vibrational state v₂ = 1f of HCN, HNC and HCO⁺ covered within the surveyed frequency range. The box corresponding to HNC 2 − 1 v₂ = 1f was left intentionally blank since its emission at 182.6 GHz falls within the telluric water transition observation gap (Sect. 2). Red lines show an attempt to fit the observed emission under LTE assuming T_ex = 300 K (see text in Sect. 5.2 for details). Nearby transitions from detected species with modeled flux densities > 10% of that of the vibrational transitions are labeled.