Observation of CH₃CN in flow at a total pressure of 25 × 10⁻³ mbar with GACELA using a load switching observing procedure. Upper panel: whole 72–116 GHz range resulting from the observation of the three sub-bands of the W-band receiver. The integration time is 11 min per sub-band. Left column: zoom of the J = 6−5 transition of CH₃CN (upper panel), (middle panel), and again together with probably the ν₇ or ν₄ + ν₈ mode of CH₃CN (bottom panel). Three panels in the middle column: J = 6−5 transition of CH₃CN in the bending mode ν₈ (upper panel), and its overtones at 2ν₈ (middle panel) and 3ν₈ (bottom panel). Three panels in the right column: J = 6 → 5 transition of the ν₄ mode of CH₃CN (upper panel), the same transition of the ground state of ¹³CH₃CN (middle panel), and of CH₃C¹⁵N (bottom panel). All the singly substituted isotopologues of methyl cyanide are in natural abundance. Intensities are expressed in K and frequencies in MHz. The kinetic temperature of the gases inside chamber is 297 K in these experiments. The difference between the observed frequencies and those measured in other laboratories, or predicted from accurate rotational constants, are less than 5 kHz for signals above 0.2 K and ∼5–20 kHz for the weakest features.