Section of the submillimeter spectrum of CH₃CN. Transitions of ¹³CH₃CN (J = 36−35) and CHCN (J = 35−34) in their ν₈ = 1 excited vibrational states were labeled with their k quantum numbers. Spin-statistics cause k = −2, −5, and 7 to appear stronger than expected, 1^∗ indicates the A₂ component of the asymmetry-like split k = + 1 transition, see Sect. 3 for further details. The clipped lines are the J = 35−34k = −8 and k = 10 transitions of the main isotopolog in ν₈ = 1. The latter line is overlapping the A₁ component of the k = + 1J = 36−35 line of CH₃C¹⁵N, ν₈ = 1. The lines appear similar to second derivatives of a Gaussian line-shape because of the 2f-modulation.