3_3, 1 − 3_2, 2 rotational transition of gg conformer of glutaronitrile showing the multiple hyperfine components completely resolved by FTMW spectrometer. The coaxial arrangement of the adiabatic expansion and the resonator axis produces an instrumental Doppler doubling, which is marked with the dashed lines. The resonances frequencies are calculated as the average of the two Doppler components. Each hyperfine component is labelled with the corresponding values of quantum numbers I′,F′-I″, F″.