Non-tunneling component of the 1₀₁ ← 0₀₀ rotational transition of doubly deuterated methyl formate recorded with the Fourier transform microwave spectrometer of Set A measurements. The quadrupole hyperfine structure associated with the two deuterium atoms can clearly be seen in the observed spectrum displayed in the upper part of the figure. The lower part of the figure shows the calculated spectrum. A Lorentzian lineshape with a half width at half height of 1.2 kHz was assumed for each hyperfine component. The experimental Doppler splitting due to the molecular beam spectrometer was taken to be equal to 66 kHz. The labeling of the transitions is presented in Sect. 3.1 and the hyperfine patterns are analyzed in Sect. 3.4.