Splitting patterns in the lowest-J rotational transitions in the ground state (v_b = 0, m = 0) of CH₃NCO. The splitting is due to the interaction of the nuclear quadrupole of the ¹⁴N nucleus with molecular rotation and is characterized by the total angular momentum quantum number F. Each component is doubled by the Doppler doubling, which is an instrumental effect specific to the FP-FTMW spectrometer. The splitting is completely resolved at the supersonic expansion conditions (T_rot = 2 K) of the FP-FTMW spectrometer. Nevertheless, the magnitude, and thus the relevance of the splitting, decreases rapidly with increasing J.