Abstract The rotational spectrum of trifluoromethyl isocyanate, CF 3 NCO, in the ground vibrational state has been measured in the region 3-15 GHz using a molecular beam Fourier transform spectrometer. The spectrum was analyzed using two theoretical models: the model of a semirigid molecule with internal rotation of the trifluoromethyl group and the quasi-symmetric-top model. The nuclear hyperfine structure of 32 a-type J+ 1← J ...