The rotational spectrum of the NI radical in the X 3Σ ground electronic state has been observed by a source modulated spectrometer with a free space cell. The NI radical was generated in the cell by a dc (direct current) glow discharge in a mixture of iodine vapor with N2 and He. A rotational transition consisted of a group of congested lines resulting from the hyperfine interactions due to both iodine and nitrogen nuclei. The molecular constants including the spin–spin and spin–rotation coupling constants and the magnetic and nuclear quadrupole hyperfine interaction constants were determined by the least-squares analysis. The spin density of the unpaired electrons was estimated from the observed hyperfine coupling constants to be 76.8% and 23.4% on the nitrogen and the iodine atoms, respectively. The quadrupole coupling constant was related with the difference in electronegativity of the two bonded atoms.

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