The rotational spectrum of the negative molecular ion NCO– has been observed both in a supersonic molecular beam and in a low-pressure glow discharge. The identification is ironclad because of the previous infrared detection of NCO–, the presence of well-resolved nitrogen quadrupole hyperfine structure, and the observation of nine harmonically related transitions in the millimeter band. The spectroscopic constants B and D are three orders of magnitude more accurate than those derived from the earlier IR measurements, and the theoretical eQq is in good agreement with that measured. The entire rotational spectrum can now be calculated well into the THz region to 1 km s–1 in equivalent radial velocity or better. NCO– is an excellent candidate for radio astronomical detection because of its high stability, polarity, and favorable partition function. The fairly high concentration of NCO– in our laboratory source implies that other molecular anions containing the NCO group may be detectable
