Two low-J transitions, N(J) = 1(0)-0(1) and 1(2)-0(1), of the HCCN radical in its 3SIGMA- ground state have been studied by a pulsed-discharge-nozzle Fourier-transform microwave spectrometer. Spectrum with well-resolved hyperfine splittings due to hydrogen and nitrogen nuclei was observed, which enabled us to determine accurate hyperfine coupling constants for both the nuclei for the first time in the present study, as well as a much improved spin-spin coupling constant. These hyperfine coupling constants suggest that the electronic structure of the HCCN radical can be described as the one with the allenic structure to be almost twice more important than the carbenic structure. The present results seems to support a conclusion that the molecular structure of the radical is almost linear with a possibility of a large amplitude bending motion.
