Detection of the triplet HC4N radical by Fourier transform microwave spectroscopy

The triplet HC4N radical in a linear carbon-chain form has been detected by Fourier transform microwave spectroscopy for the first time. Rotational transitions with fine and hyperfine structures, about 130 lines in total, were observed in a pulsed-discharge-nozzle supersonic expansion of the HC3N sample diluted in Ar buffer gas. The spectrum was assigned to the linear HC4N radical by various combinations of discharge gases, by the rotational, fine and hyperfine structures, and most decisively, by comparison of the determined molecular constants with those of the HCCN radical.