Laboratory detection of HC6N, a carbon chain with a triplet electronic ground state

A linear triplet isomer of HC6N has been detected by Fourier transform micr owave spectroscopy in a supersonic molecular beam. A total of 85 hyperfine components from six rotational transitions between 8 and 18 GHz were measur ed to an uncertainty of 5 kHz; a similar set of transitions were detected f or the N-15 isotopic species, produced using an isotopically enriched precu rsor gas sample. The spectroscopic constants for both species, including th e fine and hyperfine coupling constants, were determined to very high accur acy, and these allow calculation of the radio spectrum to a fraction of 1 k m s(-1) in equivalent radial velocity. Triplet HC6N is a highly polar, low- lying isomer; measurements show it to be about 10 times more abundant than a ring-chain isomer recently detected with the same spectrometer.