V. D. Gordon (currently with UT Austin), M. C. McCarthy, A. J. Apponi, AND P. Thaddeus are with the
Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge,MA 02138; and
Division of Engineering and Applied Sciences, Harvard University, 29 Oxford Street, Cambridge, MA 02138A linear triplet isomer of HC6N has been detected by Fourier transform microwave spectroscopy in a
supersonic molecular beam. A total of 85 hyperfine components from six rotational transitions between 8
and 18 GHz were measured to an uncertainty of 5 kHz; a similar set of transitions were detected for the
15N isotopic species, produced using an isotopically enriched precursor gas sample. The spectroscopic
constants for both species, including the fine and hyperfine coupling constants, were determined to very
high accuracy, and these allow calculation of the radio spectrum to a fraction of 1 km 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.
Subject headings : ISM: molecules -- line : identification -- methods: laboratory -- molecular data --
molecular processes -- radio lines : ISMChemistr