The rotational spectrum of HC8N, a linear molecule with a triplet ground state

By means of Fourier transform microwave spectroscopy of a supersonic molecular beam, we report here the laboratory detection of HC₈N, a carbon chain with a triplet electronic ground state. A total of 71 hyperfine components from six rotational transitions between 8 and 13 GHz were measured to an uncertainty of 5 kHz or better. Eight spectroscopic constants, including six fine and hyperfine coupling constants, were determined to high accuracy, despite a complex spectrum which consists of many closely spaced lines. Like the shorter HC_2nN chains, the linear triplet isomer detected here is highly polar and is almost certainly the ground state of HC₈N; in our molecular beam discharge source, it is 10 times less abundant than triplet HC₆N, but more than 100 times less abundant than the well-studied closed-shell cyanopolyyne HC₇N. Owing to the collapse of the hyperfine structure with increasing chain length, still longer chains may be detectable.