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 HC2nN 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.