Laboratory detection of a new carbon chain radical:: H2CCCCN

The cyanopropynyl radical, H2CCCCN, was detected in a supersonic molecular beam by Fourier transform microwave spectroscopy. For the normal and N-15 species, 89 and 40 hyperfine components, respectively, from four successive rotational transitions (N = 2 --> 1 to 5 --> 4) in the lowest energy (K-a = 0) rotational ladder were measured between 8 and 22 GHz to an uncertainty of 5 kHz. With this data, the microwave constants of cyanopropynyl, including the fine and hyperfine coupling constants, are determined to very high accuracy, and the most interesting lines in the radio spectrum from an astronomical standpoint can now be calculated to a fraction of 1 km s(-1) in equivalent radial velocity.