The radio spectrum of the phenyl radical

The phenyl radical, C6H5, derived from benzene by removal of one hydrogen atom, was detected at centimeter wavelengths in a pulsed supersonic molecular beam and subsequently at millimeter wavelengths in a low-pressure direct current glow discharge. Fourteen rotational transitions between 9 and 40 GHz and over 50 transitions between 150 and 330 GHz, each split by spin doubling, have been measured for the normal isotopic species, and a comparable number have been measured for the fully deuterated species. The spectrum of both isotopic species at millimeter wavelengths is reproduced to an uncertainty of 0.5 km s(-1) or better with seven spectroscopic constants. Rotational constants predicted from high-level molecular structure calculations are in excellent agreement with the measurements. Phenyl is a prime candidate for astronomical detection, because it is the prototypical aromatic hydrocarbon radical and a possible progenitor of other aromatic species.