The cyclic C5H radical

Cyclic C5H, the free radical formed by substituting an ethynyl group CCH fo r the hydrogen atom in the astronomical molecule c-C3H, has been detected i n the laboratory in a supersonic molecular beam by Fourier transform microw ave spectroscopy. Enough lines in the rotational spectrum have been measure d to allow a deep search for this radical with radio telescopes. Contrary t o theoretical prediction, c-C5H is apparently unsymmetrical, plausibly the result of pseudo-Jahn-Teller deformation.