LABORATORY DETECTION OF THE RING-CHAIN MOLECULE C7H2

Following the recent laboratory detection of the ring-chain carbene C5 H2, we have now detected C5H2, the next longer member of the sequence formed by substituting a carbon chain for one of the hydrogen atoms of cyclic C3H2. The strongest lines, again obtained with a diacetylene-n eon discharge, were roughly 15 times weaker than those of C5H2 Enough rotational lines were detected in both the normal and doubly deuterate d isotopic species to be certain of the identification and to obtain t he rotational and leading centrifugal distortion constants accurately enough for calculation of the entire rotational spectrum below 300 GHz to a fraction of 1 km s(-1) in equivalent radial velocity. The ring c hain here is calculated to be the most stable isomer of C7H2, and extr emely polar, so it is of considerable astronomical interest. Several o ther isomers of C7H2 may be detectable in the laboratory and in space with present techniques.