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Laboratory Detection of the Carbon Ring Chain C9H2

The ring-chain C9H2 molecule, formed by substituting the carbon chain C6H for one of the hydrogen atoms of the carbene ring C3H2, one of the more abundant astronomical molecules, has been detected in the laboratory by Fourier transform microwave spectroscopy. Fifteen a-type rotational transitions between 6 and 14 GHz were measured to 2 kHz or better, and precise values were determined for the rotational and centrifugal distortion constants. With these, the entire radio spectrum can be calculated to better than 1 km s−1 in equivalent radial velocity over the range of interest to radio astronomy. The strongest lines of C9H2, like those of the smaller ring chain C7H2, were obtained with a diacetylene-neon discharge but were about 10 times weaker. Ab initio calculations have not been done for C9H2, and isomeric energies are not known, but by analogy with the shorter members of the sequence, the ring-chain carbene here is probably the most stable structure. Of the many possible isomers of C9H2, several may lie low enough in energy to be detectable in the laboratory and in space with present techniques. Ions and radicals with ring-chain structures are also excellent candidates for laboratory and astronomical detection.