C₃H exists in two isomeric forms, a linear one and a cyclic one. The latter is considerably higher in energy. Still, both seem to be equally abundant in cold dark clouds. The cyclic isomer has three CC bonds of essentially equal lengths which correspond to a bond order of 1.5. The molecule can be viewed best as derived from cyclopropane by removing 2 × 2 plus one H atoms, yielding cyclopropanediylidenyl as the probably most appropriate name. c-C₃H was detected first in the dense cold molecular cloud TMC-1 by
S. Yamamoto, S. Saito, M. Ohishi, H. Suzuki, S.-I. Ishikawa, N. Kaifu, and A. Murakami,
Laboratory and Astronomical Detection of the Cyclic C₃H Radical,
Astrophys. J. 322, L55–L58 (1987).
Two hyperfine components each were detected for the two fine structure components of the 2₁₂ – 1₁₁ transition near 91.5 and 91.7 GHZ, respectively, employing the Nobeyama 45 m telescope.

Two hfs components of the 1₁₀ – 1₁₁ transition near 14.9 GHz were detected with the 43 m NRAO telecope in several other dense dark clouds by
J. G. Mangum and A. Wooten,
Observations of the Cyclic C₃H Radical in the Interstellar Medium,
Astron. Astrophys. 239, 319–325 (1990).

The radical was also observed in three translucent molecular clouds CB 17, CB 24, and CB 228 by
B. E. Turner, E. Herbst, and R. Terzieva,
The Physics and Chemistry of Small Translucent Molecular Clouds. XIII. The Basic Hydrocarbon Chemistry,
Astrophys. J. Suppl. Ser. 126, 427–460 (2000).

The presence of c-C₃H in the circumstellar envelopes of carbon rich late type stars was also established. E.g. CW Leo by
J.Cernicharo, M. Guélin, and C. Kahane,
A λ 2 mm Molecular Line Survey of the C-star Envelope IRC+10216,
Astron. Astrophys. Suppl. Ser. 142, 181–215 (2000).
and LL Peg by
Y. Zhang, S. Kwok, and J.-I. Nakashima,
A Molecular Line Survey of the Extreme Carbon Star CRL 3068 at Millimeter Wavelengths,
Astrophys. J. 700, 1262–1281 (2009).

H. S. Liszt, J. Pety, M. Gerin, and R. Lucas,
detected with the IRAM 30 m telescope
HCO, //c//-C₃H and CF⁺: three new molecules in diffuse, translucent and “spiral-arm” clouds
Astron. Astrophys. 564, Art. No. A64 (2014).

Contributor(s): H. S. P. Müller; 09, 2011; 10, 2017