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. l-C₃H was detected first toward the dense cold molecular cloud TMC-1 and in the circumstellar envelope of the carbon-rich AGB star CW Leo, aka IRC +10216, by
P. Thaddeus, C. A. Gottlieb, Å. Hjalmarson, L. E. B. Johansson, W. M. Irvine, P. Friberg, and R. A. Linke,
Astronomical Identification of the C₃H Radical,
Astrophys. J. 294, L49–L53 (1985).
Four hfs components of the J = 3/2 – 1/2 transition near 32.6 GHz in the lower lying ²Π_1/2 fine structure ladder were detected with the Onsala 20 m telescope. 2 parity split lines each were detected with several telescopes toward CW Leo for the 7/2 – 5/2, 9/2 – 7/2, and 13/2 – 11/2 transitions of the ²Π_1/2 ladder near 76.2, 98.0, and 141.7 GHz as well as the 9/2 – 7/2 transitions of the ²Π_3/2 ladder near 103.3 GHz.

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 higher lying (CCH) bending mode is subjected to very strong Renner-Teller interaction causing one of the Σ components to be only 27.2 cm^–1 (39.1 K) above ground. Transitions within this state were observed in the circumstellar envelope of the carbon rich late type star 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).

Propynylidyne was also detected in the circumstellar envelope of the post-AGB object (protoplanetary nebula) V353 Aur by
J. R. Pardo and J. Cernicharo,
Molecular Abundances in CRL 618,
Astrophys. J. 654, 978–987 (2007).

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

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