The butadiynyl radical in its ²Σ ground vibrational state has been detected between 85 and 115 GHz with the NRAO 11 m telescope in the circumstellar envelope of the carbon-rich late-type star CW Leo, also known as IRC +10216, in eight fine structure lines of four rotational transitions (N = 9 – 8 to 12 – 11) by
M. Guélin, S. Green, and P. Thaddeus,
Detection of the C₄H Radical toward IRC +10216,
Astrophys. J. 224 L27–L30 (1978)
before it was actually detected in the laboratory. Two rotational transitions had been reported previously as U-lines in two different publications. The highest frequency fine structure component seems to be overlapped.

Soon thereafter, it was detected with the OSO 20 m telescope toward TMC-1 by:
W. M. Irvine, B. Höglund, P. Friberg, J. Askne, and J. Elldér,
The Increasing Chemical Complexity of the Taurus Dark Clouds – Detection of CH₃CCH and C₄H,
Astrophys. J. 248 L113–L117 (1981).
Four hyperfine components of the N = 3 – 2 transition were detected near 28.55 GHz.

The radical was also observed in the diffuse medium by:
M. B. Bell, P. A. Friedman, and H. E. Matthews,
The Detection of Butadiynyl (C₄H) in Absorption against Cassiopeia A,
Astrophys. J. 273 L35–L39 (1983).

Moreover, the molecule was also detected in several PDRs, namely the Horsehead nebula (B 33), IC 63, and in the ρ Oph L1688-W cloud interface:
D. Teyssier, D. Fossé M. Gerin, J. Pety, A. Abergel, and E. Roueff,
Carbon Budget and Carbon Chemistry in Photon Dominated Regions,
Astron. Astrophys. 417 135–149 (2004).

Vibrationally excited C₄H was also detected:
M. Guélin, J. Cernicharo, S. Navarro, D. R. Woodward, C. A. Gottlieb, and P. Thaddeus,
New doublets in IRC +10216 – Vibrationally Excited C₄H?,
Astron. Astrophys. 182 L37–L39 (1987).
Subsequent laboratory work revealed these transitions to be due to the v₇ = 1 Π state and the v₇ = 2 Σ and Δ states:
S. Yamamoto, S. Saito, M. Guélin, J. Cernicharo, H. Suzuki, and M. Ohishi,
Laboratory Microwave Spectroscopy of the Vibrational Satellites for the //ν//₇ and 2//ν//₇ states of C₄H and their Astronomical Identification,
Astrophys. J. 323 L149–L153 (1987).
Unfortunately, no laboratory data is available for higher excited vibrational states !

C₄D was detected toward TMC-1 by:
B. E. Turner,
Detection of C₄D – Implications for Ion-Molecule Chemistry,
Astrophys. J. 347 L39–L42 (1989).

The detection of the ¹³C isotopic species was reported in:
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).

Contributor(s): H. S. P. Müller; 5, 2007