On the Detection of Butadiynyl, C4H, in Space

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 ν7 and 2ν7 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