Detection of the linear pentadiynylidyne radical (C5H)
in the circumstellar shell IRC +10216 was reported in
papers by Cernicharo et al. published in 1986
based on experimental measurements of the rotational spectra by
Gottlieb et al. Lines of C5H were
subsequently identified in TMC-1 in 1987.
Later confirming detections include the large scale survey of TMC-1 by
Kaifu et al. in 2004.
C5H is a
carbene species where most – but not all –
of the radical character is concentrated on the terminal carbon atom as shown in the orbital cross-section.
There is a small amount of π bonding between the third and fourth carbon atoms.