Reported in 1991 by
Guélin & Cernicharo, the cyanocarbene radical,
HCCN, is an example of a molecule that has resisted confirmation by subsequent studies. It was observed toward the
circumstellar shell IRC+10216 with the IRAM
30 meter telescope. While
Kawaguchi et al. subsequently assigned a line observed
in IRC+10216 at 43.624 GHz to the 3,2-2,1 transition of HCCN, the feature in question is barely above the baseline noise
level, making this a tentative confirmation at best.
McGonagle & Irvine performed a dedicated search for
the 1,2-0,1 transition of HCCN toward TMC-1 and several other giant molecule clouds in 1996 and did not find it.
HCCN is a quasilinear, near-prolate asymmetric top. As
Inostroza et al. noted, HCCN has a number of low-lying
singlet and triplet structures with quasilinear and cyclic geometries. Although it is something of an oversimplification,
one can view HCCN has analogous to CH2, where the 3B1 state is
slightly more stable than the 1A1 state. The
depictions of the singly occupied orbitals below are consistent with this interpretation.