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.