The propynylidyne radical was identified in space in 1985 by Thaddeus et al. toward IRC +10216 and TMC-1. C₃H is one of a number of linear polyatomic carbon chain molecules that have been detected in the circumstellar and interstellar medium. Two of the most common types are the polyynes, C_nH₂ and the cyanopolyynes, HC_nN, as well as their respective radicals, C_nH and C_nN. The backbones of these species contain one or more C≡C triple bonds.

While l-C₃H does have the arrangement of single and double bonds shown in the structure image, the terminal C atom is in an unfavorable condition because it only has a single bond with rest if the molecule. Carbon is most stable when it has four bonds. For ground state l-C₃H, charge can shift from elsewhere in the radical moves into the terminal carbon's unfilled p orbital, yielding a substantial dipole moment that strengthens its rotational spectrum. A low-lying quartet state has a very similar structure, but its dipole moment is much smaller.