January 2004

Propynylidyne radical

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, CnH2 and the cyanopolyynes, HCnN, as well as their respective radicals, CnH and CnN. 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.


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