Structure and energetics of isomers of the interstellar molecule C5H

High-level ab initio methods based on the coupled cluster approximation hav e been used to study properties of several isomers of the C5H radical, a mo lecule of significant interest in radioastronomy. The three lowest-lying is omers [the well-known linear form (1) as well as two ring-chain structures, HC2C3 (2) and C2C3H (3)] lie within 30 kcal/mol with isomer 2 approximatel y 5 kcal/mol higher than the lowest-energy isomer 1. The computed rotationa l constant for the linear isomer is within 0.7% of the value determined in previous experimental analyses. Transition states formed via simple ring-op ening mechanisms for the interconversion of the linear and ring-chain isome rs have also been located; these lie ca. 27 and 31 kcal/mol above isomers 2 and 3, respectively, indicating reasonable kinetic stability of these stru ctures to isomerization. The computed rotational constants for these isomer s should be useful for laboratory and astronomical observation of these spe cies. In addition, four other minimum-energy structures an found to lie som ewhat higher in energy. These isomers involve both three- and four-membered carbon rings, as well as a bent-chain structure.