Ab initio calculations for detectable new isomers of interstellar carbon-chain radicals CnH (n=2-8)

Detectable new isomers of interstellar carbon-chain (linear) radicals, CnH (n = 2-8), were investigated theoretically based on ab initio quantum chemical calculations. The molecular characteristics of various CnB (n = 3-8) isomers were examined, and it was found that 3-ring isomers (isomers with a three-member-carbon-ring) of C3H, C5H, and C7H are at a low energy level relative to linear isomers, and that their dipole moments are sufficiently large. The formation mechanisms of C3H and C5H were also examined, and it was found that both linear- and 3-ring-C3H and C5H can be formed via neutral-neutral reactions, C + C2H2 --> C3H + H and C + C4H2 --> C5H + H, respectively, in interstellar space. These facts strongly suggest that carbon-ring-chain radicals, C5H and C7H, are a new series of interstellar molecules that will be detectable in the laboratory and interstellar space.