| Abstract
| - High-level ab initio methods based on the coupled cluster approximation have been used to studyproperties of several isomers of the C5H radical, a molecule of significant interest in radioastronomy. Thethree lowest-lying isomers [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 approximately 5 kcal/mol higher than the lowest-energyisomer 1. The computed rotational constant for the linear isomer is within 0.7% of the value determined inprevious experimental analyses. Transition states formed via simple ring-opening mechanisms for theinterconversion of the linear and ring-chain isomers have also been located; these lie ca. 27 and 31 kcal/molabove isomers 2 and 3, respectively, indicating reasonable kinetic stability of these structures to isomerization.The computed rotational constants for these isomers should be useful for laboratory and astronomical observationof these species. In addition, four other minimum-energy structures are found to lie somewhat higher in energy.These isomers involve both three- and four-membered carbon rings, as well as a bent-chain structure.
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