Theoretical study of the isomeric cyclopropylidenemethyl and 1-cyclobutenyl cations, unusually stable vinyl cations

Abstract

Ab initio and MIND0/3 calculations were employed to explore the geometries and energies of unusual vinyl cations. In agreement with experiment, both the cyclopropylidenemethyl (8) and the cyclobutenyl (4) cations were found to be highly stabilized species. The former benefits from effective hyperconjugation involving the cyclopropyl ring while the latter is indicated to be a nonclassical ion with C₃ bridging almost equidistantly between C₁ and C₂. This 1-cyclobutenyl cation (4) is found to be significantly more stable than 8. This difference is very much reduced in the methyl-substituted isomers 10 and 11. The homopropargyl ion (9), not a minimum on the potential energy surface, should rearrange directly to the much more stable 4.