Structures and energies of isomeric C₅H₄²⁺ and C₆H₄²⁺ dications

The geometries of various C₅H₄²⁺ and C₆H₄²⁺ isomers with acetylene, cumulene, vinyl, propargyl, and three- and four-membered ring structures were optimized by ab initio molecular orbital theory employing the 3-21G basis set. Comparison of the Hartree-Fock (for C₆H₄²⁺) and MP2/6-31G* (for C₅H₄²⁺) relative energies shows clearly that there is no common minimum energy C_nH₄²⁺ structure of the CH₃-C_n-1-H²⁺ type as has been suggested from mass spectroscopic investigations. Thus, both 25 (n = 4) and 26 (n = 5) are quite high in energy relative to most other isomers examined. In addition, the cumulene dications, H₂C(C_n)CH₂²⁺ (n = 3 and 4), which favor anti-van't Hoff geometries (11a and 12a), are less stable than structures with cyclopropenium ion moieties. The vinylidenecyclopropenium dication (15a) and the triafulvene dication (18a) are the lowest energy C₅H₄²⁺ and C₆H₄²⁺ dications investigated; heats of formation of 630 ± 5 (15a) and 617 ± 5 kcal/mol (18a) are estimated.