Carbodications. I. The structures and energies of C₄H₄²⁺ isomers

Abstract

At high levels of ab initio theory (6-31G//4-31G), the most stable C₄H₄²⁺ isomer is indicated to be the nonplanar cyclobutadiene dication (1a); the planar form, 1b, is indicated to be 7.5 kcal/mol less stable. The second most stable C₄H₄²⁺ isomer, the methylenecyclopropene dication, is indicated to prefer the perpendicular (2a) over the planar (2b) arrangement by 7 kcal/mol. The "anti van't Hoff" cyclo-(HB)₂C=CH₂ system (4), isoelectronic with 2, also prefers the perpendicular conformation (4a), and retains the CC double bond. The linear butatriene dication (3) is the least stable C₄H₄²⁺ species investigated. The perpendicular (D_2d) arrangement (3a), permitting double allyl cationlike conjugation, is preferred over the planar D_2h form (3b) by 26 kcal/mol. The heat of formation of the most stable form of C₄H₄²⁺, 1a, is estimated to be 623-640 kcal/mol. This species should be thermodynamically stable toward dissociation into smaller charged fragments.