Carbodications. I. The structures and energies of C4H isomers

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Abstract
At high levels of ab initio theory (6‐31G*//4‐31G), the most stable C4H 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 C4H 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 CC double bond. The linear butatriene dication (3) is the least stable C4H species investigated. The perpendicular (D2d) arrangement (3a), permitting double allyl cationlike conjugation, is preferred over the planar D2h form (3b) by 26 kcal/mol. The heat of formation of the most stable form of C4H, 1a, is estimated to be 623–640 kcal/mol. This species should be thermodynamically stable toward dissociation into smaller charged fragments.