Carbodications. I. The structures and energies of C 4 H 4 2+ isomers

At high levels of ab initio theory (6‐31G*//4‐31G), the most stable C 4 H   4 2+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 C 4 H   4 2+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) 2 CCH 2 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 C 4 H   4 2+species investigated. The perpendicular ( D 2 d ) arrangement ( 3a ), permitting double allyl cationlike conjugation, is preferred over the planar D 2 h form ( 3b ) by 26 kcal/mol. The heat of formation of the most stable form of C 4 H   4 2+ , 1a , is estimated to be 623–640 kcal/mol. This species should be thermodynamically stable toward dissociation into smaller charged fragments.