The Nature of Doubly Charged C₃H₄²⁺ Ions: Structural and Energetic Relationships

The singlet and triplet allene dications prevail on the C₃H₄²⁺ potential hypersurface at higher levels of ab initio theory. The global minimum, the planar aliene dication 1, lies in a deep potential well with a deprotonation barrier of 82 kcal/mol. This dication has similar barriers of ca. 15 kcal/mol for scrambling of both its hydrogens and carbons. The existence of a protonated cyclopropenium ion is unlikely. The triplet propyne dication, the least stable isomer studied, is ca. 14 kcal/mol higher in energy than the perpendicular triplet aliene dication and is ca. 54 kcal/mol less stable than the planar singlet global minimum 1.