A Genuine Hydrocarbon Ion Pair More Stable Than Its Covalent Counterpart. A Computational Study.

Normally, carbocations and carbanions of hydrocarbons react to form a CC σ bond. However, if the ions are very stable with a large steric repulsion between them, one can also have the formation of an ion pair, generally much less stable than the covalent form. For an extremely large steric repulsion in the covalent form, the ion pair can become the most stable form, even in the gas phase. DFT and CASSCF calculations indicate that the tert-butylfulleride anion and the tris[1-(5-isopropyl-3,8-dimethylazulenyl)]-cyclopropenylium cation form an ion pair. This is the first study of this ion pair, although it is the building block of a salt that has already been synthesized. DFT results indicate that this ion pair is considerably more stable than its covalent counterpart. Nevertheless, several properties of the ionic and covalent forms are similar. An energy decomposition analysis indicates the polarization, electrostatic, and dispersion terms as the most important attractive terms between the ions.