Increasing the kinetic stability of a gas‐phase contact ion‐pair through enhancement of the carbocation stability

The occurrence of a hydrogen‐bonded contact ion pair (HBCIP, in the gas phase) between the dibenzotropylium cation and chloride has been suggested previously. However, the low barrier (ΔE‡, of ~1 kcal/mol) to its covalent counterpart can preclude its detection. In this work it is shown (through density functional theory calculations) that ΔE‡ can be increased significantly, based on larger but similar carbocations containing more benzene rings. Using the electrostatic potential as a measure of carbocation stability, along with its linear correlation with ΔE‡, one obtains a ΔE‡ value as high as ~3.7 kcal/mol for the HBCIP between the heptabenzotropylium cation and chloride. The trend for ΔE‡ can be explained by the deformation energy of the carbocation, from its structure in the HBCIP to that in the transition state. Increasing the kinetic stability of metastable species is a key feature to its detection. This work shows how this stability of some hydrogen‐bonded contact ion‐pairs (HBCIP) can be increased, increasing the size of the aromatic carbocation which it contains.