Stabilization of Saturated Carbocations in Condensed Phases

Based on the experimentally established mechanism of hyperconjugative stabilization of the simplest saturated carbocations [Stoyanov, E. S.; et al. PCCP, 2017, 19, 7270], the infrared spectra of t-alkyl⁺ and methyl-cyclo-pentyl⁺ carbocations were interpreted. This approach allows us to extract new information about the electronic state of (CH₃)₂C⁺R cations with R = H, CH₃, C₂H₅, C₄H₇, and CH(CH₃)₂, namely, the electron density distribution over the (CH₃)₂C group and the positive charge dispersion on the H atoms of this group. Thus, donation of the electron density to the empty 2p_z orbital of the sp² C atom occurs not only from one C-H bond oriented parallel to the 2p_z orbital but also equally from all other C-H and C-C bonds of the molecular group involved in hyperconjugation. This mechanism preserved the isoelectronic nature of this group toward the corresponding groups of the neutral alkanes. Hyperconjugation and polarization are closely linked in stabilization of carbocations: the strengthening of one effect weakens the second and vice versa without changing the efficiency of scattering of the positive charge in the carbocation. In the condensed phase, carbocations are additionally stabilized by the bulk effect and hydrogen bonding with the environment: increasing H-bonding strength increased hyperconjugation and decreased polarization. The contribution of all the effects on the stabilization of carbocations was evaluated.