Stabilization of carbocations CH₃⁺, C₂H₅⁺, i-C₃H₇⁺, tert-Bu⁺, and cyclo-pentyl⁺in solid phases: experimental data versus calculations

Abstract

Comparison of experimental infrared (IR) spectra of the simplest carbocations (with the weakest carborane counterions in terms of basicity, CHBHal, Hal = F, Cl) with their calculated IR spectra revealed that they are completely inconsistent, as previously reported for the t-Bu cation [Stoyanov E. S., et al. J. Phys. Chem. A, 2015, 119, 8619]. This means that the generally accepted explanation of hyperconjugative stabilization of the carbocations should be revised. According to the theory, one CH bond (denoted as ) from each CH/CH group transfers its σ-electron density to the empty 2p orbital of the sp C atom, whereas the σ-electron density on the other CH bonds of the CH/CH group slightly increases. From experimental IR spectra it follows that donation of the σ-electrons from the bond to the 2p C-orbital is accompanied by equal withdrawal of the electron density from other CH bonds, that is, the electrons are supplied from each CH bond of the CH/CH group. As a result, all CH stretches of the group are red shifted, and IR spectra show typical CH/CH group vibrations. Experimental findings provided another clue to the electron distribution in the hydrocarbon cations and showed that the standard computational techniques do not allow researchers to explain a number of recently established features of the molecular state of hydrocarbon cations.

Stabilization of Saturated Carbocations in Condensed Phases

“…Specific Features of Stabilization of Carbocations. Combining the results of the present work with those from our previous publications, 4,5 that is, summing up the data for all the analyzed C 1 −C 7 carbocations, we can draw the following conclusions.…”

“…The weakening of C−H bonds of the methyl group manifests itself in a significant decrease (by ∼200 cm −1 ) in the C−H stretch frequencies as compared with neutral ethane. 5 Thus, the weakening of C−H bonds under the influence of hyperconjugation significantly exceeds their strengthening because of polarization. Replacement of the two H atoms in CH 3 + with CH 3 groups can result in the involvement of both CH 3 groups in hyperconjugation with the 2p z orbital of the sp 2 C atom forming symmetrical i-Pr + sym .…”

“…Its νCH 3 expectantly decreases by a half of the decrease for C 2 H 5 + (by ∼100 cm −1 ). 5 It should be noted that an asymmetric isomer, i-Pr + as , can form as well: with one hyperconjugated CH 3 group and another presumably polarized group, CH 3 *. For both isomers, the C−H stretches of hyperconjugated CH 3 groups actually coincide (ν as CH 3 = 2831 sym and 2825 as cm −1 ); 5 this means that energies of i-Pr + sym and i-Pr + as are very similar, and positive charge dispersion over hyperconjugated CH 3 and polarized CH 3 * groups is equal.…”

“…Nonetheless, calculated infrared (IR) spectra of carbocations, in accordance with this mechanism of hyperconjugation, are completely inconsistent with empirical IR spectra of ethyl, ipropyl, t-butyl, and cyclo-pentyl cations. 4,5 Experimental data suggest that CH 3 groups of i-propyl + and t-butyl + cations are nonequivalent. One group, designated as CH 3 *, is polarized more strongly and is weakly affected by hyperconjugation.…”

“…The theory-versus-experiment discrepancies are valid for all quantum-chemical calculations of carbocations known to date. 5 Possibly, potential energy surfaces of the carbocations are more complicated than expected and cannot be analyzed by singlereference quantum-chemical methods.…”