Study of structures, energetics, IR spectra and 13C and 1H NMR chemical shifts of the conformations of isopropyl cation by ab initio calculations

The C2 conformation 3 was established to be the preferred conformation of the isopropyl cation. The calculated 13C NMR chemical shifts of C2 conformation 3 also agree very well with the experimental data. However, this is, in contrast with the recent claims by Fărcaşiu and coworkers, who found that the preferred conformation of the isopropyl cation in ion pair with trihydrofluoroborate is Cs. Computed IR spectra indicates that hyperconjugative stabilization is clearly evident due to the shorter C+–C bond lengths and larger C–C+–C bond angles in the isopropyl cation structure 3. The computed IR absorptions of 3 also indicate that the greatest degree of electron donation takes place from C–H bonds into the formally empty p-orbital of the central carbon.