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

The C-2 conformation 3 was established to be the preferred conformation of the isopropyl cation. The calculated C-13 NMR chemical shifts of C-2 conformation 3 also agree very well with the experimental data. However, this is, in contrast with the recent claims by Farcasiu and coworkers, who found that the preferred conformation of the isopropyl cation in ion pair with trihydrofluoroborate is C-5. 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. (C) 2011 Elsevier B.V. All rights reserved.