Ab Initio Calculations of the 13C NMR Spectrum of 2-Propyl Cations in Ion Pairs

Abstract

NMR chemical shift calculations (DFT-GIAO-B3LYP) on the 2-propyl cation in ion pairs were conducted for the Cs conformation (2) found earlier to be the energy minimum of the cation in the ion pair, as well as for the C2 conformation (1), which is the energy minimum of the isolated cation. The level of the agreement between the isotropic chemical shift calculated with the dzvp basis set and the experiment increased with the increase in the calculated negative charge of the anion in the series HLiH-, H3BF -, BF4-, AsF6-, and SbF6-. The calculated charge and the agreeement with experiment also increased with the basis set to tzp for HBF3-, tzp and tz2p for BF4-, and to tzp, tz2p, qz2p, and pz3d for HF2-. The value calculated for the isolated ion was off by 20 ppm and did not improve from the dzvp to the pz3d level, which indicates that even in solution the cations are mostly ion-paired or, even more likely, move in aggegates of varying sizes. From the two conformations of the cation in the ion pair, 2·X- gav...