engleski

Predictions of 13C Chemical Shifts in Tertiary Carbocations. The Use of Scaled Shieldings Calculated from GIAO-DFT Methods

13C NMR chemical shifts for static tertiary carbocations have been investigated by quantum chemical DFT methods. Geometry optimizations for all carbocations have been performed at the B3LYP/6-31G(d) level of theory. Two different structures were located on the energy surface corresponding to C-C and C-H hyperconjugative isomers, respectively. The calculated 13C chemical shift of the carbon which carries a formal positive charge is particularly informative. Using an overall scaling factor of 0.95 for the GIAO-B3LYP/6-311G(d, p) results for the carbocations having C-H hyperconjugative mode, we obtained chemical shift values within the experimental errors. For the carbocations having C-C hyperconjugative mode a scaling factor of 0.97 should be applied to the computed chemical shifts to obtain an excellent agreement between computational and experimental NMR spectra. We also found an angular dependence of the calculated 13C chemical shift of the cationic center on the dihedral angle between the C-H(C) bond and vacant p orbital at the cationic center. The same angular dependence is found for the calculated natural charges using an NBO analysis. A linear correlation between calculated 13C chemical shifts and calculated natural charges is obtained (r = 0.997). The accuracy of the GIAO-DFT method was tested in calculations of 13C chemical shifts of equilibrating carbocations. Their static chemical shifts are known experimentally from low temperature solid-state CPMAS NMR studies. The same scaling factors (0.95 or 0.97) for calculated shieldings are introduced to improve the agreement between computational and experimental NMR spectra. For carbocations in which positive charge is delocalized from the carbenium center by resonance effects (benzyl cations) the different scaling factor (1.02) is needed to get an excellent fit between computed and experimental 13C chemical shift for carbocationic center.

13C NMR chemical shift; theoretical calculations; carbocations

nije evidentirano