Computational studies of the C-13 and H-1 NMR isotropic chemical shifts using density functional optimized geometries. Adamantane and 2,4-methano-2,4-dehydroadamantane (a [3.1.1]propellane) as case studies

The C-13 and H-1 chemical shift values computed at HF, BLYP and B3LYP/6-311 G(d,p) levels of theory, for the BLYP/6-31G(d,p) optimized geometries of ad amantane and 2,4-methano-2,4-dehydroadamantane, are reported and compared w ith the available experimental data. Except for the inverted carbon atoms, the HF values are superior to the DFT ones when the isotropic shifts with r espect to TMS are in question. However, in case of the relative shifts comp uted with respect to the most deshielded center within the molecule, the DF T methods yield significantly better agreement with the experiment than the HF. The most probable reason for these findings may be the cancellation of errors arising from the inappropriate description of the paramagnetic cont ributions to the overall shielding tensor within the Kohn-Sham approach whe n an internal standard (within a molecule) is chosen, instead of an externa l one. The CSGT relative shift values correlate better with the experiment than the GIAO ones, the correlations being significantly superior at DFT th an at the corresponding HF level of theory.