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Computational Studies of the 13C and 1H 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 13C and 1H chemical shift values computed at HF, BLYP and B3LYP/6-311G(d,p) levels of theory, for the BLYP/6-31G(d,p) optimized geometries of adamantane and 2,4-methano-2,4-dehydro-adaman-ta-ne are reported and compared with the available experimental data. Except for the “inverted” carbon atoms, the HF values are superior over the DFT ones when the isotropic shifts with respect to TMS are in question. However, in case of the relative shifts computed with respect to the most deshielded center within the molecule, the DFT 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 contributions to the overall shielding tensor within the Kohn-Sham approach when an internal standard (within a molecule) is chosen, instead of an external one. The CSGT relative shift values correlate better with the experimental than the GIAO ones, the correlations being significantly superior at DFT than at corresponding HF level of theory.

chemical shifts; DFT; adamantane; 2;4-methano-2;4-dehydroadamantane

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