Computational studies of chemical shifts using density functional optimized geometries.: II.: Isotropic 1H and 13C chemical shifts and substitutent effects on 13C shieldings in 2-adamantanone

The H-1 and C-13 isotropic chemical shifts and the substituent effects thereof (with respect to adamantane), computed at the HF, BLYP, B3LYP/6-311G(d,p) as well as at MPW1PW91/6-311+G(2d,p) levels of theory with CSGT, GIAO and IGAIM algorithms, for the BLYP/6-31G(d,p) and B3LYP/G-31G(d,p) optimized geometries of 2-adamantanone are reported and compared with the experimental data. When absolute values of isotropic chemical shifts (with respect to TMS) are in question, the MPW1PW91/6-311+G(2d,p) level leads to excellent agreement with the experiment, while the HF approach is superior to the BLYP and B3LYP ones. However, the substituent effects on 13C shieldings are better reproduced at the BLYP and B3LYP levels than at the HF level, while the MPW1PW91 approach is again significantly superior to all the others, leading to excellent agreement with experimental data. 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 tenser within the Kohn-Sham approach, and the more systematic nature of errors in DFT approaches. The isotropic chemical shift values at all levels of theory, however, correlate excellently with the experimental data, the correlation being superior for DFT to the HF level of theory.