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

The H-1 and C-13 isotropic chemical shifts and the substituent effects ther eof (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 an d 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 experimenta l 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 a greement with the experiment, while the HF approach is superior to the BLYP and B3LYP ones. However, the substituent effects on 13C shieldings are bet ter reproduced at the BLYP and B3LYP levels than at the HF level, while the MPW1PW91 approach is again significantly superior to all the others, leadi ng to excellent agreement with experimental data. The most probable reason for these findings may be the cancellation of errors arising from the inapp ropriate description of the paramagnetic contributions to the overall shiel ding tenser within the Kohn-Sham approach, and the more systematic nature o f errors in DFT approaches. The isotropic chemical shift values at all leve ls of theory, however, correlate excellently with the experimental data, th e correlation being superior for DFT to the HF level of theory.