Advances in correlation between experimental and DFT/GIAO computed ~(13)C NMR chemical shifts: A theoretical study on pentacyclic terpenoids (fernenes)

摘要

The ~(13)C chemical shifts of 15 pentacyclic terpenoids (fernenes) are compared to predicted ~(13)C NMR chemical shifts obtained via empirically scaled GIAO shieldings. We report that accurate (rms error approx. 1.5ppm) predictions of ~(13)C chemical shifts can be achieved for these fernenes through the use of scaled shieldings calculated from GIAO theory with a relatively small basis set and on the basis of geometries obtained from DFT calculations. The best results (considering a reasonable cost-to-benefit ratio) were obtained from B3LYP/6-31G(d)//B3LYP/6-31G(d) computations. The chemical shifts anisotropy asymmetry (η was included in our calculations, enhancing the correlations between calculated and experimental chemical shifts. Our results indicate that the inclusion of a scaling factor allow to obtain an excellent correlation between δ_(calc) and δ_(exp). Also, the inclusion of asymmetry improves this correlation. In addition, taking into account the different conformations and the solvent effects a slight enhancement was obtained in this case.