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

Abstract

The 13C chemical shifts of 15 pentacyclic terpenoids (fernenes) are compared to predicted 13C NMR chemical shifts obtained via empirically scaled GIAO shieldings. We report that accurate (rms error approx. 1.5 ppm) predictions of 13C 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 δcalc and δexp. 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.