Scaling factors for carbon NMR chemical shifts obtained from DFT B3LYP calculations

A linear scaling of the calculated chemical shifts is used in order to improve the accuracy of the DFT predicted 13C NMR chemical shifts. The widely applied method of GIAO B3LYP/6-311+G(2d,p) using the B3LYP/6-31G(d) optimized geometries is chosen, which allows cost-effective calculations of the 13C chemical shifts in the molecular systems with 100 and more atoms. A set of 27 13C NMR chemical shifts determined experimentally for 22 simple molecules with various functional groups is used in order to determine scaling factors for reproducing experimentally measured values of 13C chemical shifts. The results show that the use of a simple relationship (δscalc = 0.95 δcalc + 0.30, where δcalc and δscalc are the calculated and the linearly scaled values of the 13C chemical shifts, respectively) allows to achieve a three-fold improvement in mean absolute deviations for 27 chemical shifts considered. To test the universal applicability of the scaling factors derived, we have used complex organic molecules such as taxol and a steroid to demonstrate the significantly improved accuracy of the DFT predicted chemical shifts. This approach also outperforms the recently recommended usage of the Hartree-Fock optimized geometries for the GIAO B3LYP/6-311+G(2d,p) calculations of the 13C chemical shifts.