Comparison of different theory models and basis sets in the calculation of 13C NMR chemical shifts of natural products.

The influence of the calculation method in mimicking experimental (13)C NMR chemical shifts of 15 low-polarity natural products singularly containing 10-20 carbon atoms was investigated by employing different quantum chemistry approaches and basis sets, both in the preliminary geometry optimizations and in the following single-point (13)C GIAO calculations of the NMR chemical shifts. The geometries of the involved species were optimized at the PM3, HF, B3LYP and mPW1PW91 levels whereas the (13)C NMR parameters were determined at the HF, B3LYP and mPW1PW91 levels. Different combinations of basis sets were also tested. The consistency and efficiency of the considered combinations of geometry optimizations and GIAO (13)C NMR calculations were thoroughly checked by the analysis of statistical parameters concerning computed and experimental (13)C NMR chemical shift values. Copyright 2004 John Wiley & Sons, Ltd.