Toward the complete prediction of the H-1 and C-13 NMR spectra of complex organic molecules by DFT methods: Application to natural substances

The NMR parameters (1H and 13C chemical shifts and coupling constants) for a series of naturally occurring molecules have been calculated mostly with DFT methods, and their spectra compared with available experimental ones. The comparison includes strychnine as a test case, as well as some examples of recently isolated natural products (corianlactone, daphnipaxinin, boletunone B) featuring unusual and/or crowded structures and, for boletunone B, being the subject of a recent revision. Whenever experimental spectra were obtained in polar solvents, the calculation of NMR parameters was also carried out with the IEF-PCM continuum method. The computed results generally show a good agreement with experiment, as judged not only by statistical parameters but by visual comparison of line spectra as well. The origin of remaining discrepancies is traced to the incomplete modeling of conformational and specific solvent effects.