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Ab initio calculations at the Hartree–Fock level with full-geometry optimization using the 6-31G(d) basis set, and GIAO (gauge including atomic orbitals) ¹³C NMR chemical shifts, are presented here as a support in the study of the stereochemistry of low-polar organic compounds having an open-chain structure. Four linear stereoisomers, fragments of a natural product previously characterized by experimental ¹³C NMR spectra, which possesses three stereogenic centers, 11 carbon atoms, and 38 atoms in total, were considered. Conformational searches, by empirical force-field molecular dynamics, pointed out the existence of 8–13 relevant conformers per stereoisomer. Thermochemical calculations at the ab initio level in the harmonic approximation of the vibrational modes, allowed the evaluation, at 298.15 K, of the standard Gibbs free energy of the conformers. The ¹³C NMR chemical shift of a given carbon atom in each stereoisomer was considered as the average chemical shift value of the same atom in the different conformers. The averages were obtained by the Boltzmann distribution, using the relative standard free energies as weighting factors. Computed parameters related to linear correlation plots of experimental ¹³C chemical shifts versus the corresponding computed average data allowed us to distinguish among the four stereoisomers. Supporting information for this article is available on the WWW under http://www.wiley-vch.de/contents/jc_2111/2002/f3743_s.pdf or from the author.

Geometry optimization and GIAO (gauge including atomic orbitals) ¹³C NMR chemical shift calculations at Hartree–Fock level, using the 6-31G(d) basis set, are proposed as a tool to be applied in the structural characterization of new organic compounds, thus providing useful support in the interpretation of experimental NMR data. Parameters related to linear correlation plots of computed versus experimental ¹³C NMR chemical shifts for fourteen low-polar natural products, containing 10–20 carbon atoms, were employed to assess the reliability of the proposed structures. A comparison with the hybrid B3LYP method was carried out to evaluate electron correlation contributions to the calculation of ¹³C NMR chemical shifts and, eventually, to extend the applicability of such computational methods to the interpretation of NMR spectra in apolar solutions. The method was tested by studying three examples of revised structure assignments, analyzing how the theoretical ¹³C chemical shifts of both correct and incorrect structures matched the experimental data. Supporting information for this article is available on the WWW under http://www.wiley-vch.de/contents/jc_2111/2002/f3744_s.pdf or from the author.