Calculation of NMR chemical shifts in carbohydrates with ONIOM: A study of the conformers of beta-D-glucopyranose

The two-layer ONIOM2(MP2-GIAO:HF-GIAO) (our own n-layer integrated molecular orbital and molecular mechanics approach, in which a small model system containing the nuclei of interest is described at the MP2-GIAO level of theory, and the rest of the moleculeusing the HF-GIAO method) ansatz is applied to the calculation of 13C, 1H, and 17O NMR chemical shifts in the 4C1 G+, 4C1 G-, 4C1 T, 1C4 G+, and 1C4 G- conformers of β-d-glucopyranose. It is shown that with an appropriate choice of the model system this construction yields chemical shifts that represent close approximations to the corresponding MP2-GIAO values for the entire molecule, which makes it suitable for post-HF NMR chemical shift calculations on higher carbohydrates. The best correlations between experimental and theoretical 13C chemical shifts are achieved using the results of the calculations on the 4C1 G+ and 4C1 G- conformers, which is in agreement with the experimental evidence about the predominance of these two forms in aqueous solution.