Calculation of NMR chemical shifts in carbohydrates with ONIOM:: A study of the conformers of β-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 molecule-using the HF-GIAO method) ansatz is applied to the calculation of C-13, H-1, and O-17 NMR chemical shifts in the C-4(1) G(+), C-4(1) G(-), C-4(1) T, C-1(4) G(+), and C-1(4) G(-) conformers Of P-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 C-4(1) G(+) and C-4(1) G(-) conformers, which is in agreement with the experimental evidence about the predominance of these two forms in aqueous solution.