| Abstrakt: |
Chemical shieldings and coupling constants in a monosaccharide Me α- d-xylopyranoside were computed by density functional theory (DFT) method. The differences between the experimental and computed chemical shifts, for both DFT and MM3 geometries, showed that the method used reliably computes these NMR parameters. The agreement with experimental values was also obtained for proton−proton and proton−carbon coupling constants across one or more bonds. Furthermore, the effect of conformation upon both NMR shielding tensors (the values and orientation of its principal components) and couplings has also been investigated. The change of conformation around the C1−O1 linkage resulted in variations of mainly anomeric proton and carbon chemical shieldings as well as both the ring and O1 oxygens. The observed variations were found similar to those in Me β-d-xylopyranoside [Hricovíni, M.; Malkina, O. L.; Bízik, F; Turi Nagy, L.; Malkin, V. G. J. Phys. Chem. 1997, 101, 9756]. Similarly, the magnitudes of 1JC-H and 3JC-H varied upon the dihedral angle φ [H1−C1−O1−CMe]. 1JC1-H1 couplings, based on DFT geometry, changed between 151.7 and 165.6 Hz with the smallest values found for φ within −60° to 60°. 3JH1-C1-O1-CMe varied between 0 and 11.2 Hz (DFT geometry) and showed the dependence comparable with the previous one for β anomer with one exception: the magnitude of 3JC-H for antiperiplanar conformation is about 3 Hz larger for the α anomer. Such differences could be important for the determination of glycosidic linkage conformation of carbohydrates and may suggest that this type of dependence should be parametrized separately for α- and β-linked carbohydrates. |
