Vibrational spectra and stereoisomerism of the hexopyranose ring in epoxysaccharides

Investigation of the potential surface of methyl 2,3-anhydro-2,3,4-trideoxy-β-D-lyxohexopyranoside has shown that the molecule has seven conformers with different frameworks. A comparative analysis of the calculated frequencies, forms, and PED of normal vibrations of the seven conformers allowed us to follow the frequency shifts of similar modes, which depend on the geometry of the epoxysaccharide framework. In the high-frequency region of the vibration spectrum, PED has considerable contributions from the deformation vibrations of methine and methylene (CCH, OCH) groups, and we can clearly observe a dependence of the frequency shift on the geometry of the ring at the site where vibrating groups are localized. The vibrations that have predominant contributions to PED from the C−O stretching vibrations depend on the overall geometry of the molecule. In the low-frequency region (below 700 cm−1), skeletal deformation modes appear. Apart from the common dominant forms of vibrations, each conformer is characterized by individual “deviations” that depend on the conformation of the hexopyranose ring as a whole. The frequency shifts in this case can be 100 cm−1 or higher.