Effect of the Pyranose Ring Conformation on the Vibrational Spectra of Sugar Epoxides

Normal vibrations of two sugar epoxides — methyl 2,3-anhydro-4-deoxy--D-ribohexopyranoside and methyl 3,4-anhydro--D-talohexopyranoside — have been carried out by the molecular mechanics method. Parametrization of the force field used has been performed and the parameters of the oxirane ring for sugar epoxides have been determined. A good agreement between the experimentally observed and calculated frequencies has been obtained. The IR spectral absorption bands of the molecules under investigation in the 1500–400-cm^–1 range have been assigned on the basis of the potential energy distribution function of normal vibrations. The investigation of the potential energy surface of these molecules has shown that their pyranose ring can take conformations close to half-chair, boat, skew boat, and twist forms. Comparative analysis of the normal vibrations of various conformers has revealed that the form of the pyranose ring influences the vibrational spectra of sugar epoxides.