Theoretical Analysis of the Conformation of the Skeletal Base of Epoxysaccharide Molecules by the Vibrational Spectra

Abstract

To explain the influence of the conformation of the skeletal base of the epoxysaccharide molecule on the frequencies and forms of normal vibrations, we have made a comparative theoretical analysis of the frequencies, forms, and potential energy distributions of normal vibrations in the 4001500 cm⁻¹ spectral range of five conformers of the epoxysaccharide methyl‐3,4‐anhydro‐α‐D,L‐allopyranoside molecule that approximate conformations of the hexapyranose rings close to the half‐chair and boat forms. It is shown that a change in the structure of the molecule leads to frequency shifts and distortions of the forms of normal vibrations practically throughout the spectral range under consideration. The character of these changes is specific for different spectral ranges. The most sensitive to conformational changes in the pyranose ring are normal vibrations localized mainly within this ring, and the least sensitive ones are those localized on the bonds lying on the molecule's periphery. The frequency shift of normal vibrations comparable in form in the low‐frequency region in the series of conformers can reach 100 cm⁻¹. It is shown that the 650800 cm⁻¹ spectral range is the most convenient for identifying the form of the epoxypyranose ring. The changes in the force field associated with the conformation transitions of the pyranose ring are analyzed.