Vibrational spectra and stereochemistry of mono- and polysaccharides. IV. α-methyl-d-glucoside, α-methyl-d-galactopyranoside, and α-methyl-d-mannopyranoside

Abstract

The IR and Raman spectra of α-methyl-D-glucoside (αMDG), α-methyl-D-galactoside (αMDGal), and α-methyl-D-mannoside (αMDM) are compared. The main distinctions between these spectra have been interpreted using experimental and theoretical data on the frequencies and modes of normal vibrations and on the potential energy distributions over particular bonds and atomic groups of the substances under investigation. Spectral characteristics that are determined by different configurations of C−O (CH) groups attached to C2 and C4 atoms and different conformations of C6H₂O6H fragments have been revealed. It has been established that replacement of hydroxyls at C1 with methoxy groups significantly increases the number of frequencies of normal vibrations that are localized mostly in the C1C2 bonds (particularly in αMDGal). For the αMDGal molecules with axial C4O4 groups, the number of frequencies of normal vibrations that show predominant contributions of these groups of PED is smaller. For the αMDM molecules, in which the conformation of CH₂OH groups (with respect to rotation around the C5C6 bonds) differs from that in αMDG and αMDGal molecules, most vibrations are localized in the C6O6 bonds rather than in the C5O5 bonds. This fact is very important in correlating the vibrational spectra and structural properties of mono- and polysaccharides.