VIBRATIONAL-SPECTRA AND STEREOCHEMISTRY OF MONOSACCHARIDES AND POLYSACCHARIDES .4. ALPHA-METHYL-D-GLUCOSIDE, ALPHA-METHYL-D-GALACTOPYRANOSIDE, AND ALPHA-METHYL-D-MANNOPYRANOSIDE

The IR and Raman spectra of alpha-methyl-D-glucoside (alpha MDG), alph a-methyl-D-galactoside (alpha MDGal), and alpha-methyl-D-mannoside (al pha MDM) are compared The main distinctions between these spectra have been interpreted using experimental and theoretical data on the frequ encies and modes of normal vibrations and on the potential energy dist ributions over particular bonds and atomic groups of the substances un der investigation. Spectral characteristics that me determined by diff erent configurations of C-O (CH) groups attached to C2 and C4 atoms an d different conformations of C6H(2)O6H fragments have been revealed. I t has been established that replacement of hydroxyls at C1 with methox y groups significantly increases the number of frequencies of normal v ibrations that are localized mostly in the C1C2 bonds (particularly in alpha MDGal). For the alpha MDGal molecules with axial C4O4 groups, t he number of frequencies of normal vibrations that show predominant co ntributions of these groups to PED is smaller. For the alpha MDM molec ules, in which the conformation of CH2OH groups (with respect to rotat ion around the C5C6 bonds) differs from that in alpha MDG and alpha MD Gal molecules, most vibrations me localized in the C6O6 bonds rather t han in the C5O5 bonds. This fact is very important in correlating the vibrational spectra and structural properties of mono- and polysacchar ides.