Harmonic dynamics of alpha- and beta-methyl-D-galactopyranoside in the crystalline state

The study of the anomeric differences observed on the spectra of methyl-alpha- and methyl-beta-D-galactopyranoside is the essential goal of this investigation. Thus, after a careful examination of the IR and Raman spectra of these two compounds, several differences in the intensities and frequency shifts are observed. This is especially noted in the region 1000-700 cm(-1). In order to make some assignments with more precision, the normal modes analyses of the two compounds are performed in the crystalline state. For this purpose, a modified Urey-Bradley-Shimanouchi force field has been combined with an intermolecular potential energy function. The initial set of force constants comes from those of alpha- and beta-D-galactopyranosyl, then the force constants have been varied, so as to obtain a good agreement between the observed and the calculated vibrational frequencies. The obtained results have finally reproduced the experimental data and have confirmed the previous assignments made for the methyl-alpha- and methyl-beta-D-galactopyranoside. The calculations have demonstrated also the transferability of the set of parameters of the initial force field of D-galactose to methyl-D-galactopyranoside.