Harmonic dynamics of α- and β-methyl- d-galactopyranoside in the crystalline state

The study of the anomeric differences observed on the spectra of methyl-α- and methyl-β- 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 α- and β- 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-α- and methyl-β- 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.