Vibrational normal modes calculation in the crystalline state of methylated monosaccharides: Anomers of the methyl-d-glucopyranoside and methyl-d-xylopyranoside molecules

Abstract

A structural investigation of the organic molecules is being carried out using vibrational spectroscopy. In this study, normal co-ordinate calculations of anomers of the methyl-d-glucopyranoside and methyl-β-d-xylopyranoside in the crystalline state have been performed using the modified Urey–Bradley–Shimanouchi force field (mUBSFF) combined with an intermolecular potential energy function. The latter includes Van der Waals interactions, electrostatic terms, and explicit hydrogen bond functions. The vibrational spectra of the compounds recorded in the crystalline state, in the 4000–500cm−1 spectral region for the IR spectra, and in the 4000–20cm−1 spectral range for the Raman spectra are presented. After their careful examination, several differences in the intensities and frequency shifts have been observed. The theoretical spectra have been obtained after a tedious refinement of the force constants. Thus, on the basis of the obtained potential distribution, each observed band in IR and in Raman has been assigned to a vibrational mode. The obtained results are indeed in agreement with those observed experimentally and thus confirm the previous assignments made for the methyl-α and β-d-glucopyranoside, as well as for the methyl-β-d-xylopyranoside.