CALCULATION OF ABSORPTION-BAND FREQUENCIES AND INTENSITIES IN THE IR-SPECTRUM OF ALPHA-D-GLUCOSE IN A CLUSTER

Coupled calculations of the vibrational frequencies and intensities of IR absorption bands of α-d-glucose have been performed taking into account the influence of intermolecular hydrogen bonds in a crystal on the intensity of the molecular absorption bands. The analysis of normal vibrations was carried out in the framework of the valence force field. The intensities were calculated by the quantum-chemical technique CNDO/2. The calculations of the α-d-glucose IR spectra were performed in two approximations: a quasi-isolated molecule (QIM) and a molecule in a cluster. As the QIM approximation, a 24-atom isolated molecule is considered. The average influence of intermolecular interactions is taken into account only in the molecule's field of forces. As the second approximation, an 84-atom cluster model of crystalline α-d-glucose is considered, which permits one to take into account the influence of intermolecular hydrogen bonds in the crystal on the electronic structure of the molecule in calculating the intensities. A detailed interpretation of the observable spectrum is given. A high sensitivity of band intensities of OH-valence vibrations in the 3600-3000 cm-1 region to the formation of hydrogen bonds and a marked sensitivity in the 600-450 cm-1 region is shown. It is found that the IR spectra of monosaccharides in the 3000-600 cm-1 range can be interpreted in the quasi-isolated molecule approximation. © 1990.