Infrared spectra of polycarbohydrates and their model systems

Abstract

AbstractThe relationship of the infrared spectra of polycarbohydrates with their specific structural properties investigated with various model systems, for example, mono‐, di‐saccharides, etc., is discussed.Obtaining the carbohydrate spectra at the temperature of liquid nitrogen has allowed to split the OR group band into distinct components and to find out peculiarities of their low‐temperature shift, and to study the relationship of OR group hydrogen bond with the configuration of certain hydroxyls more thoroughly. Due to the considerable difference in the low‐temperature shift of OR group bands in the probable frequency ranges of intra‐ and intermolecular R bonds have been established as 3350‐3560 cm.−1 and 3100‐3450 cm.−1, respectively. The frequencies and intensities of OR group bands inserted in the H bond are observed to depend strongly upon the configuration of separate hydroxyl groups, suggesting a considerable interaction between the hydroxyls of the pyranose unit. The obtained results furnish ample grounds to conclude that the frequency of OR groups inserted in the hydrogen bond is detennined not only by the energy of the R bond but by the interaction ring, depending on the configuration of separate OR groups.The establishment of a relationship between the mutual spatial arrangement of CR groups and the spectrum in the 800–950 cm.−1 region allowed us to determine the frequency intervals specific for the combinations of axial (A) and equatorial (E) CR groups EA for 810–855 cm.−1; AE for 860–855 cm,−1; AA for 885–920 cm.−1. This allows us to predict the appearance of possible frequencies in the considered region in the process of conformation transitions of the pyranose ring.The investigation of mono‐, di‐, and polysaccharide spectra in the region of CH2 group deformation vibrations and calculation of the potential function of CR2OH group inner rotations permitted us to base the essential possibility of the existence of CR2OH group rotation isomers in carbohydrates and their polymers. The investigation of the spectral feature of crystalline cellulose triesters and triethers has been carried out. It is shown that their spectra are characterized by: an intensity increase of all or the majority of the absorption bands; the selective intensity increase of separate bands; and the appearance of new absorption bands. The intensity increase for many absorption bands is explained by the prevalence of certain types of rotation isomers and the appearance of new bands by the intermolecular interaction in crystals.