Low-Frequency Vibrations of Saccharides Using Terahertz Time-Domain Spectroscopy and Ab-Initio Simulations

The vibrational properties of molecular crystals in the terahertz range are controlled by the interplay of intermolecular and intramolecular interactions. The resulting delocalized normal modes span a substantial number of atoms within the unit cell of the molecular crystals and cannot be easily attributed to a specific stretching or bending mode. Several compounds were studied to understand the nature of normal modes. However, a systematic analysis of saccharides is still missing. This study investigates the terahertz vibrational properties of various saccharides, including glucose, galactose, lactose, cellobiose, and cellulose. The terahertz spectra were measured using terahertz time-domain spectroscopy. The samples were carefully characterized, and the residual Fabry-Perot oscillations were removed to obtain the absorption coefficient and refractive index of the saccharides. Density functional theory simulations were used to obtain theoretical terahertz spectra, considering hydrogen bonding interactions with an enhanced version of the van der Waals non-local density functional. The results revealed differences in low-energy vibrational modes frequencies, influenced by molecular structure properties, hydrogen bonding networks, and water content. Librations and internal vibrations were identified as dominant dynamics in the saccharides, with significant mixing between intermolecular and intramolecular vibrations. This comprehensive analysis sheds light on the vibrational behavior of saccharides in the terahertz range.