Terahertz time-domain spectroscopy has been used to measure the characteristic spectra of the similarly structured mono-saccharides D-glucose and D-galactose in the polycrystalline state at room temperature. A number of wellresolved absorption peaks were observed in the frequency range 0.2-3.0THz, which are interpreted as originating from inter-and intra-molecular vibrational modes. In parallel with the experimental study, density functional theory (DFT) was first applied to obtain structure and vibrational frequencies of the isolated galactose molecule in terahertz region. The calculated results are fitted reasonable with the experimental data. Observed vibrational frequencies have been interpreted using DFT. The comparison between experimental and theoretical study shows that far-infrared absorption features are highly sensitive to the structure and spatial arrangement of the molecules. This work also demonstrated that THz-TDS is a potentially important tool in understanding the collective vibrational modes and conformational structures of molecules.

The high-resolution terahertz spectra of the two structural isomers, mannose and galactose, have been measured by terahertz time-domain spectroscopy (THz-TDS) in the range of 0.5–4.0 THz at room temperature. Significant differences between these similar molecules have been found in their THz characteristic spectra, implying that THz-TDS is a powerful tool for identifying isomers. Structural analyses and normal mode calculations of the two systems were performed using solid-state density functional theory (DFT) with the PBE and PW91 density functionals as well as using gas-state DFT with B3LYP hybrid functional. Among these calculations, the solid-state simulated results obtained from the PBE method exhibit a good agreement with the experimentally measured spectra. According to the calculated results of PBE, the observed spectral features were assigned as primarily external lattice translations, deformations, and rotations with lesser contributions due to intramolecular motion of pyranose ring, CH2OH group, and hydroxyl groups.