[Characterization and simulation of far-infrared spectroscopy for saturated monohydric alcohols]

Fourier transform infrared-attenuated total reflection (FTIR-ATR) was employed to measure the far-infrared (FIR) spectra in wavenumbers of 30-300 cm(-1) for six kinds of saturated monohydric alcohols, namely: methanol, ethanol, propanol, isopropanol, butanol and isobutanol. Further analysis of the FIR spectra for these monohydric alcohols with similar chemical structures reveals that absorption peaks are observed obviously for these alcohols in the 30-150 cm(-1) band, whereas not obvious peaks are measured in the 150-300 cm(-1) band. Moreover, it was found that the monohydric alcohols with higher hydroxy concentration possess lower average FIR transmission. In addition, the average FIR transmissions of linear chain monohydric alcohols are higher than those of the branched chain ones. Furthermore, the density functional theory (DFT) B3LYP/6-311G(d,p) basis set was employed to simulate the structures optimization and to calculate the responding frequencies of the methanol monomer and polymer. Simulation result indicates that no absorption peaks are found in the 30-150 cm(-1) band for the methanol monomer molecule, whereas there are obvious absorption peaks for the methanol polymers in the same band. In addition, the simulated absorption peak positions for the methanol polymers are in agreement with those experimentally measured. Both results indicate that the absorption of the methanol in Terahertz (THz) is attributed to the collective vibrations of different kinds of polymer, and that the polymer for methanol is mainly trimmer. This paper not only provides a new way to investigate the responding frequencies of organic molecule in THz band, but also is helpful for the FTIR-ATR study of other organic molecules.