Terahertz (6-18 THz) spectroscopic characterization of benzoic acid class liquid crystals using Fourier Transform Infrared technique

Terahertz spectroscopic studies of 4-octyloxybenzoic acid, 4-decyloxybenzoic acid and 4-hexylbenzoic acid liquid crystals were performed using the Fourier Transform Infrared method. The materials are characterized by distinct absorption bands in the 6–18 terahertz frequency range. A good correspondence was observed between the experimental frequencies and theoretical frequencies of the single molecule obtained by Density Functional Theory calculations for all the three compounds. The spectra of the studied compounds had three bands, which appeared at similar frequencies and with the same vibration modes. 4-Octyloxybenzoic acid, 4-decyloxybenzoic acid and 4-hexylbenzoic acid had two absorption bands that appeared at similar frequencies and exhibited different vibration modes. This study interestingly demonstrates that even though compounds of an almost similar molecular structure might output bands at the same frequencies, the correspondence might not be apparent. The single molecule model frequencies and experimental frequencies for 4-decyloxybenzoic acid and 4-octyloxybenzoic acid further revealed that all the observed bands are due to intramolecular interaction. However, the 4-hexylbenzoic acid spectrum has one apparently intermolecular band. This work addresses the limitation in the intermediate terahertz spectroscopic studies of liquid crystals and furthermore emphasizes the robustness of Fourier Transform Infrared technique in identifying materials with similar attributes.