Far Infrared/Terahertz Spectroscopy as a Complementary Method for the Analysis of the Spectral Features of Thymol and Carvacrol Structural Isomers

Abstract

Objective: The conventional method for the characterization of volatile compounds has been mostly limited to the use of gas chromatography. This work offers an alternative method to address the limitations that might be encountered with the use of the gas chromatography method. Furthermore, this work addresses the scarcity of the analysis of these materials in the far infrared/terahertz region as previous studies have been focused mainly on the mid-infrared region. Methods: The far infrared/terahertz spectra of carvacrol and thymol were analyzed using the Fourier transform infrared (FTIR) and gas chromatography-mass spectroscopy (GC-MS). Density function theory (DFT) calculations were employed for the interpretation of the FTIR experimental spectra. Results: Thymol and carvacrol spectra were characterized by distinct absorption bands in the 200-600 cm−1 (6-18 THz). There were appearances of bands at similar energies for these isomers. These bands appeared at 320.7 cm−1 (9.6 THz), 439.2 cm−1 (13.2 THz), 525.2 cm−1 (15.8 THz), and 586.8 cm−1 (17.6 THz) for thymol and at 313.6 cm−1 (9.4 THz), 419.8 cm−1 (12.6 THz), 521.2 cm−1 (15.7 THz), and 566.9 cm−1 (17.0 THz) for carvacrol. Despite these similarities, there were also significant differences in the spectra of these materials. The theoretical peak frequencies were in agreement with the experimental absorption band frequencies. Two seemingly common absorption bands were observed in the spectra of thymol and carvacrol. However, peak assignments revealed that these bands have different vibrational modes. The GC-MS results show that the retention time of thymol and carvacrol are very close at 18.5 and 18.8 respectively. Conclusion: This work underscores the ability of far infrared/terahertz waves in identifying materials with similar structural features. Furthermore, it supplements the limited studies of these isomers in the far infrared/terahertz region. The GC-MS results emphasize the need for a complementary method that can improve the characterization of these materials.