Quantum mechanical modeling of NIR spectra of thymol

Near-infrared (NIR) spectroscopy is a major analytical tool with marked importance in various applications including the qualitative and quantitative analysis of herbal medicines. Yet, the physical background for the measured spectra routinely remains un-interpreted. Quantum mechanical calculations are capable of providing deep and independent insights and form a strong support for applied spectroscopy. An example of thymol [1] is presented here, an important constituent of a traditional herbal medicine Thymi herba.

NIR spectra of thymol in dependence of the sample state (solid, melted and soluted) and concentration (for soluted sample) were analyzed. Patterned spectral changes were observed, indicating that the differences between the bands in their sensitivity to the intermolecular interactions are reflected in the quantitative models. The analysis of PLS regression coefficients vector in the quantification of thymol content in Thymi herba based on NIR spectroscopy was conducted. Surprisingly, the vibrations which are the most significant spectra forming factors and which are the most sensitive to the chemical surrounding (i.e. those of OH group) do not correlated well with the sample content.

This work was supported by the Austrian Science Fund (FWF), P32004-N28.

Fig. 1

• References

• 1 Beć KB, Grabska J, Kirchler CG, Huck CW. NIR spectra simulation of thymol for better understanding of the spectra forming factors, phase and concentration effects and PLS regression features. J Mol Liq 2018; 268: 895-902