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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.

Keywords

Essential oil isomers carvacrol thymol spectroscopy

Introduction

Carvacrol and thymol exist in many aromatic plants and hence in a number of essential oils.^1-3 These materials are structural isomers. They have the same structural backbone except for the positioning of the OH functional group. In thymol, the OH is ortho to the isopropyl group while in carvacrol it is ortho to the methyl group. These compounds fall under the classification of monoterpene phenols. These monoterpene phenols have a wide range of applications. Thymol has medicinal properties that include among others, wound healing, analgesic, antimicrobial, and antiinflammatory.^4-8 It is an active ingredient in food flavorings, topical ointments, various soaps, toothpastes, shampoos, deodorants, and mouthwashes.⁹ It can also aid in digestion by relaxing smooth muscles, reduces menstrual cramps, and alleviates respiratory difficulties.^10,11 Carvacrol on the other hand can be used as a disinfectant, fungicide, and it has been found to have insecticidal activity.^12-14 Furthermore, it is used in food as a flavor, an additive, a preservative, and in cosmetics as a scent.¹⁰ These materials have been identified as constituents of essential oils using a gas chromatography-mass spectrometer (GC-MS) and gas chromatography-flame ionization detector (GC-FID).^15,16 GC-MS has proven to be a valuable tool in the quantification of the constituents of essential oils, but this method has a limitation in that it is unable to distinguish between closely related compounds or isomers unless an internal standard is used. This calls for rapid methods that can distinguish and validate compounds that have been detected by the GC-MS.

Apart from GC methods, carvacrol and thymol isomers have been mainly characterized in the mid-infrared region.^17-19 This has resulted in limited studies of these materials in the lower frequency range, commonly known as the far infrared/ terahertz range. For an unambiguous characterization or identification of these materials, it is fundamental to understand their properties over an extended frequency range. Far infrared/terahertz spectroscopy has been widely used in material characterization/identification and has been proven to be an essential tool in this aspect. This is supported by the extensive research in this part of the electromagnetic (EM) spectrum.^20,21 THz waves are sandwiched between the infrared and microwave regions in the EM spectrum. This position indicates that the THz systems require a combination of optics and electronics, which can be employed in THz generation, detections as well as processing, but this still remains a challenge.^22,23 In definition, 1 THz corresponds to 4.1 meV of energy. It is equivalent to 33.3 cm⁻¹ and THz waves have a frequency of 10¹² hertz (Hz). THz waves possess unique properties that allow for their application in almost any field of research.^24-31 These waves have the capability of penetrating a range of materials in any form of matter.^32-37 They have a low photon energy and hence they are nonionizing. THz radiation unlike other types of EM waves is very sensitive to various resonances that include torsional, rotational, vibrational, and translational therefore, making it an important tool for providing information on molecules.³⁸ Infrared spectroscopic techniques are simple, less costly, rapid, and nondestructive and have the capability for routine analysis.¹⁷ On the other hand gas chromatography devices require complex sample preparation and have limited sensitivity.³⁹

This work presents a comparison between the spectra of thymol and carvacrol in the far infrared region with the aim of introducing a cheap and robust alternative method that can be used for the characterization of these materials. Density functional theory (DFT) calculations will be employed for the interpretation of the far infrared spectra of these isomers and spectral feature assignments will be attempted. A comparison will be made between this work and the literature.

Experimental

Carvacrol and thymol were purchased from Sigma Aldrich and used without further processing. The THz spectra of these materials were measured using the Bruker Vertex 70v vacuum spectrometer utilizing the single bounce diamond crystal attenuated total reflection (ATR) accessory. The resolution at which the spectra were collected was 1 cm⁻¹ (0.03 THz). The number of scans collected for each spectrum was 512.0 and on average it took 13 minutes for each scan. A broadband beamsplitter was employed. The tungsten lamp was used as the source of radiation and the deuterated triglycine sulfate (DTGS) was used as the detector. The ratio of the spectrum collected with the sample in the beam path to the spectrum with no sample in the beam path gave the transmission spectrum. The DFT modeling within the Orca software package⁴⁰ was utilized in the calculations of the infrared spectra. The calculations involved the use of the hybrid PBE0 functional and the def2-TZVPP basis set^41-47 for geometry optimization and calculation of the energies. The model made use of the empirical Van der Waals correction.⁴⁸

Carvacrol and thymol were analyzed using an Agilent 6890N capillary gas chromatograph connected to an Agilent G5977 mass spectrometer and a PAL 3 RSI auto-sampler (5301 Stevens Creek Blvd. Santa Clara, CA 95051, United States). The NIST library was used for the identification of compounds (NIST, Mass-spectrometry Data Center, 2017). Separation was achieved using a standard nonpolar HP-5MS capillary column (30 m length, 0.32 mm internal diameter, and 0.25 µm film thickness). Sample injections were made in a split mode using a general-purpose split/split-less liner packed with glass wool. The GC oven temperature program was started at 70 °C and held for 5 minutes, then increased to 300 °C at a rate of 5 °C min⁻¹ resulting in a total run time of 51 minutes. Sample volumes of 1 µL were injected into the instrument using helium as a carrier gas at a split ratio of 100:1. The flow rate of helium was set at a constant flow rate of 0.5 mL min⁻¹. The injector and mass transfer line temperature settings were 250 °C.

Results and Discussions

The molecular structures of carvacrol and thymol are presented in Figure 1. As can be seen, the molecular arrangements of these compounds are almost the same. The only difference is in the position of the OH functional group. The retention times for carvacrol and thymol are very close (18.5 for thymol and 18.8 for carvacrol) on the GC-MS such that it is difficult to use only the NIST library for identification without the reference standards. Figure 2 shows that the mass spectra are identical, which makes the characterization of these isomers by GC-MS challenging. Additionally, the characterization will be even more challenging in complex matrices like essential oils. Therefore, this work presents a spectroscopic method to augment the characterization of these isomers.

Figure 1.

Molecular structures of (A) carvacrol and (B) thymol.

Figure 2.

Ms structures of (A) thymol and (B) carvacrol from the NIST library.

The far infrared spectra of thymol and carvacrol isomers are displayed in Figure 3. The spectra are characterized by a number of distinct absorption bands of various intensities in the considered frequency range. The spectrum of carvacrol is made up of 10 spectral bands while that of thymol comprises 13 absorption bands. Carvacrol displayed two high-intensity bands at 462.4 cm⁻¹ (13.9 THz) and 566.9 cm⁻¹ (17.0 THz). The rest of the bands have moderate to weak intensities. Thymol displayed a strong band at 586. 8 cm⁻¹ (17.6 THz). Four medium intensity bands appeared at 342.1 cm⁻¹ (10.3 THz), 392.5 cm⁻¹ (11.8 THz), 488.5 cm⁻¹ (14.7 THz), and 525.3 cm⁻¹ (15.8 THz). The rest of the bands can be classified as weak-intensity bands. There is evidence of bands appearing at similar energies for thymol and carvacrol. This correspondence can be ascertained by determining whether the common bands share the same vibration mode. Four common bands were established between carvacrol and thymol. These bands were assigned on the basis of the appearance of the bands at similar frequencies and also displaying the same vibration mode. In addition, thymol and carvacrol had four seemingly common absorption bands that however exhibited different vibrational modes. On the other hand, the differences in the spectra of these materials are noticeable. There were nine absorption bands that were unique to thymol and six to carvacrol. Even though there is a slight difference between the molecular arrangements of these compounds, there are marked differences in the absorption spectra. This attests that far infrared/THz spectroscopy can be successfully used in the identification of materials with close structural features. It is quicker to obtain measurements using this alternative method as there is no need for extensive sample preparation. Furthermore, this result suggests that this method can be used for quality control as each material has a unique spectrum in the considered spectral range. There will however be a limitation in identifying constituents in aqueous solutions as terahertz radiation is strongly absorbed by water.

Figure 3.

Room-temperature spectra of carvacrol and thymol.

Figure 4 shows the comparison between the experimental and single molecule DFT calculations of thymol. The single molecule theoretical model produced frequencies that matched well with the observed experimental features. There was a discrepancy in experimental bands and the matching theoretical peak intensities. From the experiment, the band labeled H had a low intensity while the matching theoretical peak had a high intensity. The thymol experimental band labeled M had a high intensity and the matching theoretical peak had a low intensity. The incongruity in the intensities is attributed to single molecule calculations since intensities usually depend on crystalline fields. Absorption bands labeled C and F did not have corresponding theoretical peaks. The absence of these peaks suggests that they are a result of intermolecular interactions. The model gave two peaks at 277.4 cm⁻¹ (8.3 THz) and 513.1 cm⁻¹ (15.4 THz) both denoted x in Figure 4, which did not have matching experimental bands. This proposes that the technique used was not able to resolve these two bands. Terahertz spectra are usually associated with homogeneous and inhomogeneous broadening that leads to the concealment of spectra features at room temperature.⁴⁹ This can be counteracted by low-temperature measurements. Cooling of the samples reduces broadening and therefore can unmask spectral features that do not appear in room-temperature spectra.⁵⁰

Figure 4.

Comparison between the room-temperature experimental and theoretical spectra of thymol. The vertical lines are the calculated energies and their relative intensities.

Figure 5 shows a good agreement between the experimental and theoretical frequencies of carvacrol. All the observed experimental bands had corresponding theoretical peaks except for one band denoted S. The model further resulted in three theoretical peaks denoted x in Figure 5, which did not appear in the experiment. These peaks appeared at 258. 5 cm⁻¹ (7.8 THz), 286.4 cm⁻¹ (8.6 THz), and 332.9 cm⁻¹ (10.0 THz). Due to limited resources, low-temperature measurements that have the capability of resolving hidden spectral features could not be undertaken. The carvacrol experimental band denoted P displayed a low intensity while its corresponding theoretical peak had a high intensity. Moreover, the experimental bands denoted Q and M had medium intensities while their corresponding theoretical peaks had low intensities. This phenomenon is also observed for thymol.

Figure 5.

Comparison between the room-temperature experimental and theoretical spectra of carvacrol. The vertical lines are the calculated energies and their relative intensities.

Tables 1 and 2 present the assignment of the absorption bands for thymol and carvacrol respectively. For vibration modes assignment, the molecules were visualized using the gOpenmol program.⁵¹ The assignment was based on the vibration modes, together with approximate assignment by the vibrational frequency. From the assignment, it was discovered that thymol and carvacrol share four absorption bands. These bands denoted G, J, L, and M for both materials were found to have the same vibrational modes. Absorption bands denoted A and K for thymol seemed to correspond to carvacrol absorption bands denoted N and R, respectively. These bands, however, displayed different vibrational modes. Superficial correspondences in the absorption bands have been previously reported.⁵² The peak assignment revealed that majority of the vibrational modes in the considered frequency range are due to intramolecular interactions that encompass much of the entire molecule. It has to be noted that the other modes for phenolic compounds like thymol and carvacrol that give rise to bands below 600 cm⁻¹ (18.0 THz) may be due to intermolecular hydrogen bond stretching and bending modes. The hydrogen bonding in phenols is important in that it can be used as a model for biological systems.⁵³

Table 1.

Experimental and Theoretical Frequencies of Thymol.

Label	ThymolExperimental bandscm⁻¹ (THz)	ThymolTheoretical peakscm⁻¹ (THz)	Absorption peak description
A	212.8 (6.4)	216.8 (6.5)	Wagging of the whole molecule along -CH₃.
B	226.5 (6.8)	227.5 (6.8)	Wagging of the molecule along -OH group.
C	242.2 (7.3)	-	-
D	251.4 (7.6)	251.6 (7.6)	Rocking of the functional groups counteracted by the ring.
x	-	277.4 (8.3)	Rocking of the -CH₃ functional groups counteracted the rest of the molecule.
E	296.2 (8.9)	292.7 (8.8)	Rocking of the functional groups and minor stretch of the ring.
F	306.5 (9.2)	-	-
G	320.7 (9.6)	317.4 (9.5)	Rocking of the functional groups counteracted by the ring.
H	342.1 (10.3)	342.5 (10.3)	Rocking of the OH group counteracted by the rest of the molecule.
I	392.5 (11.8)	394.6 (11.8)	Wagging of the ring and rocking of the functional groups.
J	439.2 (13.2)	425.0 (12.8)	Stretching of the molecule along the alkyl groups.
K	488.5 (14.7)	472.2 (14.2)	Stretching of the molecule along -CH₃ group with a bit of twist.
X	-	513.1 (15.4)	Wagging of the molecule along the -CH₃ functional groups.
L	525.2 (15.8)	522.2 (15.7)	Twisting of the whole molecule.
M	586.8 (17.6)	588.2 (17.7)	Stretching of the molecule along the -OH group.

Table 2.

Experimental and Theoretical Frequencies of Carvacrol.

Label	CarvacrolExperimental bandscm⁻¹ (THz)	CarvacrolTheoretical peakscm⁻¹ (THz)	Absorption peak assignment
N	204.7 (6.1)	231.0 (6.9)	Rocking of the isopropyl group counteracted by the rest of the molecule
X	-	258.5 (7.8)	Rocking of the isopropyl group counteracted by the rest of the molecule
O	269.8 (8.1)	270.0 (8.1)	Wagging of the molecule along the -OH group
X	-	286.4 (8.6)	Rocking of the functional groups and minor stretch of the ring
G	313.6 (9.4)	303.7 (9.1)	Rocking of the functional groups counteracted by the ring
X	-	332.9 (10.0)	Rocking of the OH group counteracted by the rest of the molecule
P	377.1 (11.3)	382.4 (11.5)	Wagging of the molecule along the alkyl groups
J	419.8 (12.6)	417.2 (12.5)	Stretching of the molecule along the alkyl groups
Q	462.4 (13.9)	479.0 (14.4)	Twisting of the whole molecule
R	489.0 (14.7)	491.7 (14.8)	Rocking of the molecule along -CH₃ with a bit of stretch
L	521.2 (15.7)	523.3 (15.7)	Twisting of the whole molecule
M	566.9 (17.0)	582.9 (17.5)	Stretching of the molecule along -OH group
S	600.0 (18.0)	-	-

A further comparison was made between this work and the literature as shown in Table 3. There was correspondence between some of the observed absorption bands and the related literature studies.^54,55 The most outstanding observation is that the literature is dominated by mid-infrared studies. This has left the lower frequency range unexplored for these materials. It was found imperative to perform the low-frequency studies because the availability of an extended spectral database of these materials is crucial for a better understanding of their properties. This information will be valuable in the manufacture and identification of products derived from medicinal plants or essential oils in particular.

Table 3.

Comparison of Spectral Features of Thymol and Carvacrol with Literature.

ThymolThis studycm⁻¹ (THz)	ThymolSaraiva et al.⁵⁵cm⁻¹ (THz)	CarvacrolThis studycm⁻¹ (THz)	CarvacrolSadeq et al ⁵⁴cm⁻¹ (THz)
212.8 (6.4)		204.7 (6.2)
226.5 (6.8)		269.8 (8.1)
242.2 (7.3)		313.6 (9.4)
251.4 (7.5)		377.1 (11.3)
			411.6 (12.4)
296.2 (8.9)		419.8 (12.6)	420.8 (12.6)
306.5 (9.2)		462.4 (13.9)	461.6 (13.9)
320.7 (9.6)		489.0 (14.7)
342.1 (10.3)		521.2 (15.7)
392.5 (11.8)		566.9 (17.0)	574.1 (17.2)
439.2 (13.2)		600.0 (18.0)	633.0 (19.0)
488.5 (14.7)	493.0 (14.8)
525.2 (15.8)	525.0 (15.8)
586.8 (17.6)	588.0 (17.6)

Conclusion

The far infrared/terahertz spectra of carvacrol and thymol were measured using the FTIR-ATR technique. The difference in the spectra of these materials that have an almost similar molecular structure proves the robustness of far infrared/terahertz radiation in the identification and characterization of isomers. The absorption bands were interpreted using the DFT model. The model revealed four common absorption bands between carvacrol and thymol. The common bands were found to share the same vibrational modes. The model further suggested that there were some concealed absorption bands in the room-temperature spectra of these materials. Despite these similarities, there were peaks that were characteristic to each of the isomers. This highlights the ability of the DFT calculations in the analysis of the terahertz spectra of materials. The retention times of these isomers as shown by the GC-MS are very close and they have a similar mass spectral profile that could pose a challenge when identifying them in complex matrices. This study is important as it ushers in the possibility of characterizing compounds of similar structural features commonly found in essential oils using far infrared/THz spectroscopy as a complementary method to the conventional GC-MS. This alternative method is nondestructive, as there are no sample alterations during the analysis process. Furthermore, it offers a more rapid analysis and there is no intensive sample preparation.

Footnotes

Acknowledgments

This work was supported by the Botswana International University of Science and Technology.

Declaration of Conflicting Interests

The authors declared no potential conflicts of interest with respect to the research,authorship,and/or publication of this article.

Ethical Approval

Not applicable.

Funding

The authors received no financial support for the research,authorship,and/or publication of this article.

Statement of Human and Animal Rights

This article does not contain any studies with human or animal subjects.

Statement of Informed Consent

There are no human patients involved in this article and informed consent is not applicable.

ORCID iDs

L. M. Lepodise

T. Pheko-Ofitlhile

References

Suntres

Coccimiglio

Alipour

. The bioactivity and toxicological actions of carvacrol. Crit Rev Food Sci Nutr 2015; 55(3): 304-318.

Youssefi

Tabari

Esfandiari

, et al. Efficacy of two monoterpenoids, carvacrol and thymol, and their combinations against eggs and larvae of the west Nile vector culex pipiens. Molecules 2019; 24(10): 1867.

Sanei-Dehkordi

Heiran

Moemenbellah-Fard

, et al. Nanoliposomes containing carvacrol and carvacrol-rich essential oils as effective mosquitoes larvicides. BioNanoScience 2022; 12(2): 359-369.

Riella

Marinho

Santos

, et al. Anti-inflammatory and cicatrizing activities of thymol, a monoterpene of the essential oil from Lippia gracilis in rodents. J Ethnopharmacol 2012; 143(2): 656-663.

Ozen

Demirtas

Aksit

. Determination of antioxidant activities of various extracts and essential oil compositions of Thymus praecox subsp. skorpilii var. skorpilii. Food Chem 2011; 124(1): 58-64.

Karpanen

Worthington

Hendry

, et al. Antimicrobial efficacy of chlorhexidine digluconate alone and in combination with eucalyptus oil, tea tree oil and thymol against planktonic and biofilm cultures of Staphylococcus epidermidis. J Antimicrob Chemother 2008; 62(5): 1031-1036.

Aeschbach

Löliger

Scott

, et al. Antioxidant actions of thymol, carvacrol, 6–gingerol, zingerone and hydroxytyrosol. Food Chem Toxicol 1994; 32(1): 31-36.

Nagoor Meeran

Javed

Al Taee

, et al. Pharmacological properties and molecular mechanisms of thymol: prospects for its therapeutic potential and pharmaceutical development. Front Pharmacol 2017; 8: 380.

Sharifi-Rad

Varoni

Iriti

, et al. Carvacrol and human health: a comprehensive review. Phytother Res 2018; 32(9): 1675-1687.

10.

Shapiro

Meier

Guggenheim

. The antimicrobial activity of essential oils and essential oil components toward oral bacteria. Oral Microb Immunol 1994; 9(4): 202-208.

11.

Beć

Grabska

Kirchler

Huck

. 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.

12.

Andersen

. Final report on the safety assessment of sodium p-chloro-m-cresol, p-chloro-m-cresol, chlorothymol, mixed cresols, m-cresol, o-cresolp-cresol, isopropyl cresols, thymol, o-cymen-5–ol, and carvacrol. Int J Toxicol 2006; 25(Suppl 1): 29-127.

13.

Novato

TPL

Araújo

de Monteiro

CMO

, et al. Evaluation of the combined effect of thymol, carvacrol and (E)-cinnamaldehyde on amblyomma sculptum (acari: ixodidae) and dermacentor nitens (Acari: ixodidae) larvae. Vet Parasitol 2015; 212(3–4): 331-335.

14.

Traboulsi

Taoubi

El-Haj

, et al. Insecticidal properties of essential plant oils against the mosquito Culex pipiens molestus (Diptera: culicidae). Pest Manag Sci 2002; 58(5): 491-495.

15.

Safaei-Ghomi

Ebrahimabadi

Djafari-Bidgoli

Batooli

. GC/MS analysis and in vitro antioxidant activity of essential oil and methanol extracts of thymus caramanicus Jalas and its main constituent. Food Chem 2009; 115(4): 1524-1528.

16.

Han

G-Q

Yang

, et al. Chemical composition and antioxidant activities of essential oils from different parts of the oregano. Zhejiang Univ Sci B 2017; 18(1): 79-84.

17.

Valderrama

ACS

Rojas De

. Traceability of active compounds of essential oils in antimicrobial food packaging using a chemometric method by ATR-FTIR. Am J Anal Chem 2017; 8(11): 726-741.

18.

Friné

V-C

Hector

A-P

Manuel

N-DS

, et al. Development and characterisation of a biodegradable PLA food packaging hold monoterpene-cyclodextrin complexes against Alternaria alternata. Polymers (Basel) 2019; 11(10): 1720.

19.

Gandova

Lazarov

Fidan

, et al. Physicochemical and biological properties of carvacrol. Open Chemistry 2023; 21(1): 20220319.

20.

Lepodise

Lewis

Joseph

Horvat

. THz spectroscopic characterisation of biochar. In: IRMMW-THz 2015-40th International Conference on Infrared, Millimeter and Terahertz Waves, China.

21.

Horvat J

Lepodise

Lewis

. Absorption spectra of benzoic acid in the 5-15 THz range. In: IRMMW-THz 2014-39th International Conference on Infrared, Millimeter and Terahertz Waves. United States.

22.

In: Dexjeimer

(ed) Teraherstz spectroscopy: Principles and applications. CRC Press, 2008.

23.

Dragoman

. Terahertz fields and applications. Prog Quantum Electron 2004; 28(1): 1-66.

24.

Gao

Wang

, et al. Terahertz technology and its biomedical application. Yangtze Medicine 2019; 3(3): 157-162.

25.

Markelz

Mittleman

. Perspective on terahertz applications in bioscience and biotechnology. ACS Photonics 2022; 9(4): 1117-1126.

26.

Markl

Ruggiero

Zeitler

. Pharmaceutical applications of terahertz spectroscopy and imaging. Eur Pharm Rev 2016; 21: 45-48.

27.

Taday

. Applications of terahertz spectroscopy to pharmaceutical sciences. Philos Trans Royal Soc A 2004; 362(1815):351-364.

28.

, et al. Applications of THz spectral imaging in the detection of agricultural products. Photonics 2021; 8(518).

29.

Long

Yang

. Measurements and analysis of water content in winter wheat leaf based on terahertz spectroscopy. Int J Agric Biol Eng 2018; 11(3):178-182.

30.

Shihzad

Ullah

Ahmad

, et al. Design and analysis of dual-band high-gain THz antenna array for THz space applications. Appl Sci 2022; 12(18): 9231.

31.

Civas

Akan

. Terahertz wireless communications in space. arXiv e-prints, arXiv-2110, 2021.

32.

George

Markelz

. Terahertz spectroscopy of liquids and biomolecules. In: Peiponen

Zeitler

Kuwata-Gonokami

(eds) Terahertz Spectroscopy and Imaging. Springer Series in Optical Sciences. 171. Springer, 2012. doi:https://doi.org/10.1007/978-3-642-29564-5_9.

33.

Mou

Rubano

Paparo

. Broadband terahertz spectroscopy of imidazolium-based ionic liquids. J Phys Chem B 2018; 122(12): 3133-3140.

34.

Parrott

EPJ

Fischer

Gladden

, et al. Terahertz Spectroscopy of Crystalline and Non-Crystalline Solids. In: Peiponen

Zeitler

Kuwata-Gonokami

(eds) Terahertz Spectroscopy and Imaging. Springer Series in Optical Sciences. 171. Springer, 2012. doi:https://doi.org/10.1007/978-3-642-29564-5_8.

35.

Melinger

Harsha

Laman

Grischkowsky

. Guided-wave terahertz spectroscopy of molecular solids. J Opt Soc Am B 2009; 26(9): A79-A89.

36.

Araki

Tabata

Shimizu

Matsuyama

. Terahertz spectroscopy of CO and NO: the first step toward temperature and concentration detection for combustion gases in fire environments. J Mol Spectrosc 2019; 361: 34-39.

37.

Cuisset

Hindle

Mouret

, et al. Terahertz rotational spectroscopy of greenhouse gases using long interaction path-lengths. Appl Sci 2021; 11(3): 1229.

38.

Ghann

Uddin

. Terahertz (THz) Spectroscopy: A Cutting Edge Technology. In: Terahertz (THz) Spectroscopy- A Cutting Edge Technology. IntechOpen; 2017. doi: https://doi.org/10.5772/67031.

39.

Maštovská

Lehotay

. Practical approaches to fast gas chromatography-mass spectrometry. J Chromatogr A 2003; 1000(1–2): 153-180.

40.

Neese

. Wiley interdisciplinary reviews. Comput Mol Sci 2012; 2: 73-78.

41.

Krishnan

Binkley

Seeger

Pople

. Self-consistent molecular orbital methods. XX. A basis set for correlated wave functions. J Chem Phys 1980; 72(1):650-654.

42.

McLean

Chandler

. Contracted Gaussian basis sets for molecular calculations. I. Second row atoms, Z=11-18. J Chem Phys 1980; 72(10):5639-5648.

43.

Blandeau

J-P

McGrath

Curtiss

Radom

. Extension of Gaussian-2 (G2) theory to molecules containing third-row atoms K and ca. J Chem Phys 1997; 107(13): 5016-5021.

44.

Curtiss

McGrath

Blaudeau

J-P

, et al. Extension of Gaussian-2 theory to molecules containing third-row atoms Ga-Kr. J Chem Phys 1995; 103(14): 6104-6113.

45.

Clark

Chandrasekhar

Spitznagel

Schleyer

PVR

. Efficient diffuse function-augmented basis sets for anion calculations. III. The 3–21+G basis set for first-row elements, Li-F. J Comput Chem 1983; 4(3): 294-301.

46.

Weigend

Ahlrichs

. Balanced basis sets of split valence, triple zeta valence and quadruple zeta valence quality for H to Rn: design and assessment of accuracy. Phys Chem Chem Phys 2005; 7(18): 3297-3305.

47.

Weigend

. Accurate Coulomb-fitting basis sets for H to Rn. Phys Chem Chem Phys 2006; 8(9): 1057-1065.

48.

Grimme

. Semi-empirical GGA-type density functional constructed with a long-range dispersion correction. J Comput Chem 2006; 27(15): 1787-1799.

49.

Lepodise

. Wide temperature range studies of the low frequency THz spectrum of benzoic acid using FTIR spectroscopy. Heliyon 2020; 6(11): e05577.

50.

Laman

Harsha

Grischkowsky

Melinger

. High-resolution waveguide THz spectroscopy of biological molecules. Biophys J 2008; 94(3): 1010-1020.

51.

Laaksonen

. A graphics program for the analysis and display of molecular dynamics trajectories. J Mol Graph 1992; 10(1): 33-34.

52.

Lepodise

Horvat

Lewis

. Superficial and fundamental correspondences in the terahertz/IR (6-15 THz) absorption spectra of aspirin and benzoic acid. J Phys Chem A 2018; 122(34): 6886-6893.

53.

Fedor

Toda

. Investigating hydrogen bonding in phenol using infrared spectroscopy and computational chemistry. J Chem Edu 2014; 91(12):2191-2194.

54.

Sadeq

Kamel

Qader

. A novel preparation of thyme cream as superficial antimicrobial treatment. J Int Pharm Res 2019; 202(46): 373-381.

55.

Saraiva

AGQ

Saraiva

Albuquerque

, et al. Chemical analysis and vibrational spectroscopy study of essential oils from Lippia sidoides and of its major constituent. Vib Spectrosc 2020; 110: 103111.

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

Abstract

Keywords

Introduction

Experimental

Results and Discussions

Conclusion

Footnotes

Acknowledgments

Declaration of Conflicting Interests

Ethical Approval

Funding

Statement of Human and Animal Rights

Statement of Informed Consent

ORCID iDs

References