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Spectral and Theoretical Studies of Benzimidazole and 2-Phenyl Substituted Benzimidazoles

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A. Antony Muthu Prabhu

Submitted: 20 October 2021 Reviewed: 10 December 2021 Published: 13 July 2022

DOI: 10.5772/intechopen.101966

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Edited by Pravin Kendrekar and Vinayak Adimule

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This chapter discusses about the spectral and theoretical aspects of selected benzimidazole and 2-phenyl substituted benzimidazole molecules. The synthesis of these benzimidazoles was reported in many methods by the reaction between o-phenylenediamine with formic acid, aromatic aldehydes and N-benzylbezene-1,2-diamine in presence of oxidant tert-butyl hydroperoxide (TBHP). The spectral analysis of these molecules mainly such as UV-visible, fluorescence in solvents will be included in this chapter and discussed about the absorption, fluorescence maximum, conjugation, transition. Further the optimized structure of these molecules will be given using Gaussian 09 W (DFT 6-31G method). And also will be discussed about structural parameters, highest occupied molecular orbital (HOMO) – lowest unoccupied molecular orbital (LUMO) energy energy values, natural bond orbital (NBO), molecular electrostatic potential map (ESP). Many benzimidazole molecules having tautomers in the structure will be explained with the help of theoretical parameters to describe the structural properties.


  • benzimidazole
  • spectral properties
  • computational study
  • NBO
  • MSP

1. Introduction

This chapter will be discussed about the spectral and theoretical studies of benzimidazole and 2-phenyl substituted benzimidazoles. A series of benzimidazole molecules are very important heterocyclic compounds in organic chemistry with two nitrogen atoms in five membered ring fused with aromatic moiety having different nature in spectral and biological properties. Benzimidazoles which contain a hydrogen atom attached to nitrogen in the 1-position readily tautomerize. In this way, many benzimidazole molecules are synthesized from the basic moiety to involving in large applications especially in medicinal fields. This type of benzimidazole derivatives possess the many pharmaceutical properties such as antiviral [1], antitumor [2], antihistaminic [3], antimicrobial [4], antihelminthic [5, 6], anticancer [7], antifungal [8], antimicrobial [4], antibacterial [9], analgesic [10], anti-convulsant [11] and anti-ulcer [12] activity. Some of benzimidazole molecules are used as corrosion inhibitors for metals and alloy surfaces in industrial field [13, 14, 15].

Particularly, Albendazole, Mebendazole and Thiabendazole having benzimidazole moiety are widely used as anthelmintic drugs [16].

Some fluoroquinolones substituted benzimidazole derivatives have been reported by microwave assisted method. The synthesized compounds are reported to be the derivatives of Ciprofloxacin & Norfloxacin [17].

The structural studies of synthesized benzimidazole derivatives are characterized using the spectral techniques such as single crystal XRD, UV-visible, Infrared, 1H NMR, 13C NMR etc. [18, 19, 20, 21]. These main techniques are usually referred for characterizing many organic synthesized molecules to elucidate the presence of functional groups, conjugation and structural parameters. Particularly the absorption and fluorescence spectral properties of these benzimidazole derivatives have been changed with respect to the change in substitution in aromatic ring at o- and p-position [22, 23, 24, 25, 26, 27, 28].

Another important application of benzimidazoles is involved to exhibit the excited state intra-molecular proton transfer reaction (ESIPT) [22, 25, 29, 30, 31]. Particularly, the presence of hydroxyl group in benzimidazole at 2-position in benzene ring is exhibited this process through intramolecular hydrogen bonding between the acidic protons (-OH, -NH2) and basic centers (=N-, -C=O) in same molecule. ESIPT process for benzimidazole molecules is observed through dual fluorescence in aqueous solvent, one a normal stoke shifted fluorescence band and second large stoke shifted fluorescence band. Absorption and emission spectral study of these molecules were reported in different solvents with changing polarity. Moreover, the 2(2′-hydroxyphenyl)benzimidazole molecule are studied the enhancement of ESIPT process in aqueous β-cyclodextrin through the formation of host-guest inclusion complex [32].

The density functional theory (DFT) studies give information regarding the structural parameters, the functional groups, orbital interactions and vibrational frequencies [33]. The DFT calculations with the hybrid exchange-correlation functional B3LYP (Becke’s three parameter (B3) exchange in conjunction with the Lee–Yang–Parr’s (LYP) correlation functional) which are especially important in systems containing extensive electron conjugation and/or electron lone pairs [34, 35, 36]. The HOMO–LUMO energy, MSP map and the Mullikan population analysis will be calculated for the studied molecules. The natural bond orbital (NBO) analysis will explains the most important orbital interactions in order to clarify general structural features. The excited state potential energy surface, excited state intramolecular proton transfer of 2-(2′-Hydroxyphenyl)benzimidazole was investigated by TD-DFT method in gas phase and in solvent [37, 38, 39]. Further the theoretical calculations of Methyl-6-Nitro-1H-Benzimidazole and 1-methyl-2-phenyl benzimidazole was reported [40, 41]. Many benzimidazole molecules were reported theoretically structural properties and vibrational spectra, HOMO-LUMO, NBO analysis by ab initio HF and DFT methods [42, 43, 44, 45]. Theoretical calculations of many benzimidazole molecules in gas phase were analyzed for the structural investigation with the help of experimental results [20, 44, 46, 47, 48, 49].


2. Benzimidazole and 2-phenyl benzimidazoles

The benzimidazole molecule without any substitution was synthesized by the simple reaction between o-phenylenediamine with formic acid in the presence of alkali like sodium hydroxide, potassium hydroxide etc., [50]. The synthesis procedure of benzimidazole is given briefly. 27 g of o-phenylenediamine and 17.5 g of 90% formic acid are taken a 250 ml round bottom flask, the reaction mixture is heated at 100°C on a water bath for 2 hours. After cooling this reaction mixture, the 10% sodium hydroxide solution is added slowly to the solution with constant rotation then the reaction mixture becomes alkaline. Crude benzimidazole is filterd off at the pumb and washed with 25 ml of cold water and the crude product is dissolved in 400 ml of boiling water. Then 2 g of decolourising carbon is added to the solution and heat for 20 minutes. Finally the benzimidazole is formed after filteration at the pumb. New series of benzimidazole and its derivatives were synthesized and characterized using spectral analysis and applied for biological properties [51, 52]. For example, the series of substituted benzimidazole were reported particularly in the 2-position and 1-position replacing the hydrogen atom by both small and large size molecules (Figure 1).

Figure 1.

The reaction shows between o-phenylene diamine with formic acid in presence of sodium hydroxide.

The substituted 2-phenyl benzimidazoles were synthesized from the condensation reaction between o-Phenylenediamine and substituted aromatic aldehydes in chloroform and in the presence of ammonium chloride as a catalyst [53]. This reaction carried out at room temperature using many ammonium salts like ammonium bromide (NH4Br), ammonium chloride (NH4Cl), ammonium fluoride (NH4F), ammonium sulphate ((NH4)2SO4) and ammonium carbonate ((NH4)2CO3). Aromatic aldehydes such as benzaldehyde, m-methyl benzaldehyde, p-methyl benzaldehyde, m-methoxy benzaldehyde, p-methoxy benzaldehyde, o-hydroxy benzaldehyde, p-hydroxy benzaldehyde, o-amino benzaldehyde, p-amino benzaldehyde are used for the preparation of 2-phenyl substituted benzimidazole molecules. 1,2-phenylenediamine is added to the stirred solution of aromatic aldehydes and NH4Cl in CHCl3. This reaction mixture is stirred for 4 hours at room temperature. After completion of the reaction, the phenyl substituted benzimidazoles are formed using Thin layer chromatography, column chromatography (Figure 2).

Figure 2.

The reaction shows between o-phenylene diamine with aromatic aldehyde in chloroform and in presence of ammonium chloride.

Thus 2-phenyl substituted benzimidazole derivative has been reported by another method and the reaction scheme shows in Figure 3. The oxidative dehydrative coupling reaction of N-benzylbenzene-1,2-diamine in the presence of oxidant tert-butyl hydroperoxide (TBHP) in solvent acetonitrile to give substituted 2-phenyl benzimidazoles [54]. N-benzylbenzene1,2-diamine 1.96 g, I2 0.25 g, TBHP 1.8 g, is added in a 25 ml round bottomed flask in acetonitirle solvent and stirred at room temperature. The product is purified by column chromatography and finally the phenyl substituted benzimidazole is formed at the end of reaction.

Figure 3.

Tandem oxidative cyclization of different N-substituted benzene-1,2-diamines.


3. Absorption and emission spectral study

Thus, the molecular diagrams of studied benzimidazole derivatives are shown in Figure 4. In this section, the absorption and emission spectral study is discussed for the selected benzimidazoles in solvents. The absorption and emission maximum was observed at 273 nm, 279 nm and at 291 nm for benzimidazole [55, 56] and at 295 and 350 nm for 2-phenyl benzimidazole [57]. These maximum are similar to 2-(m-methylphenyl)benzimidazole, 2-(p-methylphenyl)benzimidazole, 2-(m-methoxyphenyl)benzimidazole, 2-(p-methoxyphenyl)benzimidazole. But the 2-(o-hydroxyphenyl)benzimidazole molecule is observed the specific property in the ground and excited states to describe the keto-enol tautomeric equilibrium through the excited state intramolecular proton transfer [58, 59, 60, 61, 62, 63, 64, 65], which property already given in introduction part and theoretical study also done for keto-enol tautomer in solvent effect. The absorption maximum of 2-(o-hydroxyphenyl)benzimidazole was observed at 335, 318 and 295 nm and fluorescence maximum at 355 and 465 nm in DMSO [32]. The absorption and emission maximum was observed at 320, 285 and 240 nm and fluorescence maximum at 417 nm in water for 2-(o-aminophenyl)benzimidazole [66] and at 313, 255, and 207 nm and at 382 nm in water for 2-(p-aminophenyl)benzimidazole [67].

Figure 4.

It shows the molecular diagram of selected molecules by DFT B3LYP 6-31G method.


4. Theoretical study

4.1 HOMO-LUMO energy parameters

Initial computational study of selected benzimidazoles has been investigated from the HOMO, LUMO energy diagram in gas phase using DFT B3LYP 6-31G method. The chemical stability of selected benzimidazoles is demonstrated with the help of explaining this energy diagram [68]. Generally, the HOMO energy picture represents the ability to donate an electron and LUMO energy picture represent the ability to obtain an electron. For selected benzimidazoles the electron density is completely localized on the whole ring for both orbitals which shows in Table 1. The higher HOMO-LUMO energy gap of benzimidazole (−5.56 eV) and lower HOMO-LUMO energy gap of 2-(o-aminophenyl)benzimidazole (−4.38 eV) is observed theoretically to describing the stability and reactivity with addition of substitution in the phenyl ring. If the substitution in the benzimidazole is clearly changed the HOMO-LUMO energy gap particularly the amino group at o-position in the phenyl ring with lower value.

Table 1.

HOMO, LUMO energy diagram and molecular electrostatic potential (MSP) diagram obtained by DFT B3LYP 6-31G method.

The theoretical physical parameters of selected benzimidazoles are determined by electronic chemical potential (μ), electronegativity (χ), absolute hardness (η), softness (S) and electrophilicity (ω) values from the HOMO as ionization energy (IE) and LUMO as electron affinity (EA) using the following equations, respectively. These parameters can be calculated using the following Eqs. (1)(5).


Theoretically calculated absolute hardness and softness are observed in the range from 1.11 to 2.78 eV. The values of μ, χ, η, S, ω for the amino group at o- and p-position leads to lesser values due to the electron donating nature of amino group. Absolute hardness and softness are important properties to measure the molecular stability and reactivity. S has been known as an indicator of the overall stability of a chemical system. A hard molecule has a large energy gap and a soft molecule has a narrow energy gap. Soft molecules are more reactive than hard molecules because they could easily offer electrons to an acceptor. For the simplest transfer of electrons, adsorption could occur at the part of the molecule where softness has the highest magnitude and hardness has the lowest [69]. The absolute hardness and softness of Albendazole molecule (benzimidazole moiety) was reported in the values of 2.56 and 0.39 eV in the gas phase [70]. Further the parameter of electrophilicity index (ω) of a organic molecule gives the information about the binding ability with biomolecules [71, 72, 73].

The calculated dipole moment values of the methoxy substituent at m- and p- are higher than those of other derivatives. But the hydroxyl group at o-position is higher than that of p-position and in amino group at p-position is higher value than that of o-position. The compound that has the highest dipole moment value is 2-(p-methoxyphenyl)benzimidazole with the value of 4.91 D. 2-(p-hydroxyphenyl)benzimidazole has the lowest dipole moment among the studied compounds (1.88 D).

Theoretically calculated energy for selected benzimidazole molecules are investigated with respect to substitution at m- and p-position of methyl, methoxy and also o- and p-position for hydroxyl and amino groups. More negative energy values are observed for 2-(m-methoxyphenyl)benzimidazole and 2-(p-methoxyphenyl)benzimidazole. Further the comparisons of substitutions at m-, p-positions and o-, p-position are not significantly changed in energy values in gas phase. Also thermodynamic parameters such as enthalpy, free energy and entropy are calculated theoretically at room temperature in the gas phase. All values of substituted benzimidazoles are higher than the parent benzimidazole (Table 2).

ParametersBenz imidazole2-phenyl benz imidazole2-(m-methyl phenyl) benz imidazole2-(m-methoxy phenyl) benz imidazole2-(p-methyl phenyl) benz imidazole2-(p-methoxy phenyl) benz imidazole2-(o-hydroxy phenyl) benz imidazole2-(p-hydroxy phenyl) benz imidazole2-(o-amino phenyl) benz imidazole2-(p-amino phenyl) benz imidazole
EHOMO (eV)−6.07−5.84−5.79−5.74−5.73−5.59−5.72−5.62−5.37−5.30
ELUMO (eV)−0.50−1.17−1.12−1.15−0.92−0.98−1.25−1.01−0.99−0.81
EHOMO-ELUMO (eV)−5.56−4.66−4.66−4.59−4.80−4.60−4.47−4.61−4.38−4.49
μ (eV)−3.28−3.50−3.45−3.44−3.32−3.28−3.48−3.31−3.18−3.05
χ (eV)3.283.503.453.443.323.283.483.313.183.05
η (eV)2.782.332.332.292.402.302.232.302.192.24
S (eV)1.391.
ω (eV)1.932.622.542.572.292.332.702.372.302.07
μ (Debye)3.613.213.594.053.434.914.071.881.924.33
E (kcal mol−1)−238299.56−383263.72−407929.73−455101.17−407929.73−455102.22−428199.43−430445.47−417989.88−417987.46
G (kcal mol−1)56.89104.71119.60123.60120.89123.44107.09106.93114.97113.24
H (kcal mol−1)80.00133.89152.48155.57152.41155.55137.32136.53145.56144.43
S (kcal mol−1 K−1)77.5397.85110.26107.22105.71107.71101.3999.26102.60104.61

Table 2.

The values of energy of HOMO, LUMO, HOMO-LUMO energy gap, structural parameters, dipole, energy, thermodynamic parameters of selected benzimidazoles obtained by DFT B3LYP 6-31G method.

4.2 Molecular electrostatic potential

Another investigating theoretical study is molecular electrostatic potential map (MSP) and displayed in Table 1. The electrophiles tend to negative ESP and the nucleophiles tend to region of positive ESP. The molecular electrostatic potential was calculated with DFT B3LYP 6-31G level of theory. The negative regions (red) are mainly contained on the nitrogen atom (=N-) and oxygen atom from methoxy, hydroxyl groups while the positive regions (blue) for the proton from –N-H, -OCH3, -OH and -NH2 group.

4.3 Natural bond orbital (NBO) analysis

Further to study of the intramolecular interactions of selected benzimidazoles, the theoretical NBO is used to calculate the stabilization energy for -C-H, -N-H and lone pair electrons in heteroatoms using second-order perturbation theory [74]. Some calculations about NBO analysis for investigating the intra and intermolecular interactions of isolated organic molecules and inclusion complexes between organic molecules and cyclodextrins [75, 76] were reported theoretically. NBO 3.1 program is applied to perform the natural bond orbital (NBO) analysis [77, 78] in the Gaussian 09 W package at the DFT/B3LYP level. In this work, DFT B3LYP 6-31G method is applied to analysis of intramolecular interactions for selected benzimidazoles. The stabilization energy E(2) for the donor and acceptor orbital delocalization is involved through the occupied Lewis-type (bond or lone pair) NBOs and formally unoccupied (antibond or Rydberg) non-Lewis NBOs within the molecules [79, 80]. The electron density of all atoms is noted in the selected benzimidazole molecules.

The second-order Fock matrix is carry out to evaluate the donor–acceptor interactions within the molecules to specify in conjugative π bonds and lone pair electrons through the NBO analysis [81]. The Eq. (6) is given below to estimate the stabilization energy E(2) for the donor and acceptor orbital delocalization within the molecule.


where qi is the donor orbital occupancy, εi and εj are diagonal elements (orbital energies), and F(i, j) is the off-diagonal NBO Fock matrix element [82, 83, 84]. This analysis reveals that the conjugative interaction, hyper-conjugative interaction, intra and intermolecular hydrogen bond in the same molecules and combine with other molecules are well described for the donor – acceptor orbitals.

Thus the interaction between π(N11-C12) and π*(C4-C7) are reveals the hyperconjugative energy about 19.04, 20.00, 20.05, 19.99, 20.10, 20.13, 19.41, 20.11, 19.96, and 20.30 kJ/mol for selected benzimidazole and 2-phenyl benzimidazoles respectively. This interaction could be revealed that the delocalization occurs in five membered ring for benzimidazole, due to the presence of C=N-C. Similarly the conjugative π bonds in the phenyl ring shows maximum delocalization during the interaction with π* acceptor bonds. It is evident from benzimidazoles that the πC2-C5, C3-C6 and C4-C7 delocalize more energy to the acceptor bond (π* acceptor). The electron density of donor bonds decreases while the acceptor (π*) bond electron density increases. Investigation of NBO analysis is described the stabilization energy for the conjugative interaction or charge transfer between the donor and acceptor bond orbitals [85, 86]. The interactions of π(C-C) with π*(C-C), π*(N-C) and π(N-C) with π*(C-C) are more responsible for the conjugation of respective π* bonds in benzimidazole and substituted 2-phenyl benzimidazole. The investigated molecules are divided into parts from the results of NBO analysis. One part is benzimidazole moiety and other benzene ring without and with substitutions. From the Table 3 these conjugative interactions are formed with close stabilization energy in the range from 17.00 to 23.00 kcal/mol. The stabilization energy values for these interactions are agreed with literature values [87]. From the Table 3, the π*(N11-C12) delocalizes the maximum energy to π(C4-C7) and (C15-C18) bond respectively for all benzimidazoles in the range from 54.62 to 190.30 kJ/mol. Similarly, the π*(C18-C19) bond transfers the high energy about 247.57 kJ/mol to (C16-C20) bond for 2-(o-hydroxyphenyl)benzimidazole. The second order perturbation energies associated with the hyperconjugative interactions in NBO basis confirms the presence of intermolecular interactions.

Donor (i)Acceptor (j)Benz imidazole2-phenyl benz imidazole2-(m-methyl phenyl) benz imidazole2-(m-methoxy phenyl) benz imidazole2-(p-methyl phenyl) benz imidazole2-(p-methoxy phenyl) benz imidazole2-(o-hydroxy phenyl) benz imidazole2-(p-hydroxy phenyl) benz imidazole2-(o-amino phenyl) benz imidazole2-(p-amino phenyl) benz imidazole
E(2) kJ mol−1
π *(C4-C7)17.7317.8317.8217.8617.8217.8518.4420.1518.1117.82

Table 3.

Second order perturbation theory analysis of Fock matrix in NBO basis for selected benzimidazoles by DFT B3LYP 6-31G method.

4.4 Mulliken atomic charges

Thus the important quantum mechanical calculations further applied to calculate the atomic charges for molecular system [88]. The charge distributions of all atoms present in benzimidazole molecules are calculated by the Mulliken method [89]. The Mulliken atomic charges of selected benzimidazole molecules are presented in Table 4 and shown in Figure 5. The Mulliken atomic charges were computed at the DFT B3LYP 6-31G method. The carbon atoms numbering C4, C7 and C12 are shown with the positive values except other carbon atoms in the whole system. These results expected that these carbon atoms are connected with electronegative nitrogen atoms in benzimidazole [90]. In benzimidazole, C7 atom is bonded with N13-H having high positive value (0.329 a.u.) and C4 atom with N11 atom having less positive value (0.032 a.u.). The other benzimidazole molecules are following the same trend. A positive charge of all the hydrogen atoms are displayed in Table 4, but H26 was gained maximum positive charge than the other hydrogen atoms, due to the presence of electronegative atom (O25) in o-hydroxy and p-hydroxyphenyl benzimidazole when compared with the hydrogen in amino group lesser values. The presence of three nitrogen atoms in o-amino and p-aminophenyl benzimidazole (N11 = −0.481 a.u., N13 = −0.804 a.u. and N25 = −0.721 a.u.) are shown in different environment because N13 atom more negative values.

AtomsBenz imidazole2-phenyl benz imidazole2-(m-methyl phenyl) benz imidazole2-(m-methoxy phenyl) benz imidazole2-(p-methyl phenyl) benz imidazole2-(p-methoxy phenyl) benz imidazole2-(o-hydroxy phenyl) benz imidazole2-(p-hydroxy phenyl) benz imidazole2-(o-amino phenyl) benz imidazole2-(p-amino phenyl) benz imidazole
2 C−0.097181−0.106860−0.107260−0.107084−0.107632−0.108005−0.097640−0.074372−0.106858−0.109524
3 C−0.066378−0.073803−0.073798−0.073607−0.074250−0.074668−0.073868−0.108222−0.073166−0.075092
4 C0.0329160.0713220.0701490.0680020.0717520.0715100.1103150.0722850.0792840.070580
5 C−0.136317−0.134176−0.134219−0.134114−0.134392−0.134414−0.135183−0.155754−0.134554−0.134574
6 C−0.156456−0.155168−0.155464−0.155447−0.155462−0.155708−0.152607−0.134226−0.155256−0.156420
7 C0.32960180.33652980.3359070.3346650.3360730.3356540.3249240.3357760.3336720.334464
10H /N0.127474110.127019110.1264260.1260320.1260640.125161−0.5605860.1352510.1291380.123999
11 N /C−0.40539712−0.43623112−0.435362−0.431475−0.437937−0.4386200.430471−0.829967−0.481336−0.440923
13 N/H/C−0.76618814−0.82816914−0.825954−0.822124−0.829186−0.8294670.3301130.359544−0.804057−0.827406
20C /H−0.13440521−0.135466−0.132738−0.157269−0.1410350.147321−0.119925−0.144329−0.143119

Table 4.

The computed Mulliken atomic charges for selected benzimidazole molecules given using DFT B3LYP 6-31G method.

Figure 5.

The computed Mulliken charges for all atoms in selected benzimidazoles.


5. Conclusion

Ten compounds of benzimidazole and 2-phenyl substituted benzimidazoles such as (1) benzimidazole, (2) 2-phenylbenzimidazole, (3) 2-(m-methylphenyl)benzimidazole, (4) 2-(p-methylphenyl)benzimidazole, (5) 2-(m-methoxyphenyl)benzimidazole, (6) 2-(p-methoxyphenyl)benzimidazole, (7) 2-(o-hydroxyphenyl)benzimidazole, (8) 2-(p-hydroxyphenyl)benzimidazole, (9) 2-(o-aminophenyl)benzimidazole and (10) 2-(p-aminophenyl)benzimidazole were selected to study for the spectral and theoretical properties. Synthesis of these molecules by many methods were discussed and given the reaction scheme. Then the absorption and fluorescence spectrum of all molecules were given with changing the wavelength respect to the substitution of groups in the benzene ring. The conjugation and maximum also involved and π-π* transition possible in the absorption spectrum. Further, DFT method was used to determine the structural parameters, energy values, HOMO-LUMO energy gap, thermodynamic parameters, molecular electrostatic potential map for all molecules. NBO analysis was revealed the hyperconjugative interaction between bonding orbitals, lone pair orbitals and antibonding orbitals and also calculated the stabilization energy of selected bonded orbitals. Finally the computed Mulliken atomic charges was determined using DFT B3LYP 6-31G method.


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Written By

A. Antony Muthu Prabhu

Submitted: 20 October 2021 Reviewed: 10 December 2021 Published: 13 July 2022