The inhibition behavior of two pyrimidine-pyrazole derivatives against corrosion in hydrochloric solution: Experimental, surface analysis and in silico approach studies

2.1 Synthesis of DPP and PP

For the preparation of B, 2-aminopyrimidine-4,6-diol (5 g, 39 mmol) was stirring at 70 °C to solubilize it in 50 ml of acetonitrile, then we put the solution of (1H-pyrazol-1-yl) methanol (3.85 g, 39 mmol) , the mixture was refluxed for 6 h, evaporated then filtered and washed with acetonitrile to obtain pink solid product. Same procedure to obtain C, but using 0.5 equivalent of 2-amino-6-methylpyrimidin-4-ol (5 g, 39 mmol) with (1H-pyrazol-1-yl) methanol (7.7 g, 78 mmol) which was stirred for 7 days under room temperature, filtered and washed with acetonitrile to obtain a pink solid product (Kaddouri et al., 2017). Fig. 1.

• DPP: 2-(((1H-pyrazol-1-yl) methyl) amino) pyrimidine-4,6-diol

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Fig. 1

General procedure for the preparation of DPP and PP.

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The structure of the new organic dye was determined by spectroscopic methods: 1H NMR, and Infrared in order to get information about inhibitors structure.

Pink solid, Yield : 88%, Mp = >250 °C, FTIR (KBr, cm-1): 3372 (C-O), 2930 (C-H), 1750 (C = O), 1630 (C = N), 1562 (C-N), 1280 (C-C), 1030 (N-N), 915 (-C-H), 645 (=C-H), RMN 1H (DMSO, 500 MHz) δ ppm: 7,6 (d, 1H, CH (5)) ; 7,31 (d, 1H, CH (3)) ; 6,26 (dd, 1H, CH (4)) ; 6,11 (t, 1H, NH) ; 5,70 (s, 2H, CH2); 4,85 (s, 1H, CH (pyrimidine)). Fig. 2.

• PP: 2-(bis((1H-pyrazol-1-yl) methyl) amino) pyrimidine-4,6-diol

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Fig. 2

The FTIR and 1H NMRspectrums of DPP: 2-(((1H-pyrazol-1-yl) methyl) amino) pyrimidine-4,6-diol.

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Pink solid, Yield: 94%, Mp = >250 °C, FTIR (KBr, cm-1): 3330 and 3260 (C-OH), 1670 (CH = CH), 1640 (C = N); 1475 (–CH2-), 1365 (C-H), 1067 (C-C); RMN 1H (DMSO, 500 MHz) δ ppm: 7,47 (s, 1H, OH); 7.29 (s, 1H, CH (4)); 6.09 (d, 1H, CH (3)); 5.76 (d, 1H, CH (5)); 4.80 (s, 2H, CH2). Fig. 3.

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Fig. 3

The FTIR and 1H NMRspetcrums of PP: 2-(bis((1H-pyrazol-1-yl) methyl) amino) pyrimidine-4,6-diol.

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2.2 Materials preparation

Mild steel undergoes before each trial a polishing of the surface on emery paper with different granulometry. Washed with distilled water to remove the mass impurities, degreasing by acetone and dry before start experiment to obtain true results. The hydrochloric acid 1 M prepared by dilution of analytical grade 37%. Table 2.

2.3 Gravimetric method

The mild steel is accurately weighted and immersed in 50 ml of hydrochloric acid 1 M without and with different concentrations of inhibitors. For the effect of concentration, the substrates immersed in the solution during 6 h at 298 K, and 2 h at temperature range 308–338 K in order to estimate their effect of temperature. When, the experience is finished the samples were taken out, washed with distilled water, dried and reweighed accurately. The inhibition efficiency (IE_Grav %) and corrosion rate (W_corr) was calculated according to the following equations (Mohamed Abdelahi et al., 2017):

2.4 Electrochemical measurements

Electrochemical measurements realized by using a potentiostat Tacussel-Radiometer PGZ 100. The different electrochemical tests carried out by three-electrode glass cell. The mild steel used as the working electrode, Ag/AgCl electrode as the reference electrode and platinum as the counter electrode. Before all tests, the potential stabilized during 30 min, in order to get the open circuit potential and have a good electrochemical result. The anodic and cathodic polarization curves were recorded at an output rate of 1 mV/sand in order to get the EIS result, the applied frequency ranged from 100 KHz to 100mHzwith 10 points perdecade.

2.5 Surface morphology analysis

Among the great method for surface morphology analysis is the scanning electron microscopy. Therefore, in order to find available information about adsorption behavior of our tested inhibitors, the scanning electron microscopy(SEM)technical were used after an immersion during 6 h with and without DPP and PP inhibitors at 10⁻³M. The materiel composition is obtained using energy dispersive X-ray (EDX) attached to the scanning electron microscopy with an acceleration voltage of 20 kV.

2.6 Quantum chemical approach

Recently, experimental researchers are convinced that the use of quantum chemical approach plays an important role in all areas of chemistry, the field of corrosion studies also can be used theoretical approach (Khaled, 2010). This approach was executed using Marvinsketch program in order to find the forms of inhibitors exist in experience conditions and Gaussian 09 program package using DFT/(B3LYP) with a basis set 6-31G (d,p) (El Assiri et al., 2019). In order to assimilate a correlation between theory and experimental studies as well as to detect the relation between inhibition efficiency and molecular properties estimated such as the highest occupied molecular orbital (HOMO), lowest unoccupied molecular properties and find the information at atomic scale by using Fukui indices calculations (Khaled, 2010).

3.1 Gravimetric measurements

3.2 Potentiodynamic polarization measurements

3.2.1

3.2.1 Effect of concentration

Figure 4 showed the Polarization curves of mild steel in acidic medium obtained in the presence and absence of different concentrations of DPP and PP. All electrochemical parameters including I_corr, E_corr, Tafel slopes (ßc, ßa) and IE% were measured by extrapolation of Tafel curves are summarized in Table 4. The percentage of inhibition efficiency (IE_PP %) values were measured using the following equation (Zarrouk et al., 2012):

(3)

$$I{E}_{\mathit{PP}}\%=\frac{i_{\mathit{corr}}-i_{\mathit{vorr}}^{\prime }}{i_{\mathit{corr}}}\ast 100$$ where i_corr and i’_corr are the values of corrosion current densities without and with inhibitors, respectively.

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Fig. 4

Polarization curves for mild steel in the absence and presence of DPP and PP at various concentrations of inhibitors at 298 K in acidic medium.

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Table 4 Kinetic parameters for mild steel in 1 M HClcontaining different concentrations of inhibitors at 298 K.

Medium	Conc.M	-EcorrmV/SCE	icorrµA.cm−2	-βcmV.dec−1	βamV.dec−1	IEPP%
HCl	1	437	983	150	110	–
DPP	10−6	401	672	170	102	32
	10−5	396	468	165	88	52
	10−4	392	279	161	72	72
	10−3	370	189	155	66	81
PP	10−6	383	578	160	75	40
	10−5	393	221	141	80	76
	10−4	385	172	158	73	83
	10−3	375	82	152	66	92

The experimental result reveals that the current densities (i_corr) values decreases gradually with the increase in concentrations of two pyrimidine-pyrazole derivatives. Alongside, the inhibition effect increase with concentration of inhibitors and attains maximum values of 81% and 92% for DPP and PP, respectively at 10⁻³M. cathodicTafel slopes ß_C, are roughly constant, it means that the inhibitors decreased the surface area for hydrogen evolution without influencing the reaction mechanism (Ech-chihbi et al., 2017). A survey literature shows that the displacement of E_corr compared to blank give information about the type of inhibitors tested: anodic or cathodic type when the displacement of E_corr is superior to 85 mV with uninhibited solution, if not the inhibitor considered as mixed type (Beniken et al., 2018). In our case, the displacement in the E_corr value is less than 85 mV, this suggest that our inhibitors classified as mixed-type inhibitor with predominant anodic (Bouoidina et al., 2019).

3.2.2

3.2.2 Effect of temperature

Generally, the increase in temperature accelerates the phenomenon of corrosion because it decreases the domains of stability of the metals and accelerates the kinetics of reactions (El-Hajjaji et al., 2019). The polarization curves obtained without and with the optimum concentration 10⁻³M of the studied inhibitors in 1 M hydrochloric acid solution at various temperatures were presented in Fig. 5.Nevertheless, the results regrouped in Table 5 showed a remarkable decrease in the densities of corrosion rate compared to the blank solution. In other side, this decreased is more pronounced in DPP inhibitors than PP inhibitor which confirming the adsorption behavior of PP inhibitor at the range temperature used.

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Fig. 5

Polarization curves in 1 M HCl obtained at 10⁻³M of inhibitors at different temperature.

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Table 5 Kinetics parameters of mild steel in 1 M HCl at different temperature.

Compounds	TemperatureK	-EcorrmV/SCE	icorrµA cm−2	-βcmVdec−1	βamVdec−1	IEPP%
Blank	298	437	983	150	110	–
	308	456	1470	127	132	–
	318	455	2200	128	137	–
	328	454	3200	118	122	–
DPP	298	370	189	155	66	81
	308	389	317	157	81	78
	318	397	544	140	86	75
	328	384	841	101	67	74
PP	298	375	82	152	66	92
	308	374	152	161	67	90
	318	381	252	140	68	89
	328	373	422	107	63	87

3.2.3

3.2.3 Activation parameters

From this study, we can also calculate the thermodynamic activation parameters of the mild steel in acidic medium by using the Arrhenius equations (3) and (4) (EL Merimi et al., 2019):

(4)

$$i_{\mathit{corr}}=A{e}^{\frac{-E_a}{\mathit{RT}}}$$

(5)

$$i_{\mathit{corr}}=\frac{\mathit{RT}}{\mathit{Nh}}{e}^{\frac{\mathrm{\Delta }{S}^{\ast }}{R}}{e}^{\frac{-\mathrm{\Delta }{H}^{\ast }}{\mathit{RT}}}$$ where A is Arrhenius factor, R is the universal gas constant, N is Avogadro’s number, T is the absolute temperature, h is Planck’s constant (h = 6.6252 10⁻³⁴ J.s), Ea, △H* and △S* are activation corrosion energy, the enthalpy and the entropy activation, respectively. Fig. 6.

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Fig. 6

Arrhenius plots of mild steel in 1 M HCL at optimal concentration of inhibitors.

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Inspection of these results reveals that the values of △H* for the dissolution reaction of mild steel in 1 M HCl in the presence of pyrimidine-pyrazole derivatives are more interesting to those obtained in the absence of inhibitors. In addition, the positive signs of the values of △H* reflect the endothermic nature of the dissolution process of mild steel and suggesting that the dissolution of steel is slow in the presence of inhibitors (Tourabi et al., 2014) Table 6. The value of activation enthalpy is more important for inhibitor PP, which confirms the higher protection character compared to inhibitor DPP, and the difficulty of the dissolution of steel in the presence of this compound (Al-amiery et al., 2014). In other hand, the values of Ea in the presence of inhibitors are superior to control. Meaning that our inhibitors adsorb on the surface of the steel by formation of electrostatic bonds (physical adsorption) according to the Radovici classifications (Rocca et al., 2019).

Table 6 Thermodynamic parameters of the activation corrosion process of mild steel in HCL 1 M in the absence and presence of inhibitors at 10⁻³M.

Activation parameters	1 M HCl	DPP	PP
Ea (KJ/mol)	32,06	40,83	44,11
△H* KJ/mol	29,47	38,23	41,51
△S* (J/mol. K)	−88.9	−73.19	−69.0

3.3 Electrochemical impedance spectroscopy (EIS)

The inhibition efficiency can also estimate by comparing the impedance values obtained in the presence and absence of inhibitors at different concentrations in acidic medium. The Nyquist plots for mild steel obtained with and without DPP and PP at 298 K are represent in Fig. 7. The inhibition efficiency in this case was calculated by the following equation:

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Fig. 7

The Nyquist plots diagrams for DPP and PP at 298 K in different concentrations.

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It can be seen from Fig. 7, the EIS diagrams are constituted of one-half loop which mean that ;the reaction of inhibitors DPP and PP on the working electrode surface were controlled by charge transfer process related to the heterogeneous and irregular surface (Zarrok et al., 2012). The parameters obtained from this measurement was illustrated in Table7.

From first observation, the data obtained from Table 7 show that inhibitor PP is more effective than DPP. Whichexplained by the inhibition efficiency reached in 10⁻³M, 88%for inhibitor PP and 77%for inhibitor DPP. Also, it can be seen that the charge transfer resistanceR_ctincrease with the concentration of inhibitor. However, the double layer capacityC_dl values reduce explicatedby adsorption of the inhibitor molecules on the surface of the metal and formation of a protective layer (El Hajjaji et al.). In addition, the factor of heterogeneityn_dl increase in inhibitor concentration related to the decrease of the surface heterogeneity and forming a protective film against the aggressive solution. It can be seen from the Nyquist plots that the shape of all semi-circle is not perfect. Therefore, the equivalent circuit presented in Fig. 8 modeled the capacitance double layer by element phase constant Q which gives us a best fitting for the Nyquist plots obtained (Beniken et al., 2018). The values of the inhibition efficiency obtained from EIS measurements showed the same trend as those obtained from the other techniques (Potentiodynamic polarization and weight loss methods).

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Fig. 8

Equivalent circuit model used in experimental impedance fit of M−steel in the presence and absence of inhibitors.

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3.4 Isotherm adsorption

The use of adsorption Isotherms is very important to determine the mechanism of adsorption of inhibitors to the surface of the metal (De Souza et al., 2014). For that, the experimental data of impedance was tested with various adsorption isotherms (Langmuir, Freundlich, El Awady, Frumkin, Flory-Huggins and Timken) Table 8. To do that, we used the following linear equations of each isotherm in order to find the model of isotherm explains the adsorption process of these inhibitors on the surface of mild steel (Beniken et al., 2018).

Table 8 Linear forms of various isotherm equations.

Isotherms	Linear equations
Langmuir (Ezeibe et al., 2019)
El-Awady (Shariatinia and Ahmadi-Ashtiani, 2019)
Flory-Huggins (Fathima et al., 2014)
Freundlich (El Hafi et al., 2018)
Frumkin (El-Hajjaji et al., 2019)
Temkin (Ghazoui et al., 2017)

Where:

X: constitutes a measure of the value of adsorbed water molecules replaced by inhibitors molecules.

1/y: the number of water molecules removed by one molecule of inhibitor compound.

Z: This parameter explains the nature of mild steel/medium interfaces (homogeneous or heterogeneous).

A and d: represent the interaction factors among adsorbed molecules (repulsion or attraction force).

On the first, Langmuir’s isotherm not verified despite the correlation coefficient very closed to unitysincethe second hypothesis of Langmuir isotherm not verified because the slope is superior to 1. On the other hand, the constant of adsorption has no significant for both Temkin and Freundlich isotherms. So, we can deduce that the adsorption process of these inhibitors follows El-Awady isotherm because is the most compatible when we compared the factor of regression obtained for the rest model traced and also, we can observe that their data were around the fitting curve compared to Frumkin and Flory-hugging isotherms Fig. 9. This model shows that one molecule of inhibitors can remove three molecules of water. In addition, the standard Gibbs energy (△G⁰_ads) values near to – 40 KJ mol⁻¹associated to chemical adsorption of inhibitors on the surface of mild steel by forming the coordination bonds (Banerjee and Chattopadhyaya, 2017). Table 9.

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Fig. 9

Regrouped various isotherms models testing of DPP and PP on mild steel surface in hydrochloric acid medium at 298 K.

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3.5 Surface analysis

The surface analysis studies of different species of mild steel in various cases with and without inhibitors after immersion during 6 h at 298 K in order to show the nature of atoms adsorbed on the surface of mild steel. SEM micrographs are shown in Fig. 10. The morphology of mild steel before immersion is smooth compared to the substrate after immersed in acidic solution, due to dissolution phenomenon. But, the presence of pyrazole derivatives inhibitors indicate the formation of a heterogeneous protecting film due to the formation of coordination bonds between inhibitors and the vacant orbital d of iron. Then, these results can be confirmed the adsorption phenomenon of inhibitors on the surface of material studied (Fig. 10 (C) (d)).

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Fig. 10

SEM micrographs of mild steel before immersion (a) after 6 h immersion in 1 M of HCl (b) after immersion in 10⁻³M of DPP inhibitor (C) and of PP inhibitor (D).

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In order to characterize the chemical composition of material used in this work and the element adsorbed on the surface of mild steel after immersed in corrosive medium with and without inhibitors. The EDX method based on the analysis of X-rays emitted during electron-matter interaction. The results are shown in Fig. 11. The percentage atomic of different elements adsorbed on the surface of mild steel are illustrated in Table 10. From the values of percentage atomic of iron for mild steel immersed only in acidic solution and after addition of DPP and PP inhibitors. Because, the value of percentage of iron in DPP inhibitor more than PP inhibitor. This result shows that PP protects the mild steel against corrosion in molar hydrochloric acid solution more than DPP by adsorption of various heteroatom’s such as: C, O and N. These results confirmed the order of efficiency obtained in experimental part by different methods (weight loss & electrochemical measurements) and theoretical study (DFT method).

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Fig. 11

EDX spectra of mild steel before immersion (a) after 6 h immersion in 1 M of HCl (b) after immersion in 10⁻³M of DPP inhibitor C after immersion in 10⁻³M of PP inhibitor D.

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3.6 In silico approach studies

Recently, experimental chemists are convinced that the use of quantum chemistry plays an important role in all areas of chemistry; the field of corrosion is no exception (Saady et al., 2019). The corrosion inhibition phenomenon was studied experimentally in acidic medium. So, it is necessary to include the effect of solvent in the computations by searching the major protonated species in the electrolytic solution, in order to realize the better approach of experimental conditions (Teixeira et al., 2015). The Marvinsketch.18 program was used for the studied compounds. Therefore,the distribution of the protonated forms as a function of pH were presented in Fig. 12 with their predominant form obtained in 1 M HCl at PH close to zero.

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Fig. 12

Percentage of the observed species in terms of pH diagrams for DPP and PP compounds and the Predominate form obtained in 1 M HCl by Marvinsketch 18 program.

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It can be noticed from Table 11 that the molecule has eight species different, each one achievesa precise value. The analyses of these resultsshow that the predominant form can be exist in 1 M HCl solution for DPP compound have two protonated sites while PP molecules have three protonated sites at pH close to zero.

Table 11 Shows the relation between the pH with the percentage of each species of DPP and PP compounds at pH = 0 obtained by Marvinsketch 18 program.

Species	DPP		PP
	Structures	Percentage at pH = 0	structures	Percentage at pH = 0
1		0.00		0.00
2		0.00		0.00
3		0.07		0.01
4		0.00		0.00
5		97.96		0.88
6		0.00		1.61
7		0.00		0.00
8		1.96		97.49

3.7 Quantum calculation

3.7.1

3.7.1 Global molecular reactivity of inhibitors compounds at protonated state

In order to obtained a good correlation between theoretical studies and experience. The computational studies of major species obtained by Marvinsketch was performed using DFT, with B3LYP/6-31G (d,p) level implemented by Gaussian 09 program.

The interesting quantum chemical parameters using in this study for comparing the order of efficiency of these inhibitors regrouped in Table 12. These parameters extracted from the result obtained by Gaussian such as: energy gap (△E), ionization potential (I), electron affinity (A), the absolute electronegativity (ᵡ), global hardness (η), softness (σ) and the fraction of electrons transferred from the inhibitor molecule to the metal surface (△N) with ηFe = 0 and ᵡ =7eV were calculated from following relations and regrouped in Table11 (Anusuya et al., 2015):

(7)

$$\mathrm{\Delta }E=E_{\mathit{LUMO}}-E_{\mathit{HOMO}}$$

(8)

$$\chi =-\frac{1}{2}(E_{\mathit{HOMO}}+E_{\mathit{LUMO}})$$

(9)

$$\eta =-\frac{1}{2}\left(E_{\mathit{HOMO}}-E_{\mathit{LUMO}}\right)$$

(10)

$$\sigma =\frac{1}{\eta }$$

(11)

$$\mathrm{\Delta }\mathit{N}=\frac{\mathit{\chi }\left(\mathit{Fe}\right)-\mathit{\chi }\left(\mathit{inh}\right)}{2\left(\eta \left(\mathit{Fe}\right)+\eta \left(\mathit{inh}\right)\right)}$$

Table 12 The principal theoretical parameters of DPP and PP in protonated state.

Inhibitors	DPP	PP
EHOMO (eV)	−8.2936	−8.5228
ELUMO (eV)	−5.9252	−6.1832
η	1.1842	1.1698
σ	0.8444	0.8548
△N	0.0462	0.1509
Χ	7.1094	7.3530
△E (eV)	2.3684	2.3396
μ(D)	14.9412	14.5190
Total energy (u.a)	−734.6990	−999.4399
IE (%) (EIS)	77	88

Firstly, the lowest value of E_LUMO of PP shows the great capacity of the inhibitory molecule to accept electrons. In addition, it has the smallest Gap energy in comparison with the DPP inhibitor which would give a better reactivity to the PP molecule, because the excitation energy from the last orbital occupied will be low (Pang et al., 2012). Also, in the field of corrosion, the dipole moment as an indicator for reactivity, does not present the reasonabletrend with the effectiveness of inhibitors. Indeed, according to some authors, it was reported that the dipole moment increases with increasing of inhibition efficiency. However, it’s shown an opposite trend in other studies. In our case, it turns out that the dipole moment does not correlate with the inhibitory efficiency (Khaled, 2010). On the other hand, it can be clear that the PP inhibitor have a less value of hardness and high value of softness than DPP inhibitor which goes with the inhibition obtained experimentally. In our study △N < 3,6 so according to Lukovist’s study it reflects the increases in electron-donating ability to the metal surface this can be decreases the corrosion rate (Ben Hmamou et al., 2013). Generally, the electronegativity parameter gives knowledge about the capacity of molecule inhibitor to attract electrons to itself. In our case, PP inhibitor show us a high capacity to attract electrons to itself which confirm the adsorption of this compound compared to DPP molecule (Zarrouk et al., 2014). Finally, it can be concluded that the quantum chemical descriptors show a good correlation with experimental results.

The table Fig. 13 indicate that the HOMO and LUMO electron density of studied inhibitors DPP and PP focused around the pyrimidine and pyrazole ring. Indicating the capacity of these inhibitors to send and accept electrons and that the adsorption phenomenon is likely to occur on reactive sites distributed along the molecule inhibitors (Zarrok et al., 2012).

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Fig. 13

The geometry optimization,HOMO and the LUMOdistributions of the studied compounds obtained by B3LYP/6-31G (d,p) level at protonated state in aqueous phase.

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