The important goal of this research is illustrating structural and electronic interactions between Cu(fcc) surface with various orientations of pyrazoloquinoline derivatives (R = Ph, 2-(NO2)C6H4, 4-(N,N-di-Me)C6H4, 2,4-di-ClC6H3, 2-(OH)C6H4, 4-(OH)C6H4, 4-(Me)C6H4, 4-(OCH3)C6H4, 4-(Cl)C6H4) as inhibitors. Electrochemical and thermodynamic principles are the original principles of corrosion behavior defining transformation of the alloys and metals into stable states. Figures 1 and 2 show all the optimized desired structures. Nitrogen and oxygen heteroatoms in pyrazoloquinoline molecule could interact with the Cu surface. Also, electrons of benzene ring on R derivatives interacted with surface atom of the metal compound in parallel and perpendicular orientations. All the parallel and perpendiculars orientations were simulated for chemical inhibitor molecules as exemplified in Fig. 4. Table 1 shows the exchanges of Gibbs free energy of Cu complexes with the chemical inhibitor pyrazoloquinoline molecule derivatives. Optimized sides of the inhibitor molecules are perpendicular to interact with Cu surface. Pyrazole ring has an effective side in the interaction field at Cu complexes. Thermodynamics leads spontaneous direction of the reaction and is applied to investigate and estimate whether corrosion behavior on metal surfaces is probable or not theoretically. According to the results presented in Table 1, all the Cu complexes had negative Gibbs free energy showing that all the derivatives can participate in spontaneous reaction. Figure 3 and Table 2 illustrate different ranges of energy in pyrazoloquinoline derivatives and Cu complexes of pyrazoloquinoline derivatives.-NO2 and -OH functions had the minimum negative value of Gibbs free energy and they had a strong interaction among the other pyrazoloquinoline derivatives. Figure 3 illustrates the pyrazoloquinoline derivative inhibitors with electron-rich groups. Spatial congestion may cause –NO2 and –OH to have minimum Gibbs free energy leading to the best stability in water phase.
Table 1
Gibbs free energy of Cu and of pyrazoloquinolines derivatives’ structure (R = Ph, 2-(NO2)C6H4, 4-(N,N-di-Me)C6H4, 2,4-di-ClC6H3, 2-(OH)C6H4, 4-(OH)C6H4, 4-(Me)C6H4, 4-(OCH3)C6H4, 4-(Cl)C6H4) and Cu complexes
Cu/R Complexes
|
Ph
|
2-(NO2)C6H4
|
4-(N,N-di-Me)C6H4
|
2,4-di-ClC6H3
|
2-(OH)C6H4
|
4-(OH)C6H4
|
4-(Me)C6H4
|
4-(OCH3)C6H4
|
4-(Cl)C6H4
|
∆G/ Kcal/mol
|
-4.87
|
-5.17
|
-4.82
|
-4.72
|
-5.35
|
-5.06
|
-4.41
|
-4.96
|
-4.76
|
Table 2
R/Compunds
|
∆G (cal/mol)
|
HOMO (a.u.)
|
LUMO (a.u.)
|
Ph
|
-780.37
|
-0.22198
|
-0.06357
|
2-(NO2)C6H4
|
-908.2
|
-0.2136
|
-0.155798
|
4-(N,N-di-Me)C6H4
|
-863.99
|
-0.16813
|
-0.05483
|
2,4-di-ClC6H3
|
-1356.75
|
-0.21018
|
-0.05595
|
2-(OH)C6H4
|
-827.14
|
-0.21086
|
-0.05429
|
4-(OH)C6H4
|
-827.14
|
-0.21137
|
-0.05684
|
4-(Me)C6H4
|
-804.62
|
-0.20843
|
-0.05402
|
4-(OCH3)C6H4
|
-851.79
|
-0.2117
|
-0.05726
|
4-(Cl)C6H4
|
-1068.32
|
-0.20883
|
-0.05656
|
Essentially, corrosion consists of two half-cell electrochemical reactions. Anodic and cathodic reactions involve leaving and taking up of the unrestricted and free electron by ionization of the metal or alloys nevertheless, cathodic reaction implicates taking up of free electrons by the dissolved oxygen in electrolyte molecules. On the oxidation reaction and solution reaction, free electrons are shaped although reduction reaction occurs later in which the electrons have been accepted. Corrosion behavior of metals and alloys can simply control the use of the electrochemical principles[34]. Corrosion inhibition properties of the alloys and metals can change because of adsorption of chemical inhibitor at solution/ metal interface significantly. Inhibition action of surface corrosion occurs at surface that includes electron transformation and adsorption of chemical inhibitors on the metal atom surface in electrolyte. DFT essences on electron density (ρ) [35] take place relatively for each electron, in place of the carrier of whole data underground state of molecule formation, as an electron wave function.
For recognizing electron donating and electron accepting roles of pyrazolo[3,4-b]quinoline-3,5-dione derivatives (R = Ph, 2-(NO2)C6H4, 4-(N,N-di-Me)C6H4, 2,4-di-ClC6H3, 2-(OH)C6H4, 4-(OH)C6H4, 4-(Me)C6H4, 4-(OCH3)C6H4, 4-(Cl)C6H4), HOMO and LUMO energies of them were calculated theoretically as shown in Table 2.
Table 3
Quantum descriptor of pyrazoloquinolines derivatives
R-Compounds
|
IP(eV)
|
EA(eV)
|
χ(eV)
|
η(eV)
|
µ(eV)
|
Dipole moment(Debye)
|
Ph
|
3.6
|
1.04
|
2.32
|
1.28
|
-2.32
|
8.44
|
2-(NO2)C6H4
|
3.5
|
2.5
|
3
|
0.5
|
-3
|
5.79
|
4-(N,N-di-Me)C6H4
|
2.7
|
0.89
|
1.79
|
0.9
|
-1.79
|
8.92
|
2,4-di-ClC6H3
|
3.44
|
0.91
|
2.17
|
1.26
|
-2.17
|
7.94
|
2-(OH)C6H4
|
3.45
|
0.88
|
2.16
|
1.28
|
-2.16
|
9.62
|
4-(OH)C6H4
|
3.46
|
0.93
|
2.19
|
1.26
|
-2.19
|
9.47
|
4-(Me)C6H4
|
3.41
|
0.885
|
2.14
|
1.262
|
-2.14
|
8.91
|
4-(OCH3)C6H4
|
3.47
|
0.93
|
2.2
|
1.27
|
-2.2
|
10.66
|
4-(Cl)C6H4
|
3.42
|
0.92
|
2.17
|
1.25
|
-2.17
|
9.48
|
Table 4
Topological parameters of pyrazoloquinolines derivatives
R-Complexes
|
BCP
|
ρ
|
ᐁ^2⍴
|
|
a
b
c
|
0.2644
0.2635
0.2639
|
-0.79
-0.783
-0.787
|
2-(NO2)C6H4
|
a
b
c
|
0.2202
0.3194
0.3198
|
-0.3293
-0.7386*10− 1
-0.7501*10− 1
|
4-(N,N-di-Me)C6H4
|
a
b
c
|
0.2422
0.2352
0.2719
|
-0.5169
-0.4886
-0.8282
|
2,4-di-ClC6H3
|
a
b
c
|
0.2957
0.1849*10− 1
0.1669
|
-0.6874
-0.7447*10− 1
-0.1449
|
2-(OH)C6H4
|
a
b
c
|
0.2340
0.2369*10− 2
0.3427
|
-0.85
+ 0.66
-0.42
|
4-(OH)C6H4
|
a
b
c
|
0.4364
0.2911*10+ 3
0.4157
|
-0.8739
-0.4015
-0.1654*10
|
4-(Me)C6H4
|
a
b
c
|
0.2689
0.2681
0.2688
|
-0.8028
-0.8028
-0.7975
|
4-(OCH3)C6H4
|
a
b
c
|
0.1879
0.2267
0.2760
|
-0.2055
-0.2934
-0.8688
|
4-(Cl)C6H4
|
a
b
c
|
0.2722
0.2748
0.3190
|
-0.8442
-0.8748
-0.8185
|
DFT is an easy technique to consider chemical molecular structure and activities of corrosion inhibitors[10][36][37] on surface. Simulation has been developed as a powerful tool to scrutinize metal complex atom of corrosion surface [38]. Mechanism of corrosion was detected by checking electron distribution Fig. 5 and Fig. 6 and molecular adsorption on metal and metal oxide surfaces.
Set orbit of linear combinations of geosynchronous functions was used for the basis aimed at electronically computing the design. Linear combination of several codgers was used due to accurate representation of atomic orbitals.
Definition of HOMO and LUMO orbital energy levels of chemical molecules is significant. Fukui [39] acknowledged that frontier orbitals are important because stereochemistry of the inhibitor coordination and stereochemistry ratio of the components in reactions are the original characteristics leading to reaction simplicity. EHOMO and corrosion resistance have a good relationship that is obtained by potential of the chemical compounds, which is important as inhibitor adsorption on the metal surface atom is the source of giving electron on chemical interaction of heteroatom electrons and π electrons of benzene ring with empty d orbitals of metal surface atoms [40]. Possibly, numerous levels of EHOMO are required to specify tendency of electron donating in the chemical compounds to the empty acceptor atom orbitals of chemical compounds where the electron accepting capacity of the molecules is determined by energy of the lower empty orbitals. As indicated in Table 2, –Ph, -OMe, -NO2, and –OH had π electron and heteroatoms and high HOMO energy to donate electron to the Cu surface. –Ph π electron caused maximum donating level of energy. Considerable inhibition efficiency has been provided by low tenets of difference in energy, because orbital of the metal surface has less contribution correctly for decreasing the energy from occupied orbital of atoms that is obtained in the previous energy level[41].
The µ of chemical molecules is a trajectory value that is most widely applied for reporting polarization of a chemical molecule. It shows the amount of departure of the two electrical charges (negative and positive). The µ is useful for scattering of the electrons among the two bonded chemical atoms. Actually, according to µ, the change in polar and non-polar chemical bonds is different. In the pure dipole moment, the molecule is actually small or zero thus, molecular bonding and molecules are deliberately non-polar and polar molecules, respectively by pure bipolar moment. According to the results presented in Table 3, -OMe and –OH functions had the biggest dipole moment that showed most polarity in chemical solutions. It means that the chemical bonds had non-similar electronegativity. Similar electronegativity of atom values produces the chemical bonds by exact smaller dipole moment. Total dipole moment, on the other hand, reproduces just the global injunction of polarization in a chemical molecule rather than a single bond notification so that, efficiency of inhibitor diminishes by decreasing efficiency. Therefore, -OH and -OMe groups act as the best inhibitor on Cu surface [42]. Positive signs of numbers of dipole moment indicate that the inhibitors could be applied to the metal surface by physical mechanism [43]. All the pyrazoloquinoline derivatives have positive dipole moment so they have physical mechanism of inhibition. DFT effectively recognizes chemical selectivity and reactivity ,according to quantum molecular properties, like chemical potential (µ) and electronegativity(χ) [44]. Another chemical parameter is the total hardness (η), which analyzes molecular selectivity and selectivity of reaction. Relationship between corrosion inhibition and quantum chemical quantities was established by the Pearson’s and Lewiss҆ theory of hard and soft acids and bases. Accordingly, a hard molecule has a high level of gap energy indicating hard natural surroundings of compounds, and a soft chemical molecule has a low level of band gap energy. Due to low band gap energy level, electrons could be provided to acceptor atom from soft compound in comparison with hard compound easily. Therefore, the reactive site is where the molecule is absorbed by the highest level of energy[45]. Table 3 shows all the results related to the pyrazolo [3,4-b]quinoline-3,5-dione derivatives. –OH, -OMe, and –NO2 had large value of hardness therefore, they were more reactive among the other pyrazoloquinoline derivatives. Chemical interactions of molecules are also covalent and/or polar electrostatically. Charge of electric force field in compounds is responsible for chemical electrostatic interactions obviously. Biological behavior of the molecules results from native electron density and/or charge of atom, which are significant for making properties of all the chemical and physico-chemical reactions. Figures 4 5 show the profile of charge density of pyrazoloquinoline derivatives and profile of electrostatic potential from nuclear charges in Cu complexes of pyrazoloquinoline derivatives. Molecular polarization is described by atomic charge of the molecules. Increase in the electron discharging power was exchanged by electron-donating molecules like (-OCH3 and –OH group) that occurred to improve prohibition but, electron-attracting group (-Cl) in pyrazoloquinoline derivatives decreases the effect of prohibition. There are two important quantum functions including electron-localization function and Laplacian density of electron exposing the electron donations linked with the amount of spatial structure arrangement of pairs in the localized electron implicitly in quantum model of VSEPR. Consequently, interpretation of both experimental and theoretical electron densities was used. Potential local energies and electronic kinetic were used for training the bonding in chemical compounds and lattices of metal crystals [46]. The power of bonding was determined by topological parameters Fig. 6. There are numerous geometrical principles for determining the distance of bonds between Cu atom surface and chemical inhibitor molecules [30]. The QTAIM was established by electron density (ρ) related to attendance of (3,-1) and (+ 3,-3) BCPs for the proton inhibitors principally. Acceptor atom of Cu surface interaction validates bonding interaction and the range of electron density is from 0.002 to 0.04 a.u. Laplacian corresponding density must be equal to 0.024–0.139 a.u. Table 4 presents the results regarding calculating the topological factors of the desired bonds like ∇2ρ and ρ. Value of + 0.66 for –OH group in the pyrazolo [3,4-b]quinoline-3,5-dione derivatives showed active BCP for making reaction.