Theoretical investigation of heterocyclic compounds on corrosion inhibition behavior of copper in hydrochloric acid
medium
S. Issaadi *, T. Douadi
Department of Engineering Process, Faculty of Technology, University of Setif-1, DZ-19000, Setif-Algeria.
issaadi2001@yahoo.fr Abstract
Quantum chemical calculations based on DFT method were performed on two schiff bases compounds, used as corrosion inhibitors for copper in HCl media to determine the relationship between the molecular structure of schiff base and inhibition efficiency. Quantum chemical parameters such as the highest occupied molecular orbital energy (EHOMO), the lowest unoccupied molecular orbital energy (ELUMO), energy gap (∆E), dipole moment (l), electronegativity (v), electron affinity (A), global ionization potential (I), the fraction of electrons transferred (∆N), and the total energy (TE), were calculated. The theoretically obtained results were found to be consistent with the experimental data reported.
Keywords: Optimization, DFT, Quantum chemical
I. INTRODUCTION
The use of corrosion inhibitor is one of the most effective measures for protecting metal surfaces against corrosion in acid environments [1]. Some organic compounds are found to be effective corrosion inhibitors for many metals and alloys. Generally, inhibitor molecules may physically or chemically adsorb on a corroding metal surface. In any case, adsorption is generally over the metal surface forming an adsorption layer that functions as a barrier protecting the metal from the corrosion [2, 3]. It has been commonly recognized that an organic inhibitor usually promotes formation of a chelate on a metal surface, by transferring electrons from the organic compounds to the metal and forming a coordinate covalent bond during the chemical adsorption. In this way, the metal
acts as an electrophile; and the nucleophile centers of inhibitor molecule are normally heteroatoms with free electron pairs that are readily available for sharing, to form a bond.
Organic compounds, containing functional electronegative groups and p-electron in triple or conjugated double bonds, are usually good inhibitors. Heteroatoms, such as sulfur, phosphorus, nitrogen and oxygen, together with aromatic rings in their structure are the major adsorption centers. The planarity and the lonely electron pairs in the heteroatoms are important features that determine the adsorption of these molecules on the metallic surface. The structural parameters, such as the frontier molecular orbital energy HOMO (highest occupied molecular orbital) and LUMO (lowest unoccupied molecular orbital), the charge distribution of the studied inhibitors, the absolute electronegativity (v) values, and the fraction of electrons (ΔN) transfer from inhibitors to iron were calculated and correlated with inhibition efficiencies.
II. THEORY AND COMPUT- ATIONAL DETAILS
DFT (density functional theory) methods were used in this study. These methods have become very popular in recent years because they can reach exactitude similar to other methods in less time and less expensive from the computational point of view. In agreement with the DFT results, energy of the fundamental state of a polyelectronic system can be expressed through the total electronic density, and in fact, the use of electronic density instead of wave function for calculating the energy constitutes the fundamental base of DFT [4]. All the calculations were done by GAUSSIAN 03 W software , using the B3LYP functional and a 6- 31G basis set. The B3LYP, a version of DFT method.
II. a. Quantum chemical calculation organic molecule (L)
Quantum chemical calculations were carried out to find a relationship between the molecular structure of the synthesized inhibitor and its inhibition effect. In the present work, the structure parameters and adsorptive performance of the synthesized inhibitor are used to elucidate the inhibition mechanism.
The corresponding molecule geometries were optimized and then, the energies were calculated at B3LYP/6-31G (d, p) level. The optimized geometry and the frontier molecule orbital density distributions of the molecule are shown in Fig. 1 and the quantum chemical parameters of the optimized molecular and protonated (HL) structures of the L inhibitor are listed in Table 1. It has been found that the HOMO is located in the region around the nitrogen and oxygen atoms. This probably indicates that the preferred active sites for an electronic attack and the favorite sites to interact with the metal surface are located within the region around the nitrogen and oxygen atoms. For the HOMO of molecule, the two nitrogen atoms (N47 and N48) of azomethine groups, also the two oxygen atoms (O33 and O41) of furan heterocyclic ring are observed to have electron densities with Mulliken atomic charges of (-0.47), (-0.47), (- 0.43) and (-0.43), respectively. Subsequently, molecule is able to adsorb on the copper surface by donating the unshared pair of electrons from N and O atoms to the vacant d- orbitals of copper, and thus prevent copper to be corroded. The calculated quantum chemical data include the energy of the fully occupied molecular orbital (EHOMO = -5.53 eV), energy of the lowest unoccupied molecular orbital (ELUMO = -1.53 eV), dipole moment (μ = 3.03 D), the energy gap, ∆E (ELUMO -EHOMO = 4.00 eV). The high value of EHOMO (-5.53 eV) is likely to indicate a tendency of the molecule to donate electrons to the appropriate acceptor molecule with low energy and empty molecular orbital, whereas the value of ELUMO (-1.53 eV) indicates the ability of the molecule to accept electrons. Therefore, the value of ∆E provides a measure for the stability of the formed complex on the metal surface. From Mulliken charge analysis (Fig. 1), it is obvious that nitrogen and oxygen atoms of the L inhibitor hold negative charges. Thus, the adsorption of L on the metal surface can occur directly involving the displacement of molecules with water from the metal surface.
The high inhibition efficiency of a molecule can be attributed to the high value of dipole
moment and to the low value of ∆E. The results of the high dipole moment and the low energy gap indicate that electron transfer from molecule to the surface takes place during adsorption on the copper surface [5].
The transferred electrons number (∆N) was also calculated depending on the quantum
chemical method [6].
N
CuCu
inhinh
2
(1) Where χCu and χinh denote the absolute electronegativity of copper and the inhibitor molecule, respectively; ηCu and ηinh are the absolute hardness of copper and the inhibitor molecule, respectively. These quantities are related to the electron affinity (A) and ionization potential (I)
2 A I
2 A I
I and A are related in turn to EHOMO and ELUMO I = -EHOMO
A = -ELUMO
The values of χ and η were calculated by using the values of I and A obtained from quantum chemical calculation. Using theoretical values of χ Cu = 4.48 eV/mol and of η Cu = 0 eV/mol for copper according to Pearson’s electronegativity scale assuming that for a metallic bulk I = A, because they are softer than neutral metallic atoms [7].
∆N, which is the fraction of electrons transferred from inhibitor to the copper surface, was calculated (Table 1). Values of
∆N showed inhibition effect resulted from electrons donation. According to Lukovits [8], if ∆N <3.6, the inhibition efficiency increased with increasing electron donating ability at the metal surface. In this study, SB was the donor of electrons, and the copper surface was the acceptor.
II. b. Quantum chemical results of the protonated organic molecule (HL) Electrochemical reactions take place in an aqueous medium in which the compound with atoms having lone pair of electrons is protonated. The protonated species have been reported to also take part in adsorption on the metal surface. Therefore, it is interesting to study the molecular properties of the protonated species and compare them with those of the neutral species to determine the ideal ones to bind to the metal surface. The protonated species are shown in Fig. 2 and their molecular properties are reported in Table
1. Since the EHOMO is higher for the neutral species than for the protonated species, this indicates that the neutral species has a greater tendency to donate electrons and therefore, to bind to the metal surface. The ∆E values suggest that the neutral species is also more reactive than the protonated species. It is therefore reasonable to infer that the protonated species of molecule are less likely to interact with metal surface as compared to the neutral species. This confirms the experimental evidence that both physisorption and chemisorption mechanisms are involved in the adsorption process of molecule on copper surface.
Fig. 1. Frontier molecule orbital density distributions using DFT at the B3LYP/6-31G (d,p). Top: HOMO, bottom:
LUMO
Fig. 2. Frontier molecule orbital density distributions of protonated structure (HL) using DFT at the B3LYP/6-31G
(d,p). Top: HOMO, bottom: LUMO
Table 1. Calculated quantum chemical parameters for molecular and protonated structure
DFT parameter L(neutral) HL
(protonated) EHOMO (eV) -5.53 -5.10 ELUMO (eV) -1.53 0.15
∆E 4.00 5.25
μ (D) 3.03 1.00
∆N (e) 0.23 0.37
III. CONCLUSION
The relationships between inhibition efficiency of copper in 1 M HCl and the EHOMO, ELUMO- EHOMO, and ∆N of L were calculated by DFT method. The inhibition efficiency increased with the increase in EHOMO and decrease in ELUMO - EHOMO. HL had the highest inhibition efficiency because it had the highest HOMO energy and ∆N values, and it was most capable of offering electrons.
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