Kỷ yếu Hội thảo “Hóa học vì sự phát triển bền vững” – Trường Đại học Thủ Dầu Một
5/2015
THE STUDY OF RELATIONSHIP BETWEEN STRUCTURE AND CORROSION INHIBITION EFFICIENCY OF SOME GREEN CORROSION INHIBITORS BY GAUSSIAN09W USING DFT Nguyen Trung Hieu Department of Chemistry, Faculty of Natural Sciences, Thu Dau Mot University Email:
[email protected]; Tel: 01288969156 ABSTRACT In this study, some of azomethine (Schiff base) were investigated by quantum chemistry method at the level of DFT/B3LYP with the 6-31+G (d, p) base sets. Quantum chemical parameters such as: HOMO, LUMO energies, orbital density distributions, energy gap, the charge distributions, absolute electronegativity values (χ), electron affinity (A), ionization potential (I), global hardness (η), softness (), the fraction of electrons transferred from inhibitors to iron (ΔN) have been calculated. The corrosion inhibition efficiency (%IE) was then regressed on the result parameters individually using one variable function. Then the primary parameters that play the most important role in %IE were determined. Keywords: Green Corrosion Inhibitor, DFT, Schiff, Quantum Mechanics, Azomethine. 1. INTRODUCTION Corrosion inhibition is one of the best methods to protect metal away from corrosion in environment medium especially when other methods show low or no efficiency. Corrosion inhibitors are widely used in many industries such as: petroleum, chemical industry, construction, acidic pickling process… However, most of corrosion inhibitors are synthesized and are often toxic to the environment. And the trend in this field is to replace the toxic inhibitors by environmentally acceptable one. Several Schiff bases have been reported as effective corrosion inhibitors for different metals and alloy in acidic media [1, 2] and have eco-friendly or low toxic properties [3, 4]. The inhibition property of Schiff bases is due to special structural R2C=NR which may provide electrons from inhibitors to metal. Table 1. The chemical structures of the investigated Schiff bases compounds Inhibitors Conformation Abbreviation 1-(4-nitrophenyl)-N-(5-phenyl1,3,4-thiadiazol-2-yl)methanimine
Schiff 1
1-(3-nitrophenyl)-N-(5-phenyl1,3,4-thiadiazol-2-yl)methanimine
Schiff 2
1-(4-chlorophenyl)-N-(5-phenyl1,3,4-thiadiazol-2-yl)methanimine
Schiff 3
Quantum chemical calculation is a good tool to investigate the compound at molecule, atom and even electron level. And then it helps to study the relationship between structure and performance of corrosion inhibitors to make faster design and evaluation. The objective of this 1
Kỷ yếu Hội thảo “Hóa học vì sự phát triển bền vững” – Trường Đại học Thủ Dầu Một
5/2015
paper is to investigate on properties of three Schiff bases inhibitors: 1-(4-nitrophenyl)-N-(5phenyl-1,3,4-thiadiazol-2-yl)methanimine (Schiff 1); 1-(3-nitrophenyl)-N-(5-phenyl-1,3,4thiadiazol-2-yl)methanimine (Schiff 2); 1-(4-chlorophenyl)-N-(5-phenyl-1,3,4-thiadiazol-2yl)methanimine (Schiff 3) (Table 1), and explore the relationship between molecular structural parameters and their inhibition efficiency. 2. CALCULATION METHOD Density Functional Theory (DFT) [5, 6] is a computational quantum mechanical modelling method used to investigate the electronic structure of atoms, molecules, and the condensed phases [7]. It is an economic and efficient chemistry computing method and widely applied in study of corrosion inhibition performance and the interaction of corrosion inhibitors and interfaces. By using DFT/B3LYP in Gaussian09W with the basis set of 6-31+G (d, p) [8, 9], we conduct geometry optimization in gas phase and water (PCM model) to find the minimal points on potential energy surface and then calculate molecular parameters such as: HOMO, LUMO energies, orbital density distributions, energy gap, the charge distributions, absolute electronegativity values (χ), electron affinity (A), ionization potential (I), global hardness (η), softness (), the fraction of electrons transferred from inhibitors to iron (ΔN). 3. RESULTS AND DISCUSSION
Where R: p-NO2, m-NO2 or p-Cl Figure 1. The molecular schematic of inhibitor Parameters as Egap, A, I, χ, η, , N were calculated as follows: Egap = ELUMO – EHOMO; I = - EHOMO; A = - ELUMO; χ = (I+A)/2; η = (I-A)/2; = 1/η; ΔN=((χFe - χinh)/2)*(ηFe+ηinh), where: χFe=7.0 eV/mol and ηFe=0 eV/mol for iron Table 2. Quantum chemical parameters of the studied inhibitors (Aqueous phase by PCM model, water solvent; *: %IE from experiment value in Reference[10], solution containing 2M HCl, 10-4M inhibitor at 298K) Phase
Gas
Aqueous
Inhibitor
EHOMO (eV)
ELUMO (eV)
Egap
Schiff 1
-6,7756
-3,6126
3,1631
5,1941 1,5815 0,6323 1,4280
31
Schiff 2
-6,7000
-3,2118
3,4882
4,9559 1,7441 0,5734 1,7826
55
Schiff 3
-6,4540
-2,9176
3,5364
4,6858 1,7682 0,5655 2,0460
80
Schiff 1
-6,6883
-3,5922
3,0961
5,1402 1,5481 0,6460 1,4395
31
Schiff 2
-6,6581
-3,2243
3,4338
4,9412 1,7169 0,5824 1,7674
55
(eV)
Schiff 3
χ
η
%IE* (%)
80 -6,5128 -2,9122 3,6006 4,7125 1,8003 0,5555 2,0591 3.1. The impact of molecular orbital energy on %IE Heteroatoms such as O, N, S and multiple bonds in the inhibitor molecule are the key factors that impact on inhibition efficiency. Through which they are adsorbed on the metal surface. The adsorption depends mainly on certain physico-chemical properties of the inhibitor group, such as functional groups, electron density at the donor atom, p-orbital character, and the electronic structure of the molecule. The inhibition efficiency of Schiff bases is much 2
Kỷ yếu Hội thảo “Hóa học vì sự phát triển bền vững” – Trường Đại học Thủ Dầu Một
5/2015
more than that of the corresponding amines and aldehyde because Schiff bases (R 2C=NR) have both the features (heteroatoms and p electrons) combined with their structure. Pi (p) electrons in the Schiff base molecule not only can locate the unoccupied orbital of the transition metal, but also can accept the electrons of the d orbital of the transition metal to form metal–inhibitor bond, which is not possible with an amine. To analyze the adsorption process, the HOMO and LUMO of inhibitor molecules are taken into consideration. EHOMO is the measurement of electron donating ability. The smaller EHOMO is, the more stable the electrons are and smaller the electron donating ability is. ELUMO, on the other hand, indicates the ability of the molecule to accept electrons. The lower the value of E LUMO, the more probable is the acceptance of electrons by a given molecule. ELUMO is closely related to molecular electron affinity. ELUMO is small means the electron’s system energy decreases much when it enters into this orbital. It also indicates that the molecule can accept electrons easily. ΔEgap= (EHOMO-ELUMO), is an important stability indicator. The bigger ΔE is, the better the stability is and the worse the activity in chemical reaction. In Fig.2, we could see that in gas or aqueous phase, the %IE has the same trend, i.e. the impact of EHOMO and Egap on %IE (gas phase: R = 0.959, 0.914; aqueous phase: R = 0.939, 0.979) is lower than that of ELUMO (gas phase: R = 0.995; aqueous phase: R = 0.998). So, in this case of these three inhibitors, the ELUMO plays more important role than EHOMO and Egap. Because of 3.0
