Estimation of the Efficiency of Corrosion Inhibition by Zn-Dithiocarbamate Complexes: a Theoretical Study

Seven Zn-dithiocarbamate complexes were suggested as corrosion inhibitors. Density functional theory (DFT) was used to predict the ability of inhibition. Room temperature conditions were applied to suggest the optimization of complexes, physical properties, and corrosion parameters. In addition, the HOMO, LUMO, dipole moment, energy gap, and other parameters were used to compare the inhibitors efficiency. Gaussian 09 software with LanL2DZ basis set was used. Total electron density (TED) and electrostatic surface potential (ESP) were utilized to show the sites of adsorption according to electron density.


Introduction
Metal complexes of dithiocarbamate compounds have been the substance of a recently growing interest because it possesses a wide range of applications in agricultural science, medicine, industry, and analytical and organic chemistry [1][2][3][4][5][6][7][8][9][10] ISSN: 0067-2904 The corrosion of metal is considered as the most serious problem in the field of industry [11]. Researchers have developed a considerable expertise in developing optimal protection for metals [12]. Many methods are being applied to prevent the corrosion process, one of them is the usage of inhibitors. The chemical compounds are a common use in this line. The most favorable types of compounds is those that contain polar atoms, such as oxygen, sulfur, phosphorus, nitrogen etc., which include a single pair of electrons that can be attracted to the metal surface by the adsorption process, forming a protective layer [13]. Dithiocarbamate is a ligand that can act as a corrosion inhibitor due to the presence of sulfur and nitrogen as electronegative atoms [14]. Recently, many studies were published that involve the utilization of metal-dithiocarbamate compounds (such as Zn(II) N-Isopropylbenzyldithiocarbamate) as corrosion inhibitors [15][16][17]. In addition, theoretical studies have been applied in this line to save time, money, and effort to suggest the best inhibitor [18]. Many researchers used dithiocarbamate compounds as corrosion inhibitors, such as sodium N,Ndiethyldithiocarbamate, ammonium pyrrolidine dithiocarbamate, ammonium (2,4-dimethylphenyl)dithiocarbamate, azepane dithiocarbamate, and others [19][20][21][22][23][24][25][26][27][28][29][30][31][32]. In this study, a theoretical study for the inhibition of corrosion by zinc compounds is conducted.

Preparation of compounds
The Zn-dithiocarbamate compounds used in this study were prepared by previously described methods [3,33].

Calculation Methods
The quantum chemical calculations were performed by Gaussian-09 software package with complete geometry optimizations [34, 35] that applied for the inhibitors which shown in (Figure-

Results and Discussion Inhibition Parameters
The prediction of absorbed inhibitor molecules was achieved by applying the limits orbital theory in respect to attraction the centers of these molecules to the metal surface [39]. This process involves the fragment molecular orbital (FMO) of the border, which gives the contribution through the inverse of the energy requirement of the stability of the difference in orbital energy (ΔE = E LUMO -E HOMO ). The ability to donate an electron to an acceptor can be expressed by the energy HOMO ( EHOMO). The high values of EHOMO for the inhibitor molecule give it a tendency to donate electrons. While the ability of the molecule to accept an electron is referred to as the energy LUMO (ELUMO), the lowest value of which means a higher electron accepting capacity. The energy gap between the orbits of the border (ΔE) is important in the explanation of the compound's activity; when the energy gap is decreased, the effectiveness of the inhibitor is improved [39]. The quantum chemical parameters are related to the efficiency of the inhibition of the molecule studied. These parameters include the highest molecular orbital energy (E HOMO ) occupied by electrons, the lowest unoccupied molecular orbital energy (E LUMO ), the energy gap (ΔE= E LUMO -E HOMO ), the dipole moment (µ), electronegativity (),the effort ionization (IP), electron affinity (EA), and the global softness (S), as shown in Tables-1 and 3. According to the theory of Koopman [39], HOMO energy is related to the ionization potential (IE) whereas LUMO energy is linked to the electron affinity (EA), as follows: …..(1) IE is the amount of energy required for removing an electron from an atom. The low ionization energy gives high efficiency for inhibition.
…..(2) EA is the quantity of energy released when an electron is added to an atom. The higher values of EA means less stability and produces high efficiency for inhibition. Hardness (η) is the second derivative of E, which measures both the stability and reactivity of the compound [40].
This value is related to HOMO and LUMO energies. The low value of electronegativity means high efficiency for inhibition. The global softness (S) is the inverse of the global hardness [41]. Softness is one of the important properties to measure the compound's stability and reactivity: Global electrophilicity index ( ) was discussed by Parr [42]. It is a measure of the stabilization in energy after a compound accepts an added number of electrons. A good inhibitor should have a lower ( ) value.
According to the parameters ( such as HOMO, ΔE , IA, and ), the final order of the complexes is: The energy gap is proportional to the final order of complexes, and it is represents the most important factor.  Figure-2 illustrates that the inhibitor compounds 4, 7 and 3 showed best inhibitory effects, based on the geometries optimization of compounds studied in the gas phase, including LUMO and HOMO density distributions. The supplementary Figure-S4 includes results of the other inhibitors. The red color indicates high electron density while the green represents low electron density [43]. High electron density area is the donating area of electrons to the metal. Low electron density area is the receiving area of electrons from the metal [47]. Hence, the two areas have very important distribution.

TED and ESP Maps
TED map presented the electron density on the complexes. The red color is due to the sites with high electron negativity; for example, the S atom and some areas of double bond in the studied molecules, which can help in the electrophilic attack. Whereas the yellow color refers to the atoms with moderate electronegativity. The blue color is assigned to the best positive area that can accept electrons from the donor compound [44], as in (Figure-3). The electrostatic surface potential (ESP) shows the adsorption direction of the complex with the metal surface, which is the same direction of the aromatic rings, S and N atoms. The green surface can be considered as the surface of the metal or alloy. Figure-4 shows the ESP values of the inhibitors (1-7).

Conclusions
The present study showed the ability of corrosion inhibition by seven compounds. According to the parameters applied, the most efficient inhibiters were 4, 7 and 3, respectively. The aromatic rings, N and S, in the compounds were allowed to donate and accept electrons with the metal. The most effective parameters were the dipole moment and energy gap, which showed values of 7.0989, 15.3546 and 1.6705 Debye and 2.6605, 2.7127 and 3.6292 eV for the inhibitors 4, 7 and 3, respectively. The compounds 1, 2, 5 and 6 had lower inhibition abilities than those of the compounds 4, 7 and 3.