Quantum chemistry study on the relationship between molecular structure and corrosion inhibition efficiency of amides
Introduction
Some organic compounds are found to be effective corrosion inhibitors for many metals and alloys. It has been commonly recognized that organic inhibitor usually promotes formation of a chelate on the metal surface, which includes the transfer of electrons from the organic compounds to metal, forming coordinate covalent bond during such chemical adsorption process [1]. In this way, the metal acts as an electrophile, whereas the nucleophile centers of inhibitor molecule are normally hetero atoms with free electron pairs which are readily available for sharing, to form a bond. The most common organic substances with these characteristics are those containing O, N and/or S atoms.
Some amides and derivatives e.g. urea (U), thiourea (TU), thioacetamide (TA) and thiosemicarbazide (TSC) have been found to be good inhibitors for mild steel in acid solutions. Relationship between molecular structures of these amides and their inhibition efficiencies have been studied in several research works [2], [3]. It was found that when the oxygen atom in a urea molecule was replaced by a sulfur atom (to form a thiourea), the corrosion inhibition efficiency increased dramatically. These effects of molecular structural change on the corrosion inhibition efficiency in the original paper was only simply explained by means of qualitative delocalized electrons model. Nevertheless, much less attention has been paid to the dependence of inhibition efficiencies (%) on the electronic properties of amide molecules. In this connection, several questions have been asked e.g. what is the effect on inhibition efficiency, when the amino group of TU is replaced with a methyl group (i.e. to be TA) and/or thiocarbamoyl group (CS) replaced by a carbonyl (CO). The latter seems especially important for sulfur containing compounds, e.g. thioamides (–CS–NH2) with excellent inhibition efficiency. Clear comprehension of above questions is helpful to understand the whole inhibition process and the mechanism of adsorption.
On the other hand, quantum chemical studies have been successfully performed to link the corrosion inhibition efficiency with molecular orbitals (MO) energy levels for some kinds of organic compounds, e.g. imidazole [4], cinnamaldehyde [5] and aniline [6], etc.
Following the theoretical analysis procedure mentioned above, we calculated the highest occupied molecular orbital (HOMO), lowest unoccupied molecular orbital (LUMO) energy levels of these amides and its derivatives. Additionally, we also studied their nucleophilic center by calculating the spatial distribution of electronic state density. Our results confirmed the strong correlation between these microscopic electronic properties and corrosion inhibition efficiencies, and gave unambiguous answers to the question about the structure–efficiency relationship of amides, as mentioned above.
Section snippets
Calculation method
All the study have been carried out using Dewar's LCAO-SCF-MO semiempirical method, MINDO/3 [7], in the commercially available computer program package (HyperChem Pro. Release 6.03 from Hypercube Inc. USA). The calculation were implemented on an Intel Pentium II 450 MHz computer. MINDO/3 develops the MO on a valence basis set. Molecular structures were optimized to a gradient norm of <0.01 in the vacuo phase. As an optimization procedure, the built-in Polak–Ribiere algorithm was used.
Results
Table 1 lists the molecular formula of amide compounds studied in the present work. Urea has a carbonyl group (CO) but other three thioamides contain thiocarbamoyl group (CS). The difference between TU and TA is that the amino group (–NH2) in TU is replaced by methyl group (–CH3) in TA. TSC is also a thioamide holding a amino group and a hydrazino (–NH–NH2) group as well.
Table 2 represents calculated energy levels (in eV) of the HOMO and LUMO for the four selected amides. The measured
Correlation between MO energy level and inhibition efficiency
According to Fukui's frontier orbital approximation [9], interactions happen only between frontier MOs. Therefore, for analysis of the chelate process during chemical adsorption, HOMO and LUMO of both reactants need to be considered. Since due to the inverse dependence of stabilization energy on orbital energy difference, terms involving the frontier MO will provide dominative contribution.
Moreover, another point regarding MO level is the gap between the HOMO and LUMO energy levels for the
Conclusions
(I) Strong relationship between HOMO, LUMO levels and corrosion inhibition efficiencies previously reported for some organic compounds was also observed in amide compounds.
(II) Substitution of the carbonyl (CO) of some amides (e.g. U) by the group of thiocarbamoyl (CS) leads to the transfer of chelation center from each pair of nitrogen atom to the sulphur atom in thiocarbamoyl of thioamide molecules, significantly resulting in the increase of HOMO level and a great reduce of HOMO–LUMO
Acknowledgements
This project is financially supported by the National Natural Science Foundation of China (NO. 20177015).
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