Quantum mechanical calculations on some 4-methyl-5-substituted imidazole derivatives as acidic corrosion inhibitor for zinc

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Abstract

Gas phase quantum chemical calculations for the efficiency of some imidazole derivatives as corrosion inhibitors, along with the relative stability and proton affinity determination were performed by means of the AM1, PM3, MINDO/3 and MNDO semi-empirical SCF molecular orbital methods. The agreement with the experimental data was found to be satisfactory.

Introduction

Organic compounds are recognized as effective inhibitors, for the corrosion of many metals and alloys. The efficiency of an organic compound as a corrosion inhibitor is closely associated with the chemical adsorption, which includes the variation in the charge of the adsorbed substance and charge transfer from one phase to the other. Thus the reaction mechanism includes the transfer of one pair of electrons from the organic compound, and the formation of the coordinate covalent bond with a metal or an alloy [1]. Under such conditions, for a given metal, the efficiency of the inhibitor depends on the stability of the formed chelate and the inhibitor molecule should have centers, which are capable of forming bonds with the metal surface via the electron transfer. In this way the metal acts as an electrophile, whereas the inhibitor molecule acts as a Lewis base, whose nücleophile centers, are normally hetero atoms with free electron pairs which are readily available for sharing, to form a bond.

Imidazole derivatives were found to act as corrosion inhibitors for copper and its alloys [2], [3], iron [4], [5], [6], [7] and for zinc in acid chloride solutions [8]. The imidazole molecule shows three different possible anchoring sites suitable for bonding; pyridine like nitrogen atom 3N, pyrrole like nitrogen 1N atom an aromatic ring itself and probably the substituent that acts as an active center. Therefore, it is of prime importance to know the tautomeric form and that of the acid dissociation constants of the studied molecules. The aim of this paper is to investigate the applicability of the quantum chemical calculations, to predict inhibition efficiencies of some imidazole derivatives for the inhibition of zinc in acid chloride solution and determine the possible tautomeric forms to elucidate the mechanism of the corrosion process.

Section snippets

Method of calculation

All the calculations were carried out with the help of complete geometry optimization. The geometry data were obtained from the pcmodel P2(3.2) packet program and subsequent AM1, PM3, MNDO and MINDO/3 calculations were performed with mopac6 packet program on IBM which is implemented on an Intel Pentium Pro 200 MHz computer.

Tautomerizm

The formulation and experimental acid dissociation constants, pKa, of the studied molecules are given in Table 1 and were evaluated in this work. As we have reported earlier [10], [11] the main tautomeric form of imidazole was found to be the 1H (a) form rather than 3H (b) unless an electron withdrawing substituent was located at 4C.

The relative stability data presented in Table 2 is a further evidence for this conclusion. As it can form the stability data without any substituent at 5C, the

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