Theoretical study of the substituent effect on corrosion inhibition performance of benzimidazole and its derivatives

The substituent effects of electron-donating and -withdrawing on the efficiency of corrosion inhibition of benzimidazole and its derivatives have been studied by density functional theory DFT and Moller-Plesset perturbation theory MP2 calculations at aqueous medium. For this investigation, the corrosion inhibition efficiencies of the protonated and non-protonated spesies of benzimidazole and its derivatives were correlated with molecular electronic properties: high occupied molecular orbital (HOMO) and low unoccuppied molecular orbital (LUMO) energies, ionization potential, electron affinity, electronegativity and fraction number of electron transfer. The dipole moment and interaction energy represent the total surface coverage and the strength of the adsorption of inhibitors on the metal surface. Natural bond orbital NBO analysis in term of the second order interaction energies were used to study the contributions of the active sites of inhibitors toward corrosion inhibition performances. The ionization potential of the inhibitors has a strong influence on the efficiency of corrosion inhibitors. It was found that the MP2 method accurately predicted the ionization potential while the DFT failed to mimic the ionization potential of the experimental results. The linear correlation was shown between electronic properties and corrosion inhibition efficiency. Electron donating substituents increase the corrosion inhibition efficiency, whereas electron withdrawing substituents give the opposite effect. The NH2 substituent contributes highest, whereas NO2 provides the weakest contribution to the corrosion inhibition efficiency for both non-protonated and protonated species of inhibitors. The second order interaction energy indicated that heteroatom at imidazole position was the main center of electron donating and they received simultaneously significant amount of electron back donation from the metal.