ABSTRACT
Inhibition and adsorption potentials of cysteine, glycine, leucine and alanine have been investigated using experimental and quantum chemical approaches. The experimental study was carried out using gravimetric, gasometric, thermometric and FTIR methods of monitoring corrosion while the quantum chemical study was carried out using semiempirical and ab-initio methods. The results obtained reveal that various concentrations of the amino acids inhibited the corrosion of mild steel in solutions of HCl through the mechanism of physiosorption with activation energy less than 80kJmol-1. The inhibition potentials of the inhibitors decreased in the order, cysteine >leucine> alanine > glycine and their adsorption was found to be exothermic, spontaneous and fitted the Langmuir adsorption model. Computational chemistry results obtained revealed excellent correlations between quantum chemical parameters (calculated for gas and aqueous phases PM6, PM3, AM1, RM1 and MNDO Hamiltonians) and experimental inhibition efficiencies. Correlations between the experimental and theoretical inhibition efficiencies were also excellent. Application of condensed Fukui and softness functions as well as relative nucleophilicity/electrophilicity (calculated for B3LYP(6-31G) and MP2(STO5G) paturbations) showed that the sites for electrophilic attack are on the amine bonds (N2-C3) of the amino acids while the sites for nucleophilic attack are on the C-3 atoms of the amino acids. The HOMO and LUMO diagrams of the amino acids as well as the electron density diagrams have also been used to support the information obtained from condensed Fukui function.
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