Part 3. Theoretical study on some amino acids and their potential activity as corrosion inhibitors for mild steel in HCl

Abstract

Quantum chemical parameters, namely energy of the highest occupied molecular orbital, energy of the lowest unoccupied molecular orbital, energy gap, dipole moment, total energy, total electronic energy, core–core repulsion, ionisation potential, cosmo area, cosmo volume and other quantum descriptors [calculated from PM6, PM3, AM1, RM1 and modified neglect of diatomic overlap (MNDO) Hamiltonians], have been used to predict the corrosion inhibition potential of asparagine, aspartic acid, glutamine and glutamic acid. The results obtained indicate that the trend for the variation of the inhibition efficiencies of the compound is in the order: glutamine>asparagine>aspartic acid>glutamic acid. There is a strong agreement between some quantum chemical parameters and the experimental inhibition efficiencies. In order to establish the sites for electrophilic and nucleophilic attacks, condensed Fukui function, condensed softness and relative nucleophilicity/electrophilicity were considered. The results reveal that the sites for nucleophilic attacks in aspartic acid and glutamine are at the nitrogen atom (N5) but at the carbon atom (C3) for asparagine and glutamic acid. The sites for electrophilic attacks are at the oxygen atom (O9, for aspartic acid), carbon atom (C6, for asparagine), oxygen atom (O10, for glutamic acid) and nitrogen atom (N9, for glutamine).