Quantum chemical studies of inhibition effect of some amino acids on corrosion of copper in nitric acid

Quantum chemical studies of inhibition effect of some amino acids on corrosion of copper in nitric acid. The experimental study of the corrosion inhibition of copper in nitric acid solution by the amino acids compounds such as arginine (Arg), cysteine (Cys), glycine (Gly), lysine (Lys) and valine (Val) was supplemented by a theoretical approach of equilibrium of protonic dissociation of these amino acids with a semi empirical method: Austin Model (AM1). The most favourable site of protonation of these compounds is the nitrogen atom of the amino group. A linear correlation between the inhibition efficiency experimentally obtaineds and the energy difference, Delta E between molecular orbitals, E-HOMO (high occupied molecular orbital energy) and E-LUMO (lowest unoccupied molecular orbital energy) was determined. This examination allowed concluding that Cys is the best inhibitor of the amino acids tested. Moreover the calculation of protonic affinities and the atomic charges of cysteine show that the functional groups of Cys (NH3+, HS, COOH) are involved in the complexation Cu (II)/Cys on the copper surface.