Quantum mechanical investigation of some N-substitued amino acids used as corrosion inhibitors

The field of corrosion inhibitors is undergoing dramatic changes from the viewpoint of environmental compatibility, due to the environmental agencies in different countries which have imposed the strict rules and regulations for the use and discharge of corrosion inhibitors, like toxicity, biodegradability and bioaccumulation criterions. Thus, the development of novel corrosion inhibitors with a minimal or zero negative effects has been considered to be more important and desirable, so called eco-friendly or green corrosion inhibitors. The amino acids which are being used usually in food and biology fields has also an area of usage in metal industry. Since amino acids act as a barrier between the metal surface and water and air through adsorption which film strips of micro-sized thicknesses are formed on the metal surface. Adsorption is simply expressed as a solid-liquid or concentration change at the solid-gas interface. Positive change in concentration increases the resistance of metal to corrosion and reduces it to a minimum. The most advantageous of using amino acids as corrosion inhibitors is that they are environmentally friendly and do not possess toxic properties when compared to the traditional acidic and basic corrosion inhibitors. Amino acids take an important place for corrosion protection of metals with scientific studies and industrial purposes. Telegdi and Beczner have synthesized novel compounds such as N-hydroxymethyl (NHM) glycine, phenylalanine, serine and glutamic acid. In this thesis study, quantum mechanical calculations are used to enlighten the corrosion inhibition efficiency of these NHM-amino acids which have been investigated experimentally and exhibit good corrosion inhibition properties. Herein, to gain insight about the properties of these derivatives in their neutral forms, a molecular orbital analysis which is widely used in the analysis of chemical interactions and provide much useful information on electronic structures of these derivatives was referred. The chemical reactivity was determined by examining the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO). Calculations were performed in order to examine the relationship between molecular structures and inhibition efficiencies of the corresponding compounds at the level of B3LYP/6-311G++(d,p) basis set using density functional theory. Quantum chemical parameters such as the highest filled molecular orbital energy (EHOMO), the lowest empty molecular orbital energy (ELUMO), energy vacancy (ΔE), dipole moment (μ), electronegativity (χ) and hardness (η) have been considered. It has been concluded that experimental inhibition efficiencies could be explained by the obtained theoretical data.