Quantum Chemical Study of Methyl Substituent's Position on Quinoline for Inhibition of Aluminium Corrosion inHydrochloric Acid Solution

Quantum chemical study of some methylquinolines on inhibition of aluminium corrosion in hydrochloric acid and effect of methyl group at 5, 7 and 8 position on quinoline was investigated theoretically with the aid of material studio using density functional theory (DFT). The simulations were performed by means of the DFT electronic program DMol<sup>3</sup> using the Mulliken population analysis in the Material Studio. DMol<sup>3</sup> permitted the analysis of the electronic structures and energies of molecules, solids and surfaces. The analysis of the quantum chemical parameters, the adsorption parameters form the simulation of the molecules, the Mulliken and Hirshfeld values of the fukui indices for the three molecules of the 5-MeQ, 7-MeQ and 8-MeQ indicated that all the three molecules exhibits high potential for inhibition of aluminium corrosion in HCl environment, with 5-MeQ being the best among all. The most popular parameters which play a prominent role are the eigen values of the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO), the HOMO-LUMO gap (ΔE), chemical hardness and softness, electro-negativity and the number of electrons transferred from inhibitor molecule to the metal surface. All the molecules showed good corrosion inhibition tendency, however, 5-MeQ molecule gives better aluminium corrosion inhibition potential than other two molecules. The orientation of the Methyl substituent on the core quinoline was found to be responsible for intra-molecular intraction which leads to weaker attraction to the aluminium surface for the 7-MeQ and 8-MeQ molecules hence lower corrosion inhibition tendency than 5-MeQ molecule despite having the same molecular mass.