The inhibition of mild steel corrosion in a 1 M HCl solution by some natural free acids and antioxidants has been investigated by weight loss measurement, potentiodynamic polarization and DFT calculations. The experimental results have shown that these compounds exhibited a good corrosion inhibition. The inhibition efficiency increased with the inhibitor concentration. The adsorption of the inhibitor molecules onto the metal surface was found to respond to Langmuir adsorption isotherm for ascorbic, oleic and stearic acids, and to Temkin adsorption isotherm for palmitic acid. Tafel plot analysis revealed that these compounds acted as mixed type inhibitors, with more polarized cathodic than anodic curves. Regarding quantum chemical calculations, parameters such as energies of highest occupied molecular orbital and lowest unoccupied molecular orbital, energy gap, dipole moment, electronegativity, global hardness, softness, global electrophilicity, fraction of transferred electrons, ∆E Backdonation, Fukui and local softness indices have been performed on the tested inhibitors to investigate their structural and electronic properties, in order to provide an adsorption mechanism, and reveal the reactivity and selectivity of the molecules' centers. The experimental results were in good agreement with theoretical results. The results for the natural acids were used to predict the linoleic acid inhibition efficiency.

In this work, we conducted a systematic search of the minimum energy of a dopamine–β-cyclodextrin complex via different ONIOM approaches using both mixed (DFT-HF) and nonmixed (DFT-DFT) combinations. Different density functionals were employed: B3LYP, MPW1PW91, M05-2X, M06-2X, and ωB97X-D. Two different basis sets were used on the dopamine–β-cyclodextrin complex; a lower basis set (3-21G*) is used on β-cyclodextrin and a higher basis set (6-31G(d)) is used on dopamine. To fulfill this task, complexation and deformation energies were determined. The relative performance of these functionals was compared with that obtained with the DFT method at the M06-2X/6-31G(d) level that is chosen as a reference method. Although we did not clearly establish an assessment of the relative performance of these density functionals, the efficacy of the ONIOM combination (DFT-HF) is shown compared with a nonmixed ONIOM combination (DFT-DFT). The intermolecular hydrogen bond interactions of the complex obtained with the full M06-2X/6-31G(d) have been analyzed with the atoms-in-molecules and natural bond rrbital methodologies.

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