SYNTHESIS OF THE PFAQ CORROSION INHIBITOR AND RESULTS OF QUANTUM CHEMICAL CALCULATIONS
Keywords:
p-phenylenediamine, formalin, alpha amino succinate, FT-IR spectroscopy, semi-empirical quantum chemical methods, HOMO–LUMO.Abstract
A novel oligomer-type corrosion inhibitor (PFAQ) based on p-phenylenediamine, formalin, and Alpha amino succinate was successfully synthesized under an inert nitrogen atmosphere. The synthesis conditions were systematically optimized, revealing that the highest product yield (≈86–88%) was achieved at a molar ratio of 1:2:2 and a temperature range of 40–65 °C, while higher temperatures led to a significant decrease in yield. The chemical structure of the synthesized inhibitor was confirmed by FT-IR spectroscopy, which revealed characteristic absorption bands corresponding to –NH, –CH, –CH₂, and C=O functional groups, indicating successful polymer formation. The reactivity and inhibition mechanism of the PFAQ molecule were investigated using semi-empirical quantum chemical methods (AM1, MNDO, PM3, RM1, and MNDO3) as well as DFT calculations at the B3LYP/6-311G(d,p) level. Mulliken charge analysis demonstrated that the highest negative effective charges are localized on oxygen atoms of C=O and C–O–C groups and on nitrogen atoms of secondary amine (N–H) groups, confirming their role as active donor centers capable of forming coordination bonds with metal surfaces. Frontier molecular orbital (HOMO–LUMO) analysis revealed significant electron density on –O–, C=O, and N–H groups, supporting strong adsorption behavior. The calculated global reactivity descriptors, including energy gap, chemical hardness and softness, electronegativity, electrophilicity index, and dipole moment, indicate high adsorption capability and effective corrosion inhibition performance. Additionally, molecular electrostatic potential analysis confirmed the presence of distinct electrophilic and nucleophilic regions, favoring stable metal–inhibitor complex formation. Overall, the obtained experimental and theoretical results demonstrate that PFAQ is a thermodynamically favorable, electronically active, and promising corrosion inhibitor for steel protection in aggressive acidic environments.
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