Quantum-Chemical Prediction of the Redox Properties of Humic Acids

Abstract

A comparative analysis of the reactivity indices of the model structure of humic acids (HAs) and a number of antioxidants was carried out based on the results of quantum-chemical calculations performed using the density functional theory (DFT) b3lyp/6-31g(d,p) method. With the use of 15 compounds as an example, it was found that the electronegativity index χ linearly correlates with the energy of the lowest unoccupied molecular orbital (ELUMO), R2 = 0.977. HAs (ELUMO = –2.52) are located close to molecular oxygen (ELUMO = –3.07), and this fact indicates a high electronegativity of these natural compounds. It was proposed to evaluate the antioxidant ability of the organic matter of HAs by the adsorption energy of molecular oxygen. By determining the local minima of O2 sensing energy in different sections of HAs using the pm6 quantum chemical method, it was established that oxygen is adsorbed by a hydroxyl group with the energy ∆Eads = –70 kcal/mol. This allows the highly reactive organic part of HAs to inhibit molecular oxygen in oxidation processes.