A computational study of the Fenton reaction in different pH ranges

Abstract

The reaction of iron(ii) and hydrogen peroxide, namely the Fenton reaction, is well-known for its strong oxidizing capability. While the Fenton reactions are ubiquitous and have wide applications in many areas, the detailed mechanism, especially the nature of the reactive intermediates responsible for oxidation, is not completely clear. In this work, the performances of various density functional theory (DFT) methods on the relative energies of key Fenton intermediates are evaluated. The DFT method selected from the benchmark study is then exploited to investigate the aqueous Fenton reactions in different pH conditions. The results show that at pH > 2.2, the major Fenton oxidants are high-valent oxoiron(iv) aquo complexes. However, depending on the pH conditions, these complexes can exist in three protonation states that display quite different oxidation reactivities. The oxidizing power of FeIV[double bond, length as m-dash]O is found to be principally determined by the total charge of the ligands and is less influenced by the axial ligand effect. Moreover, the calculations reveal that the presence of the hydronium ion can stabilize the intermediate of the hydroxyl radical and further inhibit oxoiron(iv) formation via proton transfer. The contribution of hydroxyl radicals could compete with the oxoiron(iv) species at pH below 2.2. In addition, high-level ab initio calculations question the existence of the iron(iv)-dihydroxo intermediate suggested in the literature. The implications of the computational results for the Fenton oxidation process, cytochrome P450, and catalyst design are discussed.

Why the Reactive Oxygen Species of the Fenton Reaction Switches from Oxoiron(IV) Species to Hydroxyl Radical in Phosphate Buffer Solutions? A Computational Rationale

“…Our previous benchmark study has verified that this level of theory can nicely reproduce the Brueckner coupled cluster BD(T)/Def2-TZVP results for the relative energies of key Fenton intermediates. 53 The p K a values of coordinated H 2 O of the iron complexes were evaluated by using the linear equation: p K a = 0.20847Δ G deprot – 47.59790, where Δ G deprot is the deprotonation free energy, G (A – ) – G (HA), computed by the CPCM/PBE0-D3/6-31+G(d) method. This equation has been demonstrated to be robust for predicting the p K a of the aqua complexes of the first-row transition-metal ions with the error being within 0.6 p K a unit.…”

“…This equation has been demonstrated to be robust for predicting the p K a of the aqua complexes of the first-row transition-metal ions with the error being within 0.6 p K a unit. 53…”

“…43−46 The preference for the formation of oxoiron(IV) species has also been supported by density functional theory (DFT) studies. 47−53 The DFT calculations further pointed out that the oxoiron(IV) aqua complexes can exist in three protonation states, [(H 2 O) 5 Fe IV O] 2+ , [(H 2 O) 4 Fe IV O(OH)] + , and [(H 2 O) 3 Fe IV O(OH) 2 ], depending on the pH conditions. The oxidizing power of the oxo group in these complexes was found to be quite dissimilar and principally determined by the total charge of complexes.…”

“…The oxidizing power of the oxo group in these complexes was found to be quite dissimilar and principally determined by the total charge of complexes. 53 On the other hand, when the Fenton reaction is performed in coordinating phosphate buffer solutions, the resulting ROS switches to • OH radical even in a neutral pH for some unknown reason. 44…”

West Sumatera Brown Rice Resistance to Fe

“…This radical is extremely toxic to cell metabolism, leads to oxidation of biological macromolecules such as proteins, lipids, and nucleic acids. This condition causes membrane leakage and even cell death [15] [16].…”