| Abstract
| - The formation of active intermediates from the Fenton-like reagent (a mixture of iron(III) ions and hydrogenperoxide) in aqueous solution has been investigated using static DFT calculations and Car−Parrinello moleculardynamics simulations. We show the spontaneous formation of the iron(III) hydroperoxo intermediate in afirst step. The Fenton-like reaction thus proceeds very differently compared to Fenton's reagent (i.e., theFeII/H2O2 mixture), for which we have recently shown that the first step is the spontaneous O−O lysis ofhydrogen peroxide when coordinated to iron(II) in water. For the second step in the reaction mechanism ofthe Fenton-like reagent, we compare the possibilities of homolysis and heterolysis of the O−O bond and theFe−O bond of the produced [(H2O)5FeIIIOOH]2+ intermediate. We find that concomitant hydrolysis of thereacting species plays a crucial role and, taking this into account, that O−O homolysis ([(H2O)4(OH)FeIIIOOH]+ → [(H2O)4(OH)FeIVO]+ + OH•) in vacuo is a likely second step with ΔE0k‡ = 26 kcal/mol. However,proper inclusion of the solvent effects is important, in particular, for the heterolysis reactions, in which casethe large endothermicy of the charge separations can be compensated by the hydration energies from the ionsolvation. In this work, we also calculate the free energy barrier for the O−O homolysis of the iron(III)hydroperoxo intermediate in aqueous solution at T = 300 K, using the method of constrained moleculardynamics and thermodynamic integration, resulting in ΔA300K‡ = 21 kcal/mol. Analysis of the vibrationalspectra of the high-spin (S = 5/2) and low-spin (S = 1/2) Fe(III)OOH intermediates confirm the, in theliterature, suggested effect of the spin state on the Fe−O and O−O bond strengths. In fact, we predict thatwith ligands inducing a low-spin iron(III) hydroperoxo intermediate, the barrier for the O−O homolysis willbe even significantly lower.
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