Ferrous Iron Reduction of Superoxide, A Proton-Coupled Electron-Transfer Four-Point Test

Abstract

Nelsen's four-point method of separating oxidants and reductants has been tested to evaluate its applicability to proton-coupled electron-transfer reactions. An efficient computational method was developed to determine rate-limiting steps in complex, multistep redox reactions. Geochemical redox reactions are rarely single-step, and by identifying the rate-limiting steps, computational time can be greatly reduced. The reaction of superoxide and ferrous oxide was selected as a test case for its simplicity and its importance in environmental radical generation chemistry (Fenton's reaction). Two approaches, one quantum mechanical and the other semiempirical, were compared. In both approaches, hybrid density functional theory (DFT) was used with the B3LYP/6-31+G(d,p) basis set and a polarized continuum model of the solvent to minimize the structures and determine the energies. In the quantum mechanical case, DFT was used to determine both the Gibbs free energies and the values for the intrinsic component of the reorganization energy of possible combinations of reactants and products. In the latter, experimental DeltaG(f) values were combined with calculated intrinsic reorganization energy values. The computational results matched the relative difference in rate barriers between the reduction of superoxide by ferrous iron above and below pH 4.8. In the acidic pH range, the proton is coupled to the electron transfer, whereas in the neutral case, the proton initiates the electron transfer.

Proton-Coupled Electron Transfer

“…They also showed that both an increase and a decrease in driving force result in asymmetric parabolic functions for the transferring proton, which leads to greater contributions from excited vibrational levels, leading to a maximization of the KIE when the driving force is near zero. Wander and co-workers have utilized Nelsen’s four-point methodology, which involves separate calculation of reorganization energies of oxidants and reductants to estimate relative barrier heights and isotope effects for Fe 2+ reduction of superoxide by EPT …”

Effect of Basic Site Substituents on Concerted Proton–Electron Transfer in Hydrogen-Bonded Pyridyl–Phenols

“…The solvent contributions are expected to be similar for all the reactions analyzed here because of the similar size of the phenol–pyridines. The calculations of λ i (sCPET) followed Nelson’s four-point method for ET reorganization energies, , with the proton transfer included in the reorganization energy, as we and others have done previously. The energies of the neutral molecule and radical cation (denoted E 0 and E + , respectively) are calculated at the optimized neutral and cation geometries (denoted ng and cg, respectively), and λ i (sCPET) is taken as the sum of the neutral and cation relaxation energies (denoted E relax 0 and E relax + , respectively) .…”

Proton-Coupled Electron Transfer

“…Wander and co-workers have utilized Nelsen's four-point methodology, which involves separate calculation of reorganization energies of oxidants and reductants 59 to estimate relative barrier heights and isotope effects for Fe 2+ reduction of superoxide by EPT. 60…”