Fig. 1: Marcus theory suggests separate kinetics of superoxide oxidation to 3O2 and 1O2.
From: Marcus kinetics control singlet and triplet oxygen evolving from superoxide
a, Hypothesis for how the driving force could govern 3O2 and 1O2 formation kinetics from superoxide oxidation. The left parabola results from previously measured rate constants for mediated superoxide oxidation with driving forces up to –ΔG° ≈ 1.2 eV as shown in Extended Data Fig. 1. However, a single kinetic parabola could not conclusively explain why 1O2 formed for –ΔG° ≳ 1 eV. Based on the considerations in b, individual kinetic parabolas (k3 and k1) for the reactions yielding 3O2 and 1O2 can be constructed with the full line showing their sum. The maxima are shifted by \({\Delta G}_{1\leftarrow 3}^{\circ }\approx 0.97{\rm{eV}}\) (see text), and equal prefactors and reorganization energies are assumed. The blue- and red-shaded area shows the transition from k3/(k1+3) = 0.99 to k1/(k1+3) = 0.99. b, Potential energy surfaces for mediated superoxide oxidation for different driving forces. Black, blue and red parabolas denote reactants (KO2 + RMox) and 3O2 or 1O2 in the products (3O2 + RMred or 1O2 + RMred), respectively. The cases shown are for barrierless reactions to 3O2 (i) and 1O2 (iii) and equal barriers (ii). The subscripts 3 and 1 denote triplet and singlet states, respectively.
