Molecular dynamics simulation of the first electron transfer step in the oxygen reduction reaction

We present a molecular dynamics simulation of solvent reorganization in the first electron transfer step in the oxygen reduction reaction, i.e. O2+e−→O2−, modeled as taking place in the outer Helmholtz plane. The first electron transfer step is usually considered the rate-determining step from many experimental studies. From our results, we conclude that solvent reorganization rather than inner-sphere reorganization provides the dominant contribution to the activation barrier, if no specific interaction of O2 or O2− with the electrode is included. The free energy surfaces for solvent reorganization are strongly non-linear owing to the effect of electrostriction, which is not incorporated in the classical Marcus electron transfer theory. In spite of the non-symmetric free energy barrier, the transfer coefficient at equilibrium is still close to 0.5, whereas it would be estimated to be much lower from harmonic model considerations.