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. O-2 + e(-) --> O-2(-), 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 O-2 or O-2(-) 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. (C) 2002 Elsevier Science B.V. All rights reserved.