Studies of model dependence in an ab initio approach to uncatalyzed oxygenreduction and the calculation of transfer coefficients

In a recent study [J. Am. Chem. Sec. 121 (1999) 11855], an ab initio approa ch to calculate potential dependent activation energies was applied in stud ying the outer-sphere O-2 reduction and H2O oxidation. The purpose of this paper is to examine influences of changes in the calculational methodology and the reactant structural models. The first step in the overall four-elec tron reduction of O-2 to water, O-2(g) + H+(aq) + e(-)(U) reversible arrow HOO(aq) is the focus of this work. U is the electrode potential and H+(aq) is model ed by the [HOH2(OH2)(2)](+) cluster. For an electrode potential of 0.727 V on the hydrogen scale, the findings of this study are: 1. Determining the transition state structures constrained to using the pro duct OOH angle is a satisfactory approximation. 2. The calculated activation energies are reduced for the forward reaction and increased for the reverse reaction when the hydronium ion structure is relaxed along the reaction coordinate. 3. Calculated reduction activation energies using the 6-31G** basis set are highest for the HF calculations, intermediate for MP2 calculations and low est for B3LYP density functional calculations. Adding diffuse functions low ers all of the values. 4. Increasing the model size by coordinating another water molecule to the transferring proton increases the activation energy for the forward reactio n. In addition to the above, the transfer coefficients in the Butler-Volmer eq uation relating current density to overpotential are calculated and discuss ed. (C) 2001 Elsevier Science Ltd. All rights reserved.