Ab initio studies in the changes in structures and energy the of the O~2/O~2^- system in the electron transfer process were performed by using UHF and UMP2 methods with basis sets of 3-21G, 6-31G^*, 6- 311+G^*, UMP2(full)/6-311+G^* respectively. It is shown that the UMP2 method is necessary in the calculation for this system, because the electron correlation is important. The bond lengths of O~2 and O~2^.^- calculated with UMP2(full)/6-311+G^* are in agreement with the experimental results. The calculated internal reorganization energy of the electron transfer reaction, the activation energy, and the rate constant are 77.14kJ/mol, 19.29kJ/mol and ca. 3.17 ×10^7dm^3. mol^-^1.s^-^1 respectively, for the self-exchange reaction of O~2/O~2^. ^- in the gas phase. Structures of the electron transfer activated complex of O~2/O~2^.^- system in aqueous solution were optimized. The calculated solvent reorganization energy is 146.09kJ/mol. The results show that the activation energy is 55.81kJ/mol, and the rate constant of O~2/O~2^.^- self-exchange reaction in aqueous is about 1.00× 10^2dm^3.mol^-^1.s^-^1, which is in agreement with the experimental result.

Key words: OXYGEN, AB INITIO CALCULATION, REACTION RATE CONSTANT