Activation of electron-transfer reduction of oxygen by hydrogen bond formation of superoxide anion with ammonium ion

A hydrogen bond formed between the superoxide anion and the ammonium ion (NH₄⁺) accelerates electron transfer from the C ₆₀ radical anion to oxygen significantly, whereas the tetra-n-butylammonium ion has no ability to form a hydrogen bond with the superoxidie anion, exhibiting no acceleration of the electron-transfer reduction of oxygen. The second-order rate constant of electron transfer from C ₆₀^- to O₂ increases linearly with increasing concentration of NH₄⁺. This indicates that O ₂^- produced in the electron transfer from C₆₀ to O₂ is stabilized by 1:1 complex formation between O ₂^- and NH₄⁺. The 1:1 complex formed between O₂^- and NH₄⁺ was detected by ESR. The binding of O₂^- with NH₄⁺ results in a positive shift of the reduction potential of O₂ with increasing concentration of NH₄⁺, leading to the acceleration of electron transfer from C₆₀^- to O ₂.