The reaction mechanism of H₂O₂ dismutation catalyzed by a non-heme tetraaza annulene complex [Fe(III)TMTAA] has been investigated by density functional theory using the B3LYP functional. All the stationary points on the doublet, quartet, and sextet potential energy surfaces (PES) have been fully optimized. The reaction undergoes preferentially on the quartet PES. The whole reaction can be divided into two stages. In the stage 1, the complex is oxidized by H₂O₂ through an O—O bond homolytic cleavage pathway and the intermediate IM6 and the first H₂O molecule are formed. In the stage 2, the IM6 is reduced by the second H₂O₂ molecule via two hydrogen abstraction steps and O₂ and the second H₂O are formed. The rate-determining step was found to be the O—O bond breaking step with a barrier of 63.9 kJ•mol^－1. The calculated barrier for the O—O homolytic cleavage in dismutation of free H₂O₂ is 226.7 kJ•mol^－1. The results indicate that the [Fe(III)TMTAA] complex can decrease energy barrier of the titled reaction and it may be a potential candidate for catalase mimic.

Key words: macrocyclic complex, tetraaza annulene, catalase mimic, density functional theory, reaction mechanism, intermediate, transition state