Theoretical Investigation On The Microcosmic Mechanism Of H₂O₂ Dismutation Catalyzed By Tetramethyl Dibenzotetraaza(14)Annulene Fe(III) Complexes

The microcosmic mechanism of H2O2 dismutation catalyzed by tetramethyl dibenzotetraaza[14]annulene Fe(III) complexes have been investigated by by Gaussisn03 program. The main substances are as follows:1. Theoretical investigation on the microcosmic mechanism of H2O2 dismutation catalyzed by four-coordinated tetramethyl dibenzotetraaza[14]annulene Fe(III) complex ([FeIII(tmtaa)]).2. Theoretical investigation on the microcosmic mechanism of H2O2 dismutation catalyzed by five-coordinated tetramethyl dibenzotetraaza[14]annulene Fe(III) complex ([FeIII(tmtaa)Cl]).All the reactants, intermediates, transition states, and products involved in the reaction are fully optimized by density functional theory using the B3LYP method. Ground or transition state properties are established by full-frequency calculations. Since solvation effects can be very important, the single point energies of all species involved in the reaction have been computed for water solvent by employing the CPCM continuum solvation model.From the investigation of H2O2 dismutation catalyzed by four-coordinated [FeIII(tmtaa)] complex, we can conclude that the reaction undergoes preferentially on quartet PES. The whole reaction can be divided into two stages. In the stage 1, the intermediate 4IM6 and the first H2O molecule are formed through an O-O bond homolytic cleavage pathway. In the stage 2, the second H2O yields via two hydrogen abstraction steps. The rate-determining step is found to be the O-O bond breaking step with a barrier of 63.9kJ?mol-1. The reaction mechanism of H2O2 dismutation catalyzed by five-coordinated [FeIII(tmtaa)Cl] complex is similar to the former one, but potential energy surface crossing exist in the whole reaction. The Hammond postulate and the intrinsic reaction coordinate calculations analyses used by Yoshizawa et al. have been used to locate crossing point CP1. The rate-determining step of the whole reaction is found to be the O-O bond breaking step with a barrier of 92.2 kJ?mol-1. Obviously, the activation energy of the title reaction for five-coordinated catalyst model is a little higher than that of four-coordinated catalyst model's, which indicate that the catalytic ability of five-coordinated complex is not as good as four-coordinated complex's. Compared with the 226.7 kJ?mol-1 energy barrier for O-O hemolytic cleavage of free H2O2, the activation energies for the former two is evidently lower, suggesting that four-coordinated and five-coordinated catalyst model is usable. But their catalytic ability is not as excellent as the plane one. The reason for this result is due to the change of plane feature to some extent, so we should take the planar, conjugated and azo macrocyclic complexs into account in the design of catalase mimic.