| Abstract
| - The hybrid density functional (DFT) method B3LYP was used to study the mechanism of the methanehydroxylation reaction catalyzed by a non-heme diiron enzyme, methane monooxygenase (MMO). The keyreactive compound Q of MMO was modeled by (NH2)(H2O)Fe(μ-O)2(η2-HCOO)2Fe(NH2)(H2O), I. The reactionis shown to take place via a bound-radical mechanism and an intricate change of the electronic structure of theFe core is associated with the reaction process. Starting with I, which has a diamond-core structure with twoFeIV atoms, L4FeIV(μ-O)2FeIVL4, the reaction with methane goes over the rate-determining H-abstractiontransition state III to reach a bound-radical intermediate IV, L4FeIV(μ-O)(μ-OH(···CH3))FeIIIL4, which has abridged hydroxyl ligand interacting weakly with a methyl radical and is in an FeIII−FeIV mixed valence state.This short-lived intermediate IV easily rearranges intramolecularly through a low barrier at transition state Vfor addition of the methyl radical to the hydroxyl ligand to give the methanol complex VI, L4FeIII(OHCH3)(μ-O)FeIIIL4, which has an FeIII−FeIII core. The barrier of the rate-determining step, methane H-abstraction,was calculated to be 19 kcal/mol. The overall CH4 oxidation reaction to form the methanol complex, I + CH4→ VI, was found to be exothermic by 39 kcal/mol.
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