Extended hartree-fock theory of chemical reactions. VIII. hydroxylation reactions by P450

We have investigated the reaction pathways for the primary hydroxylation reaction of trimethylmethane by a high-valent Fe(IV)=O porphyrin π-cation radical species known as compound I at the B3LYP/CEP-31G level. The isoelectronic analogy of the Fe(IV)=O core of compound I to a molecular oxygen (O₂) has been successfully used to clarify the important roles of the singlet excited state of the Fe(IV)=O core in the alkane hydroxylation, which has hitherto been neglected. The reaction is initiated by the rate-determining hydrogen-atom abstraction from the substrate to give a discrete radical intermediate complex, in accordance with the conventional radical rebound mechanism. Similar to the chemistry of O₂, however, one of the singlet excited states, i.e., the diradical component of the ¹Δ state of the Fe(TV)=O core intercepts the triplet ground state (the ³Σ state) in the region of the transition state for the hydrogen abstraction. Our findings strongly indicate that the exchange polarization or intersystem crossing for the nonradiative transition to the locally singlet state is highly important to enhance the reactivity of compound I.