A three states model for hydrogen abstraction reactions with the cytochrome P450 compound I is revisited. Isolobal and isospin analogy among Fe(IV)=O, O = O and O

Radical abstraction reactions by the cytochrome P450 compound I are investigated on the basis of the one quartet two doublet model consisted of three configurations; ⁴{²[↑• ⁺Por]–[↑• ³Fe(IV)=O •↑]} (⁴CpdI; S = 3/2), ²{²[↓• ⁺Por]–[↑• ³Fe(IV)=O •↑]} (²CpdI_a; S = 1/2) and ²{¹[Por(↑↓)]–²[↑• Fe(V)=O]} (²CpdI_b; S = 1/2) where Por denotes porphyrin and axial cysteine anion (–SR) is neglected. The dπ-pπ bond of ²CpdI_b was highly spin-polarized in the transition-state region of hydrogen abstraction reactions, providing the doublet configuration; ²{¹[Por(↑↓)]–²[↑↑•• Fe(IV)–O •↓]} (²CpdI_b,oxyl). The oxyl-radical character of ²CpdI_b,oxyl plays an important role for hydrogen radical abstraction from alkanes. The spin populations on the iron atom at the transition structures by available density functional theory (DFT) results were about 2.0 for ²CpdI_b,oxyl, whereas they were about 1.0 for ⁴CpdI and ²CpdI_a. On the other hand, the spin density population on the carbon atom of alkyl radical were negative and positive in sign for ²CpdI_b,oxyl and ⁴CpdI, respectively, in consistent with their spin structures. In order to elucidate possible reaction pathways by the cytochrome P450 compound I, relative stabilities between ⁴CpdI and ²CpdI_b were examined on the basis of available DFT and beyond DFT results. The refined DFT results obtained by the beyond DFT elucidated one of the possible reaction pathways for hydrogen abstractions; ²CpdI_a → ²Cpd_b → ²CpdI_b,oxyl → ²(TS) → (rebound). Implications of the computational results are also discussed in relation to water oxidation in artificial catalysts.