As can be seen, the QM/MM calculations show that the mechanism of the C-H amination reaction by engineered P411 is essentially similar to the C-H oxidation mechanism by the native P450 enzyme. However, whether it is completely identical to the native P450 enzyme including the involvement of porphyrin radical cation and compound II type intermediate, is not clear from the energy profile. Therefore, we calculated the spin density of the species RC, IM, and PC in Figure and detailed electronic structures of RC. The calculations reveal in Figure two unpaired electrons at the antibonding π orbitals of Fe-N bond in RC which is also supported by the Spin Natural orbital calculations shown in Figure . This electronic structure of RC (iron nitrenoid) resembles compound I except for a radical cation at the porphyrin. 56 Using the spin densities as shown in Figure we further depicted the occupation of the key orbitals throughout the reaction pathway shown in Figure . In the H-abstraction step an electron, initially in a σCH orbital of substrate, shifts to the unoccupied high energy σ * z 2 orbital of the active oxidant and produces the intermediate IM. In this species, there are three identical-spin electrons (due to orbital delocalization only 2.8 according to population analysis), while one down-spin electron is localized at the benzylic C-atom of the substrate, with a small extent of delocalization to the phenyl ring (hence, population analysis shows -0.993). In the rebound step, the substrate formally donates its electron to the Fe atom resulting in the formation of the product molecule and the ferrous heme-porphyrin complex.