To investigate the mechanisms of this nonheme Fe-catalyzed decarboxylative azidation and the origin of enantioselectivity, we performed a combined density functional theory (DFT) and classical molecular dynamics (MD) study. Based on the proposed catalytic cycle (Scheme 1D) and previous mechanistic studies, we surmised that the enantioselectivity-determining step is the radical rebound involving the photoredox-generated prochiral secondary alkyl radical and the Fe(III) azide complex. We performed DFT calculations at the (U)B3LYP-D3/def2-TZVP/SMD(diethyl ether)//(U)B3LYP-D3/SDD(Fe)-6-31G (d) level of theory to study the radical rebound pathways with Fe(III)-N3 complex 14, in which the Fe center is coordinated to an acetate as a model for glutamate, two imidazoles (Im) as a model for histidines, a water molecule, and an azide anion. Our DFT calculations indicate that the radical rebound to 14 is highly kinetically facile and exergonic (Figure )-the radical addition to the internal nitrogen atom of the Fe(III)-N3 (TS1) requires 1.7 kcal/mol with respect to a van der Waals (vdw) complex ( ), whereas the radical addition to the terminal nitrogen atom of 14 is barrierless (see Figure for reaction coordinate computed from constrained geometry optimizations). The low barriers suggest that the radical rebound may occur at either the internal or terminal N atom of the nonheme Fe(III)-N3 in the enzyme active site, depending on the preferred binding poses of the radical. The DFT-optimized transition state structure TS1 features a long forming C-N bond distance (3.31 Å), indicating an early transition state that is consistent with the high exergonicity. An OH⋯π interaction 42 between the Fe-bound water and the Ph group on the benzylic radical was observed (3.22 Å) in TS1, implying such noncovalent interaction may affect the binding pose of the benzyl radical in the active site. We also computed the radical rebound transition states with a neutral Fe(III) diazide model complex, which require higher barriers for radical rebounds with both internal and terminal azide nitrogen atoms (Figure ). Next, we performed classical MD simulations to study the nearattack conformations (NACs) of the secondary alkyl radical 15 approaching the water-bound Fe(III) azide complex within the active site of PpMPC azidase (PpMPC I291L L155F F302Y) (Figure ).