Table 2.

(Activation/Reaction) Potential Energies, Enthalpies and Free Energies of the π(S_Fe^III = 5/2)-Controlled H-Atom Abstraction from the Native Substrate by the (SyrB2)Fe^IV═O and (TMG₃tren)Fe^IV═O Species; the Intrinsic Reaction Barriers Are Also Given (All Values Are in kcal mol⁻¹)

system	ΔE≠/ΔH≠/ΔG≠	ΔE°/ΔH°/ΔG°	ΔE≠intr/ΔH≠intr/ΔG≠intr
(SyrB2)FeIV═Oa	23.1c/18.9/19.8	3.9/0.6/2.3	21.1/18.6/18.6
(TMG3tren)FeIV═Ob	25.3/21.6/21.4	5.9/3.2/0.5	22.3/20.3/21.6

^aWithin the cluster model from Figure 9, the second-shell residue Arg₂₅₄ appears to sterically destabilize the transition state/product relative to the reactant by ~ 3–4 kcal mol⁻¹. Here, the DFT results are presented for the cluster model in the absence of this Arg residue. For the energetics of the Arg-including cluster model of the active site from Figure 9, see Table S1.

^bCalculated at the same level of theory as SyrB2 (described in Computational Details) but with a dielectric constant of ε = 35.7 mimicking the solvation effect of acetonitrile.

^cThe Fe–O bond length at the TS for HAA is ~ 1.8 Å (Figure S9). For this Fe–O bond distance, the excited d_xzπ* → d_z²σ* state (active in HAA) is calculated at the CASPT2 level of theory to lie ~ 10 000 cm⁻¹ (~ 28 kcal mol⁻¹) above the ground state minimum (Figure 8). This energy difference includes (i) the ground-state Fe–O distortion that is associated with the energy increase by ~ 5000 cm⁻¹ (~ 14 kcal mol⁻¹) and (ii) the excitation from the ground state to the d_xzπ* → d_z2σ* state that requires an additional amount of ~ 14 kcal/mol. The difference of ~ 28 kcal mol⁻¹ taken from Figure 8 relative to the DFT-calculated π-controlled HAA barrier of ~ 23 kcal mol⁻¹ (ΔE^‡ for comparison to PES calculations) reflects the interaction of the Fe^IV═O moiety with the substrate C–H bond.