Figure 7.

(A) Representative scheme of tunneling (green arrow), resulting in a lower activation energy than needed to reach the transition state.¹ (B) DFT-calculated structure of F₈Cmpd-II-(LutH⁺)³⁸ showing the steric encumbrance by the lutidine molecule. (C) For HAA by iron(IV) oxo complexes, the substrate can approach at either a 180° or a 120° angle with respect to the Fe=O bond (as previously suggested by Shaik and co-workers);⁶⁶ these approaches are consistent with the sp²-hybridized oxo ligand.⁷⁸ However, in the present system with the lutidinium present, substrates can only approach at a 120° angle.