stabilizes IM1 and appears to blocks the rebound step (cf. Figure ). The second step (IM1→IM2), is a single electron transfer (SET) process wherein an electron from the carboxylate group of the substrate shifts to Fe (IV)OH and forms Fe (III)OH -. Concomitantly, CO2 splits and generates a terminal double bond across Cα-Cβ of the substrate (cf. Figs and). The energy barrier for this step is 12.4 kcal/mol relative to IM1 which is lower than the calculated rebound barrier at •CβH. In the final step, the protonated His85 relays a proton to the negatively charged IM2 via W1, thereby forming the aqua ferric low spin resting state and resets the catalytic cycle for the next turnover. The electronic structures of the intermediates in Figure were confirmed by the spin densities and Mulliken charge analysis of intermediates (See SI). Interestingly, the intermediate IM1 i.e., Fe (IV)-OH/substrate-radical is exothermic by ~5 kcal/mol relative to Cpd I/substrate (cf.