Exchange-Enhanced H-Abstraction Reactivity of High-Valent Nonheme Iron(IV)-Oxo from Coupled Cluster and Density Functional Theories

This letter reports a computational study of methane hydrogen abstraction (HAT) process by several nonheme high-valent iron(IV)-oxo model oxidants, in the quintet and triplet spin states, employing coupled-cluster (RCCSD(T)) and density functional theory (DFT) methods. The exchange-enhanced HAT reactivity of the quintet state found previously by B3LYP calculations is confirmed in this work by means of the RCCSD(T) calculations. In addition, we benchmark herein 29 functionals against the RCCSD(T) results. It is found that B3LYP gives very good triplet-state transition state (TS) geometries, while its quintet-state TS is early relative to RCCSD(T). TPSSh and M06L perform well in TS geometry optimization. However, all 29 tested DFT methods underestimate the quintet-state HAT barriers, and most of them underestimate the triplet-state HAT barriers as well. Although B3LYP, B3LYP-D, and TPSSh underestimate both quintet- and triplet-state barriers, their barriers, and quintet-triplet differences thereof, in a series of reactions correlate extremely well with the corresponding RCCSD(T) quantities