Although isomerization may occur in the Fe(IV)=O intermediate, positioning the reactive moiety away from the substrate at the cost of more sluggish but selective halogenation, it could alternately occur after hydrogen abstraction but before the radical rebound step. We thus also explore a similar isomerization RC that connects axial OH and equatorial OH where halide remains equatorial and succinate remains monodentate (Supporting Information Figure ). This RC is captured by the change in NHis-Fe-OH angle with the His trans to the axial hydroxo (Supporting Information Figure ). Consistent with oxo isomerization RCs, the hydroxo isomerization RCs are also comparable across all halides (Supporting Information Figure ). However, while an axial oxo isomer is favored along the oxo isomerization RC, here we observe that a hydroxo intermediate where OH tilts away from the axial position (NHis-Fe-OH = 154°) is favored for all halides (Figure and Supporting Information Figure ). Comparable oxo and hydroxo isomerization energy landscapes suggest that the preferred isomers before and after hydrogen atom abstraction are consistent for all halides. This indicates that the difficulty with C-H fluorination could stem from differences in halide isomerization that distinguish the behavior of fluoride and chloride/bromide intermediates.