To further distinguish across all three halide intermediates, we studied active site isomerization of monodentate Fe(IV)=O between equatorial and axial halide isomers and observed differences in equatorial halide stabilities and the halide-dependent NHis-Fe-X RC angle during isomerization, which could play a role in C-H halogenation. We also found that bidentate isomers have additional stability for fluoride relative to chloride/bromide intermediates, suggesting difficulty in isomerization between bidentate and monodentate isomers for fluoride, which could explain why C-H fluorination is not observed in non-heme iron halogenases. To understand the effect of size differences, we obtained isomerization RCs of chloride/bromide intermediates with Fe-Cl and Fe-Br bonds constrained to shorter bond lengths that revealed increased reaction barriers. Conversely, fluoride RCs where Fe-F bond was elongated resulted in slightly reduced barriers suggesting that targeted modifications in the enzyme active site to position residues nearby that can form halogen bonds with fluoride and elongate Fe-F bonds could make selective C-H fluorination possible.