Rebound reactions where the substrate radical rebounds to the hydroxo ligand are barrierless for larger halides, i.e., chloride and bromide but have a barrier of ca. 3 kcal/mol for the smaller fluoride intermediates (Figure ). Rebound reactions where the substrate radical rebounds to the halide are barrierless for bromide and exhibit a minor barrier of ca. 1 kcal/mol for chloride (Figure ). However, a significant barrier of 6.5 kcal/mol is observed for fluoride intermediates, suggesting that selective C-H fluorination could be challenging (Figure ). Consistent with the Hammond-Leffler postulate 105, 106 , the strong exothermicity corresponds to transition states that are much more similar to reactants than to the product rebound intermediate (Figure ). The moderate to significant barriers for fluoride intermediates suggest that the rebound reaction could make C-H fluorination difficult in comparison to hydroxylation, consistent with prior work on SadX. Further examination of rebound RCs reveals that the OH-rebound intermediates are energetically more favorable than halide-rebound intermediates for fluoride, chloride, and bromide intermediates, highlighting a greater thermodynamic driving force for hydroxylation (Figure ).