During QM/MM geometry optimizations, the QM region was computed using the hybrid UB3LYP functional with the def2-SVP basis set. All of the QM/MM transition states (TSs) were located by relaxed potential energy surface (PES) scans followed by full TS optimizations using the P-RFO optimizer implemented in the HDLC code. The results were further validated with single-point calculations at a higher basis set, def2-TZVP. All calculations were performed for the doublet and sextet states since recent studies showed that the pristine oxidant HM1 displays two-state reactivity (doublet and sextet spins) in gas phase .