predominantly formed a PiceH3 -/ b PiceH3 -and HO2 • . At the bottom of the panels in Figure , PT3 (370.2, 388.2) and ET2 (11.7, 13.6) were endogenous; therefore, the sequential pathway was unlikely to proceed. Thus, the only feasible pathway is a concerted PCET involving one ET and two PTs in one-step, where intermediates are not generated. We refer to this pathway as the 2PCET reaction. Notably, 2PCET must occur in a one-kinetic process among an HB complex formed from the catechol moiety of PiceH4 to O2 The ΔG° values of the PCET pathways of PiceH4, RsvH3, and pinosylvin were calculated using B3LYP, M06-2X, and TPSSh functionals for comparison (Table ). The sum of the ΔG°s of the one ET and two PTs is an energetic driving force of PCET, although that cannot embody it if PCET occurred along a pathway involving an infeasible single PT/ET. Along the plausible sequential pathway based on the electrochemical results (Figure ), the ΔG°s of PT1 and ET2 for PiceH4 or RsvH3 were endergonic obtained using each functional. Thus, 2PCET was the only feasible pathway for each compound (Table ). However, the ΔG°s of the total values for pinosylvin (B3LYP: -0. inconsistent with the electrochemical results. Thus, the ΔG° values alone cannot explain the higher reactivities of PiceH4 than those of RsvH3 and pinosylvin toward electrogenerated O2 •- observed in the loss of reversibility (Figure ). The potential energy surfaces were scanned using the DFT-(U)B3LYP/PCM/6-311+G(d,p) combined with NBO calculations, to gain mechanistic insights into the PCET of O2 •-/HO2 • scavenging by PiceH4. Three elementary steps were assumed during the reaction in DMF: i) formation of a prereactive complex (PRC) via HBs from free reactants, ii) 2PCET via a TS forming a product complex (PC), and iii) PC dissociation yielding the free products. First, we performed structural optimization of feasible PRCs formed with a combination of free reactants, O2 •-, HO2 • , PiceH4, and PiceH3 -(step i). Among these, a PRC comprising PiceH4 and O2 •- (PiceH4-O2 •-)-formed via the catechol moiety-was more stabilized (71.8 kJ mol -1 ) by two HBs than the other PRCs. Next, the intermediate complex, PC, and TS, along with the subsequent PCET, were scanned (Supporting Information, Table ). In addition, some PRCs comprising PiceH4 and HO2 • via single HB at each OH (3OH, 5OH, 3'OH, and 4'OH) were scanned, potentially leading to subsequent reactions. Consequently, only a PRC formed between the catechol moiety of PiceH4 and O2 •-(PiceH4-O2 •-) could form a PC (PiceH2 •--H2O2) through the 2PCET. And, a TS (step ii) and an intrinsic reaction coordinate (IRC) were found for the 2PCET mechanism, corresponding to moving along the red curve in Figure . Figure (a)