These changes in HOMO-LUMO energies occur during the PT between RsvH3 and O2 •-, forming RsvH2 -and HO2 • . After the subsequent ET, the HOMO-LUMO relationship between the products (i.e., a RsvH2 • / b RsvH2 • and HO2 -) is reversed and the orbital energy changes along the red dotted lines in Figure . However, as the PT-forming H2O2 has a lower HOMO (-0.27731) than that of HO2 -(-0.16694), the reverse ET cannot proceed, so the ET direction is dominantly determined by the subsequent PT. Judging from the HOMO-LUMO relationship for O2 •-scavenging by RsvH3, the net PCET involves two PTs and one ET, resulting in the formation of H2O2. For a thermodynamic analysis of the mechanism of the O2 •-scavenging by RsvH3 in DMF, standard Gibbs free energy changes (ΔG°/kJ mol -1 , 298.15 K) along the PCET were calculated using the DFT-(U)B3LYP/PCM/6-311+G(d,p) level (Supporting Information, Table ). Figure shows the equilibrium schemes and ΔG°s of the six diabatic electronic states in the PCET involving two PTs and one ET between RsvH3 and O2 •-. Along the sequential pathway in Figure , the main drivers are the ΔG°s of the individual reactions, the acid-base interaction (PT1-PT4), and the redox potentials (ET1-ET3) of the components. Since ET1 (356.5) is strongly endergonic, PT1 (30.6, 38.8) dominantly forms a RsvH2 -/ b RsvH2 -and HO2 • . In the following pathway (bottom part of the panels in Figure ), both PT3 (317.3, 309.1) and ET2 (15.5, 32.1) are endergonic, so the sequential PCET is unlikely to proceed. Efficient O2 •-scavenging then requires a concerted PCET or HAT reaction involving one ET and one PT, corresponding to moving along the diagonal of the lower panels in Figure (red arrows). Otherwise, another feasible pathway is one-step one ET concerted with sequential two-PT without generating highenergy intermediates. We refer to this pathway as the concerted two-proton-coupled electron transfer (2PCET) reaction. Notably, PT4 is exergonic (-45.8, -70.6), meaning that the second PT must be coupled to ET via a HB. Considering together that 4'OH in the para-position plays a significant role in resonance for a successful ET, the second PT coupled to ET feasibly occur in 4'OH. For a comparative study, the ΔG° values of the PCET pathways of RsvH3, pinosylvin, pterostilbene, and p-coumaric acid, were calculated using three functionals, B3LYP, ω-B97XD, and M06-2X (Table ). From a thermodynamic viewpoint, the energetic driving force of the PCET is the sum of the ΔG°s of the two PTs and one ET, although the sum cannot embody the energetic driving force if the PCET occurs along a pathway involving an infeasible single PT/ET.