As a result, the HOMO-LUMO relationship analysis suggests that the plausible PCET mechanism involves two PTs and one ET with essential dihedral rotation for successful ET eliminating HO2 • with good agreement with the electrochemical results. are endergonic, suggesting that the sequential PCET does not proceed contrary to the electrochemical results. However, the ET2 is promoted by the following PT4 (-14.0); the second PT. As shown in the HOMO-LUMO relationship (Figure ), the second PT is necessary for successful ET along the PCET. Thus, the proton and electron is transferred in one kinetic process corresponding to the diagonal of the rectangle in Figure , labeled concerted PCET (108.5), rather than a sequential pathway of ET2-PT4. (ET1-ET3), and concerted PCET were calculated using the DFT-(U)B3LYP/PCM/6-311+G(3df,2p) method.