To clarify the oxidative degradation through the PCET mechanism, potential energy surfaces were investigated. After the initial PT forming T-705 -and HO2 • as free reactants (FR), it is assumed that the reaction involves three elementary steps: i) formation of the pre-reactive complex (PRC) from the FR, ii) reaction to the product complex (PC) via a transition state (TS), and iii) dissociation of the PC yielding free products. Furthermore, the structural and electronic changes during the reaction were analyzed with the natural bond orbital (NBO) calculations at the DFT-(U)B3LYP/PCM/6-311+G(3df,2p) level of theory in DMF. We started with an analysis of potential energy scanning for the stable PRC (T-705 --HO2 • ) and PC (-2H-e-H2O2), then optimized structures of plausible PRC and PC with the TS were obtained. In Figure For a comparative study, the ΔG° and the Ea of T-1105 and catechol along the PCET were also calculated (Table ). The ΔG°s for the concerted PCET between HO2 • and deprotonated anion, To clarify the oxidative degradation mechanism of the pyrazine antiviral drugs by O2 •-through the thermodynamically unfeasible PCET, it is assumed that i) the PCET proceeds thorough the concerted manner with superior kinetics apart from the thermodynamic energetics of the chemical species, ii) the radical product of the PCET (-2H b -e) irreversibly and quickly decomposes. Regarding ii) the degradation of -2H b -e, the electrochemical and calculation analyses do not clarify its mechanism; however, since the radical product unambiguously is undetectable using in situ electrolytic UV-Vis and ESR system, -2H b -e would quickly be decomposed. Thus, since the irreversible degradation of -2H b -e occurs following the PCET, the square equilibrium of the PCET shown in Figure will be promoted by the subsequent degradation.