The formation and dissociation of the HB complex is an important process in concerted PCET, and it has a critical effect on its efficiency. Then, the authors also showed that the efficiency of 2PCET characterized by Q-H2Q π-conjugation is based on quantum tunneling. Although there are differences between the ortho and para isomers, both H2Q and catechol react with O2 •-via 2PCET (Equation ( )) through the same elementary steps (i)-(iii) (Equations ( )-( )). Whereas one catechol molecule and one O2 •-molecule forms 1:1 PRC, the formation of HB between two OH groups of H2Q is thought to lead to the formation of an extensive HB network consisting of repeating units composed of multiple H2Q and O2 •-. Since it is impossible to model this extensive HB network and analyze the 2PCET using computational methods, experimental observation of the three elementary steps is important for gaining deeper insight into the 2PCET.