, resulting in a hemibonded structure . In the latter case, the hole localizes between two oxygen ions, and the doubly oxidized water dimer forms a true O-O bond (bond distance ∼ 1.5 Å), which subsequently leads to the formation of hydrogen peroxide (H 2 O 2 ) . These findings on the simplified model of the water dimer cation provide a foundation for understanding the potential observation of hemibonded structures in the electrochemical water oxidation. We recently have demonstrated that a substantial energy barrier hinders transformation into the proton-transferred structure . This implies that hemibonded structures may have a lifespan sufficient to undergo double oxidation.