Catalytic oxidation of this molecule occurs via coupling of two radicals in the I2M mechanism. The oxidation of this molecule can be catalyzed by two bi·pyridyl·ruthenium complexes linked by a bridging oxygen atom, a complex called the “blue dimer.” In an enzymatic mechanism for the oxidation of this molecule, a tyrosine residue denoted Y-sub-z donates an electron to transition the S0 state to the S1 state. Low-temperature decomposition of this molecule can be performed by alkaline or (*) PEM (P-E-M) electrolyzers. This molecule reacts with methane to produce syngas, then high-temperature shift reactors use a reaction partly named for this molecule to produce more (10[1])hydrogen. For 10 points, name this molecule which is synthesized in fuel cells and split to produce oxygen in photosynthesis. ■END■