We derived the isotope effects of 1 O 2 formation and subsequent oxygenation or electron transfer reactions from the isotopic expression of eq 6 as documented in Section S1 of the Supporting Information. To that end, we consider the ratio of O 2 isotopologue disappearance for the two most abundant isotopologues, O 16 O and 18 O 16 O, to obtain the apparent Okinetic isotope effect, O-AKIE, in eq 10 from the ratio of the apparent rate constants k obs and 18 k obs .