From dioxygen to peroxide O2 chemistry

The most important initial reaction is (5.81), which we can also write in the generic form, where e means any kind of electron donor. The reduction potential of the donor must be smaller than -0.33 V in neutral solution or -0.125 V in acid solution (Fig. 5.13)  

The limiting step (in terms of reduction potentials Fig. 5.13) is the electron transfer to O2 from the photosensitizer. This means that an electron source adequate for the reduction of O2 will produce all the other reduced forms of dioxygen (OJ, HO2, HOOH, HO2, OH) via reduction, hydrolysis (or proteolysis) and disproportionation steps (Fig. 5.14). Thus, the most direct means to activate O2 is the addition of an electron (or hydrogen atom), which results in significant fluxes of several ROS. The idea that an electron transfer occurs in oxygen was proposed by Evans and Uri (1949) and was accepted by Leighton (1961) as a mechanism likely to be important for particulate matter  

The superoxide anion OJ is the proteolytic dissociation product of the weak acid HO2 and the perhydroxyl radical (pAT = 4.9). OJ is dominant in cloud and rainwater because of pH values normally around 4.5  

The uptake of HO2 from air (Schwartz 1984) is an important source of OJ besides photocatalytic formation (5.82). It should be considered that almost all gas phase reactions described in Chapter 5.3.2.1 also proceed in aqueous solutions, for example the H2O2 photolysis to OH radicals (5.30a) as first described by Urey et al. (1929) A threshold 379 nm. Another spontaneous radieal formation in alkaline solution was suggested (Schroeter 1963, Walling 1957)  

Finally, the following interesting pathway of OJ formation from photolysis of iron oxalate complexes was described by Zuo and Hoigne (1992)  

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