Oxygen photolytic process

Distilled rather than natural water is often used as the solvent for determination of quantum yields for two major reasons. First, the total absorbance of the solution at the wavelength of irradiation should not exceed 0.02. Second, and more important, the presence of natural water constituents (e.g., humic material, nitrate) could enhance the total photolytic transformation rate by indirect photolytic processes as described in Chapter 16. Zepp and Baughman (1978) have argued that for many chemicals d>,r obtained in distilled water is nearly the same as that observed in natural waters (at least in uncontaminated freshwaters), because concentrations of natural water constituents that could undergo reactions with or quench photolysis of excited pollutants are generally very low. Furthermore, the effects of molecular oxygen, which may act as a quencher, can also be studied in distilled water. 

Such information can be found in the work of Jaffe and Klein on the photolysis of NO2 in the presence of SO2. They measured the quantum yield of nitrogen dioxide decomposition by in situ NO2 absorptiometry. In the absence of SOj the quantum yield is 2, since each atom of oxygen formed in the primary photolytic process can react with another molecule of NO2... 

As the absorption spectrum, absorption cross sections, and potential curves has already been described in Chap. 3 (Sect. 3.2, Fig. 3.4, 3.5, Table 3.2), only photolytic processes will be discussed here. In the photolysis of O2, three different electronic states of oxygen atoms can be formed as follows. 

Chapman (1930a, b) showed successfully that the characteristics of the height of the ozone layer and ozone density in the earth s atmosphere can be described by assuming only oxygen is present as a reactive species in the atmosphere, and this reaction scheme is called pure oxygen theory or Chapman mechanism. Photolysis of O2 occurs by the solar radiation of wavelength shorter than 242 nm (see Sect. 4.3.1). Only the photolytic process. 

The Ic pattern for carbazole synthesis arises primarily in the dehydrogenative cyclization of diphenylamines. This cyclization can be accomplished photolytically, with an oxygen sensitive intermediate having been detected (equation 58) (81JA6889). Preparative work with substituted diphenylamines has resulted in rather variable yields, depending upon particular substituents. This variability may be the result of alteration in the dominant photochemical process with substituent changes. 

There is currently an international phaseout of the production and use of bromo- and chlorofluorocarbons because of the ability of photolytically produced chlorine and bromine to catalyze the conversion of ozone to oxygen through cyclic processes such as are shown in equations 22-24, in which X = Cl or Br95. This has led to some degree of depletion of the stratospheric ozone layer. 

When an oxidation reaction involves molecular oxygen, the reaction occurs spontaneously under mild conditions. It is known as autooxidation. In an autooxidation process, free radicals, formed by thermal or photolytic cleavage of chemical bonds (e.g., peroxide, ROOH) or redox processes with metal ions present in raw material impurities, are involved... 

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