Photochemical kinetics principles

Kinetic models for UV/H202 were developed based on known chemical and photochemical principles by Glaze et al. (1992), who examined the oxidation of nitrobenzene, naphthalene, and pentachlorophenol to illustrate some features of the UV/H202 process. The model took into account the effects of... 

So, reaction (and kinetic) modeling of AOPs is a laborious and difficult task that needs experience not only in water analytical techniques but also in photochemical mechanisms and the underlying photophysical principles. However, sophisticated and comprehensive investigations such as those discussed above undoubtedly contribute to an increase in our knowledge of AOP mechanisms. 

From the overview given in Chapter 1 it is clear that many chemical principles are involved in the discussion of environmental phenomena. The next two chapters are devoted to several of these principles, namely chemical equilibria, acid—base reactions, redox reactions, complexes and complex formation, chemical kinetics, photochemical processes, and radioactivity. The first three are dealt with in this chapter and the rest in the next chapter. 

In principle, the application of time-resolved techniques permits identification of intermediates by monitoring their progress to the stable products of reaction. In 1973, Lehman and Berry [25] reported the first application of time-resolved photochemical methods to the study of aryl azides. Using conventional flash photolysis, they irradiated 2-azidobiphenyl in cyclohexane solution. Time-resolved absorption spectroscopy revealed an intermediate assigned as the triplet nitrene primarily on the basis of the similarity of its spectrum to that measured by Reiser [18] in low-temperature experiments. Lehman [25] monitored the rate of carbazole formation and found it to occur by a kinetically first-order process with a lifetime of 460 /is at room temperature. These findings led them to conclude that photolysis of 2-azidobiphenyl at room temperature leads rapidly to the triplet nitrene, and that this species is the precursor to carbazole [25], However, this point of view clearly is at odds with Swenton s triplet sensitization experiments [23],... 

In principle spectroscopic methods have turned out to be preferable in kinetic analysis. Nowadays a large variety of spectroscopic methods exist which allow the progress of the photochemical reaction to be monitored. However, none of them perfectly satisfies under all conditions the requirements stated above. Even though it is difficult to state a general procedure, some generalised ideas are discussed next to find arguments for the selection of certain analytical tools. 

Thus, any successful kinetic analysis requires an optimal choice of the spectroscopic detection method, whereby special methods (e.g. pre-separation by chromatography) or modem approaches (e.g. interferometry) must also be considered. Therefore in the following sections different approaches in spectroscopy are presented, some combined irradiation and measurement devices introduced, and different set-ups classified with respect to their principle of operation (sequential, multiplex, single and double beam equipment). In addition some special devices are presented which allow an automated examination even of complex photochemical reactions (in part superimposed by thermal reactions) at a highly sophisticated level using various combinations of modem equipment and supplying data for multicomponent analysis. 

In the case of photochemical reactions superimposed by fast thermal reactions during the time of measurement the irradiation is cut off and the irradiation time stopped. But the thermal reaction proceeds simultaneously and the overall reaction time will exceed the irradiation time. This principle of two time scales will increase problems with kinetic analysis. For this reason one tries to avoid interruption of the irradiation for such photochemical reactions. 

After derivation of the principles of kinetic examinations and especially the fundamentals of photokinetics in this final chapter a large number of examples have been discussed based on the equations derived in Chapters 2 and 3. These examples cover a wide field of types of photochemical reactions that take place in various applications. By use of different types of equipment, it was demonstrated how relevant data can be obtained during the reaction. This knowledge was applied to calculate reaction constants as partial photochemical quantum yields or at least the data for a turn over, if the spectroscopic or other characteristics of the compounds involved in the reaction are not known in detail. 

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