Photodegradation, establishment

Photodegradation of DDT by the protease-liberated flavo-protein from TX-20 resulted in the formation of TDE as the major product in addition to three other minor compounds. It has been well established that DDT conversion to TDE, anaerobically, is a reductive process involving replacement of a chlorine atom by hydrogen. On the other hand, it has been suggested that photo-lytic reactions involve a charge transfer from an amine to DDT and a subsequent pickup of a proton. Thus there is a possibility that the photochemical reaction involving flavoproteins undergoes a similar reaction scheme. Much more data are, however, needed to confirm this point. 

Just as with organic combustion aerosols, the chemical and physical nature of inorganic solid substrates can have a dramatic impact on the photoreactivity of adsorbed PAH. In 1980, Korfmacher and co-workers reported that BaP, pyrene, and anthracene all pho-tolyzed efficiently in liquid solution but were resistant to photodegradation when adsorbed on coal fly ash. Subsequent studies confirmed this observation and revealed that the carbon content of the ash (and the associated darkening of color) is a key factor in establishing the photostability of these PAHs. Indeed, they were stabilized at relatively small percentages of carbon, e.g., 5% or less (Behymer and Hites, 1985, 1988 Yokley et al., 1986 Dunstan et al., 1989 Miller et al., 1990). 

Current information on the photochemical dissipation of the triazine herbicides in the atmosphere is very limited. No studies concerning the vapor-phase photolysis of these herbicides have been reported, and only two studies have investigated the phototransformation of triazine herbicides when associated with atmospheric aerosols. Photodegradation of atrazine and terbuthylazine was observed in these studies, but the significance of photodegradation in the dissipation of atmospheric concentrations of these herbicides has yet to be established. 

The lack of basic understanding and the prevalence of conflicting results in the area of photodegradation of poly(vinyl chloride) is mostly due to the deficiency in fundamental information about the sequence of events that takes place after the polymer molecule absorbs a photon. The use of PVC samples containing foreign unknown substances and the attribution of anomalies and discrepancies in experimental results to impurities further created considerable confusion in this subject. At the present stage of development in the field of photodegradation of PVC, it is critical to establish for pure PVC the identities and dynamics of the intermediates involved. 

In summary, our study of the photodegradation of poly(vinyl chloride) have established the following radical intermediates ... 

The TREPR experiments and simulations described here have provided an enormous amount of structural and dynamic information about a class of free radicals that were not reported in the hterature prior to our first paper on this topic in 2000. Magnetic parameters for many main-chain acrylic radicals have been established, and interesting solvent effects have been observed such as spin relaxation rates and the novel pH dependence of the polyacid radical spectra. It is fair to conclude from these studies that the photodegradation mechanism of acrylic polymers is general, proceeding through Norrish 1 a-cleavage of the ester (or acid) side chain. Recently, model systems have... 

Clearly, the quantum yield is a constant only if n = 0 and m = 1. Work by Emeline et al. (1998a, 2000c) has established that the reaction orders m and n can be interdependent. That is, the reaction order m varies with reagent concentration [M] and the order n depends on photon flow p. Eor example, for the photodegradation of phenol over TiOi and the photostimulated adsorption of oxygen on ZrOi, m 1 if n O, whereas n 1 if m 0. Results of kinetic studies (see Fig. 5.17) of both photostimulated processes have shown that the interdependence of the reaction rate can be generalised by eq. 5.91, which is identical with eq. 5.65. 

To establish the degradation profile of a new drug, forced or stressed conditions such as hydrolysis, oxidation, photodegradation, and solid thermal decomposition are applied to the drug to generate degradation products that may be monitored by TLC or HPLC. 

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