Photo-transformation

The initially formed ]5-dicarbonyl compounds are subject to further photo-transformations. One example is provided in the case of epoxy ketone (88), where the resulting /5-diketone (89) undergoes partly a-cleavage and acyl-alkyl... 

Photo transformations in films of copolymers of a-(2-hydroxy-4-metha-cryloyloxyphenyl)—(2,6-dimethylphenyl)nitrone (HMDN) with methyl methacrylate (MMA) showed that, on radiation at 366 nm, an increase of the steric effect in the A-aryl group leads to a decrease of the refractive index (468, 469). 

Clearly more data are needed to characterize the naphthopyran mero-form photochemistry, as it is not even unambiguously proven that single-photon excitation can bring about the full photo-transformation from the TT isomer to the TC... 

Spiropyran and the related compounds all undergo interesting photo-transformation in the crystalline state. There are examples of all these compounds undergoing both multi-photon- and single-photon-induced transformations in the crystalline state [77,106-110]. 

In the case of the single-photon transformation, this is not a general phenomenon and few examples exist. Recently, the crystalline-state photo-transformation of spiropyran [107], naphthopyran [77], and spiro-oxazine [106] chemical systems have been reported. None of these studies, however, give information about transient states except that the study on naphthopyran system described a longer-... 

Observation of the spectral evolution with time was revealing. A species with an absorption maximum around 650 nm formed in times of 10 nsec. This transient was similar to that which formed in 1-butanol within 2 psec after femtosecond excitation [26], but thereafter rapidly evolved to its final colored form. In the crystalline state, however, this transient clearly had a longer lifetime. This transient absorption change was assigned to a nonplanar photo-product transforming to a metastable merocyanine in both the solution study and the crystalline-state study. The main difference between photolysis in the crystal as compared to that in solution is that the time scales for photo-transformation are drastically increased. 

Quantum Yields for the Forward and Reverse Photo-Transformations of XIa... 

Thus, the photo transformation of peroxy radicals is accompanied by the formation of the same products as in the case of their thermal decomposition. This means that the main channel of their photo transformation is their photo dissociation accompanied by the elimination of oxygen atom ... 

One of the reasons for that is the high stability of the O-H bond in the newly formed silanol group (125-130 kcal/mol), and H-r can be not only a hydrocarbon molecule, but also H-OH, H-NH2, etc. The diamagnetic dioxasily-rane groups are also the generators of alkyl radicals (see subsection 4.2). Many intermediates can be obtained as the products of thermal or thermo oxidative transformations or photo transformations of other initial structures. For example, vinoxyl radicals were obtained by the photolysis of peroxide radicals of the vinyl type [119] ... 

Field studies, together with the compilation of chemical data, are needed in order to formulate adequate models for pollutant cycles. Such models would allow the evaluation of the transport of pollutants through air and water to regions far from the original emission sources and their possible (photo)transformations in different compartments, resulting either in less... 

As indicated in Fig. 1, light may induce the reduction of Fe(III) present as particulate iron oxides and oxyhydroxides (Processes 13 and 14). The resulting Fe(II) species could induce the formation of a mixed valence oxide at the particle surface or, more hkely, be released to solution resulting in dissolution of the solid phase. Whether the ferrous iron remains in solution or oxidises and reprecipitates as a ferric oxyhydroxide will be dependent particularly upon solution pH. If other species are adsorbed to the oxide surface, they may also undergo redox transformation as a result of reduction of the metal centre. Such photo-transformations are described in this section. 

V, = 0.6 h (summer), ty, = 1.0 h (winter) in poisoned estuarine water based on photo-transformation rate and V, = 6 d (summer), ty, = 14 d (winter) in poisoned estuarine water based on photomineralization rate (Hwang et al. 1986) ... 

C, = 1.5 to 3.0 d for direct photo-transformation from outdoor ponds (Crossland Wolff 1985) ... 

Since only absorbed light can initiate photo transformations, it may be expected that samples of different optical properties show different photoproduction rates. For comparative purposes, the rates are thus usually absorbance-normafized. However, the inconsistency when comparing results from different studies is not resolved by such normalization and the variabifity of normafized rates exceeds an order of magnitude (Table 10.1). An attempt to find a correlation between available bulk characteristics (DON, DOC, pH, absorbance) and irradiation effects proved unsuccessful (Grzybowski, 2003). Additional confusion is introduced by reports on lack of ammonium release and even its removal during irradiation, observed in apparently similar samples (Table 10.1). 

Photolysis of various chlorophenols as a function of time are shown in Fig. 1. The photo-transformation and photo-mineralization of the compounds followed a first-order equation In (Cg /C) kpt, where Cg and C are the concentrations of the compound at time zero and time t, while kp is the first-order photolysis rate constant. The photolysis half-lives of the compounds were calculated using the equation ti/2 = 0.693/kp. The relative rates of photolysis in estuarine water decreased in tne order 2,4,5-trichlorophenol, 2,4-dichlorophenol, pentachlorophenol, p-chlorophenol, phenol (Table 1). 

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