Photodecomposition processes

A UV analysis of the products formed upon photolysis of 2a at 280 nm in ethyl propionate, PMMA, and PPMA further illustrates the effect of the matrix stiffness on the photodecomposition process (Table III). The ratio 0C to b +, j [ c/( b+ d)) is determined by the ratio of absorbance of product 2c to the absorbances of products 2b and 2d [A2c/(A2b+A2d)1 n this case, since the results were tabulated from the actual absorption spectra (difference spectra), the ratio of the products formed in the solvent ethyl propionate can be directly compared to the ratios in PPMA and PMMA. From Table III, it is readily seen that the ratio increases on going from the ethyl propionate solution,... 

On the fundamental side, the research on photocatalysis has focused on several topics, including a) the primary processes involved in the production and trapping of photogenerated electrons and holes, using pulsed femtosecond or picosecond laser techniques, b) measurements on the kinetics of the photodecomposition processes on longer time scales, and c) measurements on the kinetics on small size scales. For the first topic, the reader is referred to several recent publications.69-7 This work is of great practical importance, because it helps to point out the critical factors involved in the photocatalytic materials themselves. 

Since the last reviews of carbonyl photochemistry (61,191), there have been several studies of the primary photodecomposition processes- The earlier studies have shown that there are two main primary photodissociation pathways, radical (I) and molecular (II) ... 

Photodecomposition. As reviewed by Cundall and Davies (61), both of these absorption systems have been studied, and product yields resulting from excitation from 435.8 to 238 nm have been reported (22,30,176). Since the last review by Cundall and Davies (61), most studies have dealt with the A X system, since both singlet and triplet emission have provided valuable tools in the exploration of biacetyl photochemistry (see ref. 185). Three photodecomposition processes have been proposed for biacetyl ... 

The product ratio provides information about the photodecomposition process. The yields are described by eq. 95 ... 

In order to estimate the rate of hydrogen production from the oxidation of methane by OH radicals it is convenient to assume that photodissociation is the dominant loss process for formaldehyde. From the photodissociation coefficients (see Fig. 2-19), one finds that the channel leading to H2 + CO as dissociation products contributes roughly 68% to the overall photodecomposition process. The rate of H2 formation thus is... 

Because of their close relevance to the carbohydrate investigations now to be considered, considerable attention has been given in the foregoing Sections to the photodecomposition processes which occur in water and alcohols. At the present time, there is emerging a consistent pattern which indicates that, when a carbohydrate is irradiated in aqueous solution, one or more of the following processes may operate (a) photolysis of water by radiation of wavelength lower than 2000 A., and subsequent attack on the carbohydrate by the free radicals formed (6) direct photolysis of the carbohydrate (c) sensitized decomposition of water to produce radicals which may attack the carbohydrate and (d) photosensitized decomposition of the carbohydrate by a hydrogen atom or by an electron-transfer mechanism. 

Acetone occupies a unique position in organic photochemistry. It has been one of the most intensively studied molecules for over fifty years, both in liquid solution and in the gas phase [1]. Yet, the details of its photodecomposition process, particularly of the isolated molecule, remain practically unknown. 

As shown schematically in Figure 3.47, the pollutant molecules in solution first have to be adsorbed into the carbon layer and then they have to diffuse to the photocatalyst particle surface. When the carbon layer adsorbs a large amount of pollutant, its accumulation near the surface is reasonably supposed to accelerate the photodecomposition process. Such is clearly the case for MB. When the pollutant molecules are scarcely adsorbed into the carbon layer, however, high photocatalytic activity cannot be expected for carbon-coated Ti02, as in the case of RB5. 

At very short wavelengths ( i=121.6nm) HN3 can be photolyzed via HN3-l-hv- NH(X S) + N2(B Ilg) [78]. The reactions of the electronically excited nitrogen molecules with HN3, however, will preclude this photodecomposition process, or the equivalent photoreaction of HNCO, being used as an NH(X) source. 

After each run of experiments it was also controlled that the results could be reproduced, indicating that they are not altered by photodecomposition processes. 

The type I process gives two free radicals, one polymeric and one small acyl radical. The polymeric radical can undergo a rearrangement known as )S-scis-sion which results in a break of a C—C bond in the backbone of the polymer and a consequent reduction in molecular weight. The type II reaction, however, is the major photodecomposition process that causes chains to break. In the presence of oxygen, both radical sites can induce photooxidation processes which cause chain degradation over a longer time scale. 

Pyromellitic-dianhydride based polyamide is the most photostable of the above group (4.69). The photolytic decomposition in air of polyimide films based on a dianhydride and a diarylamine with hexafluoroisopropylidene 6F bridging groups 4.71 and 4.72) is extremely rapid, leading to efficient chain cleavage and subsequent photo-oxidative decomposition. On the basis on the photo-oxidation of several model compounds and IR difference spectroscopy, it has been shown that the photodecomposition process occurs via decomposition of the arylimide linkage [996, 998] ... 

The primary photodecomposition processes in 2-methylpropanal follow the pattern of the smaller aldehydes. Both radical generation and fragmentation to form stable... 

IX-B-6.2. Quantum Fields of the Primary Photodecomposition Processes in Pentanal... 

IX-B-8.2. Quantum Yields of Primary Photodecomposition Processes in 3-Methylbutanal... 

By analogy with the photochemical behavior of the smaller aldehydes, one expects the following primary photodecomposition processes to occur ... 

Figure IX-D-7. Plot of the estimated photolysis frequencies for acetone photodecomposition for a cloudless day within the lower troposphere at 760Torr and 298 K. The black solid and dashed curves represent the maximum and minimum / -values, respectively, for the total photodecomposition [process (I) 4- process (II)] as estimated from the quantum yield data of table IX-D-2. The gray curve is the estimated maximum value for photodissociation by process (II) in the lower troposphere as estimated using the maximum quantum yield given in table IX-D-3. If the more probable, minimum estimates for lower tropospheric pressures are used in the calculation, the / -values for process (II) in acetone are very near zero.

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