Decomposition, dependence photolytic

The extent of photolytic decomposition depends on the pH, nature of the solvent(s) used, and the initid drug concentration (27). Degradation is faster in diluted solutions and at higher pHs. The photolysis is characterized in terms of first-order kinetics. 

Chemical Properties. Diacyl peroxides (20) decompose when heated or photoly2ed (<300 mm). Although photolytic decompositions generally produce free radicals (198), thermal decompositions can produce nonradical and radical iatermediates, depending on diacyl peroxide stmcture. Symmetrical aUphatic diacyl peroxides of certain stmctures, ie, diacyl peroxides (20, = alkyl) without a-branches or with a mono-cx-methyl... 

Because of the possibility of focusing laser beams, tlrin films can be produced at precisely defined locations. Using a microscope train of lenses to focus a laser beam makes possible tire production of microregions suitable for application in computer chip production. The photolytic process produces islands of product nuclei, which act as preferential nucleation sites for further deposition, and tlrus to some unevenness in tire product film. This is because the subsuate is relatively cool, and therefore tire surface mobility of the deposited atoms is low. In pyrolytic decomposition, the region over which deposition occurs depends on the drermal conductivity of the substrate, being wider the lower the thermal conductivity. For example, the surface area of a deposit of silicon on silicon is nanower dran the deposition of silicon on silica, or on a surface-oxidized silicon sample, using the same beam geomeU y. 

ESR experiments employing in situ photolytic decomposition of the peroxydisulfate anion (S20g ) have been carried out to study the reaction of S04 with aliphatic sulfoxides. In the case of dimethyl sulfoxide three radicals are detected ( CHj, CH3 S02, CH2 S(0)CH3), the proportion being pH-dependent. The reaction is assumed to proceed via an initially formed radical cation (not detected) which would be rapidly hydrated to give an intermediate identical with that generated by OH addition on the sulfoxide. Such a process parallels the rapid hydration of radical cations formed from thiophene in their reactions with SO/ and... 

Shen et al. (1995) investigated the effect of light absorbance on the decomposition of aqueous chlorophenols (CPs) by UV/H202. The photoreaction system was batch annular photoreactors with 254-nm, low-pressure UV lamps at 25°C. The light absorbance and photolytic properties of chlorophenols and H202 were found to be highly dependent on the solution pH and can be adequately described with the linear summation of the light absorbance of undissociated and dissociated species of chlorophenols ... 

Wavelength and pressure dependence of photolytic and thermal unimolecular decomposition for the internal conversion yield (51,52,137,160). 

Depending on the mode of generation, a carbene may be initially formed in either the singlet or triplet state, irrespective of its stability. Common methods used for the generation of carbenes include photolytic, thermal, or metal catalyzed decomposition of diazocompounds, elimination of halogenfrom gem-dihalides, elimination of Hx from CHX3, decomposition of ketenes, thermolysis of a-halo-mercury compounds and cycloelimination of shelf stable substrates such as cyclopropanes, epoxides, aziridines and diazirines. 

The thermal decomposition of the exodihydrotriazoles gave exo- and/or endo-aziridines, depending on the substituents on the bicyclic framework, particularly at C-7, e.g., formation of 5 or 7, whereas by the photolytic procedure (300 nm) the configuration of the starting dihydro-triazole was retained, e.g., formation of 8100-103. 

Under their conditions more than 80% of the decomposition was effected by wavelengths between 170 and 140 nm. Absorption by ground state C2O was only observed after addition of CO to the photolysis mixture. This is presumably due to formation of C2O in a three-body recombination process. Through spectroscopic comparison of the yield of CO and the consumption of C3O2 at times shorter than collision times, Braun et al. found that 2 molecules of CO were formed for each C3O2 removed. This indicates that primary process 2 dominates process 1 in this wavelength interval. An upper limit of 25% is placed on the yield of C2O. The dependence of carbon atom yields on flash intensity suggest that C( P) and C( D) are formed in a primary photolytic process, while C( S) is the result of some secondary reaction. The relative yields of C( P), C( D) and C( S) were determined to be 1.00, 0.25, <0.025, respectively. 

Because oxidative decarboxylation of carboxylic acids by lead tetraacetate depends on the reaction conditions, the co-reagents, and the structures of the acids, a variety of products such as acetate esters, alkanes, alkenes, and alkyl hahdes can be obtained. Mixed lead(IV) carboxylates are involved as intermediates as a result of their thermal or photolytic decomposition decarboxylation occurs and alkyl radicals are formed. Oxidation of alkyl radicals by lead(IV) species gives carbocations a variety of products is then obtained from the intermediate alkyl radicals and the carbocations. Decarboxylation of primary and secondary acids usually affords acetate esters as the main products (Scheme 13.41) [63]. 

Methods which involve decomposition of diazonium fluoroborates are common, with the Balz-Schiemann photolytic method offering the best results usually (30-40% yields see Section 3.02.7.1.6) <84JOCi95l>, but again these approaches are entirely dependent upon the availability of the amino-imidazole precursors. 

It is possible, on the other hand, to interpret the temperature effect in terms of a mechanism similar to that suggested (16) for the photolytic decomposition of NH3. Thus, the dependence of the yield on temperature may be attributed to the competition between Reactions 19 and 22. In this case, if the direct molecular formation of H2 is neglected, the H2 yield is given by Equation I. 

A second example of photolytic mode II cleavage is the photolysis of 5 -deoxya-denosylcobalamin which leads cleanly to cob(II)amin and a 5 -deoxyadenosyl radical, which rapidly cyclizes to give 8,5 -cyclic adenosine [84]. In this case the rate of photolytic decomposition is not substantially affected by the presence of oxygen (presumably due to a much lower rate of recombination of the radical product with the cobalt(II) species) but the products are, of course, altered to aquocobalamin and a mixture of 8,5 -cyclic adenosine and adenosine-5 -carboxaldehyde [85], the ratio of the latter two being dependent on the oxygen concentration. 

Stratospheric ozone is produced naturally as a result of the photolytic decomposition of O2. The two oxygen atoms that result each react with another 02 molecule to produce two molecules of ozone. The overall process, therefore, converts three 02 molecules to two 03 molecules. The 03 molecules produced themselves react with other stratospheric molecules, both natural and anthropogenic the balance achieved between 03 produced and destroyed leads to a steady-state abundance of 03. The concentration of stratospheric 03 varies with altitude and latitude, depending on sunlight intensity, temperature, and stratospheric air movement. 

From Figure 6.8 we see that at the Earth s surface (288 K) the lifetime of PAN against thermal decomposition is about 3h, whereas that against photodissociation is about 13 days. Because the photolytic loss of PAN is approximately independent of altitude and the rate of thermal decomposition is strongly temperature dependent, a point is reached, at about 7 km, where the two rates become equal above that altitude, photolysis is the more important loss process. At the temperature of the upper troposphere, PAN is an effective reservoir for NO because PAN is transported in the upper troposphere, this amounts to a mechanism for long-range transport of NO. ... 

Polymerization is normally dependent on an initiator, which begins the chain reactions in the polymerization mixture. Typical initiators are molecules that have rather low thermal stability and generate radicals on decomposition. Other initiator systems depend on photolytic decomposition or on the generation of radical intermediates as the result of redox reactions. The rate constants for the individual propagation steps that follow initiation are usually very large, but polymerizations are normally carried out in such a way that the concentration of the reacting chains is very low (<10 M). As a result, the overall rate of polymerization is moderate. 

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