Comparison with direct photolysis

Comparison with Direct Photolysis Process. The Ti02-mediated photocatalytic oxidation reaction involves a complex free-radical reaction mechanism in which OH radicals are responsible for the oxidation of 4-chlorophenol. The initial reaction step produces 4-chlorocatechol as the main product. In contrast, the direct photolysis of 4-chlorophenol produces a different set of reaction products. Figure 8 shows that the direct photolysis of... 

When irradiated with UV light in aqueous solution, hydrated ferric ions are photoreduced to ferrous ions with the production of hydroxyl radicals. Thus, the photolysis (>290nm) of aqueous solutions of atrazine, ametryn, prometryn, and prometon in the presence of ferric perchlorate or ferric sulfate was greatly enhanced in comparison to direct photolysis (Larson et al., 1991). In the absence of oxygen or in stream water, photoreaction rates were... 

Atrazine is successively transformed to 2,4,6-trihydroxy-l,3,5-triazine (Pelizzetti et al. 1990) by dealkylation of the alkylamine side chains and hydrolytic displacement of the ring chlorine and amino groups (Figure 1.3). A comparison has been made between direct photolysis and nitrate-mediated hydroxyl radical reactions (Torrents et al. 1997) the rates of the latter were much greater under the conditions of this experiment, and the major difference in the products was the absence of ring hydroxylation with loss of chloride. 

In order to determine the sensitivity of coumarin dimerization to heavy-atom perturbation, the direct photolysis was carried out in butyl chloride, propyl bromide, and ethyl iodide. Comparison of these results with those obtained in nonpolar solvents indicated that there was no significant change in the amount of the triplet-derived product (80) formed in the heavy-atom solvents as would be expected if heavy-atom perturbation were important.<90) More recent investigations,<93W however, of the formation of (79) in carbon tetrachloride indicate that halocarbon enhancement of the formation of this isomer exists. A direct measure of the intersystem quantum... 

Comparison of these experimental results with the calculated charge densities (S0 and Si) at the 2 and 3 positions (Table 11.5) shows that this is the expected result. Except for those compounds discussed below, the failure to observe quenching with triplet quenchers and reaction in the presence of a photosensitizer indicated singlet reactions. Compound (89) was found to also undergo benzophenone-photosensitized substitution, indicating that the triplet state of this compound is also reactive. The reaction, however, was less clean than that observed in the direct photolysis. Similarly, 1,6-dinitro-naphthalene was found to undergo both direct and benzophenone-photosensitized substitution ... 

In Table 15.7 the reaction quantum yields are given for some selected organic pollutants. As can be seen, reaction quantum yields vary over many orders of magnitude, with some compounds exhibiting very small Oir values. However, since the reaction rate is dependent on both ka and Oir (Eq. 15-34), a low reaction quantum yield does not necessarily mean that direct photolysis is not important for that compound. For example, the near-surface direct photolytic half-life of 4-nitrophenolate (Oir = 8.1 x 10 6) at 40°N latitude is estimated to be in the order of only a few hours, similar to the half-life of the neutral 4-nitrophenol, which exhibits a Oir more than 10 times larger (Lemaire et al., 1985). The reason for the similar half-lives is the much higher rate of light absorption of 4-nitrophenolate as compared to the neutral species, 4-nitrophenol (compare uv/vis spectra in Fig. 15.5 and Illustrative Example 15.3). As a second example, comparison of the near-surface photolytic half-lives (summer, 40°N... 

So far the methods described for measuring excited state lifetime, and hence reactivity, have been indirect methods that rely on a comparison with some standard le.g. actinometer quantum yield or quenching rate constant) that has already been measured. A direct method for measuring the lifetime of short-lived species produced photochemically is flash photolysis. This is a very important technique in photochemistry, though only the basic ideas as they apply to mechanistic studies are outlined here. In flash photolysis a high concentration of a short-lived species (electronically excited state or... 

Miranda MA, Perez-Prieto J, Font-Sanchis E, Scaiano JC (2000) Five-membered-ring 9-1-2 radicals direct detection and comparison with other hypervalent iodine radicals. Org Lett 1 1587-1589 Muiioz F, Schuchmann MN.OIbrich G, von Sonntag C (2000) Common intermediates in the OH-radi-cal-induced oxidation of cyanide and formamide. J Chem Soc Perkin Trans 2 655-659 Nagarajan V, Fessenden RW (1985) Flash photolysis of transient radicals. 1. X2" with X = Cl, Br, I, and SCN. J Phys Chem 89 2330-2335... 

For easier comparison the result of the thermal reaction is included for compounds 46 and 47. As can be seen in the reaction scheme above direct photolysis of the pyrazolines 46 and 47 proceeds mainly with retention of the original stereochemistry in the cyclopropanes isolated. 48,49 and 50 however lead mainly to the inverted stereochemistry in the cyclopropanes. The singlet biradical 51 formed from 46—49 is therefore clearly not on the same energy surface as a, . possible singlet diradical in the carbene cycloaddition. However one knows today that singlet carbene cycloaddition is a concerted process, so no such diradical should be detectable. 

HNCO at 248 nm have been investigated and, at this wavelength, the predominant reaction channel is direct formation of H atoms.Photolysis of DNCO at 193 and 148 nm has also been examined as a comparison with HNCO. Semiempirical (AMI) calculations have provided a potential-energy surface for dissociation of PhNCO. ... 

Photolysis of (26) in the presence of iodine and mercuric oxide gave a large amount of the expected aldehyde (27) and a low yield of the product (28) obtained by hydrogen abstraction from C-17. The structure of this product was established from its H and - C n.m.r. spectra. Direct comparison with (31), prepared by catalytic reduction of 0,N-diacetyljervine (29), showed that (28) and (31) were not identical, and so the oxygen at C-17 of (28) has the a-orientation. This configurational difference between (28) and (31) affects their reaction towards base, for whilst (31) suffers inversion of configuration at C-12, under basic conditions (28) does not. 

Figure 6.9 Comparison of calculated and observed half-lives for direct photolysis of several pesticides and pesticide derivatives quantum yields given in parentheses. 1. N-Nitrosoatrazine in water (4> = 0.30) 2. trifuralin in water (4> = 0.0020) 3. DDE analogue of methoxychlor, DMDE, in water (4> = 0.30) 4. DMDE in hexadecane (4> = 0.20) 5. DDE in hexadecane (4> = 0.26) 6. diphenylmercury in water (4> = 0.27) 7. phenylmercuric acetate in water (4> = 0.25) 8. 2,4-D-butoxyethyl ester in hexadecane (4> = 0.17) 9. carbaryl in water (4> = 0.0060) 10. 2,4-D-butoxyethyl ester in water (4> = 0.056). [Reproduced with permission from R. G. Zepp and D. M. Cline, Environ. Sci. Technol. 11, 359 (1977). Copyright 1977, American Chemical Society.]...

In comparison to water photolysis [1,2] very little research has been directed at the photoreduction of carbon dioxide and at the photooxidation of carbon monoxide. There are several reasons why chemists should be interested in these two processes. CO2 is a natural and abundant raw material it is a major atmospheric pollutant, involved in the greenhouse effect which may ultimately affect the climate and the temperature of our planet [3]. CO is used in many important industrial processes e.g. carbonylation, hydroformylation, Fisher-Tropsch reactions, and it is one of the major contaminants of industrial gases produced during catalytic processes (e.g., Haber-Bosch synthesis of NH3[4]). There are also fundamental reasons for studying CO2 and CO activation. The former is an inert molecule with carbon in its highest oxidation state and therefore its activation is difficult to achieve. Carbon dioxide could either be reduced to... 

It is noteworthy here that electrospray ionization mass spectrometry (ESI MS) is suitable for this case. To study reaction product structure, the preparative-scale photolysis is ordinary used. However, in this case the azide concentration in the solution is several orders of magnitude higher than that in the kinetic studies, and the composition of the products of the aromatic azide photolysis can change significantly with the change of the concentration [78]. ESI MS can analyze reaction mixtures at the reagents concentration of ca 10 M that enables direct comparison with the data of kinetic absorption spectroscopy. 

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