Photoreaction rates, enhancement

Kinetic data for the hydrophobic halocarbon, DDE, provide another example of a photoreaction rate enhancement attributable to sorption on NOM. The direct photoreaction rates of DDE, corrected for light attenuation effects, are increased in filtered natural water samples (0.2 pm) or a solution of a soil fulvic acid (Contech) with high DOC (Table II). As shown in Table II, the degree of rate enhancement is approximately equal to the enhancement in direct photoreaction rates in going from an aqueous to hydrocarbon solution (Table I) (38). 

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... 

Evidence is presented here that sorption to NOM enhances the photoreaction rates of halocarbons. Possible enhancement mechanisms are photoinduced electron transfer involving photoejected electrons or NOM excited states and/or formation of photoreactive complexes with NOM-associated electron donors, such as nitrogen bases. 

Negligible photoreaction was observed for p,p -dichlorobenzophenone (DCB), a DDT oxidation product, in air-saturated, distilled water (half-fife >15 h at 313 nm). Nevertheless, this halocarbon photoreacted (313 nm) with half-lives corrected for light attenuation of about 3 h in a filtered natural-water sample and a solution of Contech fulvic acid (Table II). The greater than four- to fivefold enhancement in photoreaction rate in this case probably results from hydrogen atom abstraction from the natural organic matter by the DCB in its excited triplet state (eq 14). 

The enhancement in photoreaction rates of DDE and dichlorobenzc phenone on sorption to dissolved organic matter (DOM) seem to be incor sistent with other reports that the fluorescence of aromatic hydrocarbons strongly quenched by sorption to DOM (30, 33, 34). After all, fluorescence like photoreaction, is an excited-state process, and fluorescence quenchin indicates that locally high concentrations of excited-state quenchers are pre ent in the NOM. However, most NOM is fluorescent itself, a condition ii dicating that the quenchers in NOM cannot quench all excited states efl ciently (41). Possible explanations for the difference in the effects of sorptic... 

Indirect Photoreactions. Enhancements of halocarbon photoreaction rates are not limited to compounds such as DDE and DCB, which directly absorb solar radiation. Such effects have also been observed with hah ocarbons that have little or no absorption of solar radiation. These reactions most likely occur through indirect mechanisms, in which the radiation is absorbed by natural chromophores. In this section, we provide evidence for such indirect photoreactions. Then, in subsequent sections, we discuss possibk mechanisms for indirect photoreactions of halocarbons. 

Photooxidafions are also iudustriaHy significant. A widely used treatment for removal of thiols from petroleum distillates is air iu the presence of sulfonated phthalocyanines (cobalt or vanadium complexes). Studies of this photoreaction (53) with the analogous ziuc phthalocyanine show a facile photooxidation of thiols, and the rate is enhanced further by cationic surfactants. For the perfume iudustry, rose oxide is produced iu low toimage quantifies by singlet oxygen oxidation of citroneUol (54). Rose bengal is the photosensitizer. 

If released to water, 1,3-DNB and 1,3,5-TNB may be subject to direct photolysis when exposed to sunlight because both compounds can absorb light at wavelengths greater than 290 nm (ERA 1976 Mill and Mabey 1985). However, no data were located regarding the photolysis of 1,3,5-TNB in water. The photolytic half-life of 1,3-DNB in pure water was calculated to be 23 days (Simmons and Zepp 1986). A three-to four-fold increase in the rate of photoreaction of 1,3-DNB was observed in ambient waters containing natural humic substances or in distilled water containing dissolved humic materials compared to reaction without humic substances (Simmons and Zepp 1986). This enhancement of the reaction rate has been attributed to catalysis of the photoreaction by photosensitization effects of humic substances. 

In view of the extensive delocalization of the unpaired electron, the radical ions of aromatic molecules are often relatively stable, the limitation to their lifetime being rather given by back electron transfer. They are therefore ideally suited for physical studies, just as it happens for excited states, where aromatic molecules are often chosen for photophysical experiments since they show little photochemistry. Thus, systematic studies on the rate of electron transfer have been carried out using aromatic molecules [4], and aromatic substrates are in use for enhancing the quantum yield of electron transfer photoreactions through secondary electron transfer (a typical example is biphenyl, BP, which by functioning as secondary donor slows down back electron transfer between the original radical ions and allows their chemistry to show up) [5]. 

By subtraction of the equal term from z rcr and sCs the two rates become smaller, but their ratio becomes larger and there should be a larger k. The authors searched for an enhancement of ee in the cpl-photoreaction of 2-phenylcyclohexa-none 27, 2-phenylcycloheptanone 28 and 2-phenylcyclooctanone 29, but found no magnetic field effect. 

CDx and in particular 7-CDx are known to accommodate two aromatic moieties under certain circumstances. Hence if two appropriate prochiral guest molecules are included in the same CDx cavity, regio- and enantioselective bimolecular photoreactions are expected to occur [108]. Indeed, it is known that the presence of CDx not only accelerates the rate but also modifies the product distribution of photocyclodimerizations of anthracene derivatives [109-111], coumarin derivatives [113-115], stilbene derivatives [116,117], stilbazole [118], and tranilast [119]. For instance, Tamaki and coworkers reported significantly enhanced quantum yields of photodimerization of anthracenesulfonates and anthracenecarboxyl-ates in the presence of 3- and 7-CDx [109-111]. These anthracene derivatives form 2 2 and 2 1 guest-host complexes with (3- and 7-CDx, respectively, and... 

Complexation of halocarbons with natural substances can enhance the rates of photoreactions that provide sinks. Ionizable halocarbons, such as hal-ogenated organic carboxylic acids, potentially could form complexes with pho-toreactive transition metals, such as iron. In addition, dissolved NOM and sediments are known to sorb or bind ionic and nonionic halocarbons, and sorbed halocarbons may photoreact more efficiently (eq 7). 

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