Photoproducts analysis

Friesen, K. J., M. M. Foga, and M. D. Loewen. 1996. Aquatic photodegradation of polychlorinated dibenzofurans Rates and photoproduct analysis. Environmental Science and Technology 30 2504. 

The carbene thus reacts with O2 to form an orffio-benzoquinone O-oxide, and with an aliphatic alcohol as H-donor to form a phenoxyl radical (plus an aliphatic radical not shown in Scheme 1). The ground state triplet electronic configuration of this carbene accounts for its reaction behavior, in particular for the fact that it reacts very slowly with the solvent, H2O. In agreement with the intrinsically faster intersystem crossing of 2-bromophenol compared to 2-chlorophenol, the quantum yield of the carbene pathway was higher for the former = 0.04) than for the latter compound (< = 0.003). In contrast, the quantum yields of photo contraction were comparable (< = 0.04). The transient absorption data were confirmed by photoproduct analysis, showing the formation of phenol from 4-bromophenol in the presence of H-donors [16]. 

The mechanism of the aqueous photochemistry of 4-chlorophenol has been reviewed earlier [5,8]. Its basic features are the same as those of the carbene pathway described above for 2-bromophenol. The main differences are the fact that this is the only photolytic reaction of 4-chlorophenol and that its quantum yield is considerably higher than that of the 2-sub-stituted analogues

triplet carbene, 4-oxocyclohexa-2,5-dienylidene (A.max = 384 and 370 nm) from aqueous 4-chlorophenol (see Fig. 1) [20]. Photoproduct analysis yielded p-benzo-quinone (in the presence of O2), phenol (in the presence of an alcohol), hydroquinone and isomeric chlorodihydroxybiphenyls, which could all be accounted for by carbene reactions [20]. 

The last few years have seen the emergence of projects aiming at elucidating the photobehavior of monochlorophenols in heterogeneous systems. Two studies were concerned with the behavior of 4-chlorophenol in a surface-adsorbed state, the substrates being silicalite and solid /Tcyclodextrin [40], and cellulose and silica [41]. In both cases, nanosecond transient photolysis with diffuse reflectance detection and photoproduct analysis were the experimental techniques employed. The results of these investigations are instructive in demonstrating the influence of the solid support on the outcome of the photolytic reactions. 

Dichlorprop and 2,4-D are structurally related dichlorinated phenoxy-acetic acid pesticides (see Table 1). The photochemistry of 2,4-D has been studied some time ago 2,4-dichlorophenol was found as the the main product [75]. More recent photoproduct analysis studies are available for dichlorprop [76,77]. Climent et al. identified several products corresponding to dechlorination on the one hand, and to decarboxylation on the other ho-molytic bond cleavage steps were proposed to take place [76]. In a later study, Meunier et al. were able to demonstrate that 4-chloropyrocatechol is the major primary photoproduct [77]. A heterolytic cleavage of the ether bond and of the C - Cl bond was put forward as a possible mechanism (Scheme 9). 

Other hand, when an equimolar mixture of 2,5-DSP and l OEt is recrystallized from benzene, yellow crystals, comprising 2,5-DSP and l OEt in a molar ratio of 1 2, deposit. In the DSC curve of this crystal, a single endothermic peak is observed at 166°C, which is different from the melting point of either 2,5-DSP (223°C) or l OEt (156°C). Furthermore, the X-ray powder diffraction pattern of the crystal is quite different from those of the homocrystals 2,5-DSP and l OEt. Upon irradiation the cocrystal 2,5-DSP-l OEt affords a crystalline polymer (77i h = 1.0 dl g in trifluoroacetic acid). The nmr spectrum of the polymer coincides perfectly with that of a 1 2 mixture of poly-2,5-DSP and poly-1 OEt. In the dimer, only 2,5-DSP-dimer and l OEt-dimer are detected by hplc analysis, but the corresponding cross-dimer consisting of 2,5-DSP and l OEt is not detected at all (Hasegawa et al., 1993). These observations by nmr and hplc indicate that the photoproduct obtained from the cocrystal 2,5-DSP-l OEt is not a copolymer but a mixture of poly-2,5-DSP and poly-l OEt in the ratio 1 2. 

Akhtar et al. [20] have studied the identification of photoproducts of fohc acid and their degradation pathways in aqueous solution using preparative TLC. An aqueous solution of folic acid irradiated with UV at pH 2.4 to 10.0 for 6 h was subjected to TLC analysis, which gave separation of fohc add (Rj 0.67), p-woi-nobenzolyl-L-glutamic acid (Figure 10.12). The photolyzed solutions were... 

It is clear that one of the major limitations of this analysis is the assumption of constant excited-state coverage. Deviations from the behavior described by Eq. (45) in the low frequency range have been observed at photocurrent densities higher than 10 Acm [50]. These deviations are expected to be connected to excited-state diffusion profiles similar to those considered by Dryfe et al. [see Eq. (38)] [127]. A more general expression for IMPS responses is undoubtedly required for a better understanding of the dynamics involved in back electron transfer as well as separation of the photoproducts. 

LC-tandem MS was recently used for polymer/additive characterisation. In cases of soft ionisation processes (e.g. ESI, APCI, etc.), MS/MS is often necessary to confirm the ionic species. QITMS has the potential to improve the detection limits for organotin analysis compared to QMS. HPLC-UV and LC-API-MS/MS have been employed for the characterisation of the products of photodegradation of benzotriazole-based UV absorbers (Tinuvin P/328/900) under mild conditions [642]. Among the photoproducts identified... 

An interesting alternative mechanism of activation is the photochemical reduction of Pt(IV) to Pt(II) (Fig. 3). In addition to photoreduction, photosubstitution and photoisomerization can also occur, making the photochemistry of Pt complexes difficult to predict and a careful analysis of the photoproducts imperative (21). We have been involved particularly in the development of photochemotherapeutic agents based on Pt(IV) and the study of their photodecomposition and (subsequent) interactions with... 

A thorough theoretical analysis of the bleaching kinetics for DMA and DPA in several polymers, using transmittance at 365 nm to avoid any complications due to possible absorbance in the medium or photoproducts, is being published elsewhere (J.R. Sheats, J. Phys. Chem., 1989). The reactions known from solution phase studies (20) lead (13) to the following rate equation for anthracene (A) (same for oxygen) ... 

Powder X-ray diffraction has verified that the isomerization occurs via a crys-tal-to-crystal reaction process, and that the diffraction profiles of the crystals after photoirradiation consist of overlapped patterns of diffraction due to the crystals of 10 and 11. This indicates that the crystal domains of each isomer exist simultaneously in the crystals accompanied by crystal phase separation during the photoisomerization. Single crystal structure analysis has disclosed that the crystals of 11 as the photoproduct have a symmetry different from that of the starting crystals of 10 (Fig. 14). 

Presumably it arises by the trapping of an intermediate cationic species by chloride ion. The full details of the preparation of the stable dithiet derivative (48) have appeared (see Vol. 7, p. 134). Irradiation of the lanostadiene (49) yielded the photoproduct (50) whose structure has been confirmed by X-ray analysis. 

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