Quantum yields for production

Calculated assuming a quantum yield for production of the triplet state,... 

Table 2 Quantum yields for product formation from 33 and 43...

The quantum yield for product formation from the sensitizer triplet, 0, is given by the expression ... 

Photoinitiated free radical polymerization is a typical chain reaction. Oster and Nang (8) and Ledwith (9) have described the kinetics and the mechanisms for such photopolymerization reactions. The rate of polymerization depends on the intensity of incident light (/ ), the quantum yield for production of radicals ( ), the molar extinction coefficient of the initiator at the wavelength employed ( ), the initiator concentration [5], and the path length (/) of the light through the sample. Assuming the usual radical termination processes at steady state, the rate of photopolymerization is often approximated by... 

More recent work by Richoux193 showed that these efficiencies could be further increased, as the quantum yield for product ion formation on quenching of [ZnTMPyP]44 by MV2+ increased... 

Internal hydrogen-bonding in the biradicals from 17 and 18 increases overall quantum yields for product formation by impeding disproportionation. Likewise most polar solvents enhance quantum yields but lower the cyclization/cleavage ratio, presumably because H-bonding of solvent to the hydroxyl group increases steric hindrance to bond rotations. 

Klemm (16) and Lee and coworkers (17) have examined the effect of various solvents on the photochemistry of cyclobutanone. By monitoring the quantum yields for formation of ethylene (B-cleavage product) and cyclopropane (decarbonylation product) in different solvents, they were able to demonstrate a significant reduction in the quantum yields for product formation in methanol as compared to other hydrocarbon solvents. Whereas the quantum yield of ethylene formation was found to be essentially solvent insensitive, that for cyclopropane formation was found to be somewhat solvent sensitive. This suggested that B-cleavage and decarbonylation do not result from the same immediate precursor. Since ring-expansion derivatives have not been isolated from photolyses carried out in saturated hydrocarbon solvents, the importance of this process under these conditions remains to be determined. Irradiation of cyclobutanone in the presence of 1,3-penta-diene (17,59) or 1,3-cyclohexadiene (16) did not appear to affect the quantum yields for ketone disappearance or product appearance. 

A charge transfer complex is involved in the photochemical reaction between 4-cresol and tetranitromethane. Irradiation at 350 nm yields the o-nitrated product 235 ° . Other phenols such as phenol, 2- and 4-chlorophenol and 2- and 4-cresol behave in a similar manner and irradiation yields 2- and 4-nitrated products (236, 237) ° . The quantum yields for product formation are in the range 0.12-0.31. Only the formation of 3-nitrophenol from phenol is inhibited, as might be expected from attack at the 3-position, and shows a low quantum yield. It has been reported that 2-hydroxy- or 4-hydroxybiphenyl and 4,4 -dihydroxybiphenyl are the primary products formed from the photochemical reaction of biphenyl with sodium nitrate in aqueous methanol . Apparently the hydroxybiphenyls are prone to undergo photochemical nitration as a secondary process and yield the biphenyls 238 and 239 as well as 4,4 -dihydroxy-3,3 -dinitrobiphenyl, originally reported by Suzuki and coworkers under heterogeneous conditions. 

One reason for the rather low quantum yields for product formation from Z-1,3,5-hexatriene is thought to be that the ground state conformer distribution is heavily weighted in favor of the relatively unreactive fZf-conformer . Substituents and structural constraints have quite substantial effects on the conformer distribution, and hence on the quantum yields and distribution of products obsCTved. These effects are well understood, having been worked out independently by Havinga and coworkers, who examined the photochemistry of various 2-alkyl- and 2,5-dialkyl-l,3,5-hexatrienes (120 and 121, respectively) , and by Dauben and coworkCTs with their studies of the photochemistry of substituted 6,6,9,9-tetramethyl-A -hexalins (122). Both bodies of work were predicated on the effects of substituents at the 2- and/or 5-positions of the... 

Photoreduction of aliphatic ketones may involve both singlet and triplet excited states, but the quantum yield for product formation via singlet is usually low because other competing processes, such as radical pair recombination, are involved. The rapid intersystem crossing (ISC) in aryl ketones (Section 2.1.6) allows triplet reactivity. Ketones with n,7t lowest triplets, having an unpaired electron localized in an n-orbital on oxygen, are far more reactive than those with k.k lowest triplets.863... 

In general, the quantum yield for product formation is approximately one order of magnitude less than those of aryl ketones and aldehydes undergoing similar reactions. Nevertheless, the isolated chemical yields are reasonably high. For example, simple alkenes afford oxetanesin 30-50% yield. 

Compared to Ti02-sil-Fe(TPSPP), the quantum yield for product formation globally increases when TiOj-sil-FefTDCPP) is used as catalyst. Compared to unmodified TiOa, the salient feature of the hybride catalyst is confirmed to be the production of cyclohexanol in greater amount. 

Quantum yields for production of D2 on photolysis of Cr2+ in D20 solution are lower than those for H2 liberation from H20.38 From pulse radiolysis experiments Cohen and Meyerstein39 have determined the rate constant for reaction (10) (1.5 x 10 dm3 mol-1 s-1) and they have compared their results with those obtained in earlier photochemical studies of Cr2+. 

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