Photosensitized decomposition

Because of the convenience of the technique of photosensitization in inducing decompositions and other reactions a vast amount of work has been carried out in this field. It is outside the scope of the present section to deal with the many investigations that are concerned with the nature of the energy-transfer processes in photosensitization reference is made to a review by Cvetanovic , and to a more recent theoretical discussion by Yang . Cvetanovic s article deals in some detail with hydrocarbon photosensitization, and only a brief account will be given here. 

The saturated and unsaturated hydrocarbons behave differently as far as photosensitization is concerned. The mercury-photosensitized reactions of the alkanes proceed by an initial reaction of the type 

The excited molecule produced can then be de-energized on collision, or can undergo subsequent reactions such as the elimination of a hydrogen molecule. 

The photosensitization of methane by triplet mercury atoms (6 Pi) has been 

If the light intensity is kept low the atom and radical concentrations are small so that loss of hydrogen atoms by reactions (4) and (5) is unimportant, and if concentrations of products are kept low reactions (6)-(8) are unimportant. Back and van der Auwera worked under these conditions, so that the rate of hydrogen production was a measure of the rate of reaction (1). They found that reaction (1) proceeds with a rather low efficiency, in contrast to the behaviour of other saturated hydrocarbons where the efficiency is close to unity. The efficiency also increases markedly with temperatoe. It may be that reaction (1) has a small activation energy. 

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The results of this type of experiment show that CH2 itself is usually formed as a singlet species, that can decay to the triplet state, which consequently has a lower energy (MO calculations and experimental determinations show that the difference in energy between singlet and triplet CH2 is 8-10kcal mol or 33-42 kJ moP ). However, it is possible to prepare triplet CH2 directly by a photosensitized decomposition of diazomethane.Methylene (CH2) is so reactive " that it generally reacts as the singlet before it has a chance to decay to the triplet state. 

Similar products are obtained from the photosensitized decomposition of the tertiary azides, suggesting that decomposition may result from the triplet azides under both direct and sensitized photolysis/461 Additional evidence for a discrete nitrene intermediate comes from the observation that this intermediate can be trapped by decomposition of the azides in the presence of good hydrogen donors such as tri- -butyItin hydride and jec-butyl mercaptan. Triarylamines result ... 

Baugh and co-workers exposed solutions of phenylbutazone to the light of a projector lamp in the presence of typical dyes used to colour sugar-coated tablets. Erythrosine sodium photosensitized decomposition of the drug via, it was suggested, singlet oxygen [134]. 

The photosensitized decomposition of C02 at 1849 A produces CO and a solid containing mercuric oxide and oxalates of mercury68. Oxalate formation is believed to result from dimerization on the vessel walls of a complex formed between C02 and Hg 6(1P1) atoms. 

The mercury-photosensitized decomposition of N20 was studied first by Manning and Noyes160 and later in greater detail and more quantitatively by Cveta-novic161,162. The primary reaction step is... 

Griffiths and Norrish152 have studied the chlorine-photosensitized decomposition of NC13 by following the pressure increase from the overall reaction, viz. 

Table 10. Photosensitized decomposition oi diazo compounds in the presence of olefins...

The photosensitized decomposition reduces the hydride shift, as may be expected for a triplet species. However, the stereochemical change in the cycloaddition is relatively small. Similieir results were obtained by Baer and Gutsche for the sensitized photolysis of o-n-butyl-phenyl-diazomethane 63... 

There are examples where the trialkylborane is the photochemical excited species. Lissi and Larrondo ) studied the Hg-photosensitized decomposition of... 

Acher, A.J., Saltzman, S., Brates, N., and Dunkelblum, E. Photosensitized decomposition of terbacil in aqueous solutions. /. Agric. Food Chem., 29(4) 707-711, 1981. 

Scheme 23.—Photosensitized Decomposition of N,Af-Dimethylhydrazones in the Presence of Oxygen.

Photosensitized decomposition of 9b in substituted benzenes led to similar results [83DIS(B)(44)1113]. Therefore, either spin inversion from the triplet to the singlet form of 4-diazoimidazole is faster than the decomposition of the excited diazo compound, or intersystem crossing from the triplet to singlet carbene is easier and faster than the reaction of the triplet state with substrates. 

Photosensitized decomposition of oxime oxalyl amides proceeded via carbamoyl radicals which underwent A-exo cyclizations forming four-membered /3-lactams as main products. Irradiation of solution of oxime derivative 22 and 4-methoxyacetophenone (MAP) in toluene at 100 °C with a 400 W UV lamp led to azetidinone 23 in 3 1 ratio of diastereoisomers (equation 10) . ... 

Surprisingly, the critical experiment has been done infrequently over the last one-half of a century The requirements for an experiment that truly speaks to the issue at hand are that one be able to see the results of addition of both spin states of a single carbene, and these requirements rarely have been met. For example, the direct irradiation of methyl diazomalonate leads to the stereospecific addition expected of a singlet carbene, whereas the photosensitized decomposition of the diazo compound leads to formation of the triplet carbene and loss of the stereochemical relationship originally present in the reacting alkene. Rotational equilibration in the intermediate seems to be complete, as it makes no significant difference whether cis or trans alkene is used as starting material (Scheme 7.9). ... 

Azides. In a study of the photosensitized decomposition of alkyl azides,202 an apparently linear relationship between sensitizer triplet energy and the log of the rate constant for energy transfer was obtained. Phenan-threne, however, gave a somewhat larger value than expected for this rate constant, which was determined from the variation in quantum yield of... 

Cvetanovic67 was concerned with oxygen atom reactions with unsaturated hydrocarbons. The oxygen atoms were obtained in his experiments by mercury-photosensitized decomposition of N20. Cvetanovi6 came to the conclusion that the reaction of oxygen atoms with ethylene proceeded essentially with scission of the hydrocarbon bond, while with higher olefins this was not observed. Corresponding oxides (epoxides) and carbonyl compounds were formed in the course of the reaction. 

Gunning66 has shown that disproportionation takes place when CF2C1 radicals are generated by the mercury-photosensitized decomposition of CF2CI2 and in recent studies confirming his results47 it was found that the disproportionation/combination ratio was again 0.04. 

Methylene has been prepared by the benzophenone-photosensitized decomposition of diazomethane.80 81 The mechanism is postulated to be... 

The mercury-photosensitized decomposition of ketene27 has recently been shown to yield methylene that is believed to be in a triplet state by the rule of spin conservation. 

Duncan and Cvetanovic27 studied the reaction with isobutene of methylene generated by the mercury photosensitized decomposition of CH2CO, which is believed to produce triplet methylene. Product ratios reached high-pressure limiting ratios at 200 mm. The observed yield of... 

Methylene produced by mercury photosensitized decomposition of 011200 was found to add to cfs-butene-2 to give product ratios similar to those obtained by Frey under conditions of high dilution with argon. Addition of this type of methylene to (rans-butene-2 gave similar product ratios, with the exception that the trans isomers of 1,2-dimethylcyclo-propane and pentene-2 were favored over the cis. These results were taken as proof of the nonstereospecific addition of triplet CH2 to olefins. 

Cyclohexene. The experimental results are summarized in Table IV. The product ratios are similar for CH2 generated by CH2N2 photolysis and by CH2N2 pyrolysis at 265°C., but for CH2 generated by the benzophenone-photosensitized decomposition of CH2N2 the addition product norcarane is favored. 

Most interesting and fairly successful experiment is the photosensitized decomposition of water. The decomposition of water to H2 and 02 is a highly endothermic process... 

Example of Application Large-Scale Actinometry. Neural network modelling was applied to large-scale actinometry in a continuous elliptical photochemical reactor with a concentric annular reaction chamber [2, 3,108, 148], Uranyl oxalate was used as an actinometer, which is based on the photosensitized decomposition of oxalate ions (Eq. 89) [2, 3] the experimental data were taken from the literature [108],... 

In this laboratory we have studied the Hg-photosensitized decomposition of N20, which gives N2 + O via a triplet intermediate similar to that in the N + NO reaction. In the presence of 5% propane, any 0(3P) formed is rapidly scavenged, so that Of1/)) can be inferred from the formation of NO and/or 02... 

Wright and Winkler456 postulated a sequence of reactions based on a mechanism suggested by Strausz and Gunning405 for the mercury-photosensitized decomposition of NO. Strausz and Gunning proposed... 

Under ordinary conditions, this may not be a major reaction, but for the problem of acetylene formation it is notable that Cvetanovic and Callear (19), and Mitchell and Le Roy (49), in studies of the photosensitized decomposition of ethylene, found a uni-molecular mechanism. Ingold and Stubbs (33) also found that the decomposition of propylene starts with a molecular rearrangement and decomposition ... 

The mercury-photosensitized decomposition of 1,1-difluoroethene, and of ( )- and (Z)-1.2-difluoroethene, proceeds by elimination of hydrogen fluoride via an excited molecule mechanism to produce fluoroacetylene (12).55... 

A different pathway appears to operate in the benzene-photosensitized decomposition of the sulfolene (36)38 formation of the diene (37) as the major product is in accord with the prediction of concerted conrotatory fragmentation in the excited state. Fragmentation in a vibrationally excited state would yield the alternative diene (38) as the major product. Similar observations38 in related sulfolenes support this conclusion. 

Basseches, H., The Mercury Photosensitized Decomposition of Water Vapor, Ph.D. Dissertation, The Ohio State University, 1951. 

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