Reaction quantum yield of direct

Table 15.7 Direct Photolysis Reaction Quantum Yields of Some Selected Organic Pollutants in Aqueous Solution...

Apparent Quantum Yield (0 ) of the Direct Photolysis of Chloro-phenols. The spectral data and sunlight photolysis rate constants of both actinometer valerophenone and the compound, and reaction quantum yield of valerophenone were used to calculate 0 of chlorophenols using GCSOLAR program (13). 

An interesting gas-phase study of the photolysis of ftmy-l-phenyl-2-butene has been published by Comtet.(39,40) He has found that (a) it was not possible to quench the formation of the cyclopropane product under conditions that reduced the fluorescence quantum yield, (b) sensitization by acetophenone only gave cis-trans isomerization, and (c) the quantum yield of cyclopropane formation in the direct photolysis decreases as n-butane is added to the reaction mixture. Comtet suggests that the data are consistent with a reaction from the second triplet state. 

If fluorescence and cis-trans isomerization (9.26)-(9.29) are the main competing reactions upon direct excitation, then inhibition of rotation about the central bond should produce an increase in the fluorescence quantum yield. The rigid systems (3) and (4) both have fluorescence quantum yields of 1.0 at room temperature.<44,52) While the fluorescence of /rmy-stilbene is a... 

Irradiation of [Re(CNC6Fl3-2,6-Me2)6] with UV light in CH2CI2 in the presence of halide ions yields complexes of the type [Re(CNCgH3-2,6-Me2)5X] (X = Cl, Br, or I). The quantum yield of this reaction is high and results in chemical yields between 30% and 40%. Products of the same composition have been obtained directly from the cleavage of the metal-metal bond in quadruply bonded rhenium complexes of various compositions in the presence of isocyanides. ... 

Already in his early investigations on type II direct and indirect photooxygenation reactions, Gaffron18,32 33 showed that the quantum yields of these processes are independent of the intensity as well as of the exciting wavelength of light. He also showed that in the direct photooxygenation of rubrene, two rubrene molecules have to take part in the reaction sequence which bads to the stable endoperoxide. 

Both these mechanisms involve free radicals which should cause the initiation of chain reactions.56-59 Livingston3 has discussed type I direct and indirect photooxygenation processes in detail. In the latter case benzoyl peroxide or azoisobutyronitrile are photolyzed into free radicals. These induce chain reactions, the quantum yields of which are generally greater than unity and inversely proportional to the square root of the light intensity.3... 

A direct cleavage of the ester to radicals similar to that observed in the vapor state94 and an internal hydrogen abstraction followed by radical formation98 have been proposed to explain the reaction in benzene. The internal hydrogen abstraction mechanism appears to be the more reasonable. Direct cleavage would yield ethoxy radicals, but no products derived from them are observed. The low quantum yield of ester disappearance observed for ethyl... 

---