Quantum yields for disappearance

Ddla, quantum yield for disappearance of ketone j, quantum yields for type I cleavage n, u, quantum yields for type II cleavage cb. cb> quantum yields for cydobutane formation. 

Preparative photolysis of AETSAPPE (0.25 M aqueous solution) at 254 nm (Rayonet reactor) resulted in the formation of the disulfide product 2-amino(2-hydroxy-3-(phenyl ether) propyl) ether disulfide (AHPEPED) as the primary photoproduct Photolysis of AETSAPPE at 254 nm (isolated line of medium pressure mercury lamp) resulted in rapid initial loss of starting material accompanied by formation (analyzed by HPLC) of AHPEPED (Figure 12a and 12b) (Scheme IV). Similar results were obtained for photolysis- at 280 nm. Quantum yields for disappearance of AETSAPPE and formation of AHPEPED at 254 nm and 280 nm are given in Table I. The photolytic decomposition of AETSAPPE in water was also accomplished by sensitization ( x =366 nm) with (4-benzoylbenzyl) trimethylammonium chloride (BTC), a water soluble benzophenone type triplet sensitizer. The quantum yield for the sensitized disappearance (Table I) is comparable to the results for direct photolysis (unfortunately, due to experimental complications we did not measure the quantum yield for AHPEPED formation). These results indicate that direct photolysis of AETSAPPE probably proceeds from a triplet state. 

Table 2. Quantum yields for disappearance of 4-nitropyridine (1, 7.79 X 10 8 M in 2-propanol) with varied HCl-concentration...

The behavior of cytosine and cytidylic acid during photolysis are quite different existing reports about the behavior of cytosine are contradictory. It should be noted that neither the cytosine hydrate nor the dimer has been isolated from photolyzed solutions nor identified by comparison with known substances. Early work7 reports that photolysis of cytosine in solution resulted in a decrease in absorption at 270 nm, and an increase at 240 nm. This transformation was partly reversible at room temperature,7 but the reversal was prevented by the presence of 0.1M NaCl. The quantum yield for disappearance of cytosine was about 1 to 2 x 10-3. 

Calvert has studied the competing type I and II processes in various -propyl alkyl ketones in the gas phase, and finds that increases and decreases as the alkyl group is changed from methyl through r-butyl,359 as might be expected. Perhaps the most dramatic illustration of the inherent inefficiency of type II photoelimination is provided by the fact that at 100°, where the quantum yield for type I cleavage of diethyl ketone is 1.0, the total quantum yield for disappearance of di- -propyl ketone is only 0.58.359... 

FIGURE 3. Plots of the quantum yields for disappearance of the Cp 2Mo2(CO)6 unit in polymer 5, Cp 2Mo2(CO)6 dispersed in PVC, and Cp 2Mo2(CO)6 in hexane/CCl4 (Cp = q5-C5H4CH3). 

The quantum yields for disappearance of BC-a and BC-b are identical (Fig. 9), but the rates of mesolysis for the radical anions of these compounds differ by a factor of ten. Thus the quantum yields for these compounds reflect a competition between BET and ion separation. The quantum yield for disappearance of BC-c is higher than that for BC-a or BC-b, and is now comparable... 

Fig. 9. The quantum yield for disappearance of BC as a function of [BC]. The lines represent the best fit to the following equation , and

The bridged cyclohexenediones whose photoisomerization to cyclobutanediones have been observed are summarized in Table II where it can be seen that a wide variety of compounds undergo this reaction. In all cases where the point has been checked, quantum yields for disappearance of starting material and for formation of product were identical. The series of compounds, entries 13-17, were of interest since they involve unsubstituted (entry 13), tetrachloro (14, 16) and tetrabromo (15, 17) isomers where heavy atom effects on intersystem crossing rates might be manifested in quantum yield variations. As can be seen in the Table, the results do not permit such an interpretation. 

Quantum yield for disappearance of ketones in the absence or presence of CuCl2 as scavenger. 

Since the quantum yield for disappearance of ketone (triplet ketone nor the primary radical pair PhCH2CO CH2Ph is scavenged. As stated above, the proposed mechanism for the photolysis of DBK in micellar solution is illustrated in Fig. 5 13,21). The micellar environment inhibits the diffusion of radicals to the bulk aqueous phase the radical pair s distance maximum separation is maintained to a few tens of angstroms or less. The amount of escape being reduced, the radicals can then undergo more efficient intersystem crossing and recombination. 

The photoreduction of quinones has also been studied in micellar environment 44-46) Thg relative quantum yields for disappearance of benzoquinone is various surfactants have been found to be influenced by the nature of the micelle. For example, the rate of reaction (Scheme XV) is accelerated in anionic surfactants44),... 

The results of Barltrop and Coyle , on the photolysis of aliphatic ketones in solution at 3130 A, support the explanation suggested by Wagner and Hammond for the low quantum yields. The overall quantum yield for disappearance of 2-pentanone, 2-octanone and 5-methyl 2-octanone was found to increase and approach unity in hydroxylic solvents. This increase can be attributed to the solvation of the hydroxy biradical intermediate. Since, however, the solvent effect was not observed for products originating from excited singlet molecules, it is probably only the triplet state which decomposes via biradicals. 

Quantum yields for disappearance of starting materials. Quantum yields of production formation. 

The quantum yield for disappearance of the chromophore is then given by eq. 6. 

The quantum yield for disappearance of the dimethylmalelmide chromophore via the triplet excited state at a fixed chromophore concentration was — within the experimental error — not dependent... 

Rates of quenching of excited state triplets have been measured and the influence of substituents on the phenols studied has shown that electron-donating substituents enhance the degradation process (< > 0.5) while phenol itself has a quantum yield for disappearance of only 0.1. 

The 366-nm quantum yield for disappearance of (h -C,H,)2MoH2 in degassed solution is 0.1, and for (h -C5H5)2WH2 is 0.01. Both yields are lower limits, however, being measured in sealed UV cells because of the air sensitivity of the complexes, and where the back reaction with H2 is not prevented. 

As investigations of their solution photochemistry have proliferated, it has become increasingly clear that it is nearly impossible to find a completely unreactive solvent for some diones. Thus, for example, the quantum yield for disappearance of phenanthrenequinone in degassed benzene solution at 4358 A is 0.25 131> (perfluoro compounds, which might indeed be unreactive, are extremely poor solvents for many diones). 

The quantum yield for disappearance of benzil at 3660 A in cyclohexane solution has been estimated to be not greater than 0.25. This reaction produced 37> a complex mixture of products including benzoic acid, benzaldehyde and phenyl cyclohexyl ketone which suggest that the dione underwent cleavage to benzoyl radicals. Hydrogen abstraction appears to be the major process in other solvents. 

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