Quantum yield hydrogen abstraction

In 2-propanoI, the quantum yield for photolytic conversion of benzophenone to the coupled reduction product is 2.0. The reason is that the radical remaining after abstraction of a hydrogen atom from 2-propanol transfers a hydrogen atom to ground-state benzophenone in a nonphotochemical reaction. Because of this process, two molecules of benzophenone are reduced for each one that is photoexcited ... 

The quantum yields for oxetane formation have not been determined in every case, and only a few relative rate constants are known. The reactivities of singlet and triplet states of alkyl ketones are very nearly equal in attack on electron rich olefins. 72> However, acetone singlets are about an order of magnitude more reactive in nucleophilic attack on electron-deficient olefins. 61 > Oxetane formation is competitive with a-cleavage, hydrogen abstraction and energy-transfer reactions 60 64> so the absolute rates must be reasonably high. Aryl aldehydes and ketones add to olefins with lower quantum yields, 66> and 3n-n states are particularly unreactive. 76>... 

The excited state configuration is likely to be n, -n as judged from the capability of the excited nitro group to abstract hydrogens which are in no way activated. Lack of activation, however, is overcompensated by the close proximity of the potential reaction partners. Although quantum yields of disappearance of starting material would be highly desirable, so far only quantum yields of formation of products [i.e. N-hydroxy-2-indolinones) have been obtained. 

Disappearance quantum yields for 7 were found to depend on the HCl-con-centration 77,78). in view of the anal d ical methods used, the values obtained by Cu and Testa 78) with 313 nm excitation seem to be the more reliable ones (Table 2). It is noteworthy that quantum yields obtained in air-saturated solutions seem to be less dependent on HCl than those ( deg) in degassed solutions which suggests that there might exist two different pathways for hydrogen abstraction. 

However, irradiation of biacetyl in solution gives products derived from an initial hydrogen abstraction by a photoexcited biacetyl unless the reaction is carried out in a solvent containing no abstractable hydrogen atoms.24,44 Irradiation of biacetyl in perfluorinated solvents, conditions under which no hydrogen abstraction should take place, yields small amounts of carbon monoxide and ethane, but the quantum yield for their production is much lower than is observed at low pressure in the vapor state.24 The quantum yield for the disappearance of biacetyl in perfluoro-n-octane at 100° with 366 nm exciting light is reported to be 2.4 x 10 3 this compares with a value of 0.21 for biacetyl disappearance in the vapor state at 1.8 x 10"3A/... 

This reaction can be viewed as an internal hydrogen abstraction which takes place through a highly strained four-membered transition state instead of the usual six-mem bered one. The formation of the enol is wavelength dependent and is retarded by triplet quenchers.58-80 In water, excitation into the second excited singlet of biacetyl formed the enol with a quantum yield of 0.10, while excitation into the first excited singlet formed the enol with a quantum yield of 0.01, possibly via a higher excited state formed by a triplet-triplet reaction. Therefore, the authors conclude that the second excited triplet is the state that isomerizes to the enol. 

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... 

At 0.1M acceptor, for example, the rate of energy transfer is 19.2 times the rate of abstraction. However, this means that about 5% of the benzophenone triplet will still abstract from the solvent to form ketyl radicals. A compound which is itself a poor hydrogen abstractor may show a greatly enhanced quantum yield of photoreduction under these conditions. 

In a photosensitized reaction, radical induced isomerization can occur if the sensitizer undergoes either homolytic decomposition or hydrogen abstraction, or if the system contains impurities which give radicals on irradiation. The result may be to shift the measured photostationary state in the direction of thermodynamic equilibrium and to give anomalously high values of the quantum yields for cis-trans isomerization. 

There is some contribution due to / -scission of the alkyl radical formed by the type I process, particularly in the MIPK and tBVK polymers. Loss of carbonyl occurs from photoreduction or the formation of cyclobutanol rings, and also from vaporization of the aldehyde formed by hydrogen abstraction by acyl radicals formed in the Norrish type I process. As demonstrated previously (2) the quantum yields for chain scission are lower in the solid phase than in solution. Rates of carbonyl loss are substantially different for the copolymers, being fastest for tBVK, slower for MIPK, and least efficient for MVK copolymers (Table I and Figure 1). 

A quenching study at one concentration of 2-propanol (2.6Af) in benzene indicates that the rate constant for hydrogen abstraction is about 1 x 106Af 1 sec-1,451 similar to that for benzophenone. A fact long known is that sulphonate substituents in the 2 and 6 positions do not affect quantum yields of photoreduction, whereas in the 1 and 5 positions they cut O from 1.0 down to 0.01-0.05.216... 

The quantum yield of the primary hydrogen abstraction process is unity, and is independent of irradiation wavelength, light intensity and temperature. Its wavelength range follows of course the absorption spectrum of anthraquinone so that this actinometer is particularly well suited for the UV region. 

In the course of irradiation of acetophenone in the presence of 1-phenylethanol, the actual quantum yields for pinacol formation do not exceed 50%, but rise to 71% when PhCH(OD)Me is used for photoreduction of acetophenone in acetonitrile683,684. A conclusion has been reached from this inverse DIE that half the reaction of triplet acetophenone with 1-phenylethanol involves abstraction of an OH hydrogen followed by disproportionation of the initial radical pair back to reactants. A transfer of an O-bonded hydrogen to a triplet ketone is taking place (equation 318) besides the abstraction of hydrogen from... 

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