Free radicals bimolecular electron transfer

Photopolymerization. In many cases polymerization is initiated by irradiation of a sensitizer with ultraviolet or visible light. The excited state of the sensitizer may dissociate directly to form active free radicals, or it may first undergo a bimolecular electron-transfer reaction, the products of which initiate polymerization (14). Triphenylalkylborate salts of polymethines such as (23) are photoinitiators of free-radical polymerization. The sensitivity of these salts throughout the entire visible spectral region is the result of an intra-ion pair electron-transfer reaction (101). 

Direct Electron Transfer. We have already met some reactions in which the reduction is a direct gain of electrons or the oxidation a direct loss of them. An example is the Birch reduction (15-14), where sodium directly transfers an electron to an aromatic ring. An example from this chapter is found in the bimolecular reduction of ketones (19-55), where again it is a metal that supplies the electrons. This kind of mechanism is found largely in three types of reaction, (a) the oxidation or reduction of a free radical (oxidation to a positive or reduction to a negative ion), (b) the oxidation of a negative ion or the reduction of a positive ion to a comparatively stable free radical, and (c) electrolytic oxidations or reductions (an example is the Kolbe reaction, 14-36). An important example of (b) is oxidation of amines and phenolate ions ... 

Aromatic sulfides analogous to thiophenols constitute a group of molecules that fulfils the structural conditions necessary for the observation of FET (Sec. 2.4), i.e. they exhibit a low barrier to rotation about the Qp2 S bond. Thus, the torsion motions of the substituents can be accompanied by considerable fluctuation of the electrons in the highest molecular orbitals with two extreme examples of conformers, planar and vertical. The presence of two radical cation conformers was deduced as primary products of the bimolecular free electron transfer (FET) from aromatic sulfides PhSCH2Ph, PhSCHPhj, and PhSCPhg to w-butyl chloride radical based on the nanosecond pulse radiolysis experiments. ... 

The fundamental aspects of structure-reactivity relationships in radiation-induced oxidation of substituted benzenes, bimolecular free electron transfer on the femtosecond time scale, the chemistry of sulfur-centered radicals and the radiolysis of metalloproteins are discussed in succeeding chapters. The effects of the direct and indirect mechanisms of radiation-induced DNA damage are discussed individually in two complementary chapters. The last chapter highlights the application of radiation chemical techniques to antioxidant research. 

Apart from the thermal oxidation-reduction reactions involving metal ions in different valency states, discussed in detail in relation to the initiation of autoxida-tion, one of the most important modes of formation of free radicals is by photoexcitation. The modes of formation can be generally classified as (A) bond-breaking reactions, (B) electron transfer reactions, and (C) reactions which in general form an electronically excited state of the absorbing molecule which produces atoms or free radicals in subsequent bimolecular collisions with other species present. 

The mechanism of quenching had previously been established by observing the formation of free radical ions using flash photolysis.345 Rehm and Weller proposed the empirical Equation 5.5 to fit the data, where AetG° is the free energy of photoinduced electron transfer in the contact pair (Equation 5.1), AG is the free energy of activation that accounts for the structural and solvent reorganization required for the transfer of an electron, kd and k d are the rate constants for the formation and separation of the encounter complex, respectively, Kd = kd/k d is the equilibrium constant of complex formation and Z is the bimolecular collision frequency in an encounter complex, Z 1011 s 346 A value of kd/(ZKd) = 0.25 was used. 

Reduction of the radical first in reaction (5) is consistent with the proposed electron-transfer pathway between the hemes in the CCP-yeast cyt c complex (Fig. 16). However, flash photolysis experiments involving rapid bimolecular reduction of native cyts c by free flavin semiquinones (FH ) gave different results (111). The reactions monitored were the following ... 

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