Free radicals reorganization energy

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. 

Figure 6 Electron transfer rate constants as a function of free-energy change, AG, by radical anions for intermolecular ET in rigid 2-MTHF glass (top) (17), and intramolecular ET in 2-MTHF fluid at room temperature (bottom) (18), in molecules of the form ASB, where B = biphenyl, S = 3,16-androstane, and A is one of eight acceptor molecules shown. In both parts of the figure, the rate vs. AG ° curves are of equation 1 and have identical parameters except for the temperature and solvent reorganization energy.

Table 12.2. Reorganization Energies in kcal/mol of Some Organic Free Radicals ... 

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