Free radicals electron-transfer equilibria

Elegant evidence that free electrons can be transferred from an organic donor to a diazonium ion was found by Becker et al. (1975, 1977a see also Becker, 1978). These authors observed that diazonium salts quench the fluorescence of pyrene (and other arenes) at a rate k = 2.5 x 1010 m-1 s-1. The pyrene radical cation and the aryldiazenyl radical would appear to be the likely products of electron transfer. However, pyrene is a weak nucleophile the concentration of its covalent product with the diazonium ion is estimated to lie below 0.019o at equilibrium. If electron transfer were to proceed via this proposed intermediate present in such a low concentration, then the measured rate constant could not be so large. Nevertheless, dynamic fluorescence quenching in the excited state of the electron donor-acceptor complex preferred at equilibrium would fit the facts. Evidence supporting a diffusion-controlled electron transfer (k = 1.8 x 1010 to 2.5 X 1010 s-1) was provided by pulse radiolysis. 

The thermal and photochemical activations of EDA complexes by electron transfer are both enhanced when the radical ions D+- or A--(either paired or free) undergo a facile first-order (unimolecular) transformation such as fragmentation, rearrangement, bond-formation, etc., which pulls the redox equilibrium and thus renders the competition from the energy-wasting back electron transfer less effective (compare Scheme 5). Critical to the quantitative evaluation of the reaction dynamics is the understanding that the typical [D+% A--] intermediates, as described in... 

The more acidic fluorene in tert-butyl alcohol solution, or in DMSO solution, reacts by a process that involves the carbanion in equilibrium with hydrocarbon. Thus, fluorene and 9,9-dideuteriofluorene oxidize at identical rates. We have established that the oxidation of the anion of fluorene can be catalyzed by a variety of electron acceptors (v), including various nitroaromatics (18). The catalyzed oxidation rates were found to follow the rates of electron transfer measured by ESR spectroscopy in the absence of oxygen. These results established the catalyzed reaction as a free radical chain process without shedding light upon the mechanism of the uncatalyzed reaction. 

A PET in intramolecular CPs between pyridinium ions and bromide, chloride or thiocyanate ions for polymerization initiation is described, too [137-139]. As expected, an equilibrium exists among free ions, ion pairs, and CT, which is shifted to the free ions in polar solvents and to the complex in a less polar solvent That complex serves as the photosensitive species for the polymerization (see Scheme 10). The photodecomposition of the CT yields radicals of the former anion and N-alkylpyridinyl radicals. Probably, the photopolymerization is initiated only by X- radicals, whereas latter radicals terminate the chain reaction. By addition of tetrachloromethane, the polymerization rate is increased owing to an electron transfer between the nucleophilic pyridinyl radical and CC14 (indirect PET). As a result, the terminating radicals are scavenged and electrophilic -CQ3 radicals are produced. 

The mechanism of Co(acac)2-mediated polymerization of Vac is still an open question. On the basis of an early work by Wayland and coworkers on the controlled radical polymerization of acrylates by complexes of cobalt and porphyrins, Debuigne and coworkers proposed a mechanism based on the reversible addition of the growing radicals P to the cobalt complex, [Co(II)], and the establishment of an equilibrium between dormant species and the free radicals (equation 8). Maria and coworkers, however, proposed that the polymerization mechanism depends on the coordination number of cobalt . Whenever the dormant species contains a six-coordinated Co in the presence of strongly binding electron donors, such as pyridine, the association process shown in equation 8 would be effective. In contrast, a degenerative transfer mechanism would be favored in case of five-coordinated Co complexes (equation 9). 

The free energy gap between P and P C can be calculated from measurements of the fluorescence that occurs during the lifetime of the radical-pair in reaction centers that have electron transfer to blocked by the reduction or extraction of the quinone [65,78-81]. The fluorescence emitted by P at any given time is a measure of the amount of the excited singlet state that is in equilibrium with the radical-pair. By this measure, the earliest form of P I that can be resolved lies about 0.17 eV below P in free energy, both in chromatophores and in isolated reaction centers (Fig. 1). The amplitude of the fluorescence decays in several steps, possibly because of nuclear relaxations in the radical-pair. 

Appropriate modification of the ESR spectrometer and generation of free radicals by flash photolysis enables time-resolved (TR) ESR spectroscopy [22]. Spectra observed under these conditions are remarkable for their signal directions and intensities. They can be enhanced as much as one-hundredfold and appear as absorption, emission, or a combination of both. Effects of this type are a result of chemically induced dynamic electron polarization (CIDEP) these spectra indicate the intermediacy of radicals whose sublevel populations deviate substantially from equilibrium populations. Significantly, the splitting pattern characteristic of the spin-density distribution of the intermediate remains unaffected thus, the CIDEP enhancement not only facilitates the detection of short-lived radicals at low concentrations, but also aids their identification. Time-resolved ESR techniques cannot be expected to be of much use for electron-transfer reactions from alkanes, because their oxidation potentials are prohibitively high. Even branched alkanes have oxidation potentials well above the excited-state reduction potential of typical photo-... 

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. 

Photolysis of the naphthylmethyl esters (365) gives naphthylmethyl radicals and phenylacetoxyl radical. The radical pair may transfer an electron to yield phenyl acetate and naphthylmethyl cation which is quenched by nucleophilic solvent. Alternatively, the radical pair may escape their solvent cage and so yield free radical derived products. Pincock has estimated the rate constant for the electron transfer in the radical pair for different X-substituents on the naphthalene by monitoring the ratio of free radical products to ionic products produced in the photolysis reaction, and has correlated this rate constant with the free energy of the electron transfer reaction in the radical pair. The results are discussed in terms of the Marcus theoretical relationship between reaction rate constant and equilibrium constant. 

Inactivation is accompanied by the formation of cob(III)aIamin and 5 -deoxyadenosine. (4) The inactivation rate is the same under anaerobic as under aerobic conditions. (5) Substrates are not consumed in suicide inactivation. (6) Inactivation in H20 leads to tritiated substrate and product, but not to tritiated 5 -deoxyadenosine. These facts are accounted for by the mechanism outiined in Figure 22. Catalytic intermediates 2 and 3 in Figure 20 occasionally undergo electron transfer from cob(II)alamin to a free radical center in either 2 or 3, which becomes carbanionic and is immediately quenched by a proton donor in equilibrium... 

Abel (36) has proposed a series of electron transfer reactions involving free radicals similar in principle to those suggested for the iodide-iodine kinetics. He suggests reaction through iodine cations formed for example in the equilibrium ... 

Radicals can be formed in several ways, but all involve homolytic cleavage (one electron is transferred to each adjacent atom from the bond), as depicted by X—Y, leaving two radical products. The equilibrium constant for this homolytic cleavage process depends on both the relative bond strength of X—Y and also on the relative stabilities of X and Y. Increasing the temperature of the reaction will generally shift the equilibrium toward a higher concentration of free radicals, This equilibrium makes it convenient to... 

It is believed that both pairs are in equilibrium with each other. The above ion pairs ean lead to formation of different radical pairs. The solvated ion pair can lead to back electron transfer and /or free ion formation. The hydrogen bonding character of the other components in the reaction mixture, such as monomers, solvent, or prepolymers can affect the efficiency of the proton transfer. The substituents on the nitrogen of the amine can affect the proton transfer as well. The overall reaetion can also be illustrated as follows ... 

Equilibrium in the ion source of a mass spectrometer cannot be achieved in certain ion-molecule systems because of rapid competitive reactions or because one of the species in the equilibrium of interest is an unstable species such as a free radical. In such cases, it is sometimes possible to obtain an experimental estimate of the enthalpy change of a particular reaction (charge transfer, proton transfer, etc.) by use of a technique known as bracketing, in which the ion of interest is reacted with a series of molecules selected to provide a range of values for the relevant thermochemical parameter of interest, e.g., ionization energy, electron affinity, gas-phase basicity, or acidity. Reaction is presumed to occur for exothermic processes and not to occur for endothermic processes. 

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