Free solvated radical ions

Several aromatic molecules undergo no efficient photochemistry of their own. Thus, these molecules are well suited for use as electron transfer sensitizers. According to the substrate and the conditions chosen, they may form either complexes (or tight radical ion pairs) or free, solvated radical ions (Figure 3.9). 

Apart from free solvated radical ions (FRI), evidence was gathered for two kinds of ion pairs, which are referred to as tight ion pair (or contact ion pair, CIP) and loose ion pair (or solvent separated ion pair, SSIP) (Scheme 1, Eq. (1)) [3]. It is important to point out that CIP and SSIP are not the only species in solution. There are myriads of spatial cation-anion relationships that lie between them [4]. The SSIP is a pair of two ions of opposite sign with intact solvent shells. This is lacking in the CIP, anion and cation are in direct contact, the whole aggregate being surrounded by solvent molecules. 

In 1983, Rentzepis published a paper [38] dealing with the charge-transfer interaction of chloranil (9) and aromatic hydrocarbons, e.g. naphthalene (11). Nanosecond spectroscopy of this system [39] could verify some intermediates of the proposed mechanism [21, 40] (Scheme 3) that is the triplet excited acceptor and the free solvated radical ions (A- )s and (D+ ),. 

Furthermore, on the excited surface both formation of a complex, to the stabilization of which both an exciton resonance term and an electron transfer term contribute, and full electron transfer to yield a radical ion pair may be envisaged [3], Diffusion of the charged species leads to free solvated radical ions (FRI), but for a sizeable fraction of the systems which will be discussed in the following, the reaction takes place at the stage of the contact ion pair (CIP), and then distinction between the properties of the polar exciplex and of the radical ion pair may not be unambiguous. 

The Coulombic term e/ea takes into account the mutual stabihzation exerted by the radical ions formed at an encounter distance a. Calculation of AGp is often taken as a preliminary check for assessing the potential of a PET system. Provided that the free energy is exergonic, electron transfer will lead to the formation of the radical cation of the electron donor (D) and the radical anion of the electron acceptor (A) as shown in Scheme 1. These radical ions are typically formed as a contact radical ion pair at an encounter distance of about 7 A and have the option of either diffusing apart to give free, solvated radical... 

This is called the SrnI mechanism," and many other examples are known (see 13-3, 13-4,13-6,13-12). The lUPAC designation is T+Dn+An." Note that the last step of the mechanism produces ArT radical ions, so the process is a chain mechanism (see p. 895)." An electron donor is required to initiate the reaction. In the case above it was solvated electrons from KNH2 in NH3. Evidence was that the addition of potassium metal (a good producer of solvated electrons in ammonia) completely suppressed the cine substitution. Further evidence for the SrnI mechanism was that addition of radical scavengers (which would suppress a free-radical mechanism) led to 8 9 ratios much closer to 1.46 1. Numerous other observations of SrnI mechanisms that were stimulated by solvated electrons and inhibited by radical scavengers have also been recorded." Further evidence for the SrnI mechanism in the case above was that some 1,2,4-trimethylbenzene was found among the products. This could easily be formed by abstraction by Ar- of Ft from the solvent NH3. Besides initiation by solvated electrons," " SrnI reactions have been initiated photochemically," electrochemically," and even thermally." ... 

With radical ions, the dimerization equilibrium is strongly influenced by the solvation and association of radical ions with counter ions. It has been shown that the free ions dimerize much more slowly than do the respective contact ion pairs e.g., the quinoline radical anion does not dimerize in the powerfully solvating hexamethylphosphoramide, but it does dimerize rapidly in tetra-hydrofuran (160). Thus, two equilibria should be distinguished (160), viz. 

The positive and negative ions formed by electron transfer are held together strongly by the force of electrostatic attraction. They may however separate to form free, solvated ions in polar solvents and then various secondary reactions can take place. Radical cations often undergo cycloadditions with the neutral (Figure 4.9). 

Time profiles of the formation of fullerene radical anions in polar solvents as well as the decay of 3C o obey pseudo first-order kinetics due to high concentrations of the donor molecule [120,125,127,146,159], By changing to nonpolar solvents the rise kinetics of Go changes to second-order as well as the decay kinetics for 3C o [120,125,133,148], The analysis of the decay kinetics of the fullerene radical anions confirm this suggestion as well. In the case of polar solvents, the decay of the radical ion absorptions obey second-order kinetics, while changing to nonpolar solvents the decay obey first-order kinetics [120,125,127,133,147]. This can be explained by radical ion pairs of the C o and the donor radical cation in less polar and nonpolar solvents, which do not dissociate. The back-electron transfer takes place within the ion pair. This is also the reason for the fast back-electron transfer in comparison to the slower back-electron transfer in polar solvents, where the radical ions are solvated as free ions or solvent-separated ion pairs [120,125,147]. However, back-electron transfer is suppressed when using mixtures of fullerene and borates as donors in o-dichlorobenzene (less polar solvent), since the borate radicals immediately dissociate into Ph3B and Bu /Ph" [Eq. (2)][156],... 

However, in most cases, the detailed mechanism is not yet known, i.e., whether CIP, SSIP or even free radical ions are scavenged by the nucleophile. Arnold and Snow [62] suggest an attack of methanol to solvated olefin radical cations, whereas Mariano observed a highly stereoselective example of a direct scavenging of a radical cation — radical anion pair by methanol [63]. Although this process has received relatively little attention, it is obvious that scavenging of different types of radical ion intermediates is not only possible but may be used to differentiate between the various types of radical ion pairs (CIP and SSIP). 

Under thermodynamically favourable PET reactions (AGet < 0) the radical ions are formed either as contact ion pair (CIP) or solvent-separated ion pair (SSIP). A closely related question is whether the primary intermediate is a SSIP or CIP. Gould and Farid [8] in their recent study have suggested that in polar solvents, such as acetonitrile, electron transfer quenching results in the formation of SSIP directly and in these solvents the fully solvated ions (SSIP) can separate to form free radical ions (FRI). Therefore, under these conditions the... 

The expression given in Eq. (10) for the work assumes that p = 0, where p is the ionic strength of the medium. AG is the free-energy of the equilibrated excited-state (AG AE00), rD and rA are the molecular radii of the donor and acceptor molecules, e5 is the static dielectric constant or permittivity of the solvent, and z is the charge on each ion. ss is related to the response of the permanent dipoles of the surrounding solvent molecules to an external electrical field. Equation (9), the Bom equation, measures the difference in solvation energy between radical ions in vacuo and solution. 

Anionic polymerizations are generally much faster than free-radical reactions although the A p values are of the same order of magnitude for addition reactions of radicals and solvated anionic ion pairs (free macroanions react much faster). The concentration of radicals in free-radical polymerizations is usually about 10 -10 M while that of propagating ion pairs is 10 -10 M. As a result, anionic polymerizations are lO -lO times as fast as free-radical reactions at the same temperature. 

The ion-radical pair subsequently decays as a result of back electron transfer (k et, Eq. 16), which restores the original CT ion pair. In polar solvents such as dimethyl sulfoxide, dissociation of the ion-radical pair into free, solvated ion radicals can somewhat compete with the rapid back electron transfer (A et lO s ). Thus, ion radicals are observed in small yields (Oion 0.05) on the is time scale upon 10-ns laser excitation [155]. They ultimately decay by (diffusional) back electron transfer. 

In polar solvents, polar exciplexes (contact ion pairs) dissociate into non-fluorescent radical ions (loose ion pairs or free ions) due to the stabilization of the separated ions by solvation. It has been observed that exciplex emission decreases with increasing polarity of the solvent and that at the same... 

Recent work on the role of solvated electrons in intra-DOM reduction processes has demonstrated the importance of trapped e in reactions with species adsorbed on the DOM matrix [98-100]. Modeling of DOM mediated photoreactions indicated the importance of sorption of molecules to DOM for reaction to occur [98, 99]. This is consistent with the lifetime of e" precluding escape from the aqueous DOM matrix into bulk solution. Since many important reactions with environmental implications involve binding or adsorption to DOM - see, for example, [3,101,102] - the role of matrix effects and the caged electron could be very significant. Some workers have suggested that since e remains primarily trapped within the DOM matrix, Oj must be formed by direct electron transfer from the excited triplet state of DOM to O2 [14]. However, it is equally if not more plausible that Oj may be produced by the reduction of Oj by radicals or radical ions produced by intramolecular electron transfer reactions from irradiated DOM [25]. The participation of radicals in the production of carbonyl sulfide and carbon monoxide from irradiated DOM in South Florida coastal waters was recently demonstrated by Zika and co-workers [81-83] and potential pathways for the formation of free radicals from irradiated DOM were discussed. Clearly, the relative contribution of e q and associated transients to the photochemistry of DOM has not been unequivocally resolved in the literature. 

In the equation above, the quantity kum is a measure of the diffusion-limited rate constant, and s of the free energy difference between the radical ion encounter pair (A /D + ) and the free radical ions (A + D ). The quantity s is positive, = 0.06 eV, because the former pair is less well solvated. 

Free Radical Polymerization. In situ polymerization reactions of the monomers added to metal salt solution other than Pechini process were proposed. They utilize free radical polymerization of acrylamide (Gotor, 1993 Rao, 1995 Sin, 2000, 2002) or acryUc add (Mani, 1992). The gelation in a usual synthesis occurs due to the reaction between acrylamide and N- N= methylene-bis-acrylamide. Free radicals initiating the polymerization are created by hydrogen peroxide or azobisisobutyronitrile. Just solvated copper ions strongly inhibit polymerization and sufficient amount of EDTA (Sin, 2000) or dtric add (Gotor, 1993 Rao, 1995) should be added to chelate copper and possibly other metals. The clear advantage of this method is that in contrast to Pechini-type process, which enploys reversible polyesterification reaction, the polymer formation by free radical mechanism is irreversible process that can be conducted, in addition, at low temperatures. 

In the case of acetonitrile, where the solvation of ions is imlikely to occur, pyrrole itself can solvate anions. This may explain why polypyrrole films formed from acetonitrile with perchlorate counterions are less porous because, with pyrrole solvating the counterion during synthesis, a denser structure would be expected. This also results in polymers that are more conductive and have lower capacitance and greater electrochemical reversibility than those grown from water. Similar differences in conductivity were observed between acetonitrile and water when dodecyl-sulfate (DS) was used as the counterion, although the differences in conductivity were not so marked. The presence of DS probably provides some protection from the nucleophilic solvent. In other work, we have shown that pyrrole can be reversibly oxidized in surfactant-containing media (i.e., the surfactant stabilizes the free radical produced). This reversibility could not be detected in the absence of surfactants. 

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