Radical cations kinetic considerations

As indicated already, the optical transition energy are an extremely sensitive probe for the electronic and steric properties of the three-electron-bonded species and their respective relative contributions. However, the effect of substituents on the optical transitions becomes of much lesser importance in intramolecular radical cations derived from open-chain dithianes (type 7-9). Changing the terminal substituents in R-S-(CH2)3-S-R from methyl to isopropyl results in a just 15 nm change (440 vs. 455 nm), i.e., structure clearly appears to be the dominating parameter. This is fully corroborated by the pulse radiolysis results on 2-substituted-l,3-dithiacyclopentanes.l23 As mentioned already, the radical cation (11), derived from 1,3-dithiacyclopentane (12), is very unstable if formed at all ( niax > 650 nm). The analogous radical cation generated upon oxidation of l,3-dithia-2,2-dimethylcyclopentane (13), on the other hand, exhibits a pronounced and blue-shifted absorption at 610 nm as well as a considerable kinetic and thermodynamic stability. 

The kinetic data discussed above demonstrate the effects of varying the structure of both the styrene radical cation and the alkene on the initial step in the cycloaddition reaction. However, the transient experiments do not provide any evidence that would permit one to distinguish between a concerted or stepwise mechanism. The kinetic data obtained for additions to a range of alkenes do show considerable similarities to those reported for the addition of carbenium ions to the same substrates. For example, rate constants for the addition of the bis(4-methyl-phenyl)methyi cation to a series of ring-substituted styrenes also correlate with the Hammett a and a parameters with p and p values of-5.2 and -5.0, respectively." The latter reactions are thought to proceed via a partially bridged transition slate and might, therefore, be expected to show similarities to concerted... 

Reaction 4 attempts to recover the considerable influence of electrolytes in empirical kinetics, through its discharge on the polymer. The formed radical is immediately transferred to a monomer molecule. This indirect initiation can explain the behaviour of pyrrole electropolymerization at high potentials in water [51]. The other electrolyte influences act on the metal oxide production [49], by stabilization of the monomeric radical cations [130] and through polymer oxidation. 

Confinement of ion-radicals considerably changes their reactivity. What is more important for practical applications is that the confinement increases the ion-radical stability. For instance, the cation-radicals of polyanilines (emeraldines) sharply enhance their thermodynamic and kinetic stabilities when they are formed encapsulated in cucurbituril (Eelkema et al. 2007). Emeraldines have electric condnctivity as high as 1 X 10 cm (Lee et al. 2006). Encapsulation of emer-... 

The absorptions at both 500 nm and 320 nm follow first order kinetics with a lifetime of 420 ns. This absorption species is neither the excimer of polystyrene nor free cationic species of polystyrene. Although the excimer of polystyrene has an absorption band around 500 nm, the lifetime is only 20 ns. Further the free cationic species of polystyrene should live for a longer time in this solution, and the absorption band should exist in a longer wavelength region (6). These considerations of lifetime and absorption spectrum lead us to conclude that the absorption spectrum shown in Figure 12 is due to the charge transfer-radical complex between polystyrene and Cl radical (2,4,17). A very similar... 

Besides the effect of the electrode materials discussed above, each nonaqueous solution has its own inherent electrochemical stability which relates to the possible oxidation and reduction processes of the solvent,the salts, and contaminants that may be unavoidably present in polar aprotic solutions. These may include trace water, oxygen, CO, C02 protic precursor of the solvent, peroxides, etc. All of these substances, even in trace amounts, may influence the stability of these systems and, hence, their electrochemical windows. Possible electroreactions of a variety of solvents, salts, and additives are described and discussed in detail in Chapter 3. However, these reactions may depend very strongly on the cation of the electrolyte. The type of cation present determines both the thermodynamics and kinetics of the reduction processes in polar aprotic systems [59], In addition, the solubility product of solvent/salt anion/contaminant reduction products that are anions or anion radicals, with the cation, determine the possibility of surface film formation, electrode passivation, etc. For instance, as discussed in Chapter 4, the reduction of solvents such as ethers, esters, and alkyl carbonates differs considerably in Li or in tetraalkyl ammonium salt solutions [6], In the presence of the former cation, the above solvents are reduced to insoluble Li salts that passivate the electrodes due to the formation of stable surface layers. However, when the cation is TBA, all the reduction products of the above solvents are soluble. 

The side chain can also affect electronic events of the tricyclic ring system. Electron spin resonance experiments allowed Fenner S to suggest that the influence of the side chain of phenothiazine on the formation of free radicals showed a correlation between the redox activity of the phenothiazine nucleus and dynamic aspects of stereochemistry. For example, there was a difference in the formation of cationic free radicals between promazine and alimemazine. The latter has a branched side chain, and forms cationic free radicals only under irradiation. It differs from promazine in pharmacodynamic properties, reported s to result in considerably shifted ion exchange equilibria. A kinetic study of the oxidation of dopamine by dialkylaminoalkyl phenothiazine cationic free radicals showed that a strong correlation existed between side-chain structures and oxidation rates phenothiazine free radicals with two carbon side chains had faster rates than those with three carbon side diains, albeit both were very rapid at physiological pH. 

The kinetics of cationic polymerizations are considerably more complex than those of the free-radical polymerizations, and kinetic data is difficult to interpret becanse of several reasons (92,138-140). For example, in cationic polymerizations the identity and proximity of the coimterion has a marked effect on the reactivity of the active center. An active center that is encumbered by a closely associated counterion has a dramatically lower reactivity (typically an order of magnitnde lower) than an active center that is separated from the counterion. As described in the section on photoinitiation, this consideration has lead to the development of large, nonnucleophilic coimterions however the reactivity of a cationic active center still depends on the proximity of the counterion. Therefore, at any given time, a variety of propagating species may exist, ranging from ion pairs to separated ions. For this reason, an effective propagation rate constant which inclndes contributions from all propagating species is usually adopted, as described below. Secondly, unlike free-radical polymerizations, the steady-state approximation for active center concentration is not valid since the cationic active centers are not reactive toward one another, and the rate of active center... 

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