Persistent radical, definition

A free radical (often simply called a radical) may be defined as a species that contains one or more unpaired electrons. Note that this definition includes certain stable inorganic molecules such as NO and NO2, as well as many individual atoms, such as Na and Cl. As with carbocations and carbanions, simple alkyl radicals are very reactive. Their lifetimes are extremely short in solution, but they can be kept for relatively long periods frozen within the crystal lattices of other molecules. Many spectral measurements have been made on radicals trapped in this manner. Even under these conditions, the methyl radical decomposes with a half-life of 10-15 min in a methanol lattice at 77 K. Since the lifetime of a radical depends not only on its inherent stabihty, but also on the conditions under which it is generated, the terms persistent and stable are usually used for the different senses. A stable radical is inherently stable a persistent radical has a relatively long lifetime under the conditions at which it is generated, though it may not be very stable. 

Offhand, one may be surprised. Intuitively, one could expect equal concentrations of R and Y", because the radicals are formed with equal rates, and therefore, the products R—Y and R—R should be formed in the statistical ratio of 2 1. However, this is not the case, except during a very short initial period. The simple cause is that the transient radicals R disappear by their self-termination (4) and by the cross-reaction, whereas by definition the persistent radicals Y do not self-terminate but disappear only by the cross-reaction.1 Hence, every self-termination event of R (3) causes a buildup of excess Y, and this buildup continues as the time goes on. The permanently increasing concentration of Y accelerates the cross-reaction at the expense of the self-termination... 

However, there is no definite correlation between the polymerization rate and the redox potential, partly because a higher concentration of the radical species may lead to a higher probability of bimolecu-lar termination and a higher concentration of persistent radical species.54-261 Another possibility of side reaction is due to the reduction of the radical into an anion with metal complexes with extremely low redox potential. An appropriate range for the redox potential of the metal catalysts was suggested between —0.3 and +0.6 V (versus NHE) for living radical polymerization.54... 

Russell and co-workers have obtained cation-radicals in the 1,2-dithiete system. Such radicals are cyclic, conjugated within the heterocycle, and possess (4 -P 1) electrons, with = 1 they are consequently heteroaromatic witliin the definition given in the introduction to Part I. 3,4-Dimethyl-l,2-dithiete cation radical 89 (R = R = Me) was obtained by treatment of acetoin in sulfuric acid with sulfide ion. The radical is persistent at ambient temperature and unaffected by oxygen. This evident stability, and its formation from open-chain precursors to the exclusion of acyclic possibilities such as 90, whose oxygen equivalent exists, implies that aromatic stabilization of 89 and similar radicals is a matter of fact and not merely definition. 

Griller, D. Ingold, K. U. Acc. Chem. Res. 1976, 9, 13. In the context of this review, radicals are called persistent if their lifetimes in liquid solution exceed those of reactive radical species by many orders of magnitude. They may self-terminate slowly or disappear by other reactions, but these processes do not compete with the cross coupling with usual transient radicals. Stable radicals can be isolated in pure form. They are included in our definition of persistence. 

These formulas provide an interpretation of the transience or persistence of radicals and lead to a clear definition of these concepts. 

New Acyclic Nitrone Spin Traps. - The range of new acyclic nitrone traps that have been synthesised is more limited than that of the related cyclic materials. Examples of some of these new materials are given in Scheme 3. This is due, at least in part, to the perception that these traps provide more limited data in terms of the range of hyperfine coupling constants than the cyclic nitrones, and hence decreased opportunities for definitive identification of radical adducts. Nonetheless these traps can have considerable advantages in that they often yield very persistent adducts, which can be of major benefit in determining whether a process is radical-mediated or not, before more intensive study to determine the exact nature of the species involved. This persistence is of major importance when the separation and subsequent identification (by other... 

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