Reactive intermediate lifetime

Although the existence of the stable and persistent free radicals is of significance in establishing that free radicals can have extended lifetimes, most free-radical reactions involve highly reactive intermediates that have fleeting lifetimes and are present at very low concentrations. The techniques for study of radicals under these conditions are the subject of the next section. 

Light intensity at the usual levels seldom has an effect on the primary photochemical step if all other variables are kept constant, although it may affect overall results considerably since it may control the concentrations of reactive intermediates. However, it will affect the outcome of a competition between primary one-photon and two-photon processes. The latter are still somewhat of a rarity but may be more important than is commonly realized, namely in rigid media where triplets have long lifetimes and quite a few of them are likely to absorb a second photon. The additional available energy may permit motion to new minima in Ti and thus give new products. 

Methods used for the direct characterisation of reactive intermediates must have a timescale shorter than or comparable to the lifetime of the reactive intermediate (Table 10.1). Because of the very short lifetimes of these transient intermediates, this can be brought about by ... 

Most chemical reactions can be slowed down by lowering the temperature. With low-temperature studies it is possible to prolong the lifetimes of the reactive intermediates so that they can be characterised by normal techniques. Matrix isolation allows experiments to be carried out at temperatures as low as 4K, in order to study species, such as radicals, that are produced photochemically at very low temperatures. The initial photoproduct is trapped within a rigid matrix that inhibits diffusion of the reactive species. The matrix material must be ... 

A species can be considered as an INC if its lifetime is long enough to allow for other chemical reactions than the mere dissociation of the incipient particles. This is the minimum criterion that has to be fulfilled to term a reactive intermediate an ion-neutral complex, because otherwise any transition state would also represent an INC. [171,178] In addition, the reorientation criterion [29,169] should be met,... 

Little has investigated monoactivated and doubly activated alkenes tethered to butenolide with respect to electroreductive cyclization [202]. The geminally activated systems 227 undergo cyclization to diastereomeric products 228 and 229 in an 1 1 mixture, whereas both the a,j8-unsaturated monoester and a,/ -unsaturated mononitrile fail to cyclize. Only saturation of the C-C double bond of butenolide is observed. The author explains these results by distinct reactivity and lifetime of the intermediate radical anions. The radical anions derived from the monoactivated olefins are less delocalized than those of 227 and therefore should be shorter lived and more reactive. In this case preferential saturation occurs. The radical anions derived from the doubly activated alkene 227 are comparatively long-lived and less basic and thus capable of attacking the C-C double bond of the butenolide moiety. A decrease in saturation, accompanied by a marked increase in cyclization, is observed. 

TABLE 2-4 Calculated Average Lifetimes of Several Reactive Intermediates in a Simulated Smog Mixture ... 

The radical or reactive intermediates discussed in Chapter 2 are included in the term photochemical oxidants for the purpose of this chapter. Of the reactive intermediates discussed, only the hydroperoxy, HO2, and singlet oxygen, O2 (a A), radicals have a lifetime long enough... 

The concentrations, average lifetimes, and rates of attack of the reactive intermediates can be calculated. with chemical models. 

Bacteria indigenous to Cr(VI)-polluted areas are Cr(VI) tolerant and/or resistant and have been considered as potential candidates for bioremediation of Cr(VI)-contaminated sites.16 However, the ability of bacteria to reduce Cr(VI) to the less-toxic Cr(III) compounds may produce reactive intermediates (such as Cr(V), Cr(IV), radicals), which are known to be active genotoxins and are likely to be carcinogenic.17 Therefore, the formation and lifetimes of Cr(V) intermediates, produced via bacterial reduction of Cr(VI), need to be evaluated carefully if microorganisms are to be employed as a means for remediation of chromium-polluted subsurface environments. Similarly, Cr(V) accumulation should first be monitored when considering plants and algae as biosorption materials for the bioremediation in the event of chromium pollution.18... 

Ordinarily these are treated as multiple reactions. However, if the intermediate R is highly reactive its mean lifetime will be very small and its concentration in the reacting mixture can become too small to measure. In such a situation R may not be observed and can be considered to be a reactive intermediate. 

Two chapters in this volume describe the generation of carbocations and the characterization of their structure and reactivity in strikingly different milieu. The study of the reactions in water of persistent carbocations generated from aromatic and heteroaromatic compounds has long provided useful models for the reactions of DNA with reactive electrophiles. The chapter by Laali and Borosky on the formation of stable carbocations and onium ions in water describes correlations between structure-reactivity relationships, obtained from wholly chemical studies on these carbocations, and the carcinogenic potency of these carbocations. The landmark studies to characterize reactive carbocations under stable superacidic conditions led to the award of the 1994 Nobel Prize in Chemistry to George Olah. The chapter by Reddy and Prakash describes the creative extension of this earlier work to the study of extremely unstable carbodications under conditions where they show long lifetimes. The chapter provides a lucid description of modern experimental methods to characterize these unusual reactive intermediates and of ab initio calculations to model the results of experimental work. 

Reaction rate, 46 101-102 Reactions, see specific types Reactive intermediates, 46 101-107, 164 flow characterization methods, 46 156-164 gas-phase studies, 46 107-121 lifetimes, 46 106... 

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