Transient radical intermediates

The mechanism of radical generation initially occurs via UV irradiation of the oxide producing surface trapped holes, O . Electron transfer from the adsorbed ketone to the surface trapped hole occurs, producing an adsorbed cation radical. This quickly deprotonates by H transfer to the surface oxide, forming an intermediate radical which subsequently reacts with molecular oxygen to form the adsorbed alkylperoxy radical observable by EPR [48]. 

Unlike the ionic O2 radicals, these neutral peroxy radicals are mobile and can easily diffuse across the TiO) surface, undergoing further oxidative and decompo- 

At least three sites exist for stabilization of the Ol radicals on the polycrystalline Ti02 powder, as evidenced by three different g-z values of 2.019, 2.023 and 2.026. 

The reactivity of each O2 species towards acetone is not, however, the same the species characterized by = 2.019 reacts first, followed by the second species with = 2.023. Simultaneously new signals emerge at 2.032 and 2.035. These are due to another family of organoperoxy radicals (that is the [acetone-05 surface complex), which are thermally unstable. The most reachve O5 species with spin Hamiltonian parameters of g, = 2.005, gyy = 2.011, g = 2.019, = 7.64mT 

All of these radical intermediates, classified as organoperoxy species, are thermally unstable, and cannot be observed at temperatures above 250 K. This may partly explain why they have not been widely studied to date by EPR, and at least emphasizes the need to perform the experiments at low temperatures if a complete picture of the oxidahve decomposihon pathways in heterogeneous photocatalysis is to emerge. 

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This type of polymerisation can provide direct evidence for the existence of transient radical intermediates. Transition metals in higher valence state or such metals complexed with reducible... 

Ashby, E. C. Methods of Detecting Transient Radical Intermediates in Organic Reactions Southeastem/Southvestem Regional ACS Meeting, Memphis, TN, October, 1985. 

Several ThDP-dependent enzymes, e.g. pyruvate oxidase (POX) and pyr-uvate ferredoxin oxidoreductase (PFOR), form a transient radical intermediate after one-electron oxidation by neighbouring redox-active cofactors such as flavins (in the case of POX) and Fc4S4 clusters (in the case of PFOR) (Tittmann 2009). The radical intermediates are sometimes stable even at room... 

Modem real time instmmental methods permit analyses of unstable transient species and the free-radical intermediates as well. These methods have gready expanded the scope and power of VPO studies, but important basic questions remain unresolved. Another complication is the role of surface. Peroxide decompositions and radical termination reactions can occur on a surface so that, depending on circumstances, surfaces can have either an inhibiting or accelerating effect. Each surface has varying amounts of adventitious contaminants and also accumulates deposits during reaction. Thus no two surfaces are exactly alike and each changes with time. 

In this chapter, we will consider examples of RIs characterized by a hypervalent or valency-deficient carbon, such as carbocations, carbenes, carbanions, and carbon radicals. In the first part, we will consider examples that take advantage of stabilization and persistence to determine their structures by single crystal X-ray diffraction. In the second part we will describe several examples of transient reactive intermediates in crystals. ... 

Fig. 8 Irreversible photoconversion of AvGFP. (a) Modification of the absorption spectra of AvGFP under UV light (A = 254 nm, 100 s irradiation, 12.9 mW) at 293 K, pH 8.0, showing the increase in anionic B band (maximum at 483 nm). (b) Proposed Kolbe mechanism for Glu222 decarboxylation through transient formation of a CH2 radical intermediate. Reproduced with permission from [166]...

In the absence of 1,4-CHD, the biradical 55e undergoes an intramolecular 1,5-hydrogen shift to form 57, making it possible for an intramolecular radical-radical coupling to occur to produce 58 (Scheme 20.13). The fact that 58 was produced from 54e lends support to the formation of the a,3-didehydrotoluene biradical 55e as a transient reaction intermediate. It is also worth noting that the benzylic radical center in 55 is a stabilized triarylmethyl radical. 

In the absence of either HPI or Co(acac)2, no appreciable conversion into products was obtained. EPR evidence for the formation of an aminoxyl radical intermediate was acquired, and a KIE of 3.8 determined . Generation of PINO in situ as the reactive intermediate was postulated . A subtle alternative enables the functionalization of hydrocarbons through the formation of carbocations as transient intermediates whenever PINO is formed and reacts in the presence of NO (e.g. 1 atm) . 

As with any intermediate, a transient radical can be implicated from products formed in a reaction specific to the radical of interest. Experimentally, this is the basis of so-called mechanistic probe studies. An application of this method might employ, for example, 6-bromo-l-hexene as a probe for a radical intermediate as shown in Figure 4.3. If the 5-hexenyl radical is formed as a transient with an adequate lifetime, then cyclization of this radical to the cyclopentyhnethyl radical could eventually give the cyclic product, and detection of the cyclic product provides evidence that a radical was formed. The mechanistic probe approach is deceptively simple, however. To be useful, one must exclude other possibilities for formation of the rearranged product and demonstrate that the transient was formed in the reaction of interest and not in a side reaction. The latter is especially difficult to demonstrate, and, unfortunately, some mechanistic probe studies that seemingly provided proof of radical intermediates were later found to be complicated by radical-forming side reactions. 

This reaction plays an important role in the oxidation of hydrocarbons, as illustrated in Scheme 1,2 5-267 self-reaction of the intermediate peroxyl radicals to form transient tetraoxide intermediates 44 was shown by the direct observation of these intermediates at low temperatures, and isotope scrambling when 2 and 02 was used in auto-oxidation with formation of Tetraoxides with secondary hydrogens (45) can undergo... 

SThe shape of actual potentiodynamic transients is sometimes affected by a change in the dominant crystal orientation of facets in polycrystals and by impurity adsorption. There is a conflict between the desire to make short transients (< 1 s) and thus be less affect ed by diffusion, etc. and the need for radical intermediates to attain steady state at each of the constant potentials used. 

A considerable amount of evidence has accumulated recently for the transient existence of free-radical intermediates in systems containing oxidizing catecholamines. Walaas and Walaas and their co-workers have shown that the interaction of catecholamines with cupric ion (either bound, as in ceruloplasmin, or as the free ion, cf. ref. 80) results initially in reduction of the copper atom to the... 

Conversely, the stability of compounds of oxidation state +11 increases dramatically as the atomic number of the element increases. Carbenes, CX2, and silylenes, SiX2, are well established as transient reaction intermediates, and structural data have been obtained in several cases either at high temperatures or by their generation in low-temperature matrices. However, only for germanium, tin and lead are compounds in this oxidation state stable under ordinary conditions. Compounds with the Group IV element in oxidation state +111, the formal oxidation state of the radical species, R3M, are also usually considered as unstable transients. However, when R is very bulky, these metal-centred radicals, such as for example Sn[CH(SiMe3)2]3, have extremely long, perhaps indefinite, lifetimes in solution. 

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