Radical allenes

The vibronic coupling model has been applied to a number of molecular systems, and used to evaluate the behavior of wavepackets over coupled surfaces [191]. Recent examples are the radical cation of allene [192,193], and benzene [194] (for further examples see references cited therein). It has also been used to explain the lack of structure in the S2 band of the pyrazine absoiption spectrum [109,173,174,195], and recently to study the photoisomerization of retina] [196],... 

Polymerization thermodynamics has been reviewed by Allen and Patrick,323 lvin,JM [vin and Busfield,325 Sawada326 and Busfield/27 In most radical polymerizations, the propagation steps are facile (kp typically > 102 M 1 s l -Section 4.5.2) and highly exothermic. Heats of polymerization (A//,) for addition polymerizations may be measured by analyzing the equilibrium between monomer and polymer or from calorimetric data using standard thermochemical techniques. Data for polymerization of some common monomers are collected in Table 4.10. Entropy of polymerization ( SP) data are more scarce. The scatter in experimental numbers for AHp obtained by different methods appears quite large and direct comparisons are often complicated by effects of the physical state of the monomei-and polymers (i.e whether for solid, liquid or solution, degree of crystallinity of the polymer). 

Since the n bonds of allenes are orthogonal, the rates of formation of radicals resulting... 

An analogous stepwise mechanism was also proposed by Wohrle [36] for the cation-radical-initiated cycloaddition of electron-rich allenes with pentamethyl-cyclopentadiene in the presence of tris (p-tolyl) aminium hexafluoroantimonate (TTA SbF6 ) (Equation 1.15). 

Schmittel M., Woehrle C., Bohn I. Radical Cation Initiated Cycloaddition of Electron-Rich Allenes. Evidence for a Stepwise Mechanism. Acta Chem. Scand. 1997 57 151-157... 

Keywords radical cation, stepwise mechanism, electron-rich allenes... 

The groups R2N and Cl can be added directly to alkenes, allenes, conjugated dienes, and alkynes, by treatment with dialkyl-V-chloroamines and acids. " These are free-radical additions, with initial attack by the R2NH- radical ion. " N-Halo amides (RCONHX) add RCONH and X to double bonds under the influence of UV light or chromous chloride. " Amines add to allenes in the presence of a palladium catalyst. ... 

According to Bielski and Allen , since the HO2 radical is of amphoteric character, it will be either protonated... 

Blake, D.R., Allen, R.E. and Lunec, J. (1987). Free radicals in biolr cal systems - a review oriented to inflammatory processes. Br. Med. Bull. 43, 371-385. 

Proton-coupled intramolecular electron transfer has been investigated for the quinonoid compounds linked to the ferrocene moiety by a 7r-conjugated spacer, 72 (171) and 75 (172). The complex 72 undergoes 2e oxidation in methanol to afford 74, which consists of an unusual allene and a quinonoid structure, with the loss of two hydrogen atoms from 72 (Scheme 2). The addition of CF3SO3H to an acetonitrile solution of 74 results in two intense bands around 450 nm, characteristic of a semi-quinone radical, and a weak broad band at lOOOnm in the electronic... 

Jacob, Margaret C. The radical enlightenment pantheists, Freemasons and republicans. London, Boston Allen Unwin, 1981. xiii, 312 p ISBN 0049010298... 

Fig. 23. Summary of the unimolecular dissociation processes of the ground electronic state ally 1 radical at 115kcal/mol. Two competing pathways, formation of allene and isomerization to 2-propenyl (which subsequently dissociates into allene and propyne), are observed. The experimental (bold) and RRKM (italics) rates are given. The allene formation is the dominating channel. (From Fischer et at.145)...

Butler and co-workers have taken a unique approach to study the unimolecular dissociation of the vibrationally and rotationally hot allyl radical.150-152 They have examined the secondary C-H dissociation of the allyl radicals that are produced with high internal energies above the allyl dissociation thresholds in the primary photodissociation of allyl chloride and allyl iodide at 193 nm. The production of allene versus propyne (both at mass 40) from the secondary dissociation of the hot allyl radicals are... 

Scheme 3.48. Thermolysis of benzoenynes-allenes initiating a domino radical cyclization.

The quantitative aspects of track reactions are involved some details will be presented in Chapter 7. The LET effect is known for H2 and H202 yields in aqueous radiation chemistry. The yields of secondary reactions that depend on either the molecular or the radical yield are affected similarly. Thus, the yield of Fe3+ ion in the Fricke dosimeter system and the initiation yield of radiation-induced polymerization decrease with LET. Numerous examples of LET effects are known in radiation chemistry (Allen, 1961 Falconer and Burton, 1963 Burns and Barker, 1965) and in radiation biology (Lamerton, 1963). 

Debierne (1914) was the first to suggest a radical reaction theory for water radiolysis (H and OH). In various forms, the idea has been regenerated by Risse (1929), Weiss (1944), Burton (1947, 1950), Allen (1948), and others. Platzman (1953), however, criticized the radical model on theoretical grounds and proposed the formation of the hydrated electron. Stein (1952a, b) meanwhile had suggested that both electrons and H atoms may coexist in radiolyzed water and proposed a model in which the electron digs its own hole. Later, Weiss (1953, 1960) also favored electron hydration with ideas similar to those of Stein and Platzman. In some respects, the theoretical basis of these ideas is attributable to the polaron (Landau, 1933 Platzman and... 

Discovery of the hydrated electron and pulse-radiolytic measurement of specific rates (giving generally different values for different reactions) necessitated consideration of multiradical diffusion models, for which the pioneering efforts were made by Kuppermann (1967) and by Schwarz (1969). In Kuppermann s model, there are seven reactive species. The four primary radicals are eh, H, H30+, and OH. Two secondary species, OH- and H202, are products of primary reactions while these themselves undergo various secondary reactions. The seventh species, the O atom was included for material balance as suggested by Allen (1964). However, since its initial yield is taken to be only 4% of the ionization yield, its involvement is not evident in the calculation. 

Takemura and Shida54 prepared the allene radical ion by /-radiolysis of halocarbon solid solution of allene at low temperatures and showed that the radical cation has a lower D2 structure than the precursor with a skew angle of 30-40°. Kubonzo and coworkers55 56 produced by /-radiolysis in a low-temperature halocarbon matrix several derivatives of the allene radical cation, i.e. the radical cations of 1,2-butadiene, 3-methyl-1,2-butadiene,... 

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