Free radicals diradicals

Electron paramagnetic resonance (EPR) spectroscopy (also called electron spin resonance (ESR) spectroscopy), is used to study paramagnetic species with one or more unpaired electrons, e.g. free radicals, diradicals, metal complexes containing paramagnetic metal centres, defects in semiconductors and irradiation effects in solids. While diamagnetic materials are EPR silent, paramagnetic species always exhibit an EPR spectrum. This consists of one or more lines, depending on the interactions between the unpaired electron (which acts as a probe ) and the molecular framework in which it is located. Analysis of the shape of the EPR spectrum (the number and positions of EPR lines, their intensities and line widths) provides information... 

Although these reactions are thus closely related to the acyl-alkyl diradical disproportionation to ketenes, the stereospecificity of (55) -> (56) and (57) -> (58) shows that these hydroxyketones cannot proceed through free radicals capable of rotating about single bonds prior to the intramolecular hydrogen... 

Oxygen itself (a diradical) is not reactive enough to be the species that actually abstracts the hydrogen. But if a trace of free radical (say R -) is produced by some... 

From the point of view of both synthetic and mechanistic interest, much attention has been focused on the addition reaction between carbenes and alkenes to give cyclopropanes. Characterization of the reactivity of substituted carbenes in addition reactions has emphasized stereochemistry and selectivity. The reactivities of singlet and triplet states are expected to be different. The triplet state is a diradical, and would be expected to exhibit a selectivity similar to free radicals and other species with unpaired electrons. The singlet state, with its unfilled p orbital, should be electrophilic and exhibit reactivity patterns similar to other electrophiles. Moreover, a triplet addition... 

A one-electron reduction of the bond between an aliphatic carbon and a halogen leads to a halogen anion and a carbon-free radical. A good example is the reduction of carbon tetrachloride as discussed earlier in this chapter. The first product in the reduction is the trichloromethyl-free radical. Carbon-centered radicals are not very reactive with biological molecules, but they react very rapidly with molecular oxygen (a diradical) to form a peroxy-free radical (Fig. 5.15), which is quite toxic (10). 

Some pairs of very strongly electron-donor and electron-acceptor monomers, such as p-methoxystyrene and dimethyl cyanofumarate, undergo spontaneous alternating copolymerizations without any added free-radical initiator, although heat may be required [Hall and Padias, 1997, 2001]. Initiation involves reaction of the comonomer pair to form a diradical,... 

Mendiara, S. N., E. Ghibaudi, L. J. Perissinotti, and A. J. Colussi, Free Radicals and Diradicals in the Reaction between Nitrous Acid and Bisulfite in Acid Aqueous Media, . /. Phys. Chem., 96, 8089-8091 (1992). 

Three-membered rings can also be cleaved to unsaturated products in at least two other ways. (1) On pyrolysis, cyclopropanes can undergo contraction to propenes.121 In the simplest case, cyclopropane gives propene when heated to 400 to 500°C. The mechanism is generally regarded122 as involving a diradical intermediate123 (recall that free-radical 1,2... 

Due to the involvement of free radicals, this biological strategy is poisoned by the presence of the diradical, 02. 

Figure 5.4. The polymerisation of diacetylenes, (a), (b) and (c) are various possible intermediate structures. A dot denotes a free radical and two dots a free diradical. This figure is reproduced by kind permission of the authors and publishers from Polydiacetylenes, Synthesis, Structure and Electronic Properties by Bloor, D. and Chance, R.R. 1985 (Kluwer Academic, Dordrecht).

Therefore, it seems reasonable to suppose that in the semiconductor catalyzed oxidation of DPE, the resulting radical cation of the olefin could react with the superoxide anion to give a peroxy-ethy1-1,4-diradical which subsequently reacts with the olefin and oxygen through the free radical chain processes. 

Free radicals, formed by homolytic bond cleavage, may couple to groups on a receptor in several ways (Fig. 2.4A). Diradicals (e.g. triplet excited states) probably abstract an atom from the receptor (e.g. H ) and the two radicals that are formed couple (Fig. 2.4b). 

A zwitterionic tetramethylene initiates ionic homopolymerization, while a diradical tetramethylene initiates free radical copolymerization. As initiating species, zwitterions are likely to remain in the coiled gauche-conformation and collapse to small molecules. Diradicals, on the other hand, are easily transferred to the trans-conformation. Accordingly, diradicals are more effective initiators and more radical copolymerizations occur than ionic homopolymerizations. Addition of solvent will also influence the reaction of polar tetramethylene. A polar non-donor solvent may permit carbenium ion polymerization, while a polar donor solvent impedes it. 

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