Characterization of Radicals

Experimental structure elucidation techniques can be used to characterize radicals if conditions can be found in which the radicals are produced in higher concentration and with longer lifetimes than is the case under typical reaction conditions. For example, the relative stability of the triphenylmethyl radical (4) allows it to be studied by magnetic susceptibility determination. 

Longer-lived radicals are termed kinetically stable or persistent. Extremely persistent radicals have been termed inert-. Julia, L. Ballester, M. Riera, Castaner, J. Ortin, J. L. Onrubia, C. /. Org. Chem. 1988, 53, 1267. 

Persistent free radicals have found application as image-enhancing agents for studies of pH and of molecular oxygen concentrations in biological systems Reddy, T. J. Iwama, T. Halpern, H. J. Rawal, V. H. J. Org. Chem. 2002, 67, 4635 Bobko, A. A. Dhimitruka, I. Zweier, J. L. Khramtsov, V. V. /. Am. Chem. Soc. 2007,129, 7240. 

For a discussion of substituent effects on the stability of radicals, see Creary, X. Acc. Chem. Res. 2006, 39, 761. 

In EPR spectroscopy, the sample is placed in a magnetic field and is irradiated with electromagnetic energy of an appropriate frequency. If the magnetic field is varied linearly, absorption of energy by the sample can be 

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Focsan, A. L., M. K. Bowman et al. (2008). Pulsed EPR and DFT characterization of radicals produced by photooxidation of zeaxanthin and violaxanthin on silica-alumina. J. Phys. Chem. B 112 1806-1819. 

A review considering the generation and characterization of radical ions, their reactions, formation of species with three-electron bonds, and radical cations of strained systems has been published." The redox and acidity properties of a number of substituted benzene radical cations were smdied by pulse radiolysis. ... 

Marcinek, A., Zielonka, J., Geiibicki, J., Gordon C.M., and Dunkin, I.R., Ionic liquids Novel media for characterization of radical ions, /. Phys. Chem. A, 105, 9305-9309,2001. 

The importance of e.s.r. in this field is mainly in achieving a detailed characterization of radical structures. Because the resonance line due to the unpaired electron is split by magnetic nuclei in the radical in a very characteristic way, the technique actually counts the various types of nuclei and allows in many cases an accurate determination of their respective position and geometrical orientation. Patterns of relationships between structure and e.s.r. parameters, based on many experimental results, have been developed and applied to new systems successfully. Details of e.s.r. methods are given in many books and reviews on the subject (see especially Fessenden and Schuler, 1970 Wertz and Bolton, 1972). 

The Preparation and Characterization of Radical Cation Salts Derived from Perfluorobenzene, Perfluorotoluene, and Perfluoronaphthalene... 

One-electron oxidation of aromatic compounds (ArH) leads primarily to corresponding radical cation which exist either in monomeric (ArH +) or dimeric form [(ArH)2 ] the latter usually formulated as r-dimer [70]. However, radical cations are reactive species and can undergo further reaction yielding more persistent radical cations e.g. oxidation of rert-butylbenzene or of toluene or o-xylene yielded radical cation of 4,4 -di-rerf-butyl biphenyl, 4,4 -bitoluene or 3,3, 4,4 -tetramethyl biphenyls, products of further a-coupling, proton loss and further one-electron oxidation [71]. This is a well-known pathway of biaryl dehydrodimerization, explored in anodic and metal-ion oxidation of ArH [72, 73]. Other compounds with high reactivity in (T-coupling are alkoxy and amino substituted ArH [73]. Thus a risk with characterization of radical cations is that hardy survivors and not primary radical... 

The oxidation of methyl radical with molecular oxygen is a complicated process that involves different competing reactions. The first step of CH3 oxidation is the formation, without energy barrier, of the methylperoxy radical that is a common intermediate for the subsequent reactions. In the pathway that give CH2O + OH as final products, the existence of a CH300 >CH200H isomerization process is postulated to occur. Because of the well known difficulties in the theoretical characterization of radical reactions and the importance of the role that these reactions plays in the combustion processes, we have undertaken the study of the entire reaction profile with both HF-CI and DF methods. The localization of the transition state for the above mentioned isomerization at ab-initio HF level is possible only when the correlation is considered [49]. 

Oliva et al. [23] describe the EPR characterization of radicals trapped during the photoinitiated polymerization of trimethacrylate monomer (TMA), which leads to a highly cross-linked polymer structure also, vinylmethacrylate monomer (VMA), a difunctional monomer carrying different reactive groups, has been studied. The EPR spectra obtained at room temperature from photoinitiated samples of both trimethacrylate monomer TMA and vinylmethacrylate monomer VMA consist of a nine-line EPR pattern, typical of methacrylate propagating radicals (Structure I). 

R.H. Magnuson, W.P. Giering et al. - Detection and Characterization of Radical Cations Resulting from the Oxidation of Methyl and Acetyl Iron Complexes,... 

Radical cations can be derived from aromatic hydrocarbons or olefins by reaction with one-electron oxidants. Antimony pentachloride and cobaltic ion are among the oxidants that have been used. Most radical cations have limited stability, but the sensitivity of EPR spectral parameters to structure have permitted structural characterization of radical cations despite their limited stability. 

Oyaizu K, Suga T, Yoshimura K et al (2008) Synthesis and characterization of radical-bearing polyethers as an electrode-active material for (nganic secondary batteries. Macromolecules 41(18) 6646-6652... 

Hirama, M., Akijama, K., Tanaka, T., et al. (2000) Paramagnetic enediyne antibiotic C-1027 spin identification and characterization of radical spedes. Journal of the American Chemical Society, 122(4), 720-772. 

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