ESR of radical

One way to make the short-lived intermediates amenable to study is to increase their lifetime, usually by irradiation in the solid state and/or at very low temperatures. The intermediates can be then detected at the end of the irradiation by ESR or optical absorption spectroscopy. The ESR of radicals in the solid state is carried out on single crystals, polycrystalline samples or frozen aqueous solutions. In the two latter cases the identification of radicals from the ESR spectra is frequently difficult and, for better identification, the ESR should be done on irradiated single crystals. 

In an electron spin resonance spectrometer, transitions between the two states are brought about by the application of the quantum of energy hv which is equal to g H. The resonance condition is defined when hv = g H and this is achieved experimentally by varying H keeping the frequency (v) constant. Esr spectroscopy is used extensively in chemistry in the identification and elucidation of structures of radicals. 

Solutions of colourless pyrazolidinediones give highly coloured solutions of radicals (113) when treated with lead dioxide. The ESR spectra of these radicals have been recorded (78JOC808). They dimerize to tetrazenes (114) which appear to be indefinitely stable. 

ESR can detect unpaired electrons. Therefore, the measurement has been often used for the studies of radicals. It is also useful to study metallic or semiconducting materials since unpaired electrons play an important role in electric conduction. The information from ESR measurements is the spin susceptibility, the spin relaxation time and other electronic states of a sample. It has been well known that the spin susceptibility of the conduction electrons in metallic or semimetallic samples does not depend on temperature (so called Pauli susceptibility), while that of the localised electrons is dependent on temperature as described by Curie law. 

Occasionally, equilibria between a quinoid and a diradicaloid form of tetraazafulvaleiies of type 77 have been discussed (66AG303 72NKK100 79JOC1241). Based on ESR measurements, only traces of radicals (0.1% at 200°C) could be observed and therefore 77 (Ar = Ph) exists at room temperature predominately in the quinoid structure. Other authors stated that the thermochromism of 77 mainly results from a change in intermolecular interaction, not from biradical formation (84MI1030). 

Addilion of benzophenone to the lithium derivative 2 (M = Li) proceeds in a stereorandom fashion, which is attributed to the participation of radicals, detected by ESR and produced by single-electron transfer (SET)12. The magnesium derivative reacts with 90% diastereoselectivity with no SET being recorded. Benzaldehyde as the carbonyl compound affords the [1/, 1(1S)]-and [15,1(1/ )]-diastereomers in a 70 30 mixture, with 40% de12. Enhanced selectivities are achieved with camphor-derived 2-(2-oxazolyl)isoquinolines12a. 

Tench and Coppens (Ref 13) photolyzed o-nitrobenzaldehyde, nitrobenzene, and nitroben-zoic acid with light > 3500A. ESR measurements revealed the presence of radicals for o-nitrobenzaldehyde (in solns and powder), with no radicals observed with nitrobenzene and... 

Trofimov and coworkers19 studied the yields of the radicals by ESR in the radiolysis at 100-110 K of diphenyl sulfoxide and tetramethylene sulfoxide. They found for tetramethy-lene sulfoxide a higher yield of radicals (G = 1.8) than for tetramethylene sulfide (G = 0.45). 

Rao and Symons49 studied the formation of radicals in y-radiolysis of dilute solutions of dimethyl sulfoxide in fluorotrichloromethane. By ESR studies they found the radical cation (CH3)2SOt whose ESR spectrum show considerable g anisotropy and small methyl proton hyperfine coupling. 

II. ESR SPECTROMETRY OF RADICAL ANIONS AND RADICAL CATIONS OF SULFOXIDES AND SULFONES... 

A number of radical anions of sulfur-containing aromatic compounds have been studied essentially by means of ESR spectroscopy and sometimes by electronic spectroscopy. The studied compounds include aromatic rings separated by the oxidized sulfur functionality. The effects caused by the latter depend on the geometry and topology of the aromatic systems as well as on the electron-withdrawing ability of the other substituents. 

Bowman and Symons145 probed the stability of a series of radical anions involved in the SRN1 substitution for a-substituted aliphatic nitro-compounds [Me2C(X)N02] by studying with ESR at 77 K the succession of events following electron capture by Me2C(X)N02. The radical anions were more concentrated in an ether matrix than in an... 

Benson17 has tried to collect some thermodynamic data based on a number of empirical rules for this class of radicals. He estimated heats of formation for HS02, MeSO 2) PhSO 2 and HOSO 2 as —42, —55, —37 and — 98kcalmor respectively. He also estimated a stabilization energy for the benzenesulfonyl radical of 14 kcal mol"1, which is very similar to that of the benzyl radical. However, recent kinetic studies18 (vide infra) have shown that arenesulfonyls are not appreciably stabilized relative to alkanesulfonyl radicals, in accord with the ESR studies. 

An intramolecular mechanism of the rearrangement has been shown in the special ESR study (refs. 21, 22), conducted on the model radicals, generated by abstraction of a bromine atom from T2 " the rate constant K2, equal to (5.0 + 0.3) X 104 sec- at 22°C, has been also determined. In addition, fragmentation of radical... 

Other evidence cited for SET mechanisms has been detection of radical or radical ion intermediates by ESR or CIDNP the finding that such reactions can take place at 1 -norbomyl bridgeheads and the formation of cyclic side products when the substrate has a double bond in the 5,6 position (such substrates are called radical probes). 

The use of selective deuteration is a powerful tool in electron spin resonance (ESR) experiments, in order to establish unequivocal assignments of experimental spectra of radicals. The reason for this is, as is well known, the difference in magnetic properties between the deuteron and the proton, which can be exploited to distinguish chemically inequivalent hydrogens in the molecule. 

Figure 1. Apparatus for the preparation of radical anions (11). On connection of the entire vessel to the vacuum system, traces of water and oxygen on the wall are removed by heating and discharging with a tesla coil. When the apparatus is filled with purified nitrogen through A, the weighed sample of the hydrocarbon is put into B through C, a piece of sodium is put into D, and dimethoxyethane is distilled into E, where a small amount of an Na-K alloy is added. After the system is again evacuated the solvent is distilled from E into B, the bulb E is,sealed off at F, and the sodium is sublimed to form a mirror on the wall of the bulb G. After tubes at C and H are sealed off, the apparatus is pumped to high vacuum for 1 hr and then sealed off at J. Then the solution of the hydrocarbon is poured from B into G. After a time varying from several minutes to several hours, a color is observed, and the sample is ready for optical and esr measurements.

Figure 3. Controlled-potential electrolysis cell for generation of radical ions in the cavity of esr spectrometer [from (16) by permission of the authors and the American Chemical Society]. 

Electrochemical reductions and oxidations proceed in a more defined and controllable fashion because the potential can be maintained at the value suitable for a one-electron transfer and the course of the electrolysis can be followed polarographically and by measurement of the esr or electronic spectra. In some cases, conversion is low, which may be disadvantageous. Electrolytic generation of radical ions is a general method, and it has therefore become widely used in various applications. In Figures 3 and 4, we present electrochemical cells adapted for esr studies and for measurements of electronic spectra. Recently, electrochemical techniques have been developed that permit generation of unstable radicals at low temperatures (18-21). 

These methods can give us useful information on radicals in a manner similar to that for closed-shell systems, provided the exploitation is correct. Of course, in expressions for total energy, bond orders, etc., a singly occupied orbital must be taken into account. One should be aware of areas where the simple methods give qualitatively incorrect pictures. The HMO method, for example, cannot estimate negative spin densities or disproportionation equilibria. On the other hand, esr spectra of thousands of radicals and radical ions have been interpreted successfully with HMO. On the basis of HMO orbital energies and MO symmetry... 

Direct esr evidence for the intermediacy of radical-cations was obtained on flowing solutions of Co(III) acetate and a variety of substituted benzenes and polynuclear aromatics together in glacial acetic acid or trifluoroacetic acid solution . A p value of —2.4 was reported for a series of toluenes but addition of chloride ions, which greatly accelerated the reaction rate, resulted in p falling to —1.35. Only trace quantities of -CH2OAC adducts were obtained and benzyl acetate is the chief product from toluene, in conformity with the equation given above. 

A number of methods are available for following the oxidative behaviour of food samples. The consumption of oxygen and the ESR detection of radicals, either directly or indirectly by spin trapping, can be used to follow the initial steps during oxidation (Andersen and Skibsted, 2002). The formation of primary oxidation products, such as hydroperoxides and conjugated dienes, and secondary oxidation products (carbohydrides, carbonyl compounds and acids) in the case of lipid oxidation, can be quantified by several standard chemical and physical analytical methods (Armstrong, 1998 Horwitz, 2000). 

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