Radicals detected by ESR

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. 

Composition of Parent Pitch. Once the chemical composition of the carbonizing system moves away from the comparative simplicity of polynuclear aromatic hydrocarbons to that of industrial pitches, then the pyrolysis chemistry incorporates effects caused by the presence of heteroatoms (0, N and S) and alkyl and naphthenic groups. In general terms, the system becomes more Reactive creating higher concentrations of radicals detectable by ESR. This in turn, leads to enhanced cross-linkages and polymerization of molecular constituents of any mesophase which is formed, and this causes enhanced viscosity and a reduction in size of optical texture. 

Short-lived free radicals can also be trapped chemically by so called spin trap molecules to produce a more stable radical detectable by ESR. The hyperfine structure of this radical is used to identify the radical initially trapped. The first experiments were reported in 1968-1969 [11-13]. The method is much used in biological applications. 

Free radicals formed by scission of chemical bonds are readily detected by ESR and the chemical structures of the formed radicals can be identified by analysis of the observed ESR spectrum. A successul ESR observation from any sample is positive evidence for the presence of unpaired electrons in the sample, i.e. the existence of free radicals in an organic system. The ESR intensity is always proportional to the number of broken bonds, and therefore to the number of free radicals in the sample. It is easy to estimate relative concentrations of free radicals detected by ESR. This can be done simply by comparing the observed intensities. Estimation of an absolute concentration is not so difficult if a standard sample is available. Free radical species can be identified by analyzing the ESR spectrum observed from the radicals. Formulae for spectrum analyses are well established. Variation in free radicals induced by thermal changes in ESR spectra can be detected from temperature variations and chemical reactions caused by the radicals can be followed by identification of the free radicals from the observed spectra. 

Adding a radical trap like BulNO to the reaction mixture this reacts with radicals (R ) forming nitroxide radicals Bul(R )NO that can be detected by ESR. 

As seen before, the radical cation of dimethyl sulfoxide (CH3)2SO has been detected by ESR spectroscopy among other radicals when DMSO glasses at 77 K are submitted to y-irradiation28. It has also been reported in pulse radiolysis experiments30 (Table 6). Constant current electrochemical oxidation of bis(dialkylamino)sulfoxides (R2N)2SO gives rise to radical cations which have been detected by ESR spectroscopy33. 

Detection of an Intermediate. In many cases, an intermediate cannot be isolated but can be detected by IR, NMR, or other spectra. The detection by Raman spectra of NOj was regarded as strong evidence that this is an intermediate in the nitration of benzene (see 11-2). Free radical and triplet intermediates can often be detected by ESR and by CIDNP (see Chapter 5). Free radicals [as well as radical ions and EDA complexes] can also be detected by a method that does not rely on spectra. In this method, a doublebond compound is added to the reaction mixture, and its fate traced. One possible result is cis-trans conversion. For example, cis-stilbene is isomerized to the trans isomer in the presence of RS- radicals, by this mechanism ... 

These radical anions have been detected by ESR. This mechanism is consistent with the following result when nitrosobenzene and phenylhydroxylamine are coupled, and N labeling show that the two nitrogens and the two oxygens become equivalent. Unsymmetrical azoxy compounds can be prepared by combination of a nitroso compound with an N,N-dibromoamine. Symmetrical and unsymmetrical azo and azoxy compounds are produced when aromatic nitro compounds react with aryliminodimagnesium reagents, ArN(MgBr>2. ... 

Azaquinoid structures have been detected by ESR along with several radical species formed by the in situ radiolysis of 3- and 5-hydroxy-2-pyridones in aqueous solutions (76T261 79JPC2407 90T2891). 

The presence of free radicals in solutions of the S82+ dication has been known for a long time. Initially, S4+ was detected by ESR, and its formation... 

Photolysis of dicyclopentadienyltin results in formation of the Cp- radical (again detected by ESR), along with the precipitation of some unidentified yellow solid54. In contrast, photolysis of dicyclopentadienyllead produces no Cp-, unless di-f-butyl peroxide or biacetyl are added to the reaction mixture. The trimethylstannylcyclopentadienyl radical was produced by photolysis of bis(trimethylstannyl)cyclopentadiene (reaction 35), and was detected using ESR spectroscopy57. 

The initial electron transfer to form the anion radical species seems to be reversible. For example, Allred et al. investigated the ac polarography of bis(trimethylsilyl)benzene and its derivatives which showed two waves in di-methylformamide solutions [71] the first one is a reversible one-electron wave, and the second one corresponds to a two-electron reduction. Anion radicals generated by electrochemical reduction of arylsilanes have been detected by ESR. The cathodic reduction of phenylsilane derivatives in THF or DME at — 16° C gives ESR signals due to the corresponding anion radicals [5] (See Sect. 2.2.1). 

In order to characterize electron acceptor (basic type) properties of the samples, tetracyano ethylene compound, known to be easily ionizable in TCNE radical anion, was introduced at room temperature in the samples outgassed at different temperatures up to 800°C. No ESR signal was observed. As steric hindrance could preclude the experiment, smaller molecules as SO and p-dinitro benzene were also introduced. Then too, no ESR spectrum could be detected although the ESR technique is extraordinarly sensitive. It may thus be concluded that the ZSM-5 and ZSM-11 materials did not exhibit electron donor (basic) properties as detectable by ESR. 

Broad-band irradiation from a xenon arc lamp by Felix and Sealy produced semiquinone radicals from catecholamines. They were detected by ESR spectra of their metal complexes. Semiquinones gave rise to o-quinones and... 

Chignell etal. intensely irradiated p-aminobenzoic acid with a xenon arc lamp and showed the formation of several free radicals detected by spin traps in conjunction with ESR spectroscopy. It was suggested that such radicals could cause lipid peroxidation or react via one of the excited forms of oxygen [42, 43],... 

Special spin-trapping techniques are also available for the detection of short-lived radicals in both homogeneous and heterogeneous systems. For instance, a-phenyl A-ferf-butyl nitrone (PBN), ferf-nitrosobutanc (f-NB), -(4-pyridyl A-oxidc) A-ferf-butyl nitrone (4-POBN), or 5,5-dimethyl-l-pyrroline A-oxidc (DMPO) can be made to react with catalytic intermediates to form stable paramagnetic adducts detectable by ESR [135], Radicals evolving into the gas phase can also be trapped directly by condensation or by using matrix isolation techniques [139],... 

In order to identify organic free - radicals present at quantifiable concentrations during the sonication of PCBs, we employed Electron Spin Resonance (ESR) with a spin trap, N-t-butyl-a-phenyl-nitrone (PBN). PBN reacts with the reactive free - radicals to form more stable spin-adducts, which are then detected by ESR. The ESR spectrum of a PBN spin adduct exhibits hyperfine coupling of the unpaired election with the 14N and the (3-H nuclei which leads to a triplet of doublets. The combination of the spin-adduct peak position and peak interval uniquely identifies the structure of a free-radical. 

Chiral 2-imidazoline dianions undergo one-electron oxidation in the presence of TEMPO (2,2,6,6-tetramethyl-l-piperidinyloxy) to form a radical anion that is either trapped stereoselectively by TEMPO or undergoes dimerization. Oxidation of bis-diazene oxides leads to novel (9-stabilized 4N/3e radical cations and 4N/2e dications. These were detected by ESR spectroscopy and cyclic voltammetry. B3LYR/6-31G calculations confirmed the nature of the 4N/3e and 4N/2e systems. ... 

The cation-radicals of stilbene have been detected by ESR spectroscopy. These cation-radicals are accumulated and then consumed in the course of consecutive reactions. The stereoisomeric composition of the final products occurs to be constant and does not depend on the configuration of the initial substrate. Acetoxylation of the olefinic bond in cix-stilbene is almost one order of magni-tnde slower than in trany-stilbene. This kinetic feature deserves a special explanation, because cis-stilbene is less stable thermodynamically than irani-stilbene and should react faster. The products obtained are depicted in Scheme 2.29. 

Nitroso compounds, nitrones, and other diamagnetic molecules are used as spin traps. Capturing radicals prodnced in the reaction, spin traps form the so-called spin adducts—stable nitroxyl radicals easily detectable by ESR spectroscopy. In other words, the progress of the reaction can easily be followed by an increasing intensity of the spin-adduct signal. By and large, the method of traps reveals radicals by the disappearance (or appearance) of the ESR signal. 

Although sulfur vulcanization has been studied since its discovery in 1839 by Goodyear, its mechanism is not well understood. Free-radical mechanisms were originally assumed but most evidence points to an ionic reaction [Bateman, 1963]. Neither radical initiators nor inhibitors affect sulfur vulcanization and radicals have not been detected by ESR spectroscopy. On the other hand, sulfur vulcanization is accelerated by organic acids and bases as well as by solvents of high dielectric constant. The ionic process can be depicted as a chain reaction involving the initial formation of a sulfonium ion (XI) by reaction of the polymer with polarized sulfur or a sulfur ion pair. The sulfonium ion reacts with a polymer molecule by hydride... 

The polymerization proceeds via a radical chain-reaction mechanism, judging from some features of the polymerization initiation by irradiation or upon heating, no formation of oligomers, and polymer formation irrespective of the medium or atmosphere. The propagating radicals are readily detected by ESR spectroscopy during polymerization in the crystalline state (Fig. 2), because termination between the propagating radicals occurs less frequently in the solid state [50]. 

Nitric oxide can be scavenged as a stable, ESR-detectable free radical by a quinodimethane intermediate, l,2-disopropylidene-3,5-cyclohexadiene, which can be generated by photolysis of l,l,3,3-tetramethyl-2-indanone. This approach offers the possibility of detecting and possibly quantitating NO produced by biological systems [24]. Free NO in solution is not detectable by ESR. 

A species believed to be the monomer cation radical of 9-ethylcarbazole as a green solution in acetonitrile formed by oxidation of 9-ethylcarbazole with iodine-silver(I) perchlorate, was detected by ESR spectroscopy, although the perchlorate of the cation radical could not be isolated subsequent treatment with potassium iodide gave 9,9 -diethyl-3,3 -bicarbazole. The borofluoride salts generated as crystalline materials by oxidation of carbazole or 9-methylcarbazole with tropylium borofluoride in acetonitrile followed by precipitation with methanol are not salts of the monomer cation-radicaP as originally believed. Russian workers have suggested that nitration of carbazole proceeds via a cation radical. ... 

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