3 free radicals

Free radicals are characterized by an odd number of electrons, an unpaired electron in the outer valence shell. These species are exceptionally reactive, as they are always seeking to pair off their lone election. Free radicals play a central role in atmospheric chemistry in both the stratosphere and the troposphere. Important radicals include, for example, OH and H02 (both stratosphere and troposphere) and Cl and CIO (stratosphere). 

One can represent the bonding in molecules using the Lewis dot structure, in which lines represent a pair of bonded electrons and dots represent other electrons. The hydrogen and methane molecules are represented by 

The triple bond between the two nitrogen atoms makes N2 an exceptionally stable molecule. The hydroxyl (OH) radical has the structure 

NO and N02, important throughout the atmosphere, are actually radicals. Their electronic structures are 

1a What are the lifetimes of CHF2C1 (HCFC-22) and CH2C1CF3 (HCFC-133a) by reaction with OH in the troposphere Assume an average OH concentration of [OH] = 106 molecules cm-3 and an average tropospheric temperature of T = 250 K. Reaction rate constants are (Sander et al. 2003)  

Free radicals are rttrstable molecules the instability is due to the presence of impaired or extremely available electrons. For instance, the pyrolysis of tetraethyl lead releases ethyl radicals  

The tert-butyl peroxide is a compound formed from a relatively weak central bond 0-0 (CH3)3COOC(CH3)3 It is thus a product that, even at ordinary temperature, tends to break down slowly. It therefore generates free radicals (CH3)3COo, that ate likely to initiate reactions. 

The presence of unpaired electrons shottld be symbolized by a point that follows the radical s formula. The simplest form of the radical is an atom, such as the chlorine atom represented by the symbol Ck. 

In some cases, the study of molecttlar spectnrms enables us to demonstrate the presence of radicals. Mass spectrometry also corrstitutes a valuable guide for the identification of radicals or atoms. 

The aniorts and catiorrs involve two types of reactions. The first is electrochemistry, with the iorts in sohd or liquid solutions, and gas chemistry, in 

The formation of free radicals cannot he explained on the basis of classical valency theory the stability of these molecules may however, be adequately explained by the quantum theory of valency. Ethane docs not under normal circumstances dissociate into two methyl radicals. The reaction 

Thus in the free radical several valency bond structures arc possible in addition to the normal resonance structures of benzene which are still present in /, and resonance amongst all these states will lead to an increase in the stability of the free radical. Consequently, for the dissociation of dibenzyl less energy is required than for the dissociation of ethane because 

The dissociation of hexaphcnylethane leads to the formation of two triphenylmethyl radicals. With the central carbon atom in the trivalent state eight Kckuld structures are possible the nine carbon atoms in the ortho and para positions in the three rings may also be in a trivalent state and thus in all, 36 valence bond structures are possible for the free radical. As a result of the resonance among these structures there is a gain in energy of the free radical by 50 kcals, with the result that the dissociation energy of hexaphcnylethane is as low as 11-12 kcals. 

In the hexaphcnylethane molecule the valence bonds of the two ethane carbon atoms are tetrahedral, but during the dissociation this configuration is replaced by the planar configuration which is present in the free radical. The planar structure may, however, be slightly distorted owing to the repulsion of the ortho hydrogen atoms of the benzene rings. 

Free radical formed Dissociation per cent Temperature Concentralio molsjl 

Bartlett and Nozaki receive credit as being the first to demonstrate that MA can be homopolymerized with free-radical initiators. In their work, molten monomer was heated (55 C) with 4.58 wt. % BPO to yield an uncharacterized resin having a low degree of polymerization (DP) of 29. At a reaction temperature of 79.5°C, the product obtained had DP --25. 

They also showed that MA accelerates the decomposition of which 

The radical homopolymerization of MA has also been investigated in benzene at 70°C, using lauroyl peroxide, to determine kinetics for the reaction.The reaction order versus monomer and initiator concentration, as well as the activation volume, give strong support for polymerization with degradative transfer to monomer, thereby confirming Joshi s results. The kinetic results fit the Deb equation and confirm the abnormal character of the polymerization. Kellou and Jenner ° find the enthalpy and entropy values for the polymerization to be, respectively, —A// — 14 kcal/mol and —A5 = 30-35 kcal mor comparable to other vinyl polymerizations. A plot of temperature versus rate of polymerization showed a ceiling temperature of approximately 150°C, which is low for the reported entropy value. 

Others have also looked at the kinetic characteristics of the solution homopolymerization of MA, using benzene, dioxane, and acetic anhydride solvents with BPO initiator. In dioxane, polymerization rates are proportional to initiator concentration to the power, supporting a recombination mechanism for termination of the kinetic chains. Reminiscent of the y-radiation polymerization systems, an increased rate of polymerization and greater molecular weight is observed in acetic anhydride solvent. All evidence supports the assumption that acetic anhydride is not a pure solvent for the 

Higher molecular weights and much improved conversions are achieved by the incremental addition of BPO or di-r-butyl peroxide to molten MA at temperatures Thus, 1 wt. % of initiator was initially added and 

Work prior to 1973 has been comprehensively reviewed by Kreilick. (195) A review dealing with the detection of radicals by ESR, ENDOR, and NMR methods has appeared more recently. (196) Before discu ssing the more usual H and C studies of radical species mention 

Nitroxide radicals continue to be those most widely studied by NMR. This stems from their relatively high stability, high solubility in a wide variety of solvents, and their well resolved NMR spectra when measured in concentrated solutions in which spin exchange is sufficiently rapid. H and/or data have been reported for some aryl 

The A values are given by a modified version of the Karplus-Fraenkel equation (213) 

The imino nitroxide radicals (201) exhibit a similar spin distribution to that of the nitronyl nitroxide radicals. The methyl protons on the two sides of the imino nitroxide ring are found to have different coupling constants. This is thought to be due either to different relative geometries or to an interaction between the methyl protons on the 

Electron-withdrawing groups have the reverse elfect and lead to a reduced An value. The positive sign of the OMe coupling may be explained in terms of the structures [32(a)]-[32(c)] which indicate the 

We have an interesting problem in the cause of the ready dissociation of, for example, hexaphenylethane into triphenyl-methyl radicals, so that a solution of this substance is dissociated to a considerable extent (Dissociation energy is 11 kcal, Table 18 B, p. 191). 

However the dissociation energies of the C3—H bond in propene-i and the C4—C3 bond in butene-1 are also particularly low (Table i8b Schmidt s rule). 

In these two latter cases it appears with certainty from the heat of combustion and from spectra that there is nothing special about the bonds themselves. The low dissociation energy must, therefore, find its cause in the special stability of the products of dissociation, in these cases the allyl radical. The particular stability of this radical follows from the resonance which is possible here between two equivalent configurations. H2C=CH— GH2 H2C—GH=GH2 

Still more configurations can be given for the triphenyl-methyl radical in a similar way, since the unpaired electron 

24 Gen. Disc. Faraday Soc. No. 3 (1947), The Labile Molecule W. A. Waters, The Chemistry of Free Radicals, Oxford 1946. 

Compounds that have unpaired electrons in their structures are called free radicals. These compounds also do not obey the octet rule. 

Free radicals are chemically active substances. They do not have any charge. 

Two N02 molecules may easily combine and form the N204 molecule. Explain the reason for this combinaton. 

The nitrogen atom in the N02 molecule has an incomplete octet, having a single unpaired electron. The unpaired electrons of the nitrogen atoms combine to form a single bond. 

By the combination of two N02 molecules, the nitrogen atoms complete their octet and become more stable. 

A radical (often called a free radical) is an atom or a group of atoms that have one or more unpaired electrons. Thus, carbon radicals have only seven valence electrons, and may be considered electron deficient of one electron. However, they do not in general bond 

Radicals are often uncharged (neutral) but radical cations and radical anions also exist. 

Electrically neutral groups containing unpaired electrons are designated as free radicals their names are characterized by the ending. ..yl. The way in which names of parent structures are transformed into radical names in the substituent group sense has been indicated in the corresponding 

Delocalized radicals can be named by way of their specific resonance forms or with traditional summary group designations devoid of any locants. 

The names of oxygen radicals which are mostly derived from acids, alcohols, etc. are traditionally assiged the ending...oxyl or, systematically, the suffix... oxidanyl. Radical names for other hetero groups can frequently be adopted straightforwardly from the corresponding substituent prefixes. For clarification they are usually supplemented by the descriptive term Wicar. 

C6H5-C0-0 Benzoyloxyl Benzoyloxy radical (Benzoyl-oxidanyl) 

Mono- and diradicals derived from amines are traditionally called aminyls and aminylenes, respectively. 

After the first unsuccessful attempts to record a matrix IR spectrum of the methyl radical, reliable data were obtained by the use of the vacuum pyrolysis method. IR spectra of the radicals CH3 and CD3 frozen in neon matrices were measured among the products of dissociation of CH3I, (CH3)2Hg and CD3I (Snelson, 1970a). The spectra contained three absorptions at 3162 (1 3), 1396 V2) and 617 cm (I l) belonging to the radical CH3 and three bands 2381, 1026 and 463 cm assigned to the radical CD3. Normal coordinate analysis of these intermediates was performed and a valence force field calculated. In accordance with the calculations, methyl radical is a planar species having symmetry 31,. 

Photolysis of symmetrical diacyl peroxides [109] was used for generation in inert matrices of a number of alkyl radicals (see Pacansky et al., 1991 Pacansky and Waltman, 1989, and references cited therein). Thus, ethyl. 

An informative IR spectrum of the t-butyl radical, containing 18 bands, has been recorded after freezing of the products of vacuum pyrolysis of azoisobutane [110] and 2-nitrosoisobutane [111] in an argon matrix at 10 K (Pacansky and Chang, 1981). This spectrum is in agreement with a pyramidal structure of the radical (CH3)3C (symmetry C3v) which has elongated CH bonds in positions trans to the radical centre. On the basis of experimental vibrational frequencies and ab initio calculations of the radical geometry the enthalpy value [// (300)] of its formation has been calculated as 44 kJ moP.  

Phenyl radical, side by side with methyl radical, carbon dioxide and methyl benzoate, was also stabilized in an inert matrix as a product of UV photolysis of acetyl(benzoyl)peroxide [112] (Pacansky and Brown, 1983). Of nine IR bands of the radical C6H5, intense absorption at 710 cmwhich was shifted to 519 cm for the deuterium-labelled radical C Ds, has been assigned to out-of-plane CH deformation. The bands of the phenyl radical 

An IR spectroscopic study of the radicals CF3, C2F5, C3F7 and i-C3F7 has been carried out. These radicals were formed as products of vacuum pyrolysis in a platinum reactor of the respective fluorinated iodoalkanes and were stabilized in argon matrices at 10-12 K (Snelson, 1970b Butler and Snelson, 1980a,b,c) as shown in (6). 

Structures. The methyl radical is planar and has D symmetry. Probably all other carbon-centerd free radicals with alkyl or heteroatom substituents are best described as shallow pyramids, driven by the necessity to stabilize the SOMO by hybridization or to align the SOMO for more efficient pi-type overlap with adjacent bonds. The planarity of the methyl radical has been attributed to steric repulsion between the H atoms [138]. The C center may be treated as planar for the purpose of constructing orbital interaction diagrams. 

Free-radical polymerization of a 1 1 mixture of dimethyl fumarate and vinyl acetate, resulting in a highly regular alternating copolymer, illustrates the importance of substitution on the properties of both the free radical and the olefinic substrate [139]  

Radical stabilities may be measured experimentally by the determination of homo-lytic bond dissociation energies (BDEs) in the gas phase [140] or in solution by relating them empirically to the and the oxidation potentials, E ox(A ) of weak acids, 

A quantity called the radical stabilization energy (RSE) may be defined to relate the stabilities of substituted carbon radicals to the methyl radical. The effects of adjacent X , Z, and C substituents on the RSEs of carbon-centered radicals has been widely investigated [142,143]. The expectations based on simple orbital interaction theory as espoused above are widely supported by the experimental findings, except that when the the n donor or n acceptor ability of the group is weak and the inductive electron-withdrawing power is large, as in F3C and (Me N+CHj, the net effect is to destabilize the radical relative to the methyl radical [143]. The BDE of a C—H bond of a compound R—H is another measure of stability of the product radical, R. It is related to the RSE by 

TABLE 7.1. Radical Stabilization Energies (kJ/mol) of Mono- and Disubstituted Methyl Radicals, 

Electric effects detected in semiconductor oxide films during chemi-sorbtion of atom particles have been also thoroughly studied for chemi-sorbtion of various free radicals CH2, CH3, C2H5, C6H5OH2, OH, NH, NH2, etc. [41]. It was discovered that all of these particles have an acceptor nature in relation to the electrons of dope conductivity in oxide semiconductors their adsorption, as a rule, being reversible at elevated temperatures. It is clear that we deal with reversibility of electron state of the oxide film after it has been heated to more than 250-300°C in 

Excrements show that all the alkyl, hydroxyl, and amine radicals which we have studied considerably reduce the conductivity and increase the work function of oxide semiconductors like ZnO, Ti02, CdO, WO2, M0O3, etc. during chemisorbtion. It should be noted that the revealed effects are rather profound especially if we are dealing with the effect of chemisorbtion of active particles on conductivity of a thin (less than 1 pm) sintered polycrystal semiconductor films. Thus, conductivity of such films in the presence of free CH3-radicals with the concentration of even 10 cm and less may change from initial value by dozens or hundreds percent depending on experimental conditions. 

Usually, the decrease in conductivity during chemisorbtion of alkyl radicals on semiconductor oxides of n-type at elevated temperature has a reversible nature. However, the effect value under the same conditions depends on the chemical nature of adsorbent. For example, the following adsorbent activity row can be deduced if the oxides being studied are arranged in a chemisorbtion-induced conductivity descent order. In case of, say, CH2-radicals, the other experimental conditions being the same, we obtain  

It can be easily noticed that the left-hand side of the row features oxides whose metals easily form metal-organic compounds with free radicals. 

Additionally, it was deduced from experiments that the change in conductivity of a certain oxide (e.g., ZnO) caused by chemisorbtion of various alkyl radicals (the other experimental conditions being the same) is substantially dependent on the chemical nature of free radicals. The adsorbates can be put in the following activity row provided that the simplest alkyl radicals analyzed are ordered according to their effect on the conductivity of films made of the oxide selected  

It has already been pointed out (p. 73) that the postulate of free radical chain reactions provided a reasonable explanation for early results of the radiolyses of gases. It was later suggested10 that the radiation chemistry of aqueous solutions could best be explained by the production of H atoms and OH radicals. Subsequently, the results of a large variety of radiolyses were explained in terms of radical reactions occurring therein. Although such experiments did not provide conclusive evidence for the existence of free radicals in the systems, the results obtained, e.g. product analysis, rate coefficients, were not inconsistent with the occurrence of free radical processes. 

More recently the use of esr techniques has allowed the unequivocal identification of free radicals in radiolyses. It will therefore be convenient, in this section, first to discuss this evidence and then to summarise some of the other radical characteristics, e.g. structure, spectra, reactivities, which have been deduced from systems where the supposition of free radical processes provides the simplest explanation of results. 

In general, however, the effect of phase is much less marked than for ionic species and results for different phases will not be considered separately in this section. Since, in fact, more experiments have been carried out on the radiation chemistry of liquids than of gases or solids, most of the results discussed in this section refer to the liquid state. 

That free radicals are produced in the radiolysis of liquid systems was first shown conclusively by the esr experiments of Fessenden and Schuler71. The esr spectra were observed during the continuous irradiation of the liquid samples by an electron beam from a Van de Graaff accelerator. The spectrum obtained 

The results obtained indicate that for n-alkanes and for Cs-C7 cycloalkanes most radicals arise by the loss of a hydrogen atom from the parent molecule. Although some C-C bond rupture occurs in the lower hydrocarbons, its importance decreases with increasing molecular weight. More extensive C-C bond rupture was observed in branched chain hydrocarbons. 

The mechanical shearing forces experienced during processing in extruders and mixing machinery are capable of breaking the polymer chain molecules, so that each molecule forms two highly reactive free radicals. 

Free radicals are reactive because they can be regarded as deficient . The deficiency arises because they have only a single electron where two electrons would normally be required. 

Polymers often contain hydroperoxides, which can give rise to free radicals even in the absence of the shearing forces mentioned above. 

In inert atmospheres, two free radicals often simply recombine, although other less desirable reactions involving other similar polymer molecules can also take place, such as crosslinking. 

If oxygen is present, as it often is, the free radicals tend to react with it, forming light-susceptible groups that become sites of vulnerability. 

Organometallic compounds that are not ionic but polar-covalent behave very much as if they were ionic and give similar reactions. 

A free radical (often simply called a radical) may be defined as a species that contains one or more unpaired electrons. Note that this definition includes certain stable inorganic molecules such as NO and N02, as well as many individual atoms, such as Na and Cl. As with carbocations and carbanions, simple alkyl radicals are very reactive. Their lifetimes are extremely short in solution, but they can be kept for relatively long periods frozen within the crystal lattices of other molecules.137 Many spectral138 measurements have been made on radicals trapped in this manner. Even under these conditions the methyl radical decomposes with a half-life of 10 to 15 min in a methanol lattice at 77 K.139 Since the lifetime of a radical depends not only on its inherent stability, but also on the conditions under which it is generated, the terms persistent and stable are usually used for the different senses. A stable radical is inherently stable a persistent radical has a relatively long lifetime under the conditions at which it is generated, though it may not be very stable. 

Since only free radicals give an esr spectrum, the method can be used to detect the presence of radicals and to determine their concentration. Furthermore, information concerning the electron distribution (and hence the structure) of free radicals can be obtained from the splitting pattern of the esr spectrum (esr peaks are split by nearby protons).141 Fortunately (for the existence of most free radicals is very short), it is not necessary for a radical to be persistent for an esr spectrum to be obtained. Esr spectra have been observed for radicals with lifetimes considerably less than 1 sec. Failure to observe an esr spectrum does not prove that radicals are not involved, since the concentration may be too low for direct observation. In such cases the spin trapping technique can be used.142 In this technique a compound is added that is able to combine with very reactive radicals to produce more persistent radicals the new radicals can be observed by esr. The most important spin-trapping compounds are nitroso compounds, which react with radicals to give fairly stable nitroxide radicals 143 RN=0 + R — RR N—O. 

Because there is an equal probability that a given unpaired electron will have a quantum number of + J or —4, radicals cause two lines or groups of lines to appear on an electronic spectrum, and are sometimes referred to as doublets. 

Another magnetic technique for the detection of free radicals uses an ordinary nmr instrument. It was discovered144 that if an nmr spectrum is taken during the course of a reaction, certain signals may be enhanced, either in a positive or negative direction others may be reduced. When this type of behavior, called chemically induced dynamic nuclear polarization145 (CIDNP), is found in the nmr spectrum of the product of a reaction, it means that at least a portion of that product was formed via the intermediacy of a free radical.146 For example, the question was raised whether radicals were intermediates in the exchange reaction between ethyl iodide and ethyllithium (2-39)  

18 Semenoff, Chemical Kinetics and Chain Reactions, Oxford University Press, Oxford (1935). 

19 Rice and Rice, The Aliphatic Free Radicals, The Johns Hopkins Press, Baltimore (1935). 

Paneth22 first demonstrated that the decomposition by heat of certain organic compounds furnished products which removed metallic mirrors of silver, tellurium and other metals from the walls of the tube. For example when vapor of lead tetraethyl was heated a silver coating on the inside of the exit tube was removed for a considerable distance. The results were interpreted to mean that ethyl radicals were liberated in the thermal decomposition and 

Experimental evidence of the part played by free radicals in a chemical reaction was soon forthcoming. In 1934 Frey24 found that butane decomposed very slowly at 525° but that if one per cent of dimethyl mercury was introduced the decomposition proceeded rapidly. In the same year Sickman and Allen25 found that acetaldehyde was stable at 300° but that it was decomposed completely when a few per cent of azomethane was added. The introductions of dimethyl mercury or azomethane at these temperatures apparently liberated free radicals which initiated chains. Moreover when mixed gases decomposed simultaneously they did not do so independently. The products contained groups from each in a way that could be easily explained on the assumption of the liberation and recombination of free radicals. Again the appearance of butane from the decomposition of propane is difficult to explain on any hypothesis except on the assumption that some free radicals of CH3 are split out and that they become attached to propane molecules. More direct examples will be given later in the discussion of photochemistry. 

The most definite and indisputable evidence for the existence of free radicals is obtained from spectroscopy. Physicists are able to interpret band spectra without ambiguity on the basis of such units as OH, CN, BeCl, SiO, CH2, C2 and others. There is a whole host of radicals of this type which can explain quantitatively all the lines of a complex band spectrum and there is no other way to explain them. Moreover calculations based on quantum mechanics show that many of these free radicals, which violate all rules of the classical theories of valence, are stable and do not necessarily decompose at ordinary or even at moderately high temperatures. The difficulty in finding them is not that they are too unstable but rather that they are so very reactive that they combine immedi- 

There is a fly in the ointment. EPR spectroscopy can detect only molecules that possess one or more unpaired (Sections 

The few organic molecules that do possess an unpaired electron and are paramagnetic are called free radicals. They 

Nitric Oxide Synthase (NOS) is postulated to have a role in RGC axonal toxicity. Inducible NOS is knovTi to be up regulated in the optic nerve in glaucoma (Liu and Neufeld, 2001). Inhibitors of NOS, such as 3-aminoguanidine, dea eased the RGC loss by 70% in rat eyes with raised lOP (Neufeld et al., 1999). 

Dopamine has been recognized to have neuroprotective actions in glaucoma. Retinal dopaminergic cells can be detected by the immunohistochemical staining of tyrosine hydroxylase, the rate-limiting enzyme in dopamine synthesis. It has been observed that NMDA induced RGC toxicity dramatically decreased tyrosine hydroxylase immunostaining at the June don betw een the inner nuclear layer and inner plexiform layer in glaucoma (Kitaoka and Kumai, 2004). 

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Polymerization reactions. There are two broad types of polymerization reactions, those which involve a termination step and those which do not. An example that involves a termination step is free-radical polymerization of an alkene molecule. The polymerization requires a free radical from an initiator compound such as a peroxide. The initiator breaks down to form a free radical (e.g., CH3 or OH), which attaches to a molecule of alkene and in so doing generates another free radical. Consider the polymerization of vinyl chloride from a free-radical initiator R. An initiation step first occurs ... 

The reaction between hydrogen and chlorine is probably also of this type and many organic free radical reactions (e.g. the decomposition of ethanal) proceed via chain mechanisms. 

The free radicals which have only a transient existence, like -CHa, C2H5 or OH, and are therefore usually met with only as intermediates in chemical reactions, can usually be prepared and studied directly only at low pressures of the order of 1 mm, when they may be transported from the place of preparation in a rapidly streaming inert gas without suffering... 

After the primary step in a photochemical reaction, the secondary processes may be quite complicated, e.g. when atoms and free radicals are fcrnied. Consequently the quantum yield, i.e. the number of molecules which are caused to react for a single quantum of light absorbed, is only exceptionally equal to exactly unity. E.g. the quantum yield of the decomposition of methyl iodide by u.v. light is only about 10" because some of the free radicals formed re-combine. The quantum yield of the reaction of H2 -f- CI2 is 10 to 10 (and the mixture may explode) because this is a chain reaction. 

Such reactions can be initiated by free radicals, derived from compounds (initiators) such as benzoyl peroxide, ammonium persulphate or azobis-isobutyronitrile or by ionic mechanisms... 

M.p. 296 C. Accepts an electron from suitable donors forming a radical anion. Used for colorimetric determination of free radical precursors, replacement of Mn02 in aluminium solid electrolytic capacitors, construction of heat-sensitive resistors and ion-specific electrodes and for inducing radical polymerizations. The charge transfer complexes it forms with certain donors behave electrically like metals with anisotropic conductivity. Like tetracyanoethylene it belongs to a class of compounds called rr-acids. tetracyclines An important group of antibiotics isolated from Streptomyces spp., having structures based on a naphthacene skeleton. Tetracycline, the parent compound, has the structure ... 

Polymeric vinylidene chloride generally produced by free radical polymerization of CH2 = CCl2. Homopolymers and copolymers are used. A thermoplastic used in moulding, coatings and fibres. The polymers have high thermal stability and low permeability to gases, and are self extinguishing. 

The additives for improving the cetane number, called pro-cetane, are particularly unstable oxidants, the decomposition of which generates free radicals and favors auto-ignition. Two families of organic compounds have been tested the peroxides and the nitrates. The latter are practically the only ones being used, because of a better compromise between cost-effectiveness and ease of utilization. The most common are the alkyl nitrates, more specifically the 2-ethyl-hexyl nitrate. Figure 5.12 gives an example of the... 

These materials are obtained through free-radical polymerization of acrylic or methacrylic monomers, or of fumarates. 

The lubricant oxidation mechanism is free-radical in nature and the additives act on the kinetic oxidation chain by capturing the reactive species either by decomposition of the peroxides, or by deactivation of the metal. 

Marcus R A 1952 Unimolecular dissociations and free radical recombination reactions J. Chem. Rhys. 20 359-64... 

Herzberg G 1971 The Spectra and Structures of Simple Free Radicals An Introduction to Molecular Spectroscopy (Ithaca, NY Cornell University Press)... 

FID does not die away before the deadtime has elapsed. In die case of inliomogeneously broadened EPR lines (as typical for free radicals in solids) the dephasing of the magnetizations of the individual spin packets (which all possess slightly different resonance frequencies) will be complete within the detection deadtime and, therefore, the FID signal will usually be undetectable. 

Dinse K P, Biehl R and Mdbius K 1974 Electron nuclear triple resonance of free radicals in solution J. Chem. Rhys. 61 4335—41... 

Biehl R, Plato M and Mdbius K 1975 General TRIPLE resonance on free radicals in solution. 

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