Free radicals electron spin resonance spectrum

The electron spin resonance spectrum of a free radical or coordination complex with one unpaired electron is the simplest of all forms of spectroscopy. The degeneracy of the electron spin states characterized by the quantum number, ms = 1/2, is lifted by the application of a magnetic field, and transitions between the spin levels are induced by radiation of the appropriate frequency (Figure 1.1). If unpaired electrons in radicals were indistinguishable from free electrons, the only information content of an ESR spectrum would be the integrated intensity, proportional to the radical concentration. Fortunately, an unpaired electron interacts with its environment, and the details of ESR spectra depend on the nature of those interactions. The arrow in Figure 1.1 shows the transitions induced by 0.315 cm-1 radiation. 

ESR (Electron Spin Resonance) spectrum (Fig. 1.2) recorded during electrolysis of DMPO (5.5 dimethyl-1-pyrroline-N-oxide) solution on BDD confirms the formation of OH during anodic polarization of diamond electrodes. It has been reported that the BDD-hydroxyl radical interaction is so weak (no free p or d orbitals on BDD) that the OH can even be considered as quasi-free. These quasi-ffee hydroxyl... 

A sextet has been observed in the electron spin resonance spectrum of irradiated polyethylene at 77°K [31, 32] and assigned to the central radical (II). Moreover, five hyperfine components have been detected and attributed to the terminal radical (I) [32]. According to Ranby and Yoshida [32] free radicals (I) and (II) are formed in primary photochemical processes by fission of C—C and C—H bonds. Such a conclusion seems, however, doubtful in the light of the chemical evidence presented above which supports the initiation of the photodegradation of polyethylene by adventitious impurities. Moreover Tsuji and Takeshita [33] have recently observed that alkyl radicals produced at liquid nitrogen temperature by ultraviolet irradiation of polyethylene in vacuo are transformed into transient acyl radicals when the sample is allowed to warm up to about—125°C ... 

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. 

Finally, matrix isolation combined with electron spin resonance has been used for N02 as well as for other free radicals such as HOz, ROz, and N03 (Mihelcic et al., 1985, 1990, 1993 Zenker et al., 1998). Trace gases in a sample of air (typically about 8 L) are trapped in a D20 matrix at 77 K and the ESR spectrum obtained. Any paramagnetic species present has a characteristic ESR spectrum that can be used to identify it and, using reference spectra, obtain its concentration. Since NOz is the paramagnetic species present in the largest concentration, it is easily detected and measured. 

The idea that free radicals occur in many chemical reactions is as old as the study of the mechanisms of these reactions. However, direct physical evidence for the existence of free radicals and for their presence in certain reactions is comparatively recent. Such evidence has been obtained in recent years by the methods of mass spectrometry, optical spectroscopy, and electron spin resonance spectrometry. The optical method of detecting free radicals has the advantage that it simultaneously supplies information about the structure of the radical. Indeed, in many instances the nature of the free radical has been identified by the structure of the spectrum without any assumptions about the mechanism of the reaction in which it appears.1... 

Historically, the triphenylmethyl radical (1), studied by Gomberg in 1987, is the first organic free radical. The triphenylmethyl radical can be obtained by the reaction of triphenylmethyl halide with metal Ag as shown in eq. 1.1. This radical (1) and the dimerized compound (2) are in a state of equilibrium. Free radical (1) is observed by electron spin resonance (ESR) and its spectrum shows beautiful hyperfine spin couplings. The spin density in each carbon atom can be obtained by the analysis of these hyperfine spin coupling constants as well as information on the structure of the free radical. 

Since a large number of xenobiotics are metabolized to free radicals, an overall view of this area is not obvious. By definition, free radical metabolites must exist free of the enzyme, and, therefore, enzyme-xenobiotic transition states with free radical character such as are thought to exist in the cytochrome P-450 substrate complex are excluded. It follows that if the rate of formation of the free radical is fast enough, it can be detected with electron spin resonance, and will have the same ESR spectrum as the free radical made by purely chemical means. 

From the point of view of the solvent influenee, there are three features of an electron spin resonance (ESR) speetrum of interest for an organic radical measured in solution the gf-factor of the radical, the isotropie hyperfine splitting (HFS) constant a of any nucleus with nonzero spin in the moleeule, and the widths of the various lines in the spectrum [2, 183-186, 390]. The g -faetor determines the magnetic field at which the unpaired electron of the free radieal will resonate at the fixed frequency of the ESR spectrometer (usually 9.5 GHz). The isotropie HFS constants are related to the distribution of the Ti-electron spin density (also ealled spin population) of r-radicals. Line-width effects are correlated with temperature-dependent dynamic processes such as internal rotations and electron-transfer reaetions. Some reviews on organic radicals in solution are given in reference [390]. 

A much more important technique is electron spin resonance (esr), also called electron paramagnetic resonance (epr). ° The principle of esr is similar to that of nmr, except that electron spin is involved rather than nuclear spin. The two electron spin states (m = and m = are ordinarily of equal energy, but in a magnetic field the energies are different. As in NMR, a strong external field is apphed and electrons are caused to flip from the lower state to the higher by the application of an appropriate radio-frequency (rf) signal. Inasmuch as two electrons paired in one orbital must have opposite spins which cancel, an esr spectrum arises only from species that have one or more unpaired electrons (i.e., free radicals). 

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).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. Electron spin resonance spectra have been observed for radicals with lifetimes considerably <1 s. 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... 

As is generally known, electron spin resonance reveals the presence of unpaired electrons. This is of course characteristic of free radicals and the most well known stable free radical is a, a-diphenyl-0-picrylhydrazyl(DPPH) (1,213). In the ESR spectrum it gives a narrow signal close to free spin value g 2.0036 and is used for calibrating the magnetic field (2). 

Electron spin resonance spectroscopy (ESR), also known as electron paramagnetic resonance (EPR), is based on the property that an unpaired electron placed in a magnetic field shows a typical resonance energy absorption spectrum sensitive to its environment. Recently, this technique, which was primarily developed for biological studies of membrane properties, has been adapted for the study of adsorbed polymer/surfactant layers. The mobility of the ESR probe (stable free radical incorporated into the polymer or surfactant molecule) depends of orientation of the surfactant or polymer and the viscosity of the local environment around the probe. 

Figure 6.2 Electron spin resonance, (a) Dependence of electron energy on magnetic field, (b) ERS spectrum of a simple free radical, (c) Idealized ESR spectrum of the methyl radical...

Intermediates in the radiation chemistry of high polymers include ions and trapped electrons, radicals and excited states. Free radicals trapped after irradiation have been studied mainly by electron spin resonance (ESR) and in some cases by chemical methods and by ultraviolet or infrared spectroscopy. The detection of free radicals during radiolysis has been performed by pulse radiolysis and also by ESR. Trapped ions and radical-ions were characterized by absorption spectroscopy and thermoluminescence while pulse radiolysis allows their detection during irradiation. Excited states, owing to their very short lifetime, could be observed only by pulse radiolysis or by the measurement of the luminescence spectrum and decay time during steady irradiation. 

Closely related are the 1-benzylamino-l-deoxylactitol dithiocarbamate salts developed by Eybl and co-workers316 317 for the same purpose. However, the most important application of 175 is, probably, its use as a nontoxic, water-soluble nitric oxide probe in vivo. In view of the central importance that this gaseous free-radical species plays in regulating a broad range of important biological functions, its detection and quantification near its site of production and action is of prime importance. For this purpose, the ferrous salt of MGD, which forms a stable water-soluble mononitrosyl iron-dithiocarbamate complex (176) with a characteristic electron spin resonance (ESR) spectrum at room temperature, is currently used.318-323... 

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