Spin resonance electron

Electron spin resonance was first applied to coal during the 1950s (Ingram et al., 1954 Uebersfeld et al., 1954) as a method for the determination of free-radical species in coal. Since that time, electron spin resonance has been used to compare the data for coals of different rank and to explore the potential for relating the data to the various carbon systems as well as offering valuable information about aromaticity (Toyoda et al., 1966 Retcofsky et al., 1968, 1978 Petrakis and Grandy, 1978 Kwan and Yen, 1979 Khan et al., 1988 Thomann et al., 1988 Nickel-Pepin-Donat and Rassat, 1990 Bowman, 1993 Sanada and Lynch, 1993). 

Early work on a series of carbonized coals gave 3 x 1019 free radicals per gram (1 free-radical per 1600 carbon atoms). It was also established that the free-radical content of coal at first increases slowly (in the range 70 to 90% carbon) (Ladner and Wheatley, 1965), rises markedly (90 to 94% w/w C), and then decreases to limits below detectability. Thus, in a coal having 70% carbon, there is one radical per 50,000 carbon atoms, but this is increased to one radical per 1000 carbon atoms in coal with 94% w/w carbon. 

Electron spin resonance spectra of coals usually consist of a single line with no resolvable fine structure however, the electron nuclear double resonance (ENDOR) technique can show hyperfine interactions not easily observable in conventional electron spin resonance spectra. Recently, this technique has been applied to coal, and it is claimed that the very observation of an ENDOR signal shows interaction between the electron and nearby protons and that the results indicate that the interacting protons are twice removed from the aromatic rings on which, it is assumed, the unpaired electron is stabilized. 

Adachi, Y., and Nakamizo, M. 1993 In Magnetic Resonance of Carbonaceous Solids, 

Botto and Y. Sanada (Editors). Oxford University Press, Oxford. 

Electron spin resonance (ESR), or electron paramagnetic resonance (EPR) as it is sometimes known, shares many similarities with its cousin, NMR. The origin of the phenomenon is the spin of the electron (rather than the nuclear spin) coupling with the nuclear spins of the atoms in the polymer, but much of the physics of their interactions are similar. The usual spin Hamiltonian, which is used to determine the energies of the interactions, can be written as 

A review of the application of ESR to the study of free radical polymerisation is given by Yamada and co-workers [146]. A survey of the application ESR spectroscopy spin label/probe methods in heterogeneous polymer systems is provided by Veksli and co-workers [147]. Spin probe methods allow the study of the MD of the polymer, its free volume, phase separation and phase morphology. 

Because of its sensitivity to electron spin density, EPR is an ideal tool to answer questions concerning the degree of delocalisation in conducting polymers, such as poly(aniline). EPR spectra of radical cations of oligomers, representative of a 

Askadskii, Physical Properties of Polymers Prediction and Control, Amsterdam, Gordon and Breach, 1996. 

Bicerano, Prediction of Polymer Properties, New York, Marcel Dekker, 1993. 

Electron spin resonance (ESR) has been studied in amorphous silicon for many years, long before the deliberate (or not so) introduction of hydrogen to reduce the spin signal. Since the pioneering studies of Brodsky and Title (1969), numerous measurements on ESR in a-Si have been reported. Typically, a-Si exhibits ESR spin densities of 10 cm , which have usually been attributed to Si dancing bonds. Excellent reviews of these early measurements on a-Si are available (Stuke, 1976, 1977 Solomon, 1979 Bour-goin, 1981). 

When hydrogen is added to the films the spin densities go down dramatically to the point that currently good quality films of a-Si H contain spin densities of 10 cm . The ESR measurements in these films have bwn reviewed by several authors (Voget-Grote and Stuke, 1979 Solomon, 1979 Biegelsen, 1980, 1981 Paul and Anderson, 1981 Bourgoin, 1981, Yone-zawa, 1982). 

Metastable, light-induced ESR responses have been observed (Street et al, 1981 Dersch et al., 1981a-c Pontuschka et al, 1982) in a-Si H after irradiation with band-gap light at greater intensities ( 100 mW cm ). Some of these resonances are due to unpaired spins associated with the silicon atoms and some are associated with impurities. 

There have also been several novel combinations of ESR with other experimental techniques. Such studies as spin-dependent photoluminescence, spin-dependent photoconductivity, and spin-dependent transient transport provide important information concerning the influence of spin statistics on 

As in the case of NMR, ESR measurements often provide a detailed, local probe of bonding at paramagnetic sites such as defects, impurities, or band tails in amorphous semiconductors. The important terms in the Hamiltonian for most situations of interest in ESR experiments in a-Si and a-Si H are 

Electron spin resonance (ESR) has sometimes been used to characterize electronic and structural properties of transition-metal clusters embedded in frozen rare-gas matrices. Neglecting the spin-orbit coupling, the interaction between electrons and the nuclear magnetic moment of each atom in the cluster can be expressed by the simple Hamiltonian [116, 117]  

The first term results from the Fermi contact interaction, while the second represents the long-range dipole-dipole interaction. In the equations above, ge is the free-electron g factor, /Xe the Bohr magneton, gi the nuclear gyromagnetic ratio, and /xi the nuclear moment. Moreover, the nucleus is located at position R, and the vector r has the nuclear position as its origin. Finally, p (r) = p (r) — p (r) is the electron spin density. The only nontrivial input into these equations is precisely this last quantity, i.e. Ps(r), which can be computed in the LSDA or another DFT approximation. The resulting Hamiltonian can be used to interpret the hyperfine structure measured in experiments. A recent application to metal clusters is reported in Ref. [118]. 

The application of these relations within a pseudopotential approach is confronted with the problem of evaluating the true spin density from the valence pseudocharge, which approaches the former only far from the nucleus. Unfortunately, as is apparent from the equations above, a precise determination of the spin density in the core region is crucial for an accurate determination of hyperfine parameters. Methods to reconstruct the allelectron spin density from its pseudovalence counterpart have been proposed and applied 

Electron Spin Resonance.—Nitroxide radicals of varying structiu-e have been employed in studies of micelle structure and mobility. The basic spectrum is a triplet due to N-electron coupling, which may show hyperfine coupling to 3-C-H in appropriate cases. On micelle formation or incorporation of the probe the spectrum normally broadens because of reduction in the rotational correlation time and shows enhanced broadening and change in positions of the high-field line. Cationic micelles incorporate the probe (10) with an association constant of 3 x 10 and at low surfactant concentrations there is 

Although all electrons spin, only molecules containing unpaired electrons - only free radicals - give esr spectra. Why is this (Hint Consider the possibility Ca) that one electron of a pair has its spin reversed, or (b) that both electrons of a pair have their spins reversed.) 

The signals of an esr spectrum may show splitting for the same reason that nmr signals maybe split. The esr signal will be split by n neighboring protons into n + 1 peaks. 

The protons that are responsible for splitting the signal indicates the distribution of the odd electron in the free radical. 

The Pauli exclusion principle states that no two electrons in the same atom are identical, that is, no two electrons in the same atom have an identical set of quantum numbers. Each electron of an electron pair differs only by their spin orientation they have identical principal, orbital and magnetic quantum numbers. One electron has a 

As a result, no esr spectrum is expected to arise in this manner. 

Since the magnetic moment of the electron is large (gs—2) in comparison with that of the nucleus, much higher resonance frequencies are obtained than for NMR. In commercial instruments it is customary to work at 0.34 T, corresponding to a resonance frequency of about 9.5 GHz. Thus, the experimental technique involves resonant cavities and waveguides for the microwaves used. In the presence of crystal fields the position of the resonance depends on the orientation of the crystal with respect to the magnetic field. The interaction must thus be described by vectors or tensors rather than by a scalar relation such as (7.40). 

Because of the larger energy separations in ESR the signals are much stronger than in NMR. ESR can be applied in much the same way as NMR for analytical purposes. A prerequisite is that the net spin is non-zero, which is the case for paramagnetic substances. In particular, free radicals and molecular ions can frequently be investigated. ESR techniques have been discussed in [7.47,48]. 

Because micellar systems do not have a net quantum mechanical angular momentum, a paramagnetic probe must be inserted into the sample. The use of spin probes is a useful technique for studying micellar systems provided that (1) the probe does not perturb the micelles and aggregates of the surfactant being studied, (2) the probe is stable at least for the duration of the ESR measurement, and (3) the probe is sensitive to the polarity, spatial restriction, and viscosity of its environment. The choice of the spin label is a critical step in the ESR study, as the 

Numerous nitroxide free radicals have been found to be sufficiently stable for the spin probe technique. In the ESR studies of fluorinated surfactants, N-oxyl derivatives of piperidine (I) or oxazolidine (II) have been used as the spin probes. 

The location of the probes was found to be different CAT 12 enters the micelle, whereas TempTMA is located on the surface of the micelles [156]. 

In order to avoid any possible perturbance caused by a hydrophobic chain of the probe. Ristori examined the state of water in the interlamellar regions of perfluoropolyether ammonium carboxylates by using the corresponding Cu(II) [159a] and Mn(II) [159b] salts as the spin probe. 

Normally, molecules composed of main-group elements have a ground state where all of the electrons are spin-paired. Compounds having all-spin-paired electrons show no noteworthy magnetic effects in the electronic spectrum due to spin magnetic dipoles. 

FIGURE 16.7 When / L (that is, when S 0), the splitting due to a magnetic field is more complicated and is called the anomalous Zeeman effect. The figure shows a transition in the absence and in the presence of a magnetic field. 

What is the difference between equations 16.14 and 16.11 In equation 16.14, we are considering the effect on one electron, whereas equation 16.11 is a more general case with more than one electron. For a single electron, the magnetic field effects are determined by m, whereas for multiple electrons the effects are better described by 7 and M . Therefore, for multiple electrons gj and Mj are the appropriate variables, and for a single electron g, and are the relevant variables. 

Unless otherwise noted, all art on this page is Cengage Learning 2014. 

FIGURE 16.9 A diagram of an ESR spectrometer. A sample is exposed to micro-waves of known wavelength, and a slowly varying magnetic field is applied. When the resonance condition is established, microwaves are absorbed and the transition from ground state to excited state occurs. The spectrometer detects the absorption of the microwaves to generate the spectrum. 

Carrington and Levy and Westenberg and de Haas have reviewed the early work on gas-phase reactions in this field. The apparatus involves a fast-flow system where a gas, at a relatively low pressure is passed through a microwave discharge to the resonance cavity of the spectrometer. At room temperature, it is possible to have the cavity in variable positions but at higher temperatures it is fixed °° (Figs. 62 and 63, respectively). The rate coefficients found for reactions (57)-(60) 

Coaxial Lead to stabilized and filtered diathermy unit 

The concentration of the O atoms was monitored just before and just after the RV, Nalbandyan has reviewed the use of this technique in combustion studies. Apart from investigation of reactions of hydrogen and oxygen atoms and OH radicals, it has been used to establish the site at which O attacks the complex molecule. The calibration methods and the sources of error involved in estimating the concentrations of these atomic and diatomic species are discussed by several au-thors 3°  

With good resolution of the resonance lines, it is also possible to observe hyperfine splittings caused by interactions between the electron spin and the nuclear spin, or even superhyperfine splittings due to interactions of the electron spin with nuclear spins on neighboring atoms. There are also quadrupole effects that influence the hyperfine splittings. 

Ordinarily, the resolution of superhyperfine splitting requires highly symmetric uniform sites for the localized electrons in order to reduce the number of lines. The electron-nuclear double resonance (ENDOR) technique, discussed in a later section, may be used to improve resolution. Sometimes, however, the superhyperfine splittings are large. For example, Sn02 and Sn02 exhibit splittings 

Although ESR experiments are generally carried out on transition metal ions, all that is really required is a resultant angular momentum. Hence molecules such as oxygen and nitrous oxide give signals, as do CH3 radicals, color centers, and conduction electrons. Point defects in concentrations as low as 10 spins/cm can be identified under favorable circumstances. 

In a collinear antiferromagnet, the effective field is approximated by where is the intersublattice exchange field and is 

Radicals normally do not occur in peptides but they can be generated by irradiation at low temperature with UV, electrons or X-rays, usually at the a-carbon atom, and then can be recognized by ESR. There are some radicals that are stable at ambient temperature, for instance N-oxides with the structural element C-NO-C. In biochemistry, e.g. derivatives of 2,2,5,5-tetramethyl-pyrrolidine-l-oxide are being used as spin labels, conjugated to bioactive compounds. In this way ESR can localize the position of possible receptor sites. For instance, in hormone research the distances in the neurophysin complex between spin-labeled small peptides, models of oxytocin, have been determined [27]. 

As noted in the introduction to this chapter, a very wide range of experimental methods is now available for the study of the electronic structures of earth materials. Many of the most important methods have been discussed in the preceding sections, and an attempt made in Appendix B to list all of the relevant methods, along with a very brief explanation of each technique and information on further reading. One major spectroscopic method was not discussed above but is worthy of inclusion in this chapter electron spin resonance. 

Electron spin resonance (ESR) spectroscopy, also known as electron paramagnetic resonance (EPR) or electron magnetic resonance, involves the absorption of microwave-frequency radiation by molecules, ions, or atoms possessing electrons with unpaired spins. More detailed accounts of the technique are provided by Atherton (1973), Symons (1978), and 

In a free multielectron atom or ion, the spin and orbital angular moments of the electrons couple to give a total angular momentum represented in the Russell-Saunders scheme by the quantum number J. Since J arises from vectorial addition of L (the total orbital quantum number) and 5 (total spin quantum number), it may take integral (or half-integral 

in a magnetic-resonance experiment performed on the free ion, transitions may be induced between these levels under the selection rule Mj = 1, giving the resonance condition  

The value of g and hence deviation (Ag) in its value from the free-electron value is measured in the ESR experiment. The small differences arising from location of the pareunagnetic ion in differing molecular environments are readily detectable, enabling the technique to be used as a probe in crystal chemistry and bonding studies. Also, given the above, Ag may assume different values for each molecular or crystal axis if the symmetry is lower than cubic. Hence, the g value may be anisotropic and represented by a second-rank tensor with principal axes that may, or may not, coincide with the molecular axes. In axially symmetric molecules, two values g, and g are given for lower symmetry cases, three values are determined (g, gyy, g ). 

Applications. ESR spectroscopy was used to monitor the orientation and distribution of filler particles in polymer composites, and molecular movement in filled, crosslinked material was studied.  

Several ESR spectra of thiazolyl and benzothiazolyl radicals have been recorded (667, 859), and this type of studies may be used in elucidation of the reaction mechanism. 

374 Alkyl. Aryl. Aralkyl, and Related Thiazole Derivatives 

Alkyl- and arylthiazoles rearrange undernltraviolel irradiation in different solvents to yield the corresponding isothiazoles or isomeric thiazoles. With alkylthiazoles the overall yields are very low, and it is not possible to use this method preparatively. For arylthiazoles it is possible 2-arylthiazoles. for instance, can be used to prepare 3-arylisothiazoles that are otherwise very difficult to obtain. 

TABLE III-39. RELATIVE PERCENTAGE OF ISOMERS BY IRRADIATION OF PHENYLTHIAZOLES AND ISOTHIAZOLES (217) 

PD2 signals in a Kr matrix were observed after substituting PD3 for PH3. The expected quintet pattern of each ip component (with relative intensities 1 2 3 2 1), however, was not resolved, since the spacings are six times smaller than those of the PH2 triplet due to a smaller nuclear g factor [1]. 

A better PHg spectrum (with only very weak signals from atomic P) was obtained, when an Xe matrix with 2% PH3 was y-irradiated at 4.2 K. The 1 1 1 intensity distribution was found [3]. The PH2 spectrum was observed at 4.2 and 77 K, when an Xe matrix with 1% PH3 was y-irradiated at 77 K. Spectra were studied on Xe matrices containing PH3/HI mixtures (e.g., at a ratio 100 1 0.1) and irradiated by UV light with wavelengths of A 250 nm at various temperatures between 4.2 and 100 K. The triplet intensity ratio of 1 1 1 at 4.2 K was confirmed, while at higher temperatures the expected ratio 1 2 1 was approached [5]. 

A PH2 spectrum with 1 2 1 intensity ratios of the two triplets was obtained, when PH3 sorbed in a cancrinite matrix was photolyzed at 77 K in the far UV (Ar resonance lamp). This 

An anisotropic spectrum showing a parallel ip hyperfine coupling and a near zero perpendicular coupling was observed, when PH3 in a frozen concentrated aqueous solution of sulfuric acid (-90% H2SO4) was irradiated by 0Co y rays. The parallel (apparently with the axis directed normally to the radical plane, see [7]) features became better defined when solutions of D2SO4 in D2O (giving PD2 and a trace of PHD) were used. H bonding was assumed to prevent rotation and even libration of the radicals [8]. 

Mid-IR LMR was observed for the bending vibration Vg (at 1102 cm- see p. 74) of PHg in its electronic ground state with CO2 laser radiation around 9.4 pm (from the Il-band ). The PH2 radical was generated in a flow system by the reaction PH3 + H [4]. Spectra attributable to PH2 were found for 30 out of 67 tested laser lines of and A table in the 

The ESR g values, which arise from spin-orbit coupling, are higher than 2.0023, which is the free electron value in the absence of spin-orbit coupling. The g values for vitrains and fusains 

FIGURE 10.7 Variation of unpaired electron concentration with carbon content. (From Retcofsky, H.L. et al.. Organic Chemistry of Coal, J.W. Larsen, Ed., Symposium Series No. 71, American Chemical Society, Washington, DC, 1978, p. 142.) 

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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. 

The polymer concentration profile has been measured by small-angle neutron scattering from polymers adsorbed onto colloidal particles [70,71] or porous media [72] and from flat surfaces with neutron reflectivity [73] and optical reflectometry [74]. The fraction of segments bound to the solid surface is nicely revealed in NMR studies [75], infrared spectroscopy [76], and electron spin resonance [77]. An example of the concentration profile obtained by inverting neutron scattering measurements appears in Fig. XI-7, showing a typical surface volume fraction of 0.25 and layer thickness of 10-15 nm. The profile decays rapidly and monotonically but does not exhibit power-law scaling [70]. 

Electron Spin Resonance Spectroscopy. Several ESR studies have been reported for adsorption systems [85-90]. ESR signals are strong enough to allow the detection of quite small amounts of unpaired electrons, and the shape of the signal can, in the case of adsorbed transition metal ions, give an indication of the geometry of the adsorption site. Ref. 91 provides a contemporary example of the use of ESR and of electron spin echo modulation (ESEM) to locate the environment of Cu(II) relative to in a microporous aluminophosphate molecular sieve. 

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