Spin resonance spectra, electron

Usually, electronic orbital angular momentum is zero in polyatomic molecules, and so this source of magnetic moments can often be ignored. Also, the ESR experiment is usually carried out with a liquid or solid sample, and so there is no magnetic moment arising from molecular rotation since the molecules are not freely rotating. 

The general form for the interaction Hamiltonian between an isotropically sampled magnetic field H and the electron spin is 

This equation introduces certain constants, the a/s, phenomenologically. The electron spin-nuclear spin interaction must be proportional to the dot product of the spin vectors, but the fundamental basis for the proportionality constant (a) is not considered. 

The form of the Schrodinger equation incorporating the Hamiltonian in Equation 12.19 is the same as the form of the NMR Schrodinger equation. In both, there are 2-component spin operators and spin-spin dot products. The only thing that is different in the ESR Hamiltonian is that one of the spins is electron spin and with it comes a different set of constants (e.g., versus g. Notice that gg/po is the ratio of the proton mass to the electronic mass (1836.15). 

Let us use the formyl radical HCO as an example problem for predicting high-field and low-field ESR spectra. In this molecule, the single unpaired electron gives rise to a net electronic spin of 1/2 (i.e., S = 1/2). The proton spin, I, is 1/2, and so there are four spin states for the molecule. With high fields, the Hamiltonian is broken into a zero-order part and the spin-spin perturbation  

Electron Spin Resonance Spectra.— The complexity of reaction mixtures from ceric ion oxidation of thiols is clearly demonstrated by e.s.r. in representative cases at least seven different species can be recognized. Ti -H202 oxidation of 3-hydroxy-sulphides produces both C- and S-centred radicals, while studies of syn- and anti-a-sulphonyliminoxyl radicals and alkylsulphinyl and alkylsulphonyl radicals deal with cleaner samples. 

Continuing interest in rotation barriers about single bonds in acyclic radicals, as determined by the interpretation of e.s.r. data, is illustrated in a study of R CHSR radicals and alkane-, arene-, and alkoxy-sulphonyl radicals (obtained by high-intensity u.v. irradiation of sulphinic acids in the presence of di-t-butyl peroxide at low temperatures). Further evidence for hindered rotation about the C—S bond in these species is obtained, - and an unusual order of proton hyperfine splittings a 0-H) a (a-H) a (y-H) is reported for propanesulphonyl radicals. The potential of the e.s.r. method in conformational analysis is shown in studies of radical cations (3) from 2,5-bis(alkylthio)thiophens, where the S-cis-cis conformer (3) is identified as more stable than other rotamers this is assumed to be the case too for the neutral molecule. 

Detailed reviews of ESR spectroscopy have been published (Gaffney, 1974 Vignais and Devaux, 1976 Berliner, 1976, 1979 Knowles et ai, 1976 Schreier et al., 1978 Marsh, 1981 Marsh and Watts, 1982). For those readers unfamiliar with the technique, the review by Marsh (1981) contains a good introduction to the theory and operation of ESR spectrometers. ESR spectra represent the absorption of microwaves as the function of a variable magnetic field. Due to the electrical configuration of the spectrometer itself and the method used to detect 

Comparison of equation (9.21) with equations (9.9) and (9.6) reveals the resemblance of nuclear and electron resonance methods. In equation (9.21), P is the so-called Bohr magneton, and the electron g value is a porportionality constant which is dependent upon the magnetic species and its environment. Only unpaired electrons give rise to ESR spectra, and this is why, except in the study of lipid autoxidation, lipids can only be studied through the use of spin labels, which are stable, usually nitroxide, radicals (Fig. 9.26). 

Nitroxides yield three prominent spectral lines due to hyperfine interactions with the three nuclear spin states (Fig. 9.27). The appearance of the three-lined nitroxide spectrum is determined in part by (a) the polarity of the solvent surrounding the probe (b) the type of motion exhibited by the probe (c) the rate of motion (d) the orientation 

Rates of motion are reflected in the width of the ESR lines slower motion (i.e. increased correlation times) broadens the individual lines (Seelig, 1976). Although correlation times may be obtained by line width analysis for isotropic spectra, similar interpretations of anisotropic spectra are very tedious, and require computer modelling. Owing to these complications, the rate of spin label motion is rarely taken into account in the analysis of ESR spectra. 

Other ESR studies of membrane structure include the determination of phospholipid flip-flop rates (Kornberg and McConnell, 1971), studies of membrane permeability, measurements of vesicle internal volumes and transmembrane potentials (Marsh et aL, 1976 McNamee and McConnell, 1973). 

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Adrian F J 1971 Theory of anomalous electron spin resonance spectra of free radicals in solution. Role of diffusion-controlled separation and reencounter of radical pairs J. Chem. Rhys. 54 3918-23... 

Figure 15-9. (a) IJglil-induccd electron spin resonance spectra of MDMO-PPV/PCBM upon successive illumination with 2.41 eV argon ion laser, (b) Integrated LESR intensity [ESR (illuminatcd)-ESR (dark)] of MDMO-PPV/PCBM (reproduced after Ref. 1401). 

Oxido[10]annulene closely resembles l,6-methano[lOJannulene in many of its spectral properties, particularly in its proton magnetic resonance, ultraviolet, infrared, and electron spin resonance spectra,1 but is chemically less versatile than the hydrocarbon analog due to its relatively high sensitivity toward proton and Lewis acids. 

For reviews of the use of ESR spectra to determine structures, see Walton, J.C. Rev. Chem. Intermed., 1984, 5, 249 Kochi, J.K. Adv. Free-Radical Chem., 1975, 5, 189. For ESR spectra of a large number of free radicals, see Bielski, B.H.J. Gebicki, J.M. Atlas of Electron Spin Resonance Spectra Academic Press NY, 1967. 

Despite the fact that natural elemental sulfur contains 0.75% of the isotope [4] with a nuclear spin of 7 = /2 no NMR spectra of elemental sulfur have ever been reported. Such spectra are however well-known for compounds containing just one or two sulfur atoms [5]. Electron spin resonance spectra of irradiated elemental sulfur samples and of quenched sulfur vapor have been reviewed elsewhere [6-8]. 

There have been no reports of complexes of " JV-substituted thiosemicarbazones derived from 2-formylpyridine, but 2-acetylpyridine JV-methyl-thiosemicarbazone, 3a, formed [Fe(3a-H)2]C104 and [Fe(3a-H)2]FeCl4 [117]. The nature of these two species was established by partial elemental analyses, molar conductivities, magnetic moments, electronic, infrared, mass and electron spin resonance spectra. A crystal structure of a related selenosemicarbazone complex confirmed the presence of a distorted octahedral iron(III) cation coordinated by two deprotonated anions so that each ligand is essentially planar and the azomethine nitrogens are trans to each other the pyridyl nitrogen and selenium donors are both cis. 

Copper(II) complexes of 2,6-lutidylphenylketone thiosemicarbazone, 38, have been prepared from copper(II) chloride and copper(II) bromide [186]. Similar to 2-pyridyl thiosemicarbazones, 38-H coordinates via the ring nitrogen, the azomethine nitrogen and the thiol sulfur based on infrared spectral assignments. Magnetic susceptibilities and electron spin resonance spectra indicate dimeric complexes and both are formulated as [Cu(38-H)A]2 with bridging sulfur atoms. The electronic spectra of both halide complexes show band maxima at 14500-14200 cm with shoulders at 12100 cm S which is consistent with a square pyramidal stereochemistry for a dimeric copper(II) center. 

Matsuo, M. Matsumoto, S. Electron spin resonance spectra of the chromanoxyl radicals derived from tocopherols (vitaminE) and theirrelated compounds. Lipids 1983,18, 81-86. 

Fig. 20. Electron spin resonance spectra of nitroxalkylcobalamin. (a) Spectrum before photolysis the high field line is broadened and therefore has a lower peak amplitude, (b) Expanded view of center line before photolysis showing no indication of additional hyperfine from methyl protons, (c) Spectrum of nitroxide photolysis product which has been freed from the cobalamin. (d) Expanded view of center line after photolysis now faintly showing proton hyperfine...

As with the nitroxalkylcobalamins (119) and cobinamides, the co-binamides in which nitroxide is coordinated show electron spin resonance spectra very similar to the spectrum of free nitroxide. The high field line is not broadened as much as in the spectrum of a nitroxalkyl-cobinamide. No hyperfine splitting from methyl protons in the 2 or 6 positions can be observed for the bound nitroxide. However, treatment of the coordinate spin labeled compounds with cyanide releases the nitroxide. When this happens, the proton hyperfine can be observed (Fig. 25). Thus treatment with cyanide simply displaces the nitroxide and a spectrum for free nitroxide is observed. 

The processes described and their kinetics is of importance in the accumulation of trace metals by calcite in sediments and lakes (Delaney and Boyle, 1987) but also of relevance in the transport and retention of trace metals in calcareous aquifers. Fuller and Davis (1987) investigated the sorption by calcareous aquifer sand they found that after 24 hours the rate of Cd2+ sorption was constant and controlled by the rate of surface precipitation. Clean grains of primary minerals, e.g., quartz and alumino silicates, sorbed less Cd2+ than grains which had surface patches of secondary minerals, e.g., carbonates, iron and manganese oxides. Fig. 6.11 gives data (time sequence) on electron spin resonance spectra of Mn2+ on FeC03(s). 

Less frequently used at present is electron spin resonance spectroscopy, which is based on the use of spin probes as model componnds or covalent spin labeling of drugs. Microviscosity and micropolarity of the molecnlar environment of the probe can be derived from electron spin resonance spectra. Moreover, the spectra allow us to differentiate isotropic and anisotropic movements, which result from the incorporation of the probe into liposomal structures. Quantitative distribution of the spin probes between the internal lipid layer, the snrfactant, and the external water phase is to be determined noninvasively. On the basis of the chemical degradation of drugs released from the lipid compartment, agents with reductive features (e.g., ascorbic acid) allow us to measure the exchange rate of the drugs between lipophilic compartments and the water phase [27,28]. 

Fig. 12. Electron spin resonance spectra recorded at X-band for powdered clay samples at ambient laboratory temperature (a) montmorillonite, (b) kaolin, (c) Bentonite (Aldrich Chemical), (d) Bentonite (SSP/NF), (e) Magnabrite, (f) Polargel, (g) Volclay, and (h) Fuller s Earth (68).

To elucidate some enzymatic characteristics of the isolated laccases I, II, and III, substrate specificities for several simple phenols, electrophoresis patterns, ultraviolet spectra, electron spin resonance spectra, copper content, and immunological similarities were investigated. Tyrosine, tannic acid, g c acid, hydroquinone, catechol, pyrogallol, p-cresol, homocatechol, a-naphthol, -naphthol, p-phenylenediamine, and p-benzoquinone as substrates. No differences in the specificities of these substrates was found. The UV spectra for the laccases under stucfy are shown in Figure 4. Laccase III displays three adsorption bands (280, 405, and 600nm), laccase II shows one band 280nm), and laccase I shows two bands (280 and 405 nm). These data appear to indicate differences in chemical structure. The results of the copper content analysis (10) and two-dimensional electrophoresis also indicate that these fractions are completely different proteins (10), Therefore, we may expect differences in substrate specificities between the three laccase fractions for more lignin-like substrates, yet no difference for some simple phenolic substrates. 

The electron spin resonance spectra were run in nitrogen-saturated solutions of aromatic compound ca. 10" m) and nucleophile (0-05-0-1 M) in the solvent(s) indicated. Irradiation in the cavity was effected with a high pressure mercury arc. Electrolysis was performed with the platinum cathode in the cavity, tetraethyl-ammonium perchlorate as electrolyte and electric currents of 10-250 /lA. 

Robinson JP, Wawrousek EF, McArdie JV, Coyle G, Adler I (1984) X-ray Photoelectron and Electron Spin Resonance Spectra of Iron(III) Parabactin. Inorg Chim Acta 92 L19... 

Electron spin resonance spectra of the resulting phosphoroalkyl radicals were obtained. Absolute rate constants for this process are not known, and similar abstraction by a peroxy radical in nonaqueous media is not necessarily possible. However, in a zinc salt-inhibited hydrocarbon oxidation the additional reactions in Scheme 2 might be envisaged part molecules are shown for convenience. 

Fig.5 First-derivative electron spin resonance spectra found immediately after /-irradiation of samples of 20 mg/mL DNA in 7M LiBr with various loadings of MX. The dashed spectra are simulations made by linear least-squares fits of the benchmark functions (Fig. 3a and b) to experimental spectra. The spectra clearly show that MX increases in relative amount to the DNA anion radical with increased loading of MX. At the lowest loading of MX (228 bp/1 MX) these fits suggest that 8.7% of the electrons are found on MX, whereas at the highest loading (23 bp/1 MX) 59% are captured by MX with the remainder on DNA. The fraction of electrons captured by MX increases with time [7a]. Reprinted with permission from the J. Phys. Chem. Copyright (2000) American Chemical Society...

Ciani, L., Ristori, S., Salvati, A., Calamai, L., Martini, G. (2004) DOTAP/DOPE and DC-Chol/DOPE lipoplexes for gene delivery zeta potential measurements and electron spin resonance spectra. Biochim. Biophys. Acta., 1664(1), 70-79. 

Figure 2 First-derivative electron spin resonance spectra found immediately after y-...

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