ESR spectroscopy

ESR studies have been used extensively to characterize S-N radicals that are persistent in solution at room temperature.32 Typical radicals are cyclic C-N-S systems in which the unpaired electron occupies a delocalized re-orbital. In conjunction with molecular orbital calculations, ESR spectra can provide unique information about the electronic structures of these ring systems. 

Electron spin resonance (ESR) spectra have been used to detect and characterise some paramagnetic ionic compounds which exist as intermediates. An example is l,l,4,4-tetrakis(trimethyl-silyljbutatriene [662] which can be detected only in anionic or cationic form. The anion is obtained with metallic potassium in diethyl ether at 20 °C, and the cation with aluminium chloride in dichloromethane at — 50 to — 70 °C. 

The ESR spectra of tetraorganodisilenyl radical anions (section 3.10.4) [518] are also very instructive. The tetraorganodisilenes generate tetraorganodisilenyl radical anions as they react with alkali metals (Eq. 5.1)  

The residence probability for the radical electron is divided evenly between the two silicon atoms (Eq. 5.2)  

The results of variable temperature ESR studies on methylated hexahydropyridazine radical cations are consistent with the half chair conformation, with an approximately planar CNNC structural component as the most stable conformation. For // an5-l,2,3,6-tetramethylhexa-hydropyridazine, the 3-axial, 5-equatorial conformation was more stable than the equatorial, equatorial hy about 1 kcal mol 85JCX 4749 . 

As mentioned before, ESR spectroscopy has been used extensively for the study of electrochemically generated radicals and radical ions 40 A word of caution is necessary with regard to the interpretation of such results the detection of a particular radical species is no definite proof that the radical is an intermediate in the formation of products. This can only be established by supporting the ESR studies by kinetic investigations. Also the failure to detect radicals from an electrode process does not mean that radicals are not intermediates, only that they may be too short-lived to be detectable. Generally, one can estimate the lower limit for detection of radicals from electrode reactions at a half-life of about 0.1 sec for external generation and 0.01 sec for internal generation. 

Electron spin resonance (ESR) spectroscopy is also known as electron paramagnetic resonance (EPR). spectroscopy or electron magnetic resonance (EMR) spectroscopy. The main requirement for observation of an ESR response is the presence of unpaired electrons. Organic and inorganic free radicals and many transition metal compounds fulfil this condition, as do electronic triplet state molecules and biradicals, semicon-ductor impurities, electrons in unfilled conduction bands, and electrons trapped in radiation-damaged sites and crystal defect sites. 

The principles of ESR and general applications are covered in various monographs [52]-[57]. Reviews of technique developments and applications have been published [58]. [59] industrial applications have been reviewed [60] and dosimetry [61] and biological applications described [62]-[66], 

The resonance condition for ESR, for a system having a spin value of 1/2, is  

Electron - nuclear double resonance (ENDOR) offers enhanced resolution compared with conventional ESR. This is achieved mainly because the technique is ESR-detected NMR spectroscopy. 

Another advantage is that only two ENDOR lines arise from each group of equivalent nuclei, resulting in a simplification in the spectra when compared with ESR. ENDOR has been applied to the study of organic [67] and inorganic radicals and transition metal complexes [62], [68] in both the solid (powdered and crystalline) and liquid states. 

The sublevel properties of triple excited states of organic molecules have been studied extensively also by ESR spectroscopy. In addition to the ordinary steady-state ESR spectroscopy, time-resolved ESR and optical-detection ESR spectroscopy have also been applied. 

The information that can be obtained by the ESR spectroscopy is as follows a) zero-field splitting, and b) relative populating rate constants for individual sublevels. 

Since ESR spectroscopy has been repeatedly discussed in a number of textbooks [14-16], we are not going to discuss it here any further. 

This technique is currently used very widely to study the behaviour of spin labels, especially in biological systems. Spin labels are stable nitroxide radicals (R2N0 ) which can be attached to large molecules such as proteins, lipids or drugs, which can then be monitored by ESR techniques. Generally, it is linewidth changes that are of interest, since these give a measure of correlation times [9]. 

However, the first use of ESR spectroscopy in the field of solvation was in the study of ion pairs, and this remains perhaps the most important step forward achieved by ESR spectroscopists in solvation studies. 

A series of D-glucosyl-based neoglycoUpids, both p- and o- anomers of 0-, S- and C-glycosides, have been prepared bearing spin labels. The relatively short-lived imino AT-oxyl group was used for 0-glycosides, and the J r-acylamino JV-oxyl moiety was employed for S- and C-glycosides. ESR spectra provided conformational information about the lipophilic chain. 

5 Polarimetry, Circidar IKchroisniy Calorimetry and Related Studies 

Theoretical predictions of vibrational CD spectra of D-glyceraldehyde, d-erythrose and D-threose have been made through development of a method to scale calculated force constants to agree with experimental values. 

CD spectra of tautomeric and protonated cyclic adenosine derivatives e.g. 73 have been described. A review of the chiroptical properties of cholesterol glycosides and chirality distinction has appeared (e.g. D-glucose and L-glucose lead to red and blue shifts respectively). CD has been used for chirality determination of dideoxy-g/yccro-mannonopyranose-derived 74. The C2 chir- 

Time domain reflectometry has been used to study the hydration of L-xylo- and D-orohuto-ascorbic add solutions in water and water ethanol mixtures, differential scanning calorimetry and thermogravimetric analysis have been used to study hydration of a-maltose and amylose, a new physical chemical model describes the thermodynamic properties of binary water-carbohydrate 

The ay, constants and g-values of known 1,2,3-dithiazolyl radicals are listed in Table 1. 

For lower photoelectron energies, interaction with the electronic environment of the absorber atom is much greater and each electron cannot be considered singly. The fine structure is richer and gives information such as spin state and local symmetry, but theoretical 

For EXAFS and particularly for XANES, data analysis is complex. The oscillation frequency/bond distance dependence means that extensive use is made of Fourier transform analysis. Most applications to date have been in the EXAFS region. In order to acquire sufficiently strong signals in a reasonable time, use has to be made of high-intensity photon fluxes, which are available at synchrotron facilities. These provide a broad-band tuneable source of high-intensity radiation, but the reduced number of facilities limits widespread dissemination of the technique. Reflection (fluorescent detection) mode is usually preferred to transmission. Experiments can be conducted in any phase, and the probing of electrode surfaces in situ is an important application. 

Types of electrode/solution interface studied include oxide films on metals, monolayer deposits obtained by underpotential deposition, adsorption, and spectroelectrochemistry in thin-layer cells. 

A useful illustrative example is shown in Fig. 12.8 of iron passivated by chromate and nitrite. Fourier transform of the EXAFS data to give distance-dependent signals shows the similarities and differences between the two passivation methods. 

Other X-ray surface probe methods which have not as yet found widespread use but are applicable to electrode/solution interfacial studies are based on X-ray standing waves and glancing angle X-ray diffraction. 

Electron spin resonance spectra provide direct information about paramagnetic metal complexes. Much experimental work has been done on cobalt(II) Schiff-base complexes and their 0 - adducts. Some related 

Five-coordinate cobalt(II) Schiff base complexes of the Co(salen)B 

Upon oxygenation, the ESR spectra undergo strong changes [244]. For example, the signal of Co(acacen)(py) at g 2.3 is replaced by a 

Results of 0-coupling and magnetic data show that more than 90 % of the single unpaired electron density resides on the dioxygen moiety [245]. The two antibonding electrons of free 0 are paired upon 

There has been considerable debate over the origin of the hyperfine 59 

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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. 

Jent F, Paul H and Fischer H 1988 Two-photon processes in ketone photochemistry observed by time-resolved ESR spectroscopy Chem. Phys. Lett. 146 315-19... 

Verma N 0 and Fessenden R W 1976 Time resolved ESR spectroscopy. IV. Detailed measurement and analysis of the ESR time profile J. Chem. Phys. 65 2139-60... 

The chaimel-flow electrode has often been employed for analytical or detection purposes as it can easily be inserted in a flow cell, but it has also found use in the investigation of the kinetics of complex electrode reactions. In addition, chaimel-flow cells are immediately compatible with spectroelectrochemical methods, such as UV/VIS and ESR spectroscopy, pennitting detection of intennediates and products of electrolytic reactions. UV-VIS and infrared measurements have, for example, been made possible by constructing the cell from optically transparent materials. 

Spin densities help to predict the observed coupling constants in electron spin resonance (ESR) spectroscopy. From spin density plots you can predict a direct relationship between the spin density on a carbon atom and the coupling constant associated with an adjacent hydrogen. 

The diaziridine ring exhibits a surprising stability towards strong oxidizing agents. Diaziridines unsubstituted at both N atoms can be transformed into diazirines by dichromate in acidic solution or by chlorine (Section 5.08.4.3). Radical attack by decomposing peroxide converts (136) to the diaziridinyl radical (137), as evidenced by ESR spectroscopy (76TL4205). 

Benzofurazan, 7-chloro-4-nitro-, 6, 394 as fluorigenic agents, 6, 410, 426 Benzofurazan, 4-chloro-7-sulfo-ammonium salt properties, 6, 426 Benzofurazan, 4-nitro-synthesis, 6, 408 Benzofurazans, 6, 393-426 Beckmann fragmentation, 6, 412 biological activity, 6, 425 bond angles, 6, 396 bond lengths, 6, 396 diazo coupling, 6, 409 dipole moments, 6, 400 electrochemical reduction, 5, 73 electrophilic reactions, 6, 409-410 ESR spectroscopy, 6, 400... 

K. E. Preston and L. H. Sutcliffe, ESR Spectroscopy of Eree Radicals Containing Sulfur Linked to Nitrogen, Mag. Reson. Chem., 28, 189 (1990). 

Many early studies in ESR spectroscopy were concerned with anions, cations and triplet states derived from conjugated molecules. The unpaired electron(s)... 

By means of ESR spectroscopy (68TL1843), the real structure for the 2,2 -biisobenzimidazolylidene (63JOC1931 65AS1651 69JOC234) could be assigned (Section II,D,1). 

The photolysis of 5 was investigated in neutral aqueous solution by ESR spectroscopy (98CPB1021). 

Similarly, the corresponding nickel complex, based on ESR spectroscopy, should be formulated as a nickel(II) corrole-71-radical. The iron corroles exist in the oxidation state + 111 or + IV depending on the nature of additional axial ligands. 

ESI mass spectrometry ive mass spectrometry ESR spectroscopy set EPR spectroscopy ethyl acetate, chain transfer to 295 ethyl acrylate (EA) polymerizalion, transfer constants, to macromonomers 307 ethyl methacrylate (EMA) polymerization combination v.v disproportionation 255, 262 kinetic parameters 219 tacticity, solvent effects 428 thermodynamics 215 ethyl radicals... 

Well before the advent of modern analytical instruments, it was demonstrated by chemical techniques that shear-induced polymer degradation occurred by homoly-tic bond scission. The presence of free radicals was detected photometrically after chemical reaction with a strong UV-absorbing radical scavenger like DPPH, or by analysis of the stable products formed from subsequent reactions of the generated radicals. The apparition of time-resolved ESR spectroscopy in the 1950s permitted identification of the structure of the macroradicals and elucidation of the kinetics and mechanisms of its formation and decay [15]. 

Ranby B, Rabek JF (1977) ESR Spectroscopy in polymer research, Springer, Berlin... 

Morkovnik et al. (1989) found experimentally that the addition of an equimolar amount of 4-morpholino- or 4-dimethylaminoaniline to a suspension of nitrosyl perchlorate in 100 % acetic acid, dioxan, or acetonitrile yields a mixture of the diazonium perchlorate and the perchlorate salt of the amine radical cation, with liberation of gaseous nitric oxide. Analogous results in benzene, including evidence for radicals by ESR spectroscopy and by spin trapping experiments, were obtained by Reszka et al. (1990). 

Of some relevance in this connection is a study216 on the structure of the anion radicals formed when diaryl sulphones react with n-butyllithium in hexane-HMPA solution under an argon atmosphere. Apparently, a dehydrogenative cyclization and a further one-electron reduction occurs to produce the anion radicals of substituted dibenzothiophene-S, S-dioxides. These anion radicals were studied by ESR spectroscopy. 

This review is concerned with the formation of cation radicals and anion radicals from sulfoxides and sulfones. First the clear-cut evidence for this formation is summarized (ESR spectroscopy, pulse radiolysis in particular) followed by a discussion of the mechanisms of reactions with chemical oxidants and reductants in which such intermediates are proposed. In this section, the reactions of a-sulfonyl and oc-sulfinyl carbanions in which the electron transfer process has been proposed are also dealt with. The last section describes photochemical reactions involving anion and cation radicals of sulfoxides and sulfones. The electrochemistry of this class of compounds is covered in the chapter written by Simonet1 and is not discussed here some electrochemical data will however be used during the discussion of mechanisms (some reduction potential values are given in Table 1). 

A number of radical anions of sulfur-containing aromatic compounds have been studied essentially by means of ESR spectroscopy and sometimes by electronic spectroscopy. The studied compounds include aromatic rings separated by the oxidized sulfur functionality. The effects caused by the latter depend on the geometry and topology of the aromatic systems as well as on the electron-withdrawing ability of the other substituents. 

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. 

Sulfinyl radicals studied by ESR spectroscopy have been generated photolytically either from the appropriate sulfoxide or from the corresponding chlorides, namely10,12"14,16... 

Norman and coworkers21 23 used ESR spectroscopy to study in detail the reaction of several organic compounds containing the sulfinyl moiety with the Ti(III)-H202 system in particular, dimethyl sulfoxide yields the methyl and methanesulfonyl radicals as shown in reactions 1-321. 

Kinetic ESR spectroscopy has been used to determine the rate constants for self-termination of MeS02, EtS02, PhSO 2 and 2,5-Cl2C6H3S02 radicals18. At 233 K and in cyclopropane as a solvent the values found for 2k, were in the range (4.5 1.5) x 109 m" 1 s 1 for all RSO 2 radicals. 

The photo-Kolbe reaction is the decarboxylation of carboxylic acids at tow voltage under irradiation at semiconductor anodes (TiO ), that are partially doped with metals, e.g. platinum [343, 344]. On semiconductor powders the dominant product is a hydrocarbon by substitution of the carboxylate group for hydrogen (Eq. 41), whereas on an n-TiOj single crystal in the oxidation of acetic acid the formation of ethane besides methane could be observed [345, 346]. Dependent on the kind of semiconductor, the adsorbed metal, and the pH of the solution the extent of alkyl coupling versus reduction to the hydrocarbon can be controlled to some extent [346]. The intermediacy of alkyl radicals has been demonstrated by ESR-spectroscopy [347], that of the alkyl anion by deuterium incorporation [344]. With vicinal diacids the mono- or bisdecarboxylation can be controlled by the light flux [348]. Adipic acid yielded butane [349] with levulinic acid the products of decarboxylation, methyl ethyl-... 

Kasai and McLeod (57, 58) also studied a series of bimetallic diatomics, AgM (M = Mg, Ca, Sr, Be, Zn, Cd, or Hg), by ESR spectroscopy. For all of these species, the hyperfine coupling to the Ag nucleus was found to be isotropic. It was shown that the unpaired electron resides in an orbital resulting essentially from an anti-bonding combination of the valence s orbitals of the Ag and M atoms. A typical spectrum is shown in Fig. 13. 

The compounds formed were studied by esr spectroscopy, with the magnetic parameters being used to determine the geometries. Mn02 is linear, whereas MnOa is trigonal planar (Dsh), and Mn04 is distorted tetrahedral with Csv symmetry. 

The copper atom-acetylene matrix-reaction, monitored originally by esr spectroscopy (60) has now been investigated by IR/UV-visible spectroscopy (144), and there is general agreement on the identification of two mononuclear species, CuCCaHali.. The esr/IR/UV-visible... 

The complexes of the 1,1-dithio ligands with the Group I transition metals have been studied in considerable detail, and have been extensively reviewed by Coucouvanis (1). Interest in the synthetic chemistry of these complexes has been maintained over recent years, but a large proportion of the work on these complexes has been concerned with physical studies, particularly by esr spectroscopy. 

More recently, 84 may have been identified by ESR spectroscopy of solutions of Li2S ( >6) in DMF at 303 K. The lithium polysulfide was prepared from the elements in liquid ammonia. These polysulfide solutions also contain the trisulfide radical anion ( 2.0290) but at high sulfur contents a second radical at g=2.031 (Lorentzian lineshape) was formed which was assumed to be 84 generated by dissociation of octasulfide dianions see Eq. (32) [137],... 

The species S3 (absorbing at 420 nm) and S4 (absorbing at 530 nm) have been detected by reflection spectra in the condensate but the formation of S4 is unexplained [16]. S3 and SO2 have also been observed by Raman spectroscopy in such samples [15] (the expected S4 Raman line at 678 cm was probably obscured by the SS stretching mode of S2O at 673 cm but a shoulder at the high-frequency side of the S2O line indicates that some S4 may have been present). While the reddish colors turn yellow on warming at about -120 °C, the sulfur radicals could be observed by ESR spectroscopy up to 0 °C [10]. If the condensation of S2O gas is performed very slowly at -196 °C the condensate is almost colorless and turns red only if the temperature is allowed to increase slowly. Hence, it has been suspected that S2O is actually colorless like SO2. 

On condensation at low temperatures, on dissolution in inert solvents or on raising its partial pressure substantially above 1 mbar (100 Pa) S2O polymerizes with partial disproportionation. Since sulfur radicals have been detected in such condensates by ESR spectroscopy [10] it has been proposed that a radical-chain reaction takes place according to Scheme 5. 

Oosterhuis WT (1974) The Electronic State of Iron in Some Natural Iron Compounds Determination by Mossbauer and ESR Spectroscopy. 20 59-99 Orchin M, Bollinger DM (1975) Hydrogen-Deuterium Exchange in Aromatic Compounds. 23 167-193... 

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