Electron spin resonance spectra ESR

Because of interaction between the paramagnetic moment and that from nuclear spin, very small concentrations of free radicals can be detected by their influence on the hyperfine structure of electron spin resonance spectra (ESR). This technique is now much employed in free radical studies. Concentrations of the latter down to 1 in 10 M may be detected by ESR, but many free radicals are not sufficiently long-lived to attain even this concentration. Sometimes spin-trapping techniques are employed whereby highly unstable free radicals are reacted with suitable diamagnetic molecules to form relatively long-lived radical species. 

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). 

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. 

Electron Spin Resonance. The ESR spectra of reduced and unreduced catalysts were recorded on a Bruker ER 200D-SRC X-band spectrometer with 100 kHz modulation at ambient temperature. Reduced catalysts for ESR study were prepared according to the procedure described elsewhere ( ). After hydrogen reduction at 300°C for h the sample was evacuated at the same temperature for 2 h and sealed off under vacuum. 

Takeuchi etal. (1985) have examined the HDS catalyst deposits in detail with XPS, ESR, x-ray diffraction, and electron microscopy. X-ray diffraction revealed the presence of the crystalline V3S4 phase, a nonstoichiomet-ric, polycrystalline solid with sulfur-to-vanadium ratios of 1.2 to 1.5. This polycrystalline material was observed by microscopy as 10-/um-long, rod-shaped crystals on the outer surface of the catalyst and about 0.1 /urn in length within the catalyst pores. The x-ray diffraction technique will not reveal any amorphous phases present. Electron spin resonance spectra revealed the presence of a vanadyl on the surface that was coordinated with 4S and distinctly different from the 4N coordination of the crude oil... 

Electron spin resonance spectra of ion radicals reveal a quantitative distribution of the spin density. The ESR spectrum determines the hyperfine coupling (HFC) constant for the ith... 

Electron Spin Resonance. The ESR spectra for anions from 1,5-naphthyridine... 

Fig. 3. First derivative electron spin resonance spectra. (A) ESR spectrum of an unpaired electron. (B) ESR spectrum of an unpaired electron interacting with a nitroxide resulting in a nitrogen hyperfine coupling constant aN. (C) ESR spectrum of an unpaired electron interacting with a H nucleus and a l4N nucleus as is typical for PBN radical adducts. (D) ESR spectrum of an unpaired electron interacting with the l3C nucleus, the H nucleus and the 14N nucleus of the trichloromethyl radical adduct of PBN, where the carbon tetrachloride was labeled with 13C.

The electron spin resonance spectra of the arylated phenoxyls give very many narrow lines caused by the coupling of the unpaired electron with the many aromatic protons of the substituents. By a systematic study, in which the H-atoms were substituted by D-atoms, 12C- by 13C-, and finally 160 by 170, all the coupling constants of the complicated ESR spectra of 2,4,6-triphenylphenoxyl were determined. Since all details of the results are reviewed 179) no further comment shall be added in this account. The -coupling constants agree well with those of an ENDOR spectroscopy reported by Hyde 180). 

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... 

Electron spin resonance spectroscopy (ESR) has been used by several research groups to characterize the local structure of CTB-Cu (iO, 11, 13). For the neat Ionomer, both Isolated and dimeric copper complexes, as shown In Fig. 9, have been reported. Fig. 10 compares the ESR spectra for CTB-Cu and Blend 1, PSVP/ CTB-Cu (1 1). The strong signal near 3160 G was due to Isolated copper Ions with a square planar structure as In Fig. 9a. The measured g-Lande factor and hyperflne Interaction parameters were gy" 2.320, g = 2.059, A = 145 6, and 30 + 5 G, which agreed with those reported for Isolated Cu(II) Ions In model compounds (14). 

The chemical environment of adsorbed paramagnetic ions [Cu(II), Fe(III), Mn(II)] has been studied by comparison of ESR (electron spin resonance) spectra of wet adsorbents treated with solutions of salts of these metals on the one hand and of some model compounds vide Section G) on the other. For instance, clustering or surface precipitation can be verified as the sorption mechanism. 

The solid polymeric films were removed mechanically from the walls of the borosilicate glass tubing. The infrared spectra of the neat films were obtained using a Perkin-Elmer 421 spectrophotometer. The electron spin resonance spectra of the neat films were obtained on a Varian ESR 6 spectrometer. Elemental analyses were made by Galbraith Labs. Ther-... 

ESR-spectrometer. The electron spin resonance spectra were recorded with a Varian V-4502-11 spectrometer operating at a wavelength of 3 cm. The magnetic field sweep of the Varian Fieldial unit was calibrated by means of the splitting in the 0N(S03)22" radical prepared from Fremy s salt (28). g-Factors were measured by comparison with a solid diluted sample of DPPH (a,a -diphenyl-2-picrylhydrazyl), g = 2.0037, kept in the Varian V-4532 dual cavity together with the sample under investigation. 

Electron spin resonance spectra of coals usually consist of a single Une with no resolvable fine structure however, the electron nuclear double resonance (ENDOR) technique can show hyperfine interactions not easily observable in conventional ESR spectra. Recently, this technique has been appUed 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. 

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