Hyperfine splitting, electron spin resonance

The electron-spin resonance hyperfine splitting for atomic hydrogen is given by... 

Q 8-13 TT-Electron Densities and Electron Spin Resonance Hyperfine Splitting Constants ... 

Note that the values of are orders of magnitude larger than the A values given for the hyperfine splitting of an EPR spectrum (or ENDOR spectrum) in units of the field sweep necessary to obtain electron spin resonance. These are usually on the order of only 10 mT or less ... 

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. 

Electron paramagnetic resonance (EPR) yields the location of unpaired electron density from hyperfine splitting by metals or atoms with nuclear spin.21 The S = 0 Fe(III)—O 2 state of oxy-Mb or Hb would be indicated by the absence of an EPR signal, although other results such as the IR or resonance Raman absorption of the O2 moiety would be needed for positive confirmation. 

The electron spin resonance (ESR) spectra of the radical anion of 2,2 -bipyridine, sometimes in the form of its alkali metal com-plgx, 71.175,177.299-304 radical anion of 3,3 -bipyridine, ° and the radical anion of 4,4 -bipyridine, ° ° usually obtained by reduction of the bipyridines with an alkali metal, have been measured, and hyperfine splitting constants were assigned. Related biradical species have also been investigated. The ESR spectrum of the 4,4 -bipyridinium radical cation, of which... 

The electron spin resonance (ESR) spectrum of the radical anion of 1,10-phenanthroline obtained by reduction of 1,10-phenanthroline with sodium has been measured, and hyperfine splitting constants were assigned.116... 

Electron spin resonance studies of silver(II) pyridine complexes have proved to be extremely useful in determining the nature of the spedes in solution. Since natural silver has two isotopes, 107Ag and 109Ag, in approximately the same abundance, both of spin / = J, and since their nuclear magnetic moments differ by less than 15%, interpretation of spectra is often considered in terms of a single nucleus. The forms of the hyperfine splitting patterns for IN, cis and trans 2N, 3N and 4N, would be expected to be quite different and hence the number of pyridines can be readily assessed from well-resolved spectra. Spin Hamilton parameters obtained from both solid and frozen solution spectra are collected in Table 64.497 499 501-510... 

The N//-azaindole 40 with X = 6 is paramagnetic. Its electron spin resonance (ESR) spectrum (in DMSO) shows a characteristic triplet with a 1 1 1 intensity and the hyperfine splitting constant An = 16.04 Oe. 

Cheshire, M. V., and McPhail, D. B. (1996). Hyperfine splitting in the electron spin resonance solution spectra of humic substances. Eur. J. Soil. 47,205-213. 

Fig. 7. Electron spin resonance spectra of potassium, rubidium, and cesium in (a) methylamine, (b) ethylamine, and (c) propylamine adapted from Dye and Dalton (58), with permission. Note the change in scale from pattern to pattern, as indicated by the (metal) hyperfine splittings given. Reprinted with permission from J. L. Dye and L. R. Dalton, Journal of Physical Chemistry, 71, 184 (1967). Copyright 1967 American Chemical Society.

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]. 

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