Isotropic hyperfine splitting

In this exercise, we will predict the isotropic hyperfine splitting in HNCN radical at a... 

Table 9.4 Isotropic hyperfine splittings (G) in the methyl and fluoromethyl radicals...

TABLE 28. Isotropic hyperfine splittings (Gauss) of the cyclopropyl radical computed at the UMP2/DZ + P level250... 

The anisotropy of the spin-label hyperfine splitting is a consequence of the particular charge distribution symmetry within the molecule. The analysis of spectral anisotropy provides the immobilization extent of the spin label and is sensitive to the molecular conformation. Moreover, the spin-label molecules have their charge distribution distorted by the polarity of their environment. The isotropic hyperfine splitting constant (a0) provides a relative polarity indicator (Knowles et al., 1976 Campbell and Dwek, 1984). 

Radical Temp. (K) Conditions g Factor Isotropic hyperfine splitting (G)... 

The unpaired electron is a 5s electron and consequently the isotropic hyperfine splittings are quite large, are easily identified and are sensitively subject to local environmental perturbations. 

Irradiation was therefore carried out on silver doped A-zeo-lites at 77 K and a new silver atom species was detected with a different hyperfine splitting than that of the one observed at 4K. This species is designated as Ag°(A) with an isotropic hyperfine splitting of about 1985 MHz which is very close to that of the free atom value. Species Ag°(A) is the dominant species formed by irradiation at 77 K in all of the A-zeolites studied. In addition, species Ag°(B) is also visible at 77 K. However, upon warming above 77 K species Ag°(B) decays to apparently yield Ag°(A). 

So far we have only considered liquid-phase spectra, which give isotropic hyperfine splittings and the average g-values. However, ESR spectroscopists often study frozen samples (or, if available, single... 

The constant of the isotropic hyperfine splitting a is given by the Fermi (25) approximation... 

The occurrence of hyperfine coupling for a particular isotope indicates non-zero spin density at that nucleus in accordance with the Fermi contact term (which includes the electron wave function evaluated at the nucleus) [1]. The analysis in terms of spin density then requires at least a comparison with the (calculated [5]) isotropic hyperfine splitting constant ao, ideally, results from increasingly available open-shell quantum-chemical calculation procedures [64] are employed. 

Extraction of the isotropic hyperfine splitting from an experimental spectrum, of an organic free radical in solution, is normally quite straightforward with the aid of the first order Hamiltonian,... 

Because of the large number of lines in a spectrum, overlapping of lines can lead to difficulties though these have largely been removed by the availability of computer simulation facilities. Furthermore, if one is interested in solvent effects then one obviously chooses a solute whose spectrum is capable of rapid analysis. Having obtained the isotropic hyperfine splitting constants it is necessary to relate them to the electronic structure of the radical. 

It has been found possible to relate isotropic hyperfine splitting constants to the spin densities at the nucleus of interest and at each nucleus bonded to it. Thus for the fragment... 

Isotropic hyperfine splitting constants have been calculated within the semi-empirical INDO regime for several conformations of cyclopentyl radicals. 

Fig. 1. Effect of A- and -tensor anisotropy on the line shape on the ESR spectrum of a typical nitroxide (a) Immobilized and perfectly oriented nitroxides taken at the indicated orientations of the external magnetic field with respect to the nitroxide axis system. ib) Immobilized nitroxides in a polycrystalline sample (rigid limit spectrum), (c) Fast-motional spectrum in a solvent of very low viscosity (a and g are isotropic hyperfine splitting and isotropic g-factor, respectively).

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