Isotropic Hyperfine Splitting Constants

As mentioned before, chromium(VI) reacts with NADH or NAD(P)H in the presence of rat-liver cytosolic or microsomal fractions, forming stable Cr(V)-NAD (P)H complexes, while addition of Glc6P led to stable Cr(V)-Glc6P complexes together with the Cr(V)-NAD(P)H complexes.11 The chromium(V) complexes have isotropic g values of 1.980 or 1.982, and H hyperfine splitting constants of... 

According to Table I, the small Co2+ hyperfine splitting constants indicate that the unpaired electron must be largely localized on the coordinated oxygen molecule. If the unpaired electron is localized in only one d orbital, the hyperfine tensor can be resolved into an isotropic and anisotropic part in the form ... 

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). 

Table 6-5. -Factors and isotropic nitrogen and oxygen hyperfine splitting constants, a( N) and a( 0), of 2,2,6,6-tetramethyl-4-piperidone-l-oxyl in ten solvents of increasing polarity [216],...

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. 

A wealth of data has now accumulated about the hyperfine splitting constants of organic radicals in this respect, e.s.r. studies in the liquid state have been especially valuable in that the resolution attainable permits the isotropic constants to be measured very accurately. We shall review here the theories based on these results which relate splitting constants to structural features and the extent to which it is now possible to draw conclusions about details of structure from e.s.r. spectra. 

The isotropic and anisotropic hyperfine coupling constants for 61Ni isotope (1=3/2), as well as for isotopes 14N (1=1) and 13C (1=1/2) in a relaxed cluster were calculated. The anisotropic hyperfine splitting constants for 61Ni were changed from 75 to 150 MHz depending on the nanocrystal orientation. The observations are important for future possible applications of such nanocrystals in quantum information technologies. 

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. 

Hyperfine Splitting. The isotropic value of the hfs depends on electron spin density at the nucleus in question. Since p orbitals have a node at the nucleus, it shows how much spin density resides in the atom s s orbitals. For the 2s orbital of carbon the proportionality constant is 1110 gauss/electron for the Is orbital of hydrogen, 507. ( ) The magnitude of the hfs amisotropy depends on how much the spin density in the neighborhood of the nucleus departs from spherical, and how much from axial symmetry. Since the through-space interaction is attenuated by the cube of the electron-nuclear distance, it samples primarily the immediate vicinity of the nucleus. For C-13 the anisotropic hfs commonly has an axis of symmetry and may be interpreted in terms of spin density in the 2p orbitals of the... 

The hyperfine coupling tensor (A) describes the interaction between the electronic spin density and the nuclear magnetic momentum, and can be split into two terms. The first term, usually referred to as Fermi contact interaction, is an isotropic contribution also known as hyperfine coupling constant (HCC), and is related to the spin density at the corresponding nucleus n by [25]... 

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