Hyperfine splitting in EPR spectra

A second type of structural information can be deduced from the hyperfine splitting in EPR spectra. The origin of this splitting is closely related to the factors that cause spin-spin splitting in proton NMR spectra. Certain nuclei have a magnetic moment. Those which are of particular interest in organic chemistry include H, " N, F, and P. 

There are no treatments for the pseudocontact term, since the orbital part has never been considered when dealing with magnetic coupled systems. However, Eqs. (6.7) and (6.10) hold for the hyperfine splitting in EPR spectra of both solids and solutions [1]. Therefore the same reasoning is likely to apply to the pseudocontact shift as well. The major complication arises from the point-dipolar nature of the pseudocontact shift treatment, which contrasts with the idea of a polymetallic center. 

A second type of structural information can be deduced from the hyperfine splitting in EPR spectra. The origin of hyperfine splitting is closely related to the... 

Ligand hyperfine splittings in EPR spectra, discussed in Section 12.6. 

The magnetic exchange is considerable only inside stacks, but is not extended between them. As the individual z axes of Cu ions in stacks are parallel to one another, such exchange results in averaging of the hyperfine splitting of EPR spectra, but the anisotropy of Zeeman interaction of an individual ion is completely retained. 

The information that can be obtained from an epr spectrum can be divided into three forms (i) the 0-factor, (ii) the hyperfine splitting of the spectra due to the interaction of the spin with magnetic nuclei in the radical and (iii) the shape of the observed bands. 

The above discussion defines the problem in that EPR-active metalloprotein systems often exhibit readily observable but relatively uninformative spectra, with broad linewidths and umesolved hyperfine splitting. In contrast, NMR exhibits narrow lines and nuclear-specific signals with chemical shift and spin conpling information, bnt is inherently insensitive dne to the tiny size of nuclear magnetic... 

Table 2 Dihedral angles of 3-methylene protons of tyrosyl radicals in PSII and R2 and the hyperfine splitting in the EPR spectra...

Fig. 4 Information from nitroxide CW EPR spectra, (a) Right principal axis system of electron-Zeeman and hyperfine tensors (collinear). Left the effect of rotational dynamics on the CW EPR spectra. Fast rotation (i.e., faster than a typical rotational correlation time of Tc 10 ps) leads to the averaged spectrum. The isotropic g-value gjgo determines the center of the central line and spacing between the lines that is dominated by a- a- In the intermediate motion regime 100 ns > tc > 1 ns and the rigid limit is reached at Tj, 1 ps [19]. (b) Influence of the chemical environment on CW EPR spectra. As both, hydrophilic and polar environments lead to an increased electron spin density at the nitroxide nucleus (see gray inset), and hence the line splitting in the spectra in hydrophilic and polar surroundings is larger than in non-polar and hydrophobic environments...

So far no information about details of the expansion of 3d orbitals has been obtained from the optical spectra. Valuable data on this problem may be extracted from ligand hyperfine splittings in the EPR (26) where the related relativistic nephelauxetic effect (18) is operative. 

The dipole-dipole interaction between an electron spin S= l2 with g=g and a proton has a magnitude of 79 MHz at a distance of 1 A. It scales with distance as r and is proportional to the product of the electron and nuclear g factors. Only in exceptional cases are splittings due to this interaction resolved in EPR spectra. Usually. ENDOR or ESEEM techniques are applied that measure nuclear transition frequencies with a sensitivity roughly comparable to an EPR experiment. " The resolution of the measurements is determined by the static NMR line width, which is typically up to 100 kHz for protons in solids and less for other nuclei. This indicates that distances up to 8 A between an electron spin and a proton can be measured. The precision of the distance measurement is not usually limited by the precision of the frequency measurement but rather by the spatial distribution of the unpaired electron. For a paramagnetic center with known structure, the latter contribution can be estimated by quantum-chemical computations of the hyperfine coupling and can thus be corrected. 

We have seen how radicals can easily be detected. How can we find out what they are Interactions between electron and nuclear spin magnetic moments lead to the appearance of hyperfine structure, splitting of lines, in EPR spectra, and this can provide much information about the species involved. It is often necessary to use dilute solutions to resolve these couplings. They arise in two distinct ways. The first of these is by direct dipole-dipole interaction, which depends on the angle between the vector joining the two dipoles and the... 

Thus the patterns of hyperfine splittings observed in EPR spectra provide direct information about the numbers and types of nuclei with spin coupled to the electrons this information is exactly analogous to that obtained from coupling patterns in NMR spectra. The magnitudes of the hyperfine couplings can indicate the extent to which the unpaired electrons are delocalized, while g values could also show whether unpaired electrons are based on transition-metal atoms or on adjacent ligands. Another example is described in the online supplementary material for Chapter 5 (hyperfine splitting), and many other examples of the application of EPR to chemical problems are described in a series of annual reports [5]. 

The most important features of EPR spectra are their hyperfine structure, the splitting of individual resonance lines into components. In general in spectroscopy, the term hyperfine structure means the structure of a spectrum that can be traced to interactions of the electrons with nuclei other than as a result of the latter s point electric charge. The source of the hyperfine structure in EPR is the magnetic interaction between the electron spin and the magnetic dipole moments of the nuclei present in the radical. 

In the above formula, hyperfine interactions with n-number of nuclei have been considered. Hyperfine structure (HFS) in EPR spectra is due to interaction of the electron magnetic moment with the nuclear magnetic moment (flj), which for one nucleus with quantum number I leads to splitting of the single EPR line to N=27+l lines (e.g., for 1=1 and N=3). The contribution of an additional magnetic field generated by the nucleus depends on /ij and the electron-nucleus distance. There are two mechanisms of hyperfine coupling (i) Fermi contact interaction and (ii) dipole-dipole interaction. 

In EPR spectra of transition ions, the hyperfine interaction gives rise to characteristic splittings, for example for copper I=j, 4 lines) and manganese (1= 6 lines) (Figure 2B). It can be used to identify... 

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