Hyperfine splitting, in EPR

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. 

Lineshape. The two most frequently encountered lineshapes are Lorentzian and Gaussian the latter generally arises from a large number of unresolved hyperfine splittings. In the absence of such complications, epr lines in solution almost always show a Lorentzian lineshape which is defined in the form ... 

Type 1 Cu(II) intense (e > 3000 M 1 cm ) blue (2max 600 nm) optical absorption band EPR spectrum with an uncommonly small hyperfine splitting in gn region. 

In some ways, measurement of the frequency dependence of the EPR line width (1/T2e) is simpler than very accurate g-factor determination. A precise calibration of the magnetic field sweep used to acquire the spectrum is required, which is obtained using field standards like the hyperfine splitting in Fremy s salt, in conjunction with a tracking NMR Gaussmeter. Once the data are obtained at multiple EPR frequencies, they can be analyzed to determine A2 and rv. 

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...

EPR. This trend reflects the exchange interaction between the electron and hole. For given concentrations of donors and acceptors, there is a distribution in the separations between pairs. Slow microwave-modulation frequencies can emphasize the importance of the sharper, more distant pairs (see FIGURE 2). So far, the best donor linewidth of about 1 mT has precluded the observation of N-hyperfine splitting in the 3C polytype, which EPR has shown to be 0.12mT. 

---