Paramagnetic centers location

In the general case, the sample contains a distribution of paramagnetic centers. In this case, the ESR spectrum in the presence of the magnetic field gradient is a superposition of signals from paramagnetic centers located at different... 

The experimental data in Table 7.7 indicates that the values of two g-tensor components are close for the structurally different PCs. Such is the case for the gzz component, whose value is the closest to the free-electron value g — 2.0023. The gyy component also exhibits little changes with changes in the composition of the silicon coordination sphere. This component lies in the symmetry plane of A-Si (0-Si=)2, in which the A-Si bond is located. The third component gxx proves to be most sensitive to the structure of coordination sphere. It is directed perpendicular to the symmetry plane of the paramagnetic center A-Si (Q-Si=)2 and, thus, is... 

Transformation of tetragonal zirconia phase to monoclinic phase has been studied in [53], Calcination of zirconium hydroxide ZrO(OH)2 at various temperatures produced three types of paramagnetic centers assigned to trapped electrons located in oxygen vacancies of Zr02 (g = 2.0018), to adsorbed 02"... 

The results presented in the previous sections demonstrate the importance of point defects at the surface of oxide materials in determining the chemical activity of deposited metal atoms or clusters. A single Pd atom in fact is not a good catalyst of the cyclization reaction of acetylene to benzene except when it is deposited on a defect site of the MgO(lOO) surface. A detailed analysis of the reaction mechanism, based on the calculation of the activation barriers for the various steps of the reaction, and of a study of the preferred site for Pd binding, based on the MgO/Pd/CO adsorption properties, has shown that the defects which are most likely involved in the chemical activation of Pd are the oxygen vacancies, or F centers, located at the terraces of the MgO surface and populated by two (neutral F centers) or one (charged paramagnetic F centers) electrons. 

As lanthanide-induced shifts often have values of up to 20 ppm, the increased dispersion, and hence interpretability, of NMR spectra afforded by this method is enormous. Moreover, pseudo-contact shifts diminish regularly with the distance from the paramagnetic center, r in Figure 12.14, and since this can be approximately located (the lanthanide complex is attached to the lone pair), their magnitudes give valuable structural information. 

To extract detailed hyperfine information from an ESE spectrum, however, it is necessary to calculate the expected modulation pattern for a certain assumed nuclear geometry. By varying the assumed hyperfine interaction for a simulated modulation pattern until it fits the observed modulation pattern, the weak hyperfine parameters may be determined. The general analysis procedure is to analyze the ESE spectrum in terms of a number of equivalent nuclei located at an average distance from the paramagnetic center. The assumption is then that there is a small overlap of the unpaired electron wave function on the closest nuclei to give an isotropic hyperfine coupling constant... 

Dopant carbon centers in a-BN (where the C atom is located above the plane of the BN layer) form paramagnetic centers which can be studied by ESR spectroscopy. A model for the respective fragment of the BN layer has been developed and the ESR spectrum has been... 

It is known (Blumberg 1960, Tse and Hartmann 1968, McHenry et al. 1972) that the kinetics (308) is typical for disordered systems in which a strong inhomogeneous NMR line broadening takes place and nuclear spin diffiision is hampered due to the Larmor frequencies differences of the nuclear spins located in the neighbouring crystal lattice sites. In such systems the nuclear spins at low temperatures relax directly via paramagnetic centers, well coupled with phonons. When paramagnetic centers with 5 = are randomly distributed in a crystal lattice, the nuclear spin-lattice relaxation rate is determined as follows ... 

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