Paramagnetic centres

The sharpness of the frequency response of a resonant system is conunonly described by a factor of merit, called the quality factor, Q=v/Av. It may be obtained from a measurement of the frill width at half maxuuum Av, of the resonator frequency response curve obtained from a frequency sweep covering the resonance. The sensitivity of a system (proportional to the inverse of tlie minimum detectable number of paramagnetic centres in an EPR cavity) critically depends on the quality factor... 

PARAMAGNETIC CENTRES OF DISPERSE SORBENTS IN ANALYTICAL DETERMINATION OF ECOTOXICANTS... 

The intensity of the EPR resonance absorption is a measure of the number of paramagnetic centres present [346], while the type of line observed and the measured g factor are indications of the interactions of the paramagnetic particles and of their distribution within the matrix. Such spectra are much more sensitive to changes in crystal field and atomic orientations than X-ray diffraction and are not dependent upon crystallinity [347]. The nature of the paramagnetic particles may be discerned from the superfine structure of the spectrum. 

A major advantage of studying pure compounds is that single crystals can be used, and hence e.s.r. parameters, which are generally anisotropic, can be accurately extracted. Furthermore, if the crystal structure is known, and if, as is frequently the case, the paramagnetic centres retain the orientation of the parent species, the directions of the g- and electron-nuclear hyperfine tensor components can be identified relative to the radical frame. 

Group VIA (Cr, Mo, W). y-Radiolysis studies have been carried out for the simple hexacarbonyls (31,32) and for certain carbonyl iodides (33) and cyclopentadienyl carbonyl iodides (34). In the case of the hexacarbonyls (31,32), two free-radical products have been detected and characterized for Cr(C0)6, weak unidentified EPR signals have been observed for Mo(C0)6, but irradiated W(C0)6 apparently contains no detectable paramagnetic centres. 

A way out of this dilemma is to perform an NMR-type experiment on the paramagnetic centre. NMR provides an inherently higher resolution than EPR, since only one line pair at frequences v is obtained for each nucleus (or set of magnetically equivalent nuclei). The lines appear, to first order, symmetrically spaced around the Larmor frequency v of the respective nucleus for nuclei with I = i and a hfc A the resonance condition is... 

It is of interest that atomic hydrogen centres, which are unstable even at 4 K in quartz, were observed in both coesite and stishovite at 77K.66 The electron centre associated with Ti impurity was observed in stishovite, but not clearly in coesite. Electronic structures of these paramagnetic centres were calculated with the discrete variational (DV)-Xa method to establish a model for centres having substitutional impurities of Al, Ge and Ti.67... 

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