Electron spin resonance , definition

Methods such as nuclear magnetic resonance (NMR), electron spectroscopy for chemical analysis (ESCA), electron spin resonance (ESR), infrared (IR), and laser raman spectroscopy could be used in conjunction with rate studies to define mechanisms. Another alternative would be to use fast kinetic techniques such as pressure-jump relaxation, electric field pulse, or stopped flow (Chapter 4), where chemical kinetics are measured and mechanisms can be definitively established. 

Despite of this inherent limitation, several spectacular results have been obtained. It should be noted that the initiation mechanism of the cationic polymerization of styrene described above was also deduced from the results of pulse radiolysis experiments. The pulse radiolysis combined with other techniques, such as the matrix isolation technique, the electron spin resonance technique and usual polymerization techniques, definitely provides a powerful means for investigating fundamentals of polymerization. 

The properties are most useful when there are several closely overlapping peaks, and higher order derivatives are commonly employed, for example in electron spin resonance and electronic absorption spectroscopy, to improve resolution. Figure 3.11 illustrates the first and second derivatives of two closely overlapping peaks. The second derivative clearly indicates two peaks and fairly accurately pinpoints their positions. The appearance of the first derivative would suggest that the peak is not pure but, in this case, probably does not provide definitive evidence. It is, of course, possible to continue and calculate the third, fourth, etc., derivatives. 

Since a large number of xenobiotics are metabolized to free radicals, an overall view of this area is not obvious. By definition, free radical metabolites must exist free of the enzyme, and, therefore, enzyme-xenobiotic transition states with free radical character such as are thought to exist in the cytochrome P-450 substrate complex are excluded. It follows that if the rate of formation of the free radical is fast enough, it can be detected with electron spin resonance, and will have the same ESR spectrum as the free radical made by purely chemical means. 

Various techniques have been utilized to determine the existence of a metal-metal bond in the solid state. Single crystal x-ray analysis and neutron diffraction (17) are the most accurate methods available and are often facilitated by the strong scattering of the metal atoms. Polarized electronic spectroscopy (430), electron spin resonance, magnetic susceptibility, and dc conductivity have been used to indicate some solid state interaction. These techniques are, however, not definitive (144). Recent work indicates that resonance enhancement in Raman spectroscopy may provide a useful tool (101, 382) in elucidating metal-metal interactions. 

Electron spin resonance (ESR) operates at somewhat higher frequencies in the GHz range and is sometimes called electron paramagnetic resonance (EPR). ESR is like NMR but uses electron spins rather than nuclear spins. By definition, FT-ESR studies free radicals, and it is more sensitive than FT-NMR but of less general applicability. Related to... 

Deep state experiments measure carrier capture or emission rates, processes that are not sensitive to the microscopic structure (such as chemical composition, symmetry, or spin) of the defect. Therefore, the various techniques for analysis of deep states can at best only show a correlation with a particular impurity when used in conjunction with doping experiments. A definitive, unambiguous assignment is impossible without the aid of other experiments, such as high-resolution absorption or luminescence spectroscopy, or electron paramagnetic resonance (EPR). Unfortunately, these techniques are usually inapplicable to most deep levels. However, when absorption or luminescence lines are detectable and sharp, the symmetry of a defect can be deduced from Zeeman or stress experiments (see, for example, Ozeki et al. 1979b). In certain cases the energy of a transition is sensitive to the isotopic mass of an impurity, and use of isotopically enriched dopants can yield a positive chemical identification of a level. 

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