Paramagnetic transition metal ions

A review of all of the different types of EPR spectrum that can be obtained from the various paramagnetic transition metal ions is beyond the scope of this chapter. However, a few examples will be presented below to illustrate the type of information that can be obtained. 

Paramagnetic transition metal ions partially exchanged into... 

A second way to overcome this spin conservation obstacle is via reaction of 302 with a paramagnetic (transition) metal ion, affording a superoxometal complex (Fig. 4.1, Reaction (3)). Subsequent inter- or intramolecular electron-transfer processes can lead to the formation of a variety of metal-oxygen species (Fig. 4.2) which may play a role in the oxidation of organic substrates. 

Because of their quasi-isostructural nature, trinuclear clathrochelates are unique for the study of intramolecular exchange interaction between three paramagnetic transition metal ions as a function of their d electronic configuration by magnetic susceptibility measurements and EPR spectra. 

The third principal application of the electron spin resonance technique is to the study of paramagnetic transition metal ions in biochemical systems. Most examples are complexes of copper, iron, manganese, chromium, cobalt and molybdenum. Other metals such as titanium, vanadium and nickel are sometimes employed as structural probes. Only four of these ions, Cu ", Mn, Gd " and VO ", are seen in ESR spectroscopy at room temperature under virtually all conditions. Therefore, they are of special importance. 

The radical species located on the PPy chains has been studied extensively using EPR spectroscopy [1,39,51,54,80,81] a sharp resonance is usually observed in the g = 2.0 region. The latter originates from a combination of polarons and neutral n-radicals. (This topic has been discussed in detail elsewhere [1]) EPR spectra of PPy containing paramagnetic transition-metal ions exhibit signals originating from the metal ion in addition to the g = 2.Q resonance [6,8,10,11]. Therefore this technique is suited to a study of the nature of the counter-anion and its environment within these materials. 

The large enhancement of fluorescence in L1-L3 in the presence of a paramagnetic transition metal ion is unprecedented. To confirm that the... 

Consider a paramagnetic transition metal ion in a crystal when a magnetic field is applied. The total Hamiltonian may in this case be represented by ... 

The real power of electron spin resonance spectroscopy for structural studies is based on the interaction of the impaired electron spin with nuclear spins. This interaction splits the energy levels and often allows determination of the atomic or molecular structure of species containing unpaired electrons, and of the ligation scheme around paramagnetic transition-metal ions. The more complete Hamiltonian is given in equation 2 for a species containing one unpaired electron, where the summations are over all the nuclei, n, interacting with the electron spin. 

Spin probes are free radicals or paramagnetic transition metal ions that are admixed to the system of interest, while spin labels are stable free radicals that are covalently bound to a macromolecule of interest. Spin labeling thus involves modification of the synthesis of the material and some modification of its structure. Suitable spin probes are often commercially available, so that spin probing typically requires less effort. If a question can be answered either by labeling or probing, probing is thus the technique of choice. 

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