Electron Paramagnetic Resonance EPR

The EPR (or ESR) technique allows the study of the radicals that accompany the coke formation, and thus estimate roughly the amount of coke and obtain information regarding its nature. One of the advantages of this technique, is that it can be used both under static or under on-stream conditions. 

This technique has been used to study the carbonization of polyethylene over acidic zeolites 

Coke formation on H-Mordenite during olefins reaction was studied both under static and under on-stream conditions . Under static conditions, it was found that the carbonization of ethylene and propylene could be separated into two processes, depending upon the temperature. Below 227°C, the radicals of a low-temperature coke are formed and subsequently annihilated. These radicals do not appear to be precursors of the high-temperature coke. Above 227°C, highly unsaturated radicals of high-temperature coke are formed. The presence 

In contrast to a number of other Cu enzymes, the erythrocuprein copper was fully detectable by EPR (3,81, 114). Exposure of the protein to 6 M guanidine hydrochloride caused no measurable changes in the 

EPR spectrum. Thus, detection of the number of transferable electrons was possible (Fig. 14). Aqueous dithionite effectively reduced erythrocuprein. Plotting the height of the g signal versus dithionite concentra- 

As in the CD measurements, dramatic changes occurred in the EPR spectrum when the pH was raised above 12. A copper biuret-type EPR spectrum appeared (gN =2.179 and An =0.019 cm-1) and a completely new copper singnal was observed after the back titration to neutral pH (gN =2.235, g =2.056 and Ah =0.0175 cm-1), indicating irreversible denatursLtion of the native copper protein (81). 

Short-time treatment of erythrocuprein with cyanide was found to produce considerable changes in the visible absorption spectrum and CD properties. EPR measurements of cyanide-, cyanate-, thiocyanate-, and azide-treated erythrocuprein revealed some interesting changes in the EPR parameters (81,113,161,162). These are summarized in Table 5. 

In addition, these anion-binding studies have been supported by optical titrations and by nuclear magnetic relaxation dispersion (NMRD) measurements. An example is presented of cyanide-treated erythrocuprein (Fig. 16). 

The effect of resonant absorption of electromagnetic waves by electron paramagnetic centers in a substance in a permanent magnetic field is referred to as electronic paramagnetic resonance (EPR). 

In the EPR method, the resonance condition for electrons looks like that for nuclei (refer to eq. (8.3.1)  

The basic EPR circuit is similar to that for NMR though the technique of EPR measurements differs significantly from that of NMR. 

Intraatomic superfine interaction is described differently depending on the aggregative state of the substance investigated. In particular, it has the same nature in liquids as that described in Section 8.3 and is referred to as contact interaction it is described by the term  

It can be seen that contact interaction is caused mostly by s-electrons, because only for s-electrons is the wave function on the nucleus distinct from zero. However, contact interaction of the same character can be observed in some cases in the absence of a noncoupled s-electron it can arise to the account of 7i-electron of aromatic hydrocarbons belonging, for example, to anion-radicals contact interaction of a nucleus with s-electrons is rather sensitive even to small electron excitation at hybridization. 

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It can be seen from Table 1 that there are no individual steps that are exothermic enough to break carbon—carbon bonds except the termination of step 3a of —407.9 kJ/mol (—97.5 kcal/mol). Consequentiy, procedures or conditions that reduce the atomic fluorine concentration or decrease the mobiUty of hydrocarbon radical intermediates, and/or keep them in the soHd state during reaction, are desirable. It is necessary to reduce the reaction rate to the extent that these hydrocarbon radical intermediates have longer lifetimes permitting the advantages of fluorination in individual steps to be achieved experimentally. It has been demonstrated by electron paramagnetic resonance (epr) methods (26) that, with high fluorine dilution, various radicals do indeed have appreciable lifetimes. 

The reaction of bis(benzene)vanadium [12129-72-5] with TCNE affords an insoluble amorphous black soHd that exhibits field-dependent magnetization and hysteresis at room temperature, an organic-based magnet (12). The anion radical is quite stable in the soHd state. It is paramagnetic, and its intense electron paramagnetic resonance (epr) spectmm has nine principal lines with the intensity ratios expected for four equivalent N nuclei (13) and may be used as an internal reference in epr work (see Magnetic spin resonance). 

Electron spin is the basis of the experimental technique called electron paramagnetic resonance (EPR), which is used to study the structures and motions of molecules and ions that have unpaired electrons. This technique is based on detecting the energy needed to flip an electron between its two spin orientations. Like Stern and Gerlach s experiment, it works only with ions or molecules that have an unpaired electron. 

The [NiFe] hydrogenase from D. gigas has been used as a prototype of the [NiFe] hydrogenases. The enzyme is a heterodimer (62 and 26 kDa subunits) and contains four redox active centers one nickel site, one [3Fe-4S], and two [4Fe-4S] clusters, as proven by electron paramagnetic resonance (EPR) and Mosshauer spectroscopic studies (174). The enzyme has been isolated with different isotopic enrichments [6 Ni (I = I), = Ni (I = 0), Fe (I = 0), and Fe (I = )] and studied after reaction with H and D. Isotopic substitutions are valuable tools for spectroscopic assignments and catalytic studies (165, 166, 175). 

Since cupric ions are paramagnetic, it is possible by electron paramagnetic resonance (EPR) to obtain information on the status and the environment of the Cu ions adsorbed on uronic acids [4, 5]. Nitella cell walls with uronate charges compensated to 9 or 100% with copper in equilibrium with mixed copper and zinc chloride solutions had their EPR spectra recorded at two different temperatures, 93 and 293 °K (Fig. 3.a, b). 

Compared with the extensive application of NMR procedures, electron paramagnetic resonance (EPR) is used less frequently except for analysis of the state of metals in enzymes and coenzymes. There has, however, been increasing awareness of the role of radical-mediated reactions and some examples are used to illustrate its potential. 

Shortly after, Doetschman and Hutchison reported the first example of a reactive carbene in the crystalline solid state, by preparing diphenylcarbene from diphenyldi-azomethane in mixed crystals with 1,1-diphenylethylene 84 (Scheme 7.23). When the mixed crystals were irradiated, carbene 85 was detected by electron paramagnetic resonance (EPR) and the disappearance of the signal was monitored to determine its kinetic behavior. Two reactions were shown to take place under topochemical... 

Up to date, several experimental techniques have been developed which are capable of detecting some of these particles under ordinary thermodynamic conditions. One can use these methods to keep track of transformations of the particles. For instance, it is relevant to mention here the method of electron paramagnetic resonance (EPR) with sensitivity of about 10 particles per cm [IJ. However, the above sensitivity is not sufficient to study physical and chemical processes developing in gaseous and liquid media (especially at the interface with solids). Moreover, this approach is not suitable if one is faced with detection of particles possessing the highest chemical activity, namely, free radicals and atoms. As for the detection of excited molecular or atom particles... 

Local Structure of the Eu2+ Impurity. From the experimental perspective, the doping of lanthanide ions into solid state materials can be probed by different instrumental technics such as nuclear magnetic resonance (NMR),44 extended X-ray absorption fine structure (EXAFS),45,46 or electron paramagnetic resonance (EPR),47 which instead of giving a direct clue of the local geometry offers only data that can be corroborated to it. From the theoretical point of view,... 

In general, several spectroscopic techniques have been applied to the study of NO, removal. X-ray photoelectron spectroscopy (XPS), electron paramagnetic resonance (EPR), nuclear magnetic resonance (NMR), extended X-ray absorption fine structure (EXAFS) and X-ray absorption near-edge structure (XANES) are currently used to determine the surface composition of the catalysts, with the aim to identify the cationic active sites, as well as their coordinative environment. 

However, reduced ceria is able, alone, to dissociate NO. Martinez-Arias et al. [85] have first investigated by electron paramagnetic resonance (EPR) and FTIR spectroscopies NO reaction on ceria pre-outgassed at different temperatures and showed the role of superoxides differentially coordinated in the formation of hyponitrites species further decomposed into NzO. Later Haneda et al. [86] have demonstrated that reduced ceria and reduced praseodymium oxide dissociate NO even though the presence of a noble metal (Pt) significantly increases the formation of N2 or N20. The main results of this study are summarized in Table 8.9. 

Electron spin resonance (ESR), or electron paramagnetic resonance (EPR) as it is sometimes known, shares many similarities with its cousin, NMR. The origin of the phenomenon is the spin of the electron (rather than the nuclear spin) coupling with the nuclear spins of the atoms in the polymer, but much of the physics of their interactions are similar. The usual spin Hamiltonian, which is used to determine the energies of the interactions, can be written as... 

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