Intermediates paramagnetic

Zeolites are attractive host materials as they present regular channel systems, providing crystallographic defined locations for metal clusters. Hence, the size of the clusters inside the zeolites can be limited by the dimensions of their channels and cages. Zeolite-supported noble metal catalysts were extensively studied due to their wide application as catalysts in petroleum industry.Reduction of silver in zeolite hosts has been carried out by /-irradiation.The intermediate paramagnetic (charged) silver clusters formed in /-irradiated silver-exchanged zeolites have been studied by electron spin resonance (ESR)... 

Reaction (5) results in the formation of an intermediate paramagnetic complex (Mo "...NO ) which arises from a two-electron transfer from Mo " ion to a NO molecule. This complex has been detected by EPR (see below). Reaction (6) is the interaction of the... 

The reaction mechanism of the NO photocatalytic reduction by CO discussed above suggests that N2O is formed via intermediate paramagnetic complexes (Mo. ..NO ) by reactions (5) and (6). 

Potential and demonstrated applications of electron spin resonance are ubiquitous. The technique is particularly useful because it is sensitive only to those species that are paramagnetic. If these are important reaction intermediates, one has a selective analytical technique to look only at those specific types of reaction intermediates. Paramagnetic species are probably much more widespread than is generally believed. Radicals are typically reactive species. 

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 difference between the two extremes is essentially that, in the former, the Re retains its valence electrons in its d orbitals whereas in the latter it loses 6 of them to delocalized ligand orbitals. In either case paramagnetism is anticipated since rhenium has an odd number of valence electrons. The magnetic moment of 1.79 BM corresponding to 1 unpaired electron, and esr evidence showing that this electron is situated predominantly on the ligands, indicates that an intermediate oxidation state is involved... 

Planar-octahedral equilibria. Dissolution of planar Ni compounds in coordinating solvents such as water or pyridine frequently leads to the formation of octahedral complexes by the coordination of 2 solvent molecules. This can, on occasions, lead to solutions in which the Ni has an intermediate value of jie indicating the presence of comparable amounts of planar and octahedral molecules varying with temperature and concentration more commonly the conversion is complete and octahedral solvates can be crystallized out. Well-known examples of this behaviour are provided by the complexes [Ni(L-L)2X2] (L-L = substituted ethylenediamine, X = variety of anions) generally known by the name of their discoverer I. Lifschitz. Some of these Lifschitz salts are yellow, diamagnetic and planar, [Ni(L-L)2]X2, others are blue, paramagnetic, and octahedral, [Ni(L-L)2X2] or... 

Monomer-oligomer equilibria. [Ni(Me-sal)2], mentioned above as a typical planar complex, is a much studied compound. In pyridine it is converted to the octahedral bispyridine adduct (/zsoo = 3.1 BM), while in chloroform or benzene the value of is intermediate but increases with concentration. This is ascribed to an equilibrium between the diamagnetic monomer and a paramagnetic dimer, which must involve a coordination number of the nickel of at least 5 a similar explanation is acceptable also for the paramagnetism of the solid when heated above 180°C. The trimerization of Ni(acac)2 to attain octahedral coordination has already been referred to but it may also be noted that it is reported to be monomeric and planar in dilute chloroform solutions. 

Simultaneous ESR and electrochemical measurements on a polypyrrole film give convincing evidence that the charging process in this film involves the generation of paramagnetic species which are obviously intermediates in the process of switching from the neutral to the oxidized state In any case, independent of all other findings,... 

Is the paramagnetic adduct between CO and Cluster A a kinetically intermediate in acetyl-CoA synthesis Questions have been raised about whether this adduct is a catalytic intermediate in the pathway of acetyl-CoA synthesis 187, 188) (as shown in Fig. 13), or is formed in a side reaction that is not on the main catalytic pathway for acetyl-CoA synthesis 189). A variety of biochemical studies have provided strong support for the intermediacy of the [Ni-X-Fe4S4l-CO species as the precursor of the carbonyl group of acetyl-CoA during acetyl-CoA synthesis 133, 183, 185, 190). These studies have included rapid ffeeze-quench EPR, stopped flow, rapid chemical quench, and isotope exchange. 

What determines the way in which the spins couple Parallel orientation always occurs when the corresponding atoms act directly on one another. This is the case in pure metals like iron or nickel, but also in EuO (NaCl type). Antiparallel orientation usually occurs when two paramagnetic particles interact indirectly by means of the electrons of an intermediate particle which itself is not paramagnetic this is called superexchange mechanism. That is the case in the commercially important spinels and garnets. 

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