Metal-radical interactions

Vibrational frequencies measured in IR experiments can be used as a probe of the metal—ligand bond strength and hence for the variation of the electronic structure due to metal—radical interactions. Theoretical estimations of the frequencies are obtained from the molecular Hessian, which can be straightforwardly calculated after a successful geometry optimization. Pure density functionals usually give accurate vibrational frequencies due to an error cancellation resulting from the neglect of... 

Detailed analysis of the line widths in the NMR spectra produced the main parameters of dynamic processes in the complexes under discussion (Table IV). The results obtained show that high sensitivity of the relaxation rate of free radicals to the metal-radical interaction may be useful in the application of spin labels for investigating metal-containing biomolecules. 

This reaction is one example of several possible radical transition-metal ion interactions. The significance of this and similar reactions is that radicals are destroyed and are no longer available for initiation of useful radical reactions. Consequentiy, the optimum use levels of transition metals are very low. Although the hydroperoxide decomposes quickly when excess transition metal is employed, the efficiency of radical generation is poor. 

Whereas the quasi-chemical theory has been eminently successful in describing the broad outlines, and even some of the details, of the order-disorder phenomenon in metallic solid solutions, several of its assumptions have been shown to be invalid. The manner of its failure, as well as the failure of the average-potential model to describe metallic solutions, indicates that metal atom interactions change radically in going from the pure state to the solution state. It is clear that little further progress may be expected in the formulation of statistical models for metallic solutions until the electronic interactions between solute and solvent species are better understood. In the area of solvent-solute interactions, the elastic model is unfruitful. Better understanding also is needed of the vibrational characteristics of metallic solutions, with respect to the changes in harmonic force constants and those in the anharmonicity of the vibrations. 

To explain the particles that formed in both the ethylene/oxygen and hydrogen/oxygen mixtures, it was postulated that they form in the gas phase and that the overall etching process takes place in three steps. First, free radicals are formed homogeneously in a boundary layer adjacent to the surface. Second, these radicals interact with metal atoms in the surface. This interaction results in the formation of volatile intermediates. Third, the metastable, volatile intermediates interact in the gas phase so that metal particles are formed and stable product molecules released. Individual metastable species presumably interact with each other and also with particles formed from multiple collisions. The larger particles interact with each other as well. 

Rapid metal (or semiconductor) etching involving loss of material is encountered in several areas of technology and science. Similar mechanisms account successfully for these processes. Vapor-phase free radicals interact with the solid surface to form surface complexes which are subsequently volatilized. This... 

In metal peroxide chemistry, the heterolytic or homolytic nature of catalytic oxidation seems to be strongly dependent on the heterolytic or homolytic dissociation mode of the peroxide intermediate, for which the triangular coordination mode of the peroxide moiety of the metal appears to be a key feature. Heterolytic oxidations require attainable coordination sites on the metal, involve strained metallacyclic reaction intermediates, and are highly selective. In contrast, homolytic oxidations involve bimolecular radical processes with no metal-substrate interactions and are less selective. In the important field of palladium oxidation chemistry, hydroperoxo... 

Radicals themselves are also subject to oxidation and reduction. Radicals interact with oxidizing and reducing transition metal complexes with formation, respectively, of carbocation and carbanion products.111, 112... 

The superoxide oxide radical interacts with nitric oxide to produce peroxynitrite at a rate which three times faster than the rate at which superoxide dismutase utilizes superoxide (Beckman, 1994). Peroxynitrite is capable of diffusing to distant places in neural cells where it induces lipid peroxidation and may be involved in synaptosomal and myelin damage (Van der Veen and Roberts, 1999). After protonation and decomposition, peroxynitrite produces more hydroxyl radicals. This mechanism of hydroxyl radical generation is not dependent on redox active metal ions and may be involved in initiating lipid and protein peroxidation in vivo (Warner et al., 2004). 

Reactions 68-73 (Table IV) represent the radical interactions with metal complexes radical addition (Reaction 68), electron transfer from a radical to a metal (Reaction 70) or from a metal to a radical (Reaction 69), homolytic displacements at the metal or at a ligand (Reactions 71 and 72), and fi-radical fragmentations (Reaction 73). The final reactions (74-79) concern the reactivity of radicals with organic reagents. 

The free radical polymerization is probably initiated by the reaction of the peroxide with a metal—carbon bond which has been modified through complexation, solvation, or even chemical interaction with a proper monomer. This "site is interacted with the peroxide molecule, which is then decomposed in a metal-catalyzed manner to form a free radical terminus on the polymer chain along with an inert metal—peroxide interaction product. Whether the metal in question is aluminum, titanium, or a complex of the two is uncertain since the mechanism of Ziegler type reactions is still uncertain and since all three have been found in separate studies to promote the polymerization of methyl methacrylate in the presence of peroxides. However, the complex between AlEt>Cl and TiCl3 has been observed to have a much greater effect in accelerating the polymerization of methyl methacrylate than either component by itself hence, the complex appears to be the most likely species. 

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