Radical association

We have also measured the rate constant for the association reaction of two Mn(C0)5 radicals generated on photolysis of Mn2(CO)io- With appropriate assumptions regarding the absorption coefficient for Mn(C0)5, the rate constant for this reaction was determined to be (2.7 0.6) x 10 1 mole l s [6,10]. This is compatible with the diffusion limited rate constant for this reaction that has been measured in solution and is within an order of magnitude of a gas kinetic rate constant as would be expected for an essentially unactivated radical-radical association reaction [33a]. 

In conclusion, we have shown that PMj 5 contains stable radicals that can be detected by EPR. The EPR parameters, persistence in air, and DNA-damaging capacity of the PM2 5 radicals are similar to those of the radicals in cigarette tar. Therefore, we propose that the radicals associated with PMj 5 include semiquinone-type radicals that, like the... 

The results presented here suggest a new mechanism of toxicity for PM 5 based on sustained hydroxyl radical generation by the semiquinone radicals present in PM 5. Because a substantial fraction of the fine particles in the atmosphere arises from combustion sources (9), it is possible that the deleterious health effects associated with PM2 5 can be at least partially ascribed to radicals associated with combustion-generated particulate matter. 

Still, one can use standard packages for attractor states by actually finding a stationary geometry. The problem is that the standard theory does not conceive of confined states. This element is new. It qualitatively simplifies the analyses. But it is not a practical tool yet. However, confinement underlies a severe limitation of the LCAO bonding orbitals as they dissociate in a manner inconsistent with the general confinement property. Barrier less profiles for radical association process can be traced back to a numerical artifact. 

Comparisons of classical trajectory calculations with various versions of VTST have also been performed for quite a series of other reaction systems such as neutral radical association/dissociation and radical-surface association processes [27-30], In these studies, the various treatments employed... 

Plasma malondialdehyde-like material, an indicator of lipid peroxidation, is increased in conditions of ischaemia, such as stroke [83, 84] and myocardial infarction [85]. Mitochondria extracted from hearts of vitamin-E-deficient rabbits showed a decreased mitochondrial function and an increased formation of oxygen radicals associated with a reduced superoxide dismutase activity. This was partially reversed by addition of vitamin E in vitro [86]. Measurement of in vitro susceptibility to lipid peroxidation in cardiac muscle from vitamin-E-deficient mice showed a highly significant negative correlation between the concentration of vitamin E and in vitro lipid peroxidation. The results indicate that short-term vitamin E deficiency may expose cardiac muscle to peroxidation injuries [ 87 ]. In rats, treatment for 2 days with isoprenaline increased lipid peroxide activity, as measured by malondialdehyde levels, in the myocardium. Vitamin-E-deficient animals were even more sensitive to this effect, and pretreatment with a-tocopheryl acetate for 2 weeks prevented the effect induced by isoprenaline. The authors [88] propose that free-radical-mediated increases in lipid peroxide activity may have a role in catecholamine-induced heart disease. 

Scavenging of free radicals associated with a significant enhancement in glutathione peroxidase activity... 

The CHj groups taken as planar rc-c is the most probable value for the methyl radical association complex at 400°K. value depends somewhat on assumed polarizability.21... 

Figure 8B shows /(c) and K(t) for ethane formed by the association of two methyl radicals [using complex 3, Table I, to evaluate the 22 P(e rf) ] and for ethane thermally activated at 300°C. The average energy of the ethane molecule formed by methyl radical association is 3.6 kcal. mole-1 higher than for the molecule formed by thermal activation. Therefore, if... 

Fig. 3. Nature of free radical associated with compound I in peroxidases/catalases. Structure of first intermediate, following peroxide addition to ferric peroxidases and catalases. Boxes denote porphyrin ring. The amino-acid free radicals are depicted as protonated (tryptophan) and deprotonated (tyrosine), although this is yet to be conclusively determined.

The authors cited a combination of ischemia and exci-totoxicity due to cocaine exposure as the possible cause of the brain injury. Oxidative stress and free radicals associated with cerebral hypoxia contribute to cell damage and death. 

Note This reaction is somewhat complicated by the cage effect on the escape of the newly formed free radicals. = association constant for 2NO2 N2O4... 

Solutions of sulfur in more dilute oleum, e.g., 15%, give ESR spectra with a second signal (g = 2.027). This signal is also obtained from a solution of in fluorosulfuric acid. It seems reasonable, therefore, to attribute this ESR signal to a radical associated with Sjg, presumably Sa, formed by dissociation [Bq. (32)]. 

Rate coefficients for recombination reactions are related to those for dissociation via the equilibrium constant, which can generally be calculated from thermodynamical information with a high degree of precision, although the accuracy depends on the quality of the thermodynamic data. The rate coefficients are pressure dependent and the theoretical framework of unimolecular reactions can therefore be used to describe them. Because there is little or no activation energy for the recombination process, rates of radical association reactions can be measured over a wide range of temperatures and can be used, in combination with thermodynamic information, to calculate rate coefficients for unimolecular dissociations. The availability of data for a number of radical recombination reactions over a wide range of pressures and temperatures makes these reactions excellent test beds for theoretical models of pressure dependent reactions. 

Various thermodynamic properties of 178 have been studied. Lucken early recognized that the radical associates to a dimeric form in solid salts and certain solutions. This association has been the object of recent scrutiny. Optical spectra of 178 free, in ion-paired form with perchlorate, and in a dimeric form associated with two perchlorate ions have been recognized. The exact nature of the forms present in a solution depends upon the solvent equilibrium constants and activation parameters for the association were determined. 

Equation (33)), which then oxidize substrate. A prominent example of Equations (29), (31), and (32) is formation of a POM-bound oxygen atom donor, which transforms to a POM oxometal intermediate, which then oxidizes the substrate. The last mode (generation of metal-free oxidizing intermediates) can involve oxygen donors, such as iodosylarenes, peroxides (formation of alkyl-peroxy, alkoxy radicals, etc.), or 02 (formation of all the radicals associated with radical chain oxidation or autoxidation ).335-338... 

These reactions are defined as the association of one nucleophile with a radical that leads to the displacement of another nucleophile. Several examples of these reactions are shown in Table 9.7. It is generally possible to rewrite these reactions as two component reactions of the radical association with nonradical type. For example, the reaction... 

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