Dissociation mechanisms radical-induced

This nuance of the original Sr I mechanism may thus occur in quite a number of cases. Nomenclature purists may consider it necessary to find other symbols to name this mechanism and, presumably, to question the adequacy of the 1 in this case. Beyond symbols, if the Sr I mechanism is viewed as an outer sphere electron-transfer-induced nucleophilic substitution , a possible designation of the mechanism under discussion might be dissociative electron-transfer-induced nucleophilic substitution . The original designation of these reactions as nucleophilic reactions proceeding via anion radical intermediates (Komblum, 1975) would still apply to both nuances of the mechanism since, in the present case, RNu is an essential intermediate in the reaction, even if RX is not. 

In the case of the iron and cobalt porphyrins discussed above, RDox/RDred - - Dox/Dred 0 with cobalt(i) and iron(o) whereas the opposite is true for iron(i). RDox/RDred Dox/Dred s the difference in driving forces between reactions (147) and (145) and also that between reactions (144) and (148). Thus, kc > k c for the isoelectronic Co(l) and Fe(0), which matches the radical character of Co(n) and Fe(i) (Lexa et al., 1981), and kc > k t for Fe(i) hence the occurrence of the chain mechanism is unlikely in all cases. In the case of iron(o) and iron(i), this conclusion falls in line with the observed effect of steric constraints which should not appear in this outer sphere, dissociative electron-transfer-induced chain mechanism. 

The degree of dissociation is very small but the diphenylcyanomethyl radical is sufficiently reactive to induce polymerization in styrene. Methyl radicals or hydrogen atoms bring about polymerization of vinyl monomers in the gas phase.Hydrogen peroxide in the presence of ferrous ions initiates polymerization in the aqueous phase or in aqueous emulsions through generation of hydroxyl radicals according to the Haber-Weiss mechanism... 

The above examples should suffice to show how ion-molecule, dissociative recombination, and neutral-neutral reactions combine to form a variety of small species. Once neutral species are produced, they are destroyed by ion-molecule and neutral-neutral reactions. Stable species such as water and ammonia are depleted only via ion-molecule reactions. The dominant reactive ions in model calculations are the species HCO+, H3, H30+, He+, C+, and H+ many of then-reactions have been studied in the laboratory.41 Radicals such as OH can also be depleted via neutral-neutral reactions with atoms (see reactions 13, 15, 16) and, according to recent measurements, by selected reactions with stable species as well.18 Another loss mechanism in interstellar clouds is adsorption onto dust particles. Still another is photodestruction caused by ultraviolet photons produced when secondary electrons from cosmic ray-induced ionization excite H2, which subsequently fluoresces.42... 

On the other hand, in accord with the free radical mechanism peroxynitrite is dissociated into free radicals, which are supposed to be genuine reactive species. Although free radical mechanism was proposed as early as in 1970 [111], for some time it was not considered to be a reliable one because a great confusion ensued during the next two decades because of misinterpretations of inconclusive experiments, sometimes stimulated by improper thermodynamic estimations [85]. The latest experimental data supported its reliability [107-109]. Among them, the formation of dityrosine in the reaction with tyrosine and 15N chemically induced dynamic nuclear polarization (CIDNP) in the NMR spectra of the products of peroxynitrite reactions are probably the most convincing evidences (see below). 

In 1957, Otsu and coworkers reported that the polymer obtained from St with 13 could induce the radical polymerization of second monomers leading to block copolymers [70-74]. Poly(St)-hZock-poly(MMA), poly(St)-hZock-poly (AN), poly(St)-Z Zock-poly(VAc), and poly(St)-hZock-poly(VA) were prepared from the end-functional poly(St) [75], In the photopolymerization of St and MMA with 13, it was also confirmed that the molecular weight of the polymers produced linearly increased with the reaction time, although the reaction mechanism was not ascertained at that time. Thereafter, the poly(St) produced with 13 was confirmed to have two DC end groups, which can further dissociate pho-tochemically [76]. 

If the charge transfer (CT) complex is sufficiently strong, the ion-radical pair would dissociate to induce ionic and/or radical reactions. The mechanism of this photoexcitation is different from the n — n or n — n excitations. The later process is the excitation of isolated molecules whereas the CT excitation requires two molecules in contact. Surprisingly, rather limited attention has been directed to this field of photosensitized CT process from the viewpoint of organic reactions. 

As mentioned above, the secondary reaction in the system is caused by a new reaction of free radical interaction (produced in the elementary dissociation of the radical initiator) with the initiator. In this case, one more active intermediate particle (free radical), not observed at usual peroxide decomposition, is generated in the system. Owing to formation of this active site, conjugated reaction proceeds by the radical-chain mechanism. Thus products formed may be analogous to those obtained at usual initiator decomposition, or different products may be formed—this circumstance is of no importance for detection of induced decomposition. 

In this context, works [20, 21] should be mentioned, in which 0 2 ion-radicals were ESR detected in all samples of solution quickly frozen after initiation of H202 catalytic dissociation on metals and oxides (applied on A1203). The ion-radicals mentioned occur on the surface and desorb to the liquid phase, where, with high probability, they induce free radical processes. These results conform to the Weiss mechanism (6.3) of H202 dissociation on heterogeneous catalysts. 

Kinetic measurements were made by monitoring the laser-induced fluorescence of CH following the excitation in the (0-0) band of the X — A transition as a function of the time delay after the ArF laser dissociation. In the absence of any added reactants, CH had a decay time of 100 to 300 /isec at a total pressure of 30 to 100 torr (CHBr3 pressures of 1 to 10 mtorr) which can be attributed mainly to the CH + CH reaction. The addition of the reactants listed in Table I shortened the CH radical decay times considerably, indicative of some removal process involving a bimolecular mechanism since the total pressure was always maintained constant. Least squares plots of the inverse lifetimes of CH radicals versus the partial pressure of the added reactant yielded... 

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