Radical-induced dissociation

Nonehilbel and Walton [13] called such reactions producing free radicals induced dissociation processes. Hence, ROOH and ROOR dissociation is considered as induction of OH and other radicals by free radicals, generated in the chemical system by self-hemolytic break of their—O O-bond with formation of an active site (alkoxide). 

FIGURE 9.48 Examples of radical-induced dissociations of distonic ions. 

As described earlier, superoxide is a well-proven participant in apoptosis, and its role is tightly connected with the release of cytochrome c. It has been proposed that a switch from the normal four-electron reduction of dioxygen through mitochondrial respiratory chain to the one-electron reduction of dioxygen to superoxide can be an initial event in apoptosis development. This proposal was supported by experimental data. Thus, Petrosillo et al. [104] have shown that mitochondrial-produced oxygen radicals induced the dissociation of cytochrome c from bovine heart submitochondrial particles supposedly via cardiolipin peroxidation. Similarly, it has been found [105] that superoxide elicited rapid cytochrome c release in permeabilized HepG2 cells. In contrast, it was also suggested [106] that it is the release of cytochrome c that inhibits mitochondrial respiration and stimulates superoxide production. 

Nizigiyimana, L. Rajan, P.K. Haemers, A. Claeys, M. Derrick, P.J. Mechanistic Aspects of High-Energy Collision-Induced Dissociation Proximate to the Charge in Saturated Fatty Acid n-Butyl Esters Ca-tionized With Lithium. Evidence for Hydrogen Radical Removal. Rapid Commun. Mass Spectrom. 1997, 11, 1808-1812. 

Kranenburg M, Ciriano MV, Cherkasov A, Mulder P (2000) Carbon-oxygen bond dissociation enthalpies in peroxyl radicals. J Phys Chem A 104 915-921 Lai M, Rao R, Fang X, Schuchmann H-P, von Sonntag C (1997) Radical-induced oxidation ofdithio-threitol in acidic oxygenated aqueous solution a chain reaction. J Am Chem Soc 119 5735— 5739... 

It is also known that the induced dissociation in various solvents proceeds with the participation of the R radical and is sufficiently dependent on both the origin of radical and solvent. Approximately the same interpretation of induced dissociation processes are caused by charge transfer, photosensitizers, etc. [13, 15],... 

This is proved by the type of kinetic equation (1.7), from which it follows that the first term describes usual monomolecular dissociation, and for the second process value n may become equal 2/3 or 1 with respect to the solvent, i.e. induced dissociation of radical initiator obeys kinetic laws. 

Table 21-4. The relative electronic energies (AEr) and free energies (A( I)) at 298 K of stationary points on the reaction path leading from the radical anions (Y -dXOMPII Y = 3, 5, X=C,T) via transition states (TS) to the C3 -0 and C5 -0 bond broken complexes (Product complex) for electron induced dissociation of pyrimidine nucleotides. All values given in kcal/mol. (Table 2 of ref. [79] (Reprinted with permission. Copyright (2006) National Academy of Sciences, U.S.A.) and Table 1 of ref. [80] (Reprinted with permission. Copyright 2006 American Chemical Society.))...

Bamford and Mullik (66) showed that irradiation of Mn(CO)5CH3 in methyl methacrylate brought about free radical-induced polymerization of the monomer. The ketone methyl 2-methyl-4-oxopentanoate (73) was also a major product of this reaction [Eq. (78)]. Polymerization was strongly inhibited by CO which led the authors to suggest that the primary photochemical step was CO dissociation to give the coordinatively unsaturated complex 74, [Eq. (79)]. This, they suggested, decomposed thermally... 

The latter observations with methyl oleate, together with thermodynamic considerations and EPR evidence for free radical intermediates, suggest an alternative explanation for the dramatic increase in oxidation rates once hydroperoxides accumulate, namely that bimolecular decomposition may be specific to allylic hydroperoxides and proceed via LOO radical-induced decomposition rather than by dissociation of hydrogen-bonded dimers (280). Reaction sequence 63 is analogous to Reactions 49 and 50a, where one slowly reacting radical reacts with a... 

Despite its high heat of formation (AH = 224 kcalmoP ) the next higher homo-logue of 1, i. e. the cation radical of methylcyclopropane (107) appears to be formed as a substantial fraction of the C4H8 product ions generated from 1- and 2-chlorobutane (121, 122), 1-butanethiol (123), -butyl vinyl ether (124) and w-butyl formate (125), as evidenced by labelling studies, kinetic energy release measurements and the analysis of collision induced dissociations (Scheme 18). 

As would be predicted from Equation 1, the rate of dissociation of free radical initiators is decreased by the application of pressure. Thus azobisisobuty-ronitrile dissociates with a rate constant equal to 4.47 X 1(H sec." at 1500 atm. but at 1 atm. the dissociation rate constant is 5.5 X 10 sec. (8). Studies concerning the effect of pressure on the decomposition of benzoyl peroxide reveal that the rate of this reaction also decreases with increasing pressure (II, 18). The extent to which the radical-induced decomposition of this peroxide at high pressures affects the rate is not clear, but it appears that some complications arise from this cause. 

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