Electron spin resonance peroxy

Determination of Rate Constants for the Self-Reactions of Peroxy Radicals by Electron Spin Resonance Spectroscopy... 

Rate constants for the self-reactions of a number of tertiary and secondary peroxy radicals have been determined by electron spin resonance spectroscopy. The pre-exponential factors for these reactions are in the normal range for bi-molecular radical-radical reactions (109 to 1011 M"1 sec 1). Differences in the rate constants for different peroxy radicals arise primarily from differences in the activation energies of their self reactions. These activation energies can be large for some tertiary peroxy radicals (—10 kcal. per mole). The significance of these results as they relate to the mechanism of the self reactions of tertiary and secondary peroxy radicals is discussed. Rate constants for chain termination in oxidizing hydrocarbons are summarized. 

Electron spin resonance spectra of the resulting phosphoroalkyl radicals were obtained. Absolute rate constants for this process are not known, and similar abstraction by a peroxy radical in nonaqueous media is not necessarily possible. However, in a zinc salt-inhibited hydrocarbon oxidation the additional reactions in Scheme 2 might be envisaged part molecules are shown for convenience. 

The problem of bringing a large magnet into the field for ambient measurements has been overcome in electron paramagnetic resonance (EPR, also called electron spin resonance, ESR) by Mihelcic, Helten, and coworkers (93-99). They combined EPR with a matrix isolation technique to allow the sampling and radical quantification to occur in separate steps. The matrix isolation is also required in this case because EPR is not sensitive enough to measure peroxy radicals directly in the atmosphere. EPR spectroscopy has also been used in laboratory studies of peroxy radical reactions (100, 101). 

Bauer G (2000) Reactive oxygen and nitrogen species efficient, selective and interactive signals during intercellular induction of apoptosis. Anticancer Res 20 4115-4140 Beckwith AU, Davies AG, Davison IGE, Maccoll A, Mruzek MH (1989) The mechanisms of the rearrangements of allylic hydroperoxides 5a-hydroperoxy-3p-hydrocholest-6-ene and 7a-hydro-peroxy-3(1-hydroxycholest-5-ene. J Chem Soc Perkin Trans 2 815-824 Behar D, Czapski G, Rabani J, Dorfman LM, Schwarz HA (1970) The acid dissociation constant and decay kinetics of the perhydroxyl radical. J Phys Chem 74 3209-3213 Benjan EV, Font-Sanchis E, Scaiano JC (2001) Lactone-derived carbon-centered radicals formation and reactivity with oxygen. Org Lett 3 4059-4062 Bennett JE, Summers R (1974) Product studies of the mutual termination reactions of sec- alkylper-oxy radicals Evidence for non-cyclic termination. Can J Chem 52 1377-1379 Bennett JE, Brown DM, Mile B (1970) Studies by electron spin resonance of the reactions of alkyl-peroxy radicals, part 2. Equilibrium between alkylperoxy radicals and tetroxide molecules. Trans Faraday Soc 66 397-405... 

This leads to a hydroperoxide and an alkyl radical, which can again react with oxygen according to Reaction (4.2). The rate of Reaction (4.3) is slow, ks = 10 -10 1 mor s at 30°C, dependent on the type of hydrocarbon, when compared with Reaction (4.2), and is therefore the rate-determining step for chain propagation. Due to their low reactivity, peroxy radicals are present in relatively high concentration in the system when compared with other radicals, determined via electron spin resonance. 

Figure I. Electron spin resonance spectra of peroxy radical formation at room temperature (5.0 Mrads). (a) Primary radical R under vacuum,...

In polymers, the oxidative degradation is caused by the free radical reactions that lead to the formation of the peroxy radicals, viz. (R-22). The role of the peroxy radicals has been elucidated by several groups, e.g., by the luminescence and electron spin resonance methods, ESR [Partridge, 1972 Dole, 1973b Kadir et al., 1989 Kashiwabara and Seguchi, 1992 Williams, 1992 Jipa et al.,... 

Radicals, generated in polypropylene fibers during irradiation as well as the transition frcan these reidicals (R ) to peroxy radicals (ROa ) are monitored by electron spin resonance spectroscopy. Experimental data exhibit an anomaly in the temperature dependence of RO, concentration, around the glassy transition temperature (Tq). The dependence of RO, concentration on temperature, around T, is described by a Hil-liam-Landel-Perry equation rather than by an Arrhenius one. Consequently, the transition consists of two steps the first is associated with diffusion processes and the second with the proper chemical reaction. 

The effect of gamma irradiation on the physical properties of PTFE film are shown in Table 52.10. The faUoff in physical properties is dramatic, even after irradiation in vacuo followed by exposure to air. The radicals produced by irradiation have been shown to have a long lifetime even after heating to 300 °C [103], By looking at the electron spin resonance spectrum, radical I is detected for irradiation in vacuo and peroxy radical II is detected after exposure to air [ 105]. 

The oxidative degradation of isotactic PP in the presence of air (as well as the postradiation oxidation) takes place frequently in industrial irradiation processes. The estimation of peroxy radicals, formed as a direct product of radiolytic oxidation reactions between oxygen and carbon-centred free radicals, is very useful in understanding the oxidation process. The application of electron spin resonance (ESR) is a practical method to follow the quantitative effect of this process [2],... 

Kuzuya M, Kondo S, Sugito M, Yamashiro T. Peroxy radical formation from plasma-induced surface free radicals of polyethylene as studied by electron spin resonance. Macromolecules 1998 31 3230-34. 

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