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Radical trapping sites

Mechanically relevant information has been obtained from an analysis of radical trapping sites. Thus it has been concluded in previous sections of this chapter that [Pg.170]

The authors analyzed the effect of quenching, annealing, and cold-drawing on the mosaic-block structure of their films. [Pg.171]

From a change in line shape parameter of the PE sextet spectrum and of the temperature dependence of radical recombination rates the authors determined the increase in unoriented material content during mechanical fatigue [51]. For the study of the surfaces of as-grown and annealed PE single crystals Kusumoto et al. [Pg.171]

The ESR technique has permitted to identify initial rupture points in mechanically degraded copolymers. Lazar and Szocs [53] investigated random, block, and graft copolymers of methylmethacrylate (MMA) and styrene (S). They found in a degraded mixture of equal parts MMA and S homopolymers equal concentrations of MMA and S radicals. In a random copolymer (MMA S = 1 1) the styrene radical prevailed clearly. By comparing the reactions of mechano-radicals in ball-milled PP with those of 7-radicals Kurokawa et al. [54] concluded that the mechano-radicals were produced and trapped on the fresh surfaces created by the breaking-up of solid polypropylene. [Pg.171]

Hassell M. S. Thesis, Department Mechanical Engng. Univers. of Utah (June 1973). [Pg.171]


The mere exposure of diphenyl-polyenes (DPP) to medium pore acidic ZSM-5 was found to induce spontaneous ionization with radical cation formation and subsequent charge transfer to stabilize electron-hole pair. Diffuse reflectance UV-visible absorption and EPR spectroscopies provide evidence of the sorption process and point out charge separation with ultra stable electron hole pair formation. The tight fit between DPP and zeolite pore size combined with efficient polarizing effect of proton and aluminium electron trapping sites appear to be the most important factors responsible for the stabilization of charge separated state that hinder efficiently the charge recombination. [Pg.377]

The broad emission and low-fluorescence quantum yield of PPS suggested a distribution of trapping sites in the Si skeleton, which were also considered responsible for the lower-than-expected conductivity. The far-IR spectrum of PPS suggested the existence of cyclohexasilane rings connected by linear chains.361,362 Subsequent investigations by Irie et al. on the electronic absorption spectra of radical ions of poly(alkylsilyne)s were taken to indicate the presence of various cyclic silicon species, in corroboration of this conclusion.363 The large Stokes shift and broadness of the fluorescence emission indicate a range of fluorophore structures, different from the chromophore structures. This is... [Pg.631]

The sequence of events following the reaction of metMb with hydrogen peroxide has also been investigated through the use of spin trapping agents. Initial studies of this type with 5,5-dimethylpyrroline N-oxide (DMPO) led to the identification of Y103 as the primary site of DMPO adduct formation, a modification that was blocked by specific iodination of this residue 187). The identity of the radical trapped in this reaction... [Pg.24]

Recently, alkylation of alkyl aromatic hydrocarbons such as toluene, ethylbenzene, cumene, and xylenes with ethene, propene, and 1,2-diphenylethene was investigated by Kijenski et al. (245), who used superbasic K-MgO and K-AI2O3 catalysts at low temperature at atmospheric and elevated pressures. The reaction kinetics, EPR measurements of adsorbed intermediates, and the effects of poisoning determined by the radical trap TEMPO (2,2,6,6-tetramethyl-l-piperidinyloxyl, free radical) led the authors to conclude that sites are the catalytically active centers. To demonstrate the importance of strong one-electron donor sites (F ) for the alkylation and the inactivity of strong two-electron donor centers, the ethylation of cumene, ethylbenzene, and toluene was carried out with MgO-10%NaOH. On this catalyst, strong basic two-electron donor sites (27 33) were found, along... [Pg.284]

The low-temperature EPR experiments used to determine the DNA ion radical distribution make it very clear that electron and hole transfer occurs after the initial random ionization. What then determines the final trapping sites of the initial ionization events To determine the final trapping sites, one must determine the protonation states of the radicals. This cannot be done in an ordinary EPR experiment since the small hyperfine couplings of the radicals only contribute to the EPR linewidth. However, detailed low-temperature EPR/ENDOR (electron nuclear double resonance) experiments can be used to determine the protonation states of the low-temperature products [17]. These proto-nation/deprotonation reactions are readily observed in irradiated single crystals of the DNA base constituents. The results of these experiments are that the positively charged radical cations tend to deprotonate and the negatively charged radical anions tend to protonate. [Pg.436]

The proposal that holes are detrapped at lower temperatures than the excess electrons is based on the observations discussed above in Very Shallow Traps and Shallow Traps (Sec. 4.3.2). One expects that the activation energy needed to detrap the hole from Gua in duplex DNA is relatively small, an order of magnitude less than that needed to detrap the electron. This fits well with the observation that upon warming 4 K irradiated crystalline DNA to 77 K, 10-30% of the radicals anneal out, i.e., at least one of the trapping sites [fide infra Gua(N3-H) ] is very shallow. [Pg.463]

The use of iodoacetic acid as an aryl radical trapping agent has confirmed the intermediacy of aryl radicals in some hydrodediazoniation reactions, whether these are initiated or not.4 Spontaneous hydrodediazoniation of aryldiazonium fluoroborates occurs in warm dimethylformamide (DMF). Detailed study5 of the conversion of the 4-nitro derivative into nitrobenzene indicates a homolytic mechanism in which H-atom abstraction occurs from both sites in DMF with a formyl methyl preference of 3.5 1.0. High yields of mixed perfluorinated biaryls may be obtained by the catalytic dediazoniation of pentafluorobenzenediazonium ions in acetonitrile containing aromatic substrates and small amounts of iodide salts. The catalytic role of iodide and the isomeric product distributions indicate that arylation proceeds through the pentafluorophenyl radical in an efficient homolytic chain process.6... [Pg.242]

Some of the different chemistry of DNA as compared to its low-molecular-weight model compounds is due to the fact that DNA is a polymer, and some aspects of polymer free-radical chemistry are dealt with in a separate chapter. The special properties of dsDNA allow hole and electron transfer to trapping sites. This is an area that attracts very strong attention at present, and the level of understanding is already very high. [Pg.6]


See other pages where Radical trapping sites is mentioned: [Pg.140]    [Pg.141]    [Pg.170]    [Pg.171]    [Pg.140]    [Pg.141]    [Pg.170]    [Pg.171]    [Pg.1114]    [Pg.421]    [Pg.189]    [Pg.192]    [Pg.406]    [Pg.712]    [Pg.227]    [Pg.614]    [Pg.454]    [Pg.35]    [Pg.201]    [Pg.758]    [Pg.44]    [Pg.103]    [Pg.125]    [Pg.188]    [Pg.614]    [Pg.38]    [Pg.274]    [Pg.275]    [Pg.309]    [Pg.112]    [Pg.228]    [Pg.1114]    [Pg.225]    [Pg.444]    [Pg.330]    [Pg.7]    [Pg.184]    [Pg.187]    [Pg.787]    [Pg.15]    [Pg.16]    [Pg.24]    [Pg.200]    [Pg.978]   
See also in sourсe #XX -- [ Pg.121 , Pg.170 ]




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