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EPR methods

In this article only the most important and frequently applied EPR methods will be introduced. For more extensive treatments of CW and pulsed EPR the reader is referred to some excellent review articles that will be specified in the respective sections of this article. A good starting point for fiirther reading is provided by a number of outstanding textbooks which have been written on the various aspects of EPR in general [2, 3, 4, 5, 6, 7 and 8]. Interested readers... [Pg.1548]

As a sununary it may be of interest to point out why TREPR spectroscopy and related methods remain important in the EPR regime, even though pulsed EPR methods are becoming more and more widespread. (1) For the case of an inliomogeneously broadened EPR line the time resolution of TREPR compares favourably with pulsed teclmiques. [Pg.1566]

Stehlik D and MObius K 1997 New EPR methods for investigating photoprooesses with paramagnetio intermediates Annu. Rev. Phys. Chem. 48 745-84... [Pg.2968]

It can be seen from Table 1 that there are no individual steps that are exothermic enough to break carbon—carbon bonds except the termination of step 3a of —407.9 kJ/mol (—97.5 kcal/mol). Consequentiy, procedures or conditions that reduce the atomic fluorine concentration or decrease the mobiUty of hydrocarbon radical intermediates, and/or keep them in the soHd state during reaction, are desirable. It is necessary to reduce the reaction rate to the extent that these hydrocarbon radical intermediates have longer lifetimes permitting the advantages of fluorination in individual steps to be achieved experimentally. It has been demonstrated by electron paramagnetic resonance (epr) methods (26) that, with high fluorine dilution, various radicals do indeed have appreciable lifetimes. [Pg.275]

The diffusion of H and D atoms in the molecular crystals of hydrogen isotopes was explored with the EPR method. The atoms were generated by y-irradiation of crystals or by photolysis of a dopant. In the H2 crystals the initial concentration of the hydrogen atoms 4x 10 mol/cm is halved during 10 s at 4.2 K as well as at 1.9 K [Miyazaki et al. 1984 Itskovskii et al. 1986]. The bimolecular recombination (with rate constant /ch = 82cm mol s ) is limited by diffusion, where, because of the low concentration of H atoms, each encounter of the recombinating partners is preceded by 10 -10 hops between adjacent sites. [Pg.112]

EPR methods that allow a more direct determination of kv have been developed. These enable absolute radical concentrations to be determined as a function of conversion. With especially sensitive instrumentation, this can be done by direct measurement/57 160 An alternative method, applicable at high conversions, involves trapping the propagating species in a frozen matrix361 362 by rapid cooling of the sample to liquid nitrogen temperatures. [Pg.217]

The radical concentration, when coupled with information on the rate of polymerization, allows k (and k,) to be calculated. The EPR methods have been applied to various polymerizations including those of B, DMA, MMA,361 566 S 67 368 and VAc.369 Values for kp are not always in complete agreement with those obtained by other methods (e.g. PLP, SIP) and this may reflect a calibration problem. Problems may also arise because of the heterogeneity of the polymerization reaction mixture,365 and insufficient sensitivity for the radical concentrations in low conversion polymerizations 63 or very low molecular weights. Some data must be treated with caution. However, the difficulties are now generally recognized and are being resolved. 60... [Pg.217]

Such techniques imply analysis of chemical products of photolysis. Application of mass-spectrometers of various types is often hampered by a number of circumstances. These difficulties will be discussed later on. The EPR method, which is currently the most extensively employed technique, features low sensitivity and is usually used for analysis of primary fragments of photolysis. For this purpose, the radicals produced are frozen on the walls of a quartz pin and are thus accumulated inside the device. On one hand, this approach allows one to overcome the sensitivity threshold of the device. However, on the other hand, this excludes the possibility of direct kinetic measurements. The SS technique permits the use of weak light sources for detecting active particles under... [Pg.226]

Since studies bearing on the role of molybdenum in milk xanthine oxidase have depended heavily on the EPR method, it is convenient to precede detailed discussion by a general description of the various molyb-denum(V) EPR signals which have been obtained from the enzyme. [Pg.119]

Due to the rapidity of the spontaneous dismutation reactions, the steady-state concentrations of 02 achieved by chemical or by enzymatic reactions are usually quite low. The physical methods for detecting 02, although direct and unequivocal, are restricted to measurements of steady-state concentrations and are thus often found to lack of sensitivity. For distince, due to the reason mentioned above, when the EPR method was employed for studying the 02 production by xanthine oxidase, it was necessary to use a high concentration of the reactants and to work at elevated pH so... [Pg.169]

Catalysis by nitroxyl radicals in hydrocarbon oxidation was discovered and studied recently [82-89], The introduction of /V-hydroxyphthalimide into oxidized alkylaromatic hydrocarbon was found to accelerate the oxidation. The formation of the stable phthalimide-/V-oxyl (PINO) radical was evidenced by the EPR method [90]. The following kinetic scheme was put forward to explain the accelerating effect of PINO on the chain oxidation of hydrocarbons [82-84]. [Pg.236]

The sufficiently higher values of bre (brc 14.82 (kJ mol-1)172 and Ee0 ( e0 = 66.7kJ mol-1) were estimated from the experimental rate constants of the reaction of the 1,1-dimethylethyl peroxyl radical with /V-diethylethaneamine. This reaction was studied by Tavadyan et al. [21] using EPR method at low temperatures (210-260 K). This discrepancy in Ee0 values for reactions of one group is the result of a great difference in the temperature of the experiment (210-260 K [21] and 330-380 K for other experiments). [Pg.362]

By the EPR method in oxidized cumene with metal oxides (Mn02, Co203, CuO, and NiO [259]) there were fixed cumylperoxyl radicals. [Pg.422]

Several experimental evidences of this mechanism were obtained. The rate of chain oxidation of a substrate (RH) was proved to be proportional to [ArOH]-1 (see Chapter 14) [1-6], The phenoxyl radicals were identified by the EPR method as intermediate products of the reaction [6]... [Pg.513]

The rate has been measured in 1 1 acetone/CHCl3 by a stopped-flow/epr method. The epr signals of Cu(mnt)2 and Cu(mnt)(Et2dsc) are sufficiently different to allow analysis. (Fig. 4.10). A seeond-order rate law is usually observed, but this belies a simple mechanism,... [Pg.241]

Due to plasma treatment the radicals (R, free spin) are generated on polymer chain. Not only C-H but also C-C bonds are likely to brake, the later leading to fragmentation of polymer chain. The radical concentration determined by EPR method is in Table 3. The concentration of free radicals R decreases during the aging to about quarter of the initial after 80 days. The decrease of R is a result of radical recombination [h]. Detailed comment of EPR observation ( aging of radicals) is described too [79]. [Pg.32]

Table 3.C (Is) a O (Is) concentrations (at. %) determined from XPS measurements and radical densities (R, 10 g ) determined by EPR method. The concentrations and densities were determined on the pristine PE, plasma-treated PE (1 or 24 hours after the treatment) and plasma-treated and water etched PE (see left hand side of the table). Before the measurement the samples were stored under standard laboratory conditions [78]... Table 3.C (Is) a O (Is) concentrations (at. %) determined from XPS measurements and radical densities (R, 10 g ) determined by EPR method. The concentrations and densities were determined on the pristine PE, plasma-treated PE (1 or 24 hours after the treatment) and plasma-treated and water etched PE (see left hand side of the table). Before the measurement the samples were stored under standard laboratory conditions [78]...
Jezowska-Bojczuk et al. studied the interaction of Cu + ion with kanamycin B (5), which has an additional 2 -amino group as compared to kanamycin A (4), and in particular explored the involvement of this group in metal coordination by use of potentiometric, UV-vis, CD, and EPR methods. The presence of five amino groups in kanamycin B results in a significant lowering of the Ka value of one of its amino nitrogens (pA a = 5.74) with the Ka values for other amino... [Pg.241]

Three slightly different octahedrally coordinated centers have been found in topaz by the EPR method (Schott et al. 2003), thus its participation in topaz luminescence is possible. [Pg.199]

Vanin, A. F. (1967). Identification of divalent iron complexes with cysteine in biological systems by the EPR method. Biochemistry (USSR) 32, 228-232. [Pg.174]

A review has been given on paramagnetic resonance spectroscopy of metal centers. It comprises both paramagnetic NMR as well as high frequency EPR methods applied mostly to copper proteins of the blue type but also to iron and manganese containing proteins.42... [Pg.121]


See other pages where EPR methods is mentioned: [Pg.1547]    [Pg.1548]    [Pg.1564]    [Pg.1574]    [Pg.1584]    [Pg.111]    [Pg.88]    [Pg.238]    [Pg.437]    [Pg.114]    [Pg.132]    [Pg.159]    [Pg.165]    [Pg.197]    [Pg.199]    [Pg.427]    [Pg.191]    [Pg.128]    [Pg.135]    [Pg.120]    [Pg.417]    [Pg.428]    [Pg.214]    [Pg.224]   
See also in sourсe #XX -- [ Pg.88 ]




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