Specific trapping rate

The specific trapping rate vnv for small (A < 10) spherical clusters of N vacancies may be approximated as... 

The specific trapping rates vd for dislocations were obtained by correlating positron lifetime measurements and data from transmission electron microscopy (TEM) or other techniques capable of determining dislocation density (e.g., X-ray diffraction profiles) [118]. In metals, vd lies in the range 10 to 10 " m s ... 

Table 4.17 Lifetimes to and specific trapping rates Vd for positrons trapped at dislocations...

The experimental lifetimes of positrons trapped at dislocations and the specific positron trapping rates for dislocations at room temperature are collected in Table 4.17. The different lifetimes for screw and edge dislocations in single-crystal Fe were reported in [118]. According to this work, edge dislocations exhibit larger specific trapping rates vd and lifetimes than screw ones. Similar to vacancies in the... 

Figures 4.53 and 4.54 present results for k (the trapping rate) obtained using the procedure described above. The plausible value of 10 s [129] was assumed for the specific trapping rate Kspec- Because of slight surface oxidation, this evaluation of K results in a systematic underestimate. A lower limit of 10 ns for k may be derived within the framework of the STM, since we have observed saturation trapping at defects.

Laser flash photolysis methods have also been applied to the study of nitrenium ion trapping rates and hfetimes. This method relies on short laser pulses to create a high transient concentration of the nitrenium ion, and fast detection technology to characterize its spectrum and lifetime The most frequently used detection method is fast UV-vis spectroscopy. This method has the advantage of high sensitivity, but provides very little specific information about the structure of the species being detected. More recently, time-resolved infrared (TRIR) and Raman spectroscopies have been used in conjunction with flash photolysis methods. These provide very detailed structural information, but suffer from lower detection sensitivity. 

The assumption that one radical is an appropriate model for another is most sound when one is using a clock to calibrate a bimolecular reaction and the local environment of the clock is similar to that of the radical of interest. For example, the rate constant found for reaction of the 5-hexenyl radical with a specific trapping agent should be a good approximation of the rate constant for reaction of another primary alkyl radical, especially one without substituents at C2. For most synthetic applications, the small errors in rate constants from this assumption will be unimportant. 

If the two steps are of about equal rates, only a small eoncentration of the intermediate will exist at any time. It is sometimes possible to intemipt such a reaction by lowering the temperature rapidly or adding a reagent that stops the reaction and isolate the intermediate. Intermediates can also be trapped. A eompound which is expected to react specifically with the intermediate is added to the reaction system. If trapping occurs, the intermediate is diverted from its normal eourse, and evidence for the existence of the intermediate is obtained if ftie structure of the trapped product is consistent with expectation. 

The rate of oxidation/reduction of radicals is strongly dependent on radical structure. Transition metal reductants (e.g. TiMt) show selectivity for electrophilic radicals (e.g. those derived by tail addition to acrylic monomers or alkyl vinyl ketones - Scheme 3.89) >7y while oxidants (CuM, Fe,M) show selectivity for nucleophilic radicals (e.g. those derived from addition to S - Scheme 3,90).18 A consequence of this specificity is that the various products from the reaction of an initiating radical with monomers will not all be trapped with equal efficiency and complex mixtures can arise. 

Transition metal salts trap carbon-centered radicals by electron transfer or by ligand transfer. These reagents often show high specificity for reaction with specific radicals and the rates of trapping may be correlated with the nucleophilicity of the radical (Table 5.6). For example, PS radicals are much more reactive towards ferric chloride than acrylic propagating species."07... 

The results enumerated above indicate that the quinone methide species must be protonated, by either a specific or general acid, to afford a cation before it can trap a nucleophile. The pK.A determined from pH-rate profile (pKA = 6.66) is consistent with (9-protonated quinone methide pKA values of 6-7 discussed in Section 7.3.5. 

An alternative system proved to be both simpler and more user friendly (Unger et al., 2004 Machtejevas et al., 2006). Thus far we have used this configuration to analyze human plasma, sputum, urine, cerebrospinal fluid, and rat plasma. For each particular analysis we set up an analytical system based on a simple but specific strategy (Figure 9.5). The analysis concept is based on an online sample preparation and a two-dimensional LC system preseparating the majority of the matrix components from the analytes that are retained on a RAM-SCX column followed by a solvent switch and transfer of the trapped peptides. The SCX elution used five salt steps created by mixing 20 mM phosphate buffer (pH 2.5) (eluent Al) and 20 mM phosphate buffer with 1.5 M sodium chloride (eluent Bl) in the following proportions 85/15 70/30 65/45 45/55 0/100 with at the constant 0.1 mL/min flow rate. Desorption of the... 

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