Trapping rate constant

Two possible approaches are indicated in Schemes 4 and 5. In the first, a reactive radical R> is spin-trapped in competition with its pseudo-first order reaction with a substrate SH, which occurs at a known rate to give RH and S. The growth of both spin-adducts (ST—R ) and (ST—S ) is monitored, and simple analysis leads to the trapping rate constant kT. In the second approach, R-does not react with a substrate, but undergoes unimolecular rearrangement or fragmentation at a known rate to give a new species R. This latter procedure... 

TABLE 13.7. Absorption Maxima (nm), Lifetimes, and Trapping Rate Constants (M s ) of Nitreninm Ions Characterized by Laser Flash Photolysis of A-Aminopyridininm Salts"... 

The aminopyridinium route has been employed in flash photolysis studies of aryl as well as diarylnitrenium ions. Several examples of nitrenium ion species, along with their absorption maxima and some trapping rate constants are given in Table 13.7. To the extent the data are comparable, there is good agreement with the behavior of nitrenium ions generated by the azide route. For example, the 4-biphenylyl systems from the azide protonation and /-aminopyridinium routes both give absorption maxima at 460 nm and live for several microseconds in water. Likewise, the 4-methoxyphenyl systems show maxima at 300 nm (from azide) and 320 nm (from aminopyridinium ion). The discrepancy in this case can be attributed to the A -methyl substituent, present in the aminopyridinium route, but absent in the azide experiment. 

Because of the time-dependence, the PALS spectrum is no longer constituted of a sum of decaying exponentials its mathematical expression depends on whether the lifetime of the product of the reaction is longer (e.g., Ps trapping in micelles) [98] or shorter (e.g., Ps oxidation) [103] than that of o-Ps. In the former case for instance, neglecting the time-dependence effect on the p-Ps trapping rate constant, the continuous PALS spectrum, S(t), comes [98] ... 

The trapping rate constant obeys the Arrhenius equation, as ... 

Winter, A. H., Gibson, H. H., Falvey, D. E., Carbazolyl Nitrenium Ion Electron Configuration and Antiaromaticity Assessed by Laser Flash Photolysis, Trapping Rate Constants, Product Analysis, and Computational Studies, J. Org. Chem. 2007, 72, 8186 8195. 

Cyanox 1790 and Lowinox WSP exhibited the next best stabilization efficiencies (of Irganox 1076). This can be related with the combination of substitution in both ortho positions of a methyl-phenyl group and a tert-butyl group, which increases the radical trapping rate constant [40]. These two products are similar from the point of view of their substitution in the para position of the phenolic nucleus [41] and they will give the same type of stable quinone methide. Lowinox TBP6 exhibited a lower... 

Another surprise was that interstrand cross-Unks were also formed independently of O2. However, this was initially rationalized by demonstrating that O2 reacted reversibly with 102 (Scheme 43). Nonlinear regression analysis of the ratio of thymidine to oxygenated products (eg 107) as a function of GSH concentration provided an estimated rate constant for GSH trapping of 102 (fecsH = 6.9 X 10 M s ) consistent with expectations for reaction of an alkyl radical with the thiol.The accuracy of the thiol trapping rate constant validated the extracted rate constant for O2 loss from 106 to reform 102 (feo2 = d.4 s ), which is consistent with rate constants reported for O2 loss from other peroxyl radicals. 

If at appropriate concentrations PBN can trap radicals quantitatively the rate constant of trapping must be quite large. By means of competitive kinetics estimates of the spin trapping rate constants have been obtained ... 

We thus have the following summary of PBN spin trapping rate constants in benzene at room temperature. 

Spin Trapping Rate Constants of Other Nitrones... 

By competitive experiments spin trapping rate constants of other nitrones were obtained for t-butoxy and benzoyloxy radicals (5, J6). The values found are given in Table 1. Two features of the rate data are noteworthy (1) Phenyl or methyl substitution on the carbon atom of the nitronyl function slows the reaction ... 

For benzoyloxy radical electron-withdrawing substituents on the phenyl group decrease the trapping rate constant whereas electron-donating substituents increase the rate constant although the effect is small. For t-butoxy radical the substituent effect is not clear. 

Probably a major factor is resonance although some steric hindrance to approach may also exist depending on the strucutre of these nitrones. X-ray diffraction studies of PEN crystals are planned. It would of course be of interest to compare the spin trapping rate constant of N-t-butyl nitrone with N-phenyl nitrone but this compound has never been synthesized ... 

Spin Trapping Rate Constants with Nitroso Compounds... 

A quantitative explanation of this behavior has been somewhat difficult to obtain, however. There have been several proposals to identify the experimentally resolved exponentials with physical entities, such as multiple monomer states [17-20] or multiple excimer states [15,21]. It is quite probable that the kinetics of the aryl vinyl pol)nners are indeed more complex than the simple Birks scheme would allow. However, recent theoretical studies on electronic excitation transport in random systems of donors and traps have shown that the fluorescence decays are in general, nonexponential [4,5, 22-26]. A key feature of these analyses is that the trapping dynamics in one dimensional and quasi one dimensional polymeric systems require that a trapping rate function k(t) rather than a trapping rate constant be used. In Section 2.1 we give the relationship of k(t) to the observables in a trapping experiment and provide the connection with G (t), which is obtained from theory. 

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