Nitrone spin trapping rate constants

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 ... 

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 ... 

Rate constants for spin trapping of alkyl radicals measured by the procedures outlined here, are collected with other spin-trapping rate data in Table 5. It will be seen that most nitrone and nitroso traps scavenge reactive radicals of diverse types with rate constants generally in the range 10s-10 1 mol-1 s l. Of the nitroso-compounds, the nitroso-aromatics (except for the very crowded TBN) are particularly reactive, whilst MBN and DMPO are the most reactive nitrones. Much of the data for spin trapping by nitrones has been accumulated by Janzen and his colleagues, who have discussed in a short review how steric and electronic factors influence these reactions (Janzen et ai, 1978). 

TABLE 8.8 Rate Constants for the Reaction of 02" with Several Nitrone Spin Traps... 

Using in situ radiolysis TR EPR the reaction rate constants of the nitrone spin trap 5,5-dimethyl-1-pyrroline N-oxide (DMPO) with a number of small alkyl and sigma parent radicals in dilute aqueous solution have been measured. The rate constants determined are all in the order of 10 -10 s for methyl,... 

Very recently, rate constants for scavenging of hydroxyl radicals by DMPO, and by the nitrone [18c], have been determined (Marriott et al., 1980) (see Table 5). As might be expected, the figures are close to the diffusion-controlled limit. The report of this work includes a concise and informative discussion of some of the difficulties with, and limitations of, the spin trapping method, especially where these relate to reactions involving hydroxyl radicals. 

Lauricella R, Allouch A, Roubaud V, Boutellier J-C (2004) Anew kinetic approach to the evaluation of rate constants for the spin trapping of superoxid/hydroperoxyl radical by nitrones in aqueous media. Org Biomol Chem 2 1304-1309... 

Since the rate constant for hydrogen atom abstraction from cyclohexane by t -butoxy radicals has been estimated W (namely 1x10 sec ll at ambient temperatures) a rate constant of 5.5x10 sec for the trapping of t-butoxy radicals can be determined. (Actually a cyclic nitrone trap, 5,5-dimethylpyrroline-l-oxide (DMPO) was used for this study since a serious peak overlap problem was encountered with the two spin adducts of PBN (j6). 

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