Practice of Kinetics (Comprehensive Chemical Kinetics, Volume 1)

8 TRAPPED RADICALS 33 1flame containing an alkali and a few percent of a halogen. Such a flame shouldcontain a large preponderance of heavy ions over electrons and both concentrationsand collision frequencies should be measurable.8. Trapped radicalsThe great chemical reactivity of most free radicals results in relatively lowsteady concentrations of the species, even under conditions favouring their formation.Methods available for the estimation and detection of radicals may not besensitive enough to detect the small concentrations encountered, and techniquesmust be sought which enable radicals to be accumulated without reaction until aconcentration has been built up sufficient for detection by the instruments available.A brief discussion of work on “trapped” radicals therefore concludes thepresent chapter. The application of the technique of trapping has, indeed, beenmentioned already in the section describing electron spin resonance (Section 4),and it is intended now to amplify the remarks made in that section with particularreference to optical spectroscopy of trapped species. The customary caveat will beissued at this stage: except for systems which are self-trapping, theresultsof trappedradicalstudies refer to a system which differs from the normal reaction system,and care must be exercised in the extrapolation of results to the non-trapped case.A reduction in temperature will decrease the rate of any radical reaction whichhas an appreciable activation energy, and it has long been understood that radicalreactions become increasingly simple at lower temperatures. However, radicalscombine with little or no activation energy, and except at temperatures veryclose to the absolute zero, recombination of radicals will be a significant process.By trapping radicals within a solid matrix, their diffusion towards one another maybe impeded and recombination made unimportant. Such effects are observed inpolymeric systems, where radicals may become trapped within a high polymermatrix even at room temperature (a survey is given in Chapter 12 of reference 293).An estimate of the maximum concentration of radicals that may be obtained bytrapping may be arrived at by assuming that each reactive species must be isolatedfrom its neighbours by a matrix cage. Jackson and M ~ntroll~~~ and Goldenzg5have concluded that the radical concentration could not exceed 10 to 14 %of the total, depending upon the packing. In the event, the highest radical concentrationsobseA-ved have been generally less than one-tenth of this theoreticalestimate. One factor which may place a limit on the maximum concentrationattainable is the effect of the heat released on those occasions when two radicalsdo recombine: local diffusion may be initiated to an extent sufficient to set off a“chain” of recombination events. F ~ntana~~~ has shown that atomic nitrogen isstable in a molecular nitrogen matrix up to concentrations of 0.04 mole per cent,although at higher concentrations the sample becomes unstable.References pp. 336-34.2