Practice of Kinetics (Comprehensive Chemical Kinetics, Volume 1)

310 DETECTION AND ESTIMATION OF INTERMEDIATESHence the highest sensitivity is obtained with large AE (equivalent to large H) andlow temperature. At the energy corresponding to X-band resonance, and at roomtemperature, the ratio is about 0.998. It may be shown that the ultimate sensitivityof a spectrometer working in the X-band at room temperature is approximately10l2 AH.z-* spins where AH is the width of the absorption line, and z is the timetaken to sweep through the line. The actual sensitivity is at least ten times less thanthis theoretical value; AH may be taken to be about 10 gauss and z about 10 seconds,so that the minimum concentration of free radicals which may be detectedis about lo-' molar. Thus ESR spectrometers are normally not sufficiently sensitivefor the estimation of intermediates present at their steady state concentrations inphotochemical and thermal systems, although in specially favourable circumstancesradicals may be detected. For example, Harle and tho ma^'^' followed the concentrationof radicals formed during the oxidation of octadecene by freezing sampleswithdrawn from the reaction mixture at different time intervals. The inhibitingaction on the reactions of phenyl a-naphthylamine could also be studied.One of the most successful techniques which has been used to produce concentrationsof radicals measurable by ESR methods is that of 'trapping' (cf. Section 8).In condensed phases, the rate of radical recombination may be limited by diffusion,and at sufficiently high viscosities, the rate of production of radicals may exceedthe rate of destruction, and the concentration of radicals will then build up. X-rayand y-ray irradiation of glasses may lead to production of fragments with enoughkinetic energy to escape from the 'cage' and thus avoid primary recombination.Schneider ef al. 160 performed the first investigation by ESR spectrometry using X-ray irradiation. They irradiated polymethylmethacrylate, and obtained a resonancespectrum similar to that observed in polymerization studies of methylmethacrylate.Defects in crystals, produced by radiation damage, may also be studied by theresonance techniques161; work has been done on F-centres162, V-~entres'~~''~~,U-~entres'~~ and interstitial atoms165, and such defect centres may be of importancein determining the course of chemical reaction in irradiated solids.Fragmentation of molecules by ultraviolet radiation is perhaps of greater interestthan fragmentation by X-rays or y-rays, since the energy of the radiation maybe controlled more closely, and the number of alternative reaction paths is muchreduced. As a direct result of the smaller energy available, the kinetic energy ofthe photolytic fragments is less, and primary recombination may limit the concentrationof free radicals if the medium is too viscous. Low temperature hydrocarbonglasses have proved useful for trapping studies of photolytically formed radicals,since the viscosity of the glass may be changed by alteration of the constitution ofthe hydrocarbon mixture or by variation of the temperature. An ether-isopentaneethanol(EPA) mixture is well suite& to these experiments. The earliest ESR spectraof radicals produced photochemically were made by Ingram et al. 166. Ultravioletphotolysis at h = 2537 A of ethyl iodide, benzylamine and benzyl chloride in ahydrocarbon glass produced radicals (although irradiation at h = 3650 A did not).