Practice of Kinetics (Comprehensive Chemical Kinetics, Volume 1)

4 ESR SPECTROMETRY 309ates. Muller et a1.'s5*'56 first used the method in a study of the dissociation ofhexaphenylethane in benzene solution at different temperatures; the techniquemay be extended to the investigation of triplet states produced in fluorescence processes's'. Unfortunately, direct determination of paramagnetic susceptibility hassome important drawbacks. Correction must be made for the inherent diamagnetismof the species studied, and traces of ferromagnetic impurity will increasegreatly the apparent paramagnetic susceptibility. The measurements do not inthemselves yield any information about the nature of the paramagnetic species.Electron spin resonance (ESR) techniques overcome each of these difficulties: thediamagnetic component of the susceptibility appears only as a perturbation, ferromagneticresonance is readily resolved from paramagnetic resonance, and thehyperfine structure and position of the resonance spectrum can lead to the identificationof the paramagnetic species. Absolute concentrations may be measured bycalibration of the spectrometer with a stable free radical such as diphenyl-picrylhydrazyl(DPPH) or molecular oxygen.In ESR spectrometry, the sample under investigation is placed in a magneticfield. The energy leveb for a free electron become split, corresponding to the twoorientations of the electron spin with respect to the field. The situation is morecomplex in a real free radical, since coupling may exist between electrons andneighbouring nuclei of non-zero spin: such coupling leads to the observed hyperfinestructure, but to a first approximation, the energy levels are similar to thosefor a free electron. Transitions between the energy levels (subject to the selectionrule AM, = k 1, AM, = 0) may occur, and can be observed by the absorptionof radiation. The energy separation, AE, between states is given bywhere H is the magnetic field, Be is the Bohr magneton for the electron (eh/472mc)and gc is a molecular constant analogous to the Land6 splitting factor for theelectron in an atom (= 2.0023 for an electron uncoupled to any orbital angularmomentum). Since AE = hv, the frequencies at which absorption is to be expectedmay be calculated at any field strength. Most spectrometers have operated withmagnetic fields of around 3000 gauss, which correspond to resonant frequencies of9,000 Mc.sec-' (Le. a wavelength of 3 cm-the so-called X-band), although lowfieldinstruments are sometimes used'58 in cases where the sample would absorbmicrowave radiation too strongly. The frequency at which resonance is made tooccur affects also the sensitivity of the apparatus. If the two levels are in thermalequilibrium, then the relative populations of the two states are given bynupper -- - exp (-AE/kT)nlOWerReferences pp. 336-342