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               344                                                                               Electron Spin Resonance


               Steady-state electron spin resonance can be used to study  so appropriate time-domain or trapping techniques must
               kinetics over periods of milliseconds to hours. Thermo-  be used to detect them before they decay into nonparam-
               dynamic information can also be obtained by measuring  agnetic species. In such studies, electron spin resonance
               reaction intermediates as a function of temperature.  plays a major role.
                 Another major area of application involves magnetic
               energy transfer. By measuring magnetic relaxation times
               of paramagnetic species, one can determine the degree of  XI. NEW DEVELOPMENTS
               interaction of paramagnetic species or free radicals with
               the “lattice” in which they are embedded. This lattice may  In the past few years electron spin resonance has seen
               be a liquid or a solid. The mechanism of the magnetic  impressive developments in the area of time-domain tech-
               energy transfer is an important question and has been ad-  niques. Improved solid-state microwave components, fast
               dressed in many studies. Applications of this type relate  digitizers, and computer systems have made possible new
               to the diffusion of paramagnetic species in liquids and in  types of pulsed electron spin resonance experiments. The
               solids and to the coupling of the spin system to the elec-  sensitivity of these new experiments also makes them
               tronic system of the lattice.                     practical for many new chemical, physical, and biological
                 One simple application of magnetic energy transfer that  problems. A second-generation general-purpose pulsed
               is often neglected is to use microwave power saturation  electron spin resonance spectrometer has been commer-
               to distinguish overlapping radicals. Radicals of different  cially introduced which makes these pulsed techniques,
               chemical types often have quite different spin–lattice re-  including pulsed ENDOR, widely available. Fourier trans-
               laxation times. For example, alkyl radicals are typically  form and two-dimensional electron spin-echo techniques
               much more easily saturated than peroxy radicals. Thus, if  are also included. A newer commercial development is
               both alkyl and peroxy radical spectra are superimposed,  pulsed ESR and pulsed ENDOR at the higher frequency
               one can detect the presence of at least two different types  of 95 GHz. This has particular advantages of better sen-
               of radicals by carrying out selective power-saturation mea-  sitivity for small samples and enhancement of ENDOR
               surements. Similar distinctions can be made between rad-  sensitivity for low gyromagnetic ratio nuclei.
               ical cations and radical anions, where one may have a  In continuous-wave electron spin resonance, extended
               much shorter spin–lattice relaxation time than the other.  multifrequency capabilities from 0.3 to over 100 GHz
               An example involves the photoionization of chlorophyll in  have been developed based on loop-gap and other types of
               vesicle systems with an electron scavenger such as a halo-  resonators. The lower frequencies seem particularly use-
               genated quinone present. Typically one detects the chloro-  ful for some biological applications. Very high frequency
               phyll cation radical signal superimposed on the electron  spectrometers have also been developed up to 700 GHz,
               acceptoranionradical signal, butthey can be distinguished  withcommercialinstrumentationavailableat95GHz.The
               by their different responses to microwave power satura-  higher frequencies are based on Fabry-Perot resonators
               tion. The radical anion involving a halogen atom will typ-  and give superior g-anisotropy resolution, suppression
               ically saturate with much more difficulty than will the  of second-order effects, and better sensitivity for small
               chlorophyll cation radical, which is more characteristic of  samples.
               an organic free radical.
                 Magnetic relaxation measurements can also be used to
               determine the spatial distribution of paramagnetic species  SEE ALSO THE FOLLOWING ARTICLES
               in a solid matrix. This is important if the spatial distribu-
               tion is nonuniform. Often when radicals or paramagnetic
                                                                 ATOMIC PHYSICS • ATOMIC SPECTROMETRY • CHEMI-
               species are produced by photolysis or by radiolysis they
                                                                 CAL KINETICS,EXPERIMENTATION • NUCLEAR MAG-
               are trapped in a nonuniform manner, and this can be de-
                                                                 NETIC RESONANCE • PERTURBATION THEORY • QUAN-
               tected by a careful analysis of the magnetic relaxation
                                                                 TUM MECHANICS
               characteristics of the radical.
                 Potentialanddemonstratedapplicationsofelectronspin
               resonance are ubiquitous. The technique is particularly
                                                                 BIBLIOGRAPHY
               useful because it is sensitive only to those species that are
               paramagnetic. If these are important reaction intermedi-
               ates, one has a selective analytical technique to look only  Atherton, N. M. (1993). “Principles of Electron Spin Resonance,” Ellis
                                                                   Horwood, London.
               at those specific types of reaction intermediates. Paramag-
                                                                 Berliner, L. J., and Rueben, J., eds. (1989). “Biological Magnetic Reso-
               netic species are probably much more widespread than is  nance. Spin Labeling: Theory and Applications,” Vol. 8, Plenum, New
               generally believed. Radicals are typically reactive species,  York.
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