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Encyclopedia of Physical Science and Technology EN005E-212 June 15, 2001 20:32
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.