Multiple Electron Resonance Spectroscopy: An Introduction
Since its discovery,(1) electron paramagnetic resonance (epr) has flourished to the point where several hundred papers per year are now published. This has resulted in the concomitant increase in review articles, which now appear regularly.(2–4) “It is doubtful that ... any chemist would have been so bold as to predict the great diversity of systems which have proved amenable to study by EPR spectroscopy.”(5) Epr has, of course, proven itself useful because of its high sensitivity in the identification and study of a variety of paramagnetic species. Systems studied range from single crystals containing unpaired electrons to complex biological and polymeric matrices. However, the limitations of the spectral interpretations have become increasingly apparent as the user population has become more sophisticated in the types of problems to which epr was applied. Epr linewidth changes, for example, may occur over the range of rotational correlation times from approximately 1 × 10−9 to 1 × 10−7 sec. Hence, epr is less applicable at regions of much slower or faster molecular motions. Further, a scientist is frequently confronted with significantly overlapped epr spectra due to the presence of multiple radical species. Again, interpretations of the spectral displays are difficult. The study of molecular energy transfer and relaxation processes was also quite difficult from epr spectra alone.
KeywordsElectron Paramagnetic Resonance Pump Power Electron Paramagnetic Resonance Spectrum Double Resonance Electron Paramagnetic Resonance Spectroscopy
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