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Electron nuclear double resonance has been performed on six free radicals in solution. Polar and nonpolar solvents were employed, and both neutral and ionic radicals investigated. The instrumentation is described in detail. Features of the instrument include: pulsed nuclear radio-frequency magnetic fields, rf fields as high as 40 G in the rotating frame, and provision to observe changes in the EPR spectrum caused by nuclear resonance with the ordinary EPR spectrum subtracted (ENDOR-induced EPR). ENDOR mechanisms and adjustment of experimental conditions to achieve optimum signal intensity are discussed. It is established that successful ENDOR experiments can be performed on a large number of free radicals in solution, resulting in improved resolution, simplification of spectra and more accurate hyperfine splittings.
James S. Hyde (1965) studied this question.