ELECTRON-ELECTRON DOUBLE RESONANCE OF FREE RADICALS IN SOLUTION

The technique of electron-electron double resonance, in which one part of the EPR spectrum of a paramagnetic sample is irradiated with an intense microwave field and the effect of this intense field on other parts of the spectrum is determined utilizing a second weak microwave field, has been applied to free radicals in solution. EPR signals detected by the weak microwave source are reduced in intensity when the two frequencies are separated by an integral number of hyperfine intervals. In some cases this reduction is as much as 40%. A bimodal cavity of novel design capable of supporting the two resonant microwave modes is described. A nitroxide radical has been investigated in greatest detail. Two mechanisms have been identified: rapid nuclear relaxation induced by electron-nuclear dipolar (END) interaction, which is dominant at low concentrations and temperatures, and Heisenberg exchange (HE), dominant at high concentrations and temperatures. A theoretical analysis of the relaxation processes is presented which permits extracting the critical relaxation parameters from the experimental data. The main theme of the paper is a description of the basic effects, but there would appear to be a number of analytical or structural applications of this technique.