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Abstract

A new ELDOR spectrometer linked with a computer was constructed which permits one to scan the magnetic field, the pumping frequency, or both simultaneously, the latter being equivalent to scanning the observing frequency. The advantages of this last method are discussed. Using this spectrometer we have investigated the intra- and intermolecular ELDOR effect as a function of the rotational correlation time τrot using mixtures of nitroxide radicals which contain the nitrogen isotopes 14N and 15N. The variation of rrot was achieved either by varying the temperature or by selecting solvents of different viscosity at constant temperature.

It can be shown theoretically that the intermolecular part of the ELDOR reduction factor due to exchange interactions is a function of the exchange frequency ωex, while the intramolecular part due to dipolar interactions depends as well on the nuclear relaxation rates Wn and Wn′. Since the former always contributes to the ELDOR reduction and the latter only if the pumped and the observed hyperfine lines belong to the same nitrogen isotope, it is possible to differentiate between the rotational correlation time τrot and the translational correlation time τrans if one takes into account the fact that Wn and Wn′ depend on τrot while τtrans is inversely proportional to the exchange frequency ωex.