General TRIPLE resonance on free radicals in solution. Determination of relative signs of isotropic hyperfine coupling constants

Abstract

For the first time a general type of electron–nuclear–nuclear TRIPLE resonance has been performed on radicals in liquid solution. In this method two inequivalent nuclei are irradiated simultaneously at their respective NMR frequencies, the resonance being detected by an enhancement of a saturated ESR line. This general TRIPLE experiment, the analog of which is known already in the solid state (DOUBLE ENDOR), gives direct information about relative signs of hyperfine coupling constants. Several important aspects of the experimental setup are described. The experimental results are discussed on the basis of a simple relaxation model.