EPR, ELDOR, and ENDOR studies of alkali metal- o-dimesitoylbenzene radical complexes in solution. I. Rubidium and cesium complexes

Abstract

A combined EPR, ENDOR, and ELDOR study on ion pairs of o-dimesitoylbenzene anions with Rb or Cs cations in solution is reported. With respect to the Rb compound the signal reductions by ELDOR, together with additional more detailed ENDOR results, are incompatible with the explanation of an earlier ENDOR study in terms of a dominant W x1 relaxation mechanism originating in modulation of the isotropic hyperfine coupling of the metal nucleus. The combined action of quadrupolar and dipolar relaxation provides a new view of the explanation of ENDOR line intensities and restores the consistency of all ENDOR and ELDOR results. Experimental ENDOR features, such as the temperature dependence and rf power dependence of line intensity ratios, are qualitatively reproduced. The ELDOR results in the Cs complex point to dominating dipolar relaxation. The ENDOR results are in agreement with this conclusion and confirm that the contribution of a W x1 process is vanishingly small. The EPR and ELDOR results together yield a variety of structural information: spectral densities, rotational correlation time of the complex, anisotropic magnetic interactions of the alkali atom and of the two equivalent protons, and the assignment of the hyperfine coupling of 7.5 MHz to positions 2 and 3 of the benzene ring. Internal motions appear to be of no importance in the dynamical picture of these ion pairs.