Electron spin resonance investigation of biradical metal chelates in the slow tumbling region

The ESR spectra of a series of N,N′-disubstituted 1,4-diazabutadiene and of the tetrachloro-o-benzoquinone metal chelates in methyltetrahydrofuran have been recorded at different temperatures. The spectra which displayed the typical pattern of slow tumbling triplet species were reproduced by the discrete jump model of Norris and Weissmann. The model was extended to include the case of molecule having a non-vanishing E parameter. The logarithm of the product of the absolute temperature and of the rotational diffusion rates ln(K_rT) shows a linear dependence on 1/T like that observed for the viscosity of the solvent. However, the slope was found to vary with the molecular species and to be quite different from the corresponding value of the solvent. These results are discussed in terms of recent modifications of the Debye—Stokes—Einstein relation.