The motional model in the electron spin relaxation of large Mn(II) complexes: Rotational or fluctuational process?

The solution ESR spectra of Mn(II) bound to ATP have been analyzed at three microwave frequencies. The liquid-type relaxation process induced by rapid fluctuations of the crystal-field symmetry does not account for the frequency dependence of the linewidth. The ESR lineshapes are better explained in terms of a static distribution of crystalfield sites which induces field dependent inhomogeneous broadening.