PROTON-ENHANCED RELAXATION IN LOW-SYMMETRY PARAMAGNETIC-COMPLEXES (S = 1) - BEYOND THE SOLOMON-BLOEMBERGEN AND MORGAN THEORY .1. THE SMOLUCHOWSKY DISTORTION MODEL OF THE ZFS INTERACTION

A model describing paramagnetic enhanced proton relaxation (PER) in low-symmetry complexes is developed. The theoretical framework valid in the slow-motion region for the electron spin system is reviewed and related to the Solomon-Bloembergen-Morgan theory. The dynamic model of a low-symmetry paramagnetic complex comprises a static and a transient ZFS interaction. Calculations of NMRD dispersion curves are presented for complexes of paramagnetic ions with electron spin quantum number S = 1. The dipole-dipole cross-correlation functions are shown to play an important role in the low-field region where the ZFS interaction is larger than the Zeeman interaction. The calculations of dispersion curves cover the range from the Redfield region to the slow-motion region and are compared with analogous results obtained using the Bloembergen-Morgan approach. The results indicate that an appropriate model for many low-symmetry complexes must include a model similar to the restricted pseudo-rotation of the principal frame of the ZFS interaction in order to average the electron spin - nuclear spin dipole-dipole coupling, implying a low and constant PER effect in the low-field region. This fact may explain the partial success of fitting the Bloembergen-Morgan expressions to the experimental dispersion curves for low-symmetry complexes.