NUCLEAR-SPIN RELAXATION IN PARAMAGNETIC-COMPLEXES IN THE SLOW-MOTION REGIME FOR THE ELECTRON-SPIN - THE ANISOTROPIC PSEUDOROTATION MODEL FOR S=1 AND THE INTERPRETATION OF NUCLEAR MAGNETIC-RELAXATION DISPERSIONRESULTS FOR A LOW-SYMMETRY NI(II) COMPLEX

This work presents the anisotropic pseudorotation (APR) model, which i s a new dynamic model for the interpretation of experimental nuclear s pin-lattice relaxation times in paramagnetic (S=1) complexes of low sy mmetry. It comprises two dynamic processes active in modulating the ze ro-field splitting interaction (ZFS). Reorientation of the complex mod ulates a static zero-field splitting, defined as a measure of the asym metry in the equilibrium geometry at the paramagnetic site. Local moti ons of the ligands surrounding the paramagnetic site further contribut e a rapidly fluctuating (transient) zero-field splitting interaction. This dynamic model is evaluated within a general theoretical framework capable of dealing with the electron-spin system in the low- and high -magnetic field limits for both Redfield and slow-motion cases, i.e., where the motions: inducing electron-spin relaxation and the electron- spin relaxation itself are characterized by the same time scale. The d ynamic model is characterized and discussed by calculating results for a large number of parameter sets. The obtained results are compared w ith the traditional theory, the Solomon-Bloembergen-Morgan equations ( SBM), by least-squares fitting the SBM equations to the APR model. Res ults show that in most cases the SBM model can fit the nuclear magneti c relaxation dispersion (NMRD) profiles from the APR model at the expe nse of using a different parameter set. For both models, a restricted fit to-experimental NMRD data, from amethyl-3,5-heptanedionato)Ni(II)( aniline-d(5))(2) (abbreviated Ni(dpm)(2)(aniline-d(5))(2) or simply NI DPM) in solution, has been performed. The parameters obtained suggest that NIDPM is a slow-motion case comprising a static contribution to i ts zero-field splitting, so that the SBM model is inapplicable.