Some aspects of the slow motion problem in the NMR-relaxation theory : a theoretical study of high spin systems

Abstract: A theory of nuclear spin relaxation in paramagnetic complexes, allowing for the electron spin relaxation to be in the slow motion regime, is presented. The lattice i3 described in terms of the electron Zeeman interaction, a zero field splitting (ZFS) and a Markov operator for the relevant classical degrees of freedom. Several dynamic models for the motions in the lattice are discussed.A reorientation model which includes only isotropic rotational diffusion of the complex is presented. Based on this model numerical calculations of nuclear spin-spin and spin-lattice relaxation rates are reported for electron spin systems (S=3/2 and S=5/2) coupled to the nuclear spin system via dipole-dipole and scalar spin-spin interaction.The nuclear magnetic relaxation of protons in Ni(II) hexa-aquo complexes is discussed in detail. Three different dynamic models for the transient ZFS-interaction are presented. It is found that at least two different models is consistent with the available experimental data . A combined distortion-reorientation model for the ZFS-interaction is presented.Within the slow motion theory the so called quadrupolar dip is discussed and numerical calculations is presented for the case of ^H-'^ spin pairs.Analytical expressions for NMR lineshape of spin 5/2 and 7/2 nuclei are derived assuming quadrupolar relaxation and non-extreme narrowing conditions. The effects of both chemical exchange and dynamic shifts are included in the description.