FIELD-DEPENDENT PROTON NMR RELAXATION IN AQUEOUS-SOLUTIONS OF NI(II) IONS - A NEW INTERPRETATION

A new model is presented for nuclear-spin relaxation in paramagnetic t ransition metal complexes in solution, allowing the electron-spin rela xation to be outside the Redfield limit. The novel feature is that the transient zero-field splitting (ZFS), modulated by distortions of the solvation shell, is allowed to be of rhombic rather than cylindrical symmetry. The model, which assumes that the static ZFS is absent, is a pplicable to aqueous solutions of transition metal ions. The magnetic- field dependence of the proton spin-lattice relaxation rate enhancemen t in aqueous solution has been investigated, and calculations are pres ented for an S = 1 system such as Ni2+(aq), using different degrees of rhombicity of the ZFS and different motional conditions (Redfield lim it and slow-motion regime). The new model is also applied to fit the p reviously reported data for the field dependence of proton relaxation in aqueous solution of Ni(ClO4)(2) at low pH [J. Kowalewski, T. Larsso n, and P.-O. Westlund, J. Magn. Reson. 74, 56 (1987)]. The inclusion o f rhombicity, motivated by recent theoretical work, provides a model w hich performs as well as the earlier ad hoc model. (C) 1996 Academic P ress, Inc.