Field-Dependent Proton NMR Relaxation in Aqueous Solutions of Ni(II) Ions. A New Interpretation
✍ Scribed by J. Svoboda; T. Nilsson; J. Kowalewski; P.-O. Westlund; P.T. Larsson
- Publisher
- Elsevier Science
- Year
- 1996
- Tongue
- English
- Weight
- 155 KB
- Volume
- 121
- Category
- Article
- ISSN
- 1064-1858
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✦ Synopsis
A new model is presented for nuclear-spin relaxation in paraspin relaxation was assumed to be due to distortional modumagnetic transition metal complexes in solution, allowing the eleclation of the zero-field splitting (ZFS) interaction. They also tron-spin relaxation to be outside the Redfield limit. The novel carefully specified the validity limits of the theory (the elecfeature is that the transient zero-field splitting (ZFS), modulated tron-spin relaxation time must not be shorter than the correlaby distortions of the solvation shell, is allowed to be of rhombic tion time for the modulation of the relevant interaction). rather than cylindrical symmetry. The model, which assumes that These conditions are sometimes denoted as ''strong narthe static ZFS is absent, is applicable to aqueous solutions of transirowing conditions'' or Redfield limit [a slightly more gention metal ions. The magnetic-field dependence of the proton spineral formulation of the strong narrowing conditions, in terms lattice relaxation rate enhancement in aqueous solution has been of interaction strengths and correlation times, has been disinvestigated, and calculations are presented for an S Å 1 system cussed in a series of papers from our laboratories ( 4-6)]. such as Ni 2/ (aq), using different degrees of rhombicity of the ZFS and different motional conditions (Redfield limit and slow-The set of simple equations, describing the enhancement of motion regime). The new model is also applied to fit the previously the nuclear-spin relaxation rates in the region where the reported data for the field dependence of proton relaxation in Bloembergen-Morgan theory is valid, is commonly called aqueous solution of Ni(ClO 4 ) 2 at low pH [J. Kowalewski, T. Larsthe modified Solomon-Bloembergen (MSB) or Solomonson, and P.-O. Westlund, J. Magn. Reson. 74, 56 (1987)]. The Bloembergen-Morgan (SBM) equations. Bloembergen and inclusion of rhombicity, motivated by recent theoretical work, pro-Morgan (3) issued a warning that the case of aqueous Ni(II) vides a model which performs as well as the earlier ad hoc model. could possibly be beyond the strong narrowing conditions.