Effect of zero field splitting interactions on the paramagnetic relaxation enhancement of nuclear spin relaxation rates in solution

The enhancement of nuclear spin relaxation rate R1m that is produced by paramagnetic metal ions in solution (the NMR-PRE) has been investigated for electron spin systems with S=1 using recently developed relaxation theory that incorporates both Zeeman and zero field splitting (zfs) interactions of arbitrary magnitude in the electron spin Hamiltonian. The zfs interaction gives rise to important qualitative features which have no analog in the Zeeman-limit theory. The three principal physical phenomena responsible for these effects are (1) alterations in the geometry of the magnetic dipole–dipole coupling energy due to requantization of the electron spin from laboratory to molecular axes; (2) the crossing or ‘‘pinching’’ of spin energy levels that occurs in the regime of field strengths between the zfs and Zeeman limits; and (3) an effective magnetic field dependence in the reorientational correlation time that results from a change in the appropriate definition of this quantity in the intermediate regime. In the zfs limit and in the intermediate regime, the field dispersion profile depends strongly on the position of the nuclear spin with respect to the molecular coordinate axes. For equatorial positions of the nuclear spin, the principle qualitative feature of the dispersion profile is a strong increase in R1m with increasing field strength coupled, in most cases, with a shallow local R1m maximum; both features are centered near the cross-over field between the limits. For axial positions, the profile exhibits a feature that is superficially similar to those characteristic of Zeeman-limit theory, but which is fundamentally different in quantitative properties and in physical origin. As a test of theoretical predictions, the experimental magnetic field profile of the NMR-PRE of the hexaquo-Ni(II) cation, an S=1 model system that has previously been studied extensively, has been reinterpreted. It is shown that the major qualitative features of the experimental field profile result specifically from physical effects of the zfs interaction and are closely related to the phenomenon of requantization of the electron spin in the intermediate regime.