NUCLEAR-SPIN RELAXATION DUE TO PARAMAGNETIC SPECIES IN SOLUTION - EFFECT OF ANISOTROPY IN THE ZERO-FIELD SPLITTING TENSOR

The NMR (nuclear magnetic resonance) paramagnetic relaxation enhanceme nt (NMR-PRE) that is produced by paramagnetic solutes in solution has been investigated theoretically with respect to the influence of zero field splitting (zfs) interactions in the electron spin Hamiltonian, i n particular with respect to the effects of anisotropy in the zfs tens or. These effects are a physical consequence of the influence of the z fs on the motion of the electron spin vector SBAR. When the zfs energy is large compared to the Zeeman energy (the zfs limit), the precessio nal motion of SBAR is quantized in the molecule-fixed coordinate syste m that diagonalizes the zfs tensor. The uniaxial portion of the zfs te nsor influences the NMR-PRE primarily through its influence on the qua ntization axes of SBAR; the characteristic behavior of the NMR-PRE und er the influence of a uniaxial zfs has been described in detail previo usly. Anisotropy in the zfs tensor induces oscillatory motion in S(z). This motion has a profound influence on the NMR-PRE, the major part o f which normally arises from low frequency components of the local mag netic field that are associated with S(z) rather than from the rapidly precessing local fields that are associated with the transverse compo nents S+/-. For this reason, the NMR-PRE is a sensitive function of zf s anisotropy, which acts to lower the NMR-PRE below the value that occ urs in the uniaxial situation. The magnitude of this effect depends on the ratio (E/D) of the anisotropic and uniaxial zfs parameters, on th e reduced dipolar correlation time, and on the location of the nuclear spin in the molecular coordinate frame. A second physical effect of z fs anisotropy on the NMR-PRE arises from a resonance between the elect ron spin precessional motion in the transverse plane with the precessi onal motion that is perpendicular to the transverse plane (the latter due to zfs anisotropy). Resonance of these motions, which occurs spin energy levels crossings, gives rise to low frequency transverse compon ents of SBAR which result in a resonant increase in the NMR-PRE within a restricted range of E/D ratios.