Outer-sphere nuclear spin relaxation in paramagnetic systems: a low-field theory

A low-field theory for paramagnetic relaxation enhancement (PRE), appropria te for the outer-sphere relaxation, is presented for the electron spin quan tum number S = 1, 3/2, 2, 5/2, 3 and 7/2. The theory is used to calculate t he PRE at low magnetic field, as a function of the translational diffusion coefficient, for various values of the electron spin quantum number, for sm all and fairly large values of the static zero-field splitting (ZFS), and f or a given set of parameters determining the electron spin relaxation. We h ave found earlier that the static ZFS has a profound influence on the elect ron spin relaxation; such effects are also evident in the present study. Co mparisons are made with other existing models for the outer-sphere PRE, and significant differences are found for slowly diffusing systems with large ZFS. The theory is also used to obtain a novel interpretation of experiment al data for an acetone solution of a Mn(III) complex.