Paramagnetic relaxation in dipolar-coupled homonuclear spin systems

Abstract

Measurements of the paramagnetic contribution to the longitudinal relaxation of nuclei interacting with a paramagnetic probe can be complicated by effects of chemical exchange and cross relaxation. A theoretical investigation is presented of the relaxation behavior of the longitudinal nuclear magnetization, following either selective or non-selective inversion of the equilibrium magnetization, in a dipolar-coupled homonuclear two-spin system that undergoes exchange between free and bound (to a paramagnetic probe) states. Approximate solutions are derived for a number of limiting cases characterized in terms of the relative rates of exchange, cross relaxation, and spin-lattice relaxation. It is shown that unless the cross-relaxation rate (CRR) is slow compared to the other rates that affect the longitudinal relaxation, reliable and accurate paramagnetic relaxation rates that can yield useful structural information can be obtained only from measurement of the initial rate of recovery of the longitudinal magnetizations. Some advantages of selective over nonselective spin perturbations are pointed out.