EFFECTS OF PARAMAGNETIC CATIONS ON THE NONEXPONENTIAL SPIN-LATTICE RELAXATION OF RARE SPIN NUCLEI IN SOLIDS

Nonexponential spin-lattice relaxation is often observed for rare spin nuclei in the solid state. Deviation from single-component decay may be amplified by the coupling of rare spin nuclei to paramagnetic cente rs. Nonexponential spin-lattice relaxation was observed in derivatized silica gels resins. This phenomenon was localized and enhanced when p aramagnetic transition metal cations were bound to surface functional groups. A stretched exponential analysis method was determined to be r obust in fitting nonexponential relaxation curves for silica gels both with and without bound paramagnetic ions. Spin-lattice relaxation rat es (T-1(-1)) for functional group nuclei increased as a function of pe rcent surface coverage with metal ion. The magnitude of the relaxation rate increase was dependent upon internuclear distances from the para magnetic center. At low surface coverages, a semi-random distribution of paramagnetic centers increased the degree of stretching of spin-lat tice relaxation decays, as measured by decreases in the calculated str etching parameter beta. At higher surface coverages, calculated beta v alues reached a limiting value, indicating that while the spin-diffusi on mechanism in metal-exchanged silica gels is restricted, it is not c ompletely diminished. (C) 1997 Elsevier Science B.V.