Effects of paramagnetic cations on the nonexponential spin-lattice relaxation of rare spin nuclei in solids

✍ Scribed by M.H. Alaimo; I.E. Roberts

• Publisher: Elsevier Science
• Year: 1997
• Tongue: English
• Weight: 998 KB
• Volume: 8
• Category: Article
• ISSN: 0926-2040
• DOI: 10.1016/s0926-2040(97)00006-4

✦ Synopsis

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 centers. Nonexponential spin-lattice relaxation was observed in derivatized silica gels resins. This phenomenon was localized and enhanced when paramagnetic transition metal cations were bound to surface functional groups. A stretched exponential analysis method was determined to be robust in fitting nonexponential relaxation curves for silica gels both with and without bound paramagnetic ions. Spin-lattice relaxation rates (T1(-1)) for functional group nuclei increased as a function of percent surface coverage with metal ion. The magnitude of the relaxation rate increase was dependent upon internuclear distances from the paramagnetic center. At low surface coverages, a semi-random distribution of paramagnetic centers increased the degree of stretching of spin-lattice relaxation decays, as measured by decreases in the calculated stretching parameter beta. At higher surface coverages, calculated beta values reached a limiting value, indicating that while the spin-diffusion mechanism in metal-exchanged silica gels is restricted, it is not completely diminished.