title: Effects of paramagnetic cations on the nonexponential spin-lattice relaxation of rare spin nuclei in solids.
abstract: 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.
authors.affiliation: Department of Chemistry, Lehigh University, Bethlehem, PA 18015, USA.
authors.forename: M H
authors.initials: MH
authors.lastname: Alaimo
authors.forename: J E
authors.initials: JE
authors.lastname: Roberts
mesh_terms: D002412:Cations / Q000737:chemistry*; D055598:Chemical Phenomena; D002627:Chemistry, Physical; D004578:Electron Spin Resonance Spectroscopy / Q000379:methods*; D007554:Isotopes; D009682:Magnetic Resonance Spectroscopy / Q000379:methods*; D012825:Silicon
chemical_list: D002412:Cations; D007554:Isotopes; D012825:Silicon