Longitudinal Si-29 nuclear magnetic relaxation in aqueous alkali-metal silicate solutions revisited

We report the results of a wide ranging Si-29 longitudinal (spin-lattice) relaxation study of aqueous alkalimetal silicate solutions. Measurement and interpretation of relaxation rates for the many chemically distinct Si centers found in these solutions is complicated by (a) Si-Si chemical exchange during the inversion-recovery pulse sequences, (b) persistent paramagnetic impurities, and (c), in the case of moderately concentrated low-temperature solutions, the inapplicability of the extreme narrowing condition. The situation is therefore complex but nonetheless amenable to analysis. Our findings indicate that the primary causes of Si-29 longitudinal relaxation are interactions (dipolar and contact) with adventitious paramagnetic ions, dipole-dipole relaxation by solvent protons, and an apparent spin rotation interaction, The relative importance of these mechanisms at the individual Si centers is a function of solution composition and purity, along with the size and structure of the corresponding silicate anions. Analysis of the Si-29-H-1 dipole-dipole contribution to relaxation for the hydrated silicate monomer anion yielded an activation energy for isotropic tumbling (between 275 and 375 K) of 18 +/- 3 kJ mol(-1). Contrary to an earlier suggestion, there was no measurable contribution from dipole-dipole interactions with alkali-metal nuclei.