NMR linewidth, relaxation and diffusion studies of hexamethyldisilane confined in porous silica

The rotational and translational dynamics of hexamethyldisilane confined within silica pores of diameter 4, 6 and 20 nm were studied as a function of temperature by measuring H-1 and C-13 NMR linewidths, self-diffusion coefficients, spin-spin relaxation times and spin-lattice relaxation times, and the results were compared with values obtained for the bulk material. The confinement in the pores gives rise to substantial changes in the molecular dynamics and phase behaviour. The lineshape, T-2 and self-diffusion measurements revealed a two-phase system consisting of a liquid-like component at the surface and a crystalline solid in the centre of the pore. The highly mobile surface layer is observable far below the reduced transition temperature of the confined hexamethyldisilane. A high diffusion rate of the adsorbed hexamethyldisilane is observed over a wide temperature range, even well below the region of the depressed transition point. The diffusion coefficient of the molecules in the surface layer is three orders of magnitude larger than in the plastic phase of bulk hexamethyldisilane. (C) 1998 John Wiley & Sons, Ltd.