NMR investigations of hexamethyldisilane confined in controlled pore glasses: Pore size distribution and molecular dynamics studies

摘要

In this work, the melting-point depression and molecular dynamics of hexamethyldisilane confined within five controlled pore glasses, with mean diameters ranging from 7.9 to 23.9 run, are studied by high-field (9.4 T) nuclear magnetic resonance (NMR), and the results are discussed with reference to the bulk substance. The melting-point depression in pores with radius R follows the simplified Gibbs-Thompson equation DeltaT = k(p)/(R - s) with a k(rho) value of 74 K . nm and an value of I nm. To our knowledge, this is the first time the k(rho) value of hexamethyldisilane is reported. Proton spin-lattice relaxation times (T), spin-spin relaxation times (T-2), and diflusivities (D) are reported as a function of temperature. The confinement in the pores gives rise to substantial changes in the molecular dynamics and the phase behavior. The line-shape measurements reveal a two-phase system assigned to a relatively mobile component at the pore walls and a crystalline solid at the center of the pores. However, the T, measurements show that the mobile phase also embraces a minor component attributed to nonfrozen liquid in pockets or micropores. The diffusivity of the major narrowline component is approximately three orders of magnitude larger than that in the plastic bulk phase, reflecting fast diffusion of mobile molecules. Below the melting region, T, of the narrow line is significantly shorter than T-1 of the broad line, suggesting that the molecular reorientation is more hindered close to the surface than at the center of the pore. [References: 24]