Dynamic 1H and 2H NMR investigations of acetonitrile confined in porous silica

Abstract

In this work, the molecular dynamics of acetonitrile and acetonitrile-d₃ confined in silica pores of nominal diameter 6 and 20 nm are studied by high field ¹H and ²H NMR, and the results are discussed with reference to the bulk substances. NMR line-shapes, spin–lattice relaxation times (T₁), spin–spin relaxation times (T₂) and diffusivities (D) are reported as a function of temperature. The line-shape and relaxation measurements clearly reveal a two-component system in the solid phase, consisting of a narrow line superimposed on a broad complex resonance. The melting process takes place over a certain temperature region, due to an appreciable pore size distribution. Below the melting region, the narrow line originates solely from the surface layer, while the broad line is attributed to the crystalline solid at the interior of the pore. The confining geometry restricts the mobility of the liquid probe molecules and gives rise to significantly shorter T₁ and T₂ relaxation times and reduced self-diffusion rates, as compared to the bulk substance. This observation is also reflected in increased activation energies of the reorientational and translational motions. True intra-grain diffusivities were obtained for the probe molecules by using the short diffusion time model, and extrapolating to zero observation time.