The dynamics of liquid and plastic tert-butyl cyanide in bulk and confined in porous silica studied by high-field NMR

The phase behaviour and molecular dynamics of tert-butyl cyanide (TBCN) confined within silica pores of diameter 6 and 20 nm were studied as a function of temperature by measuring the NMR line widths, self-diffusion coefficients and T-1 and T-2 relaxation times, and the results are compared with the new results obtained for the bulk material. The NMR line width data of the bulk TBCN reveal the existence of two crystalline phases between 160 K and the melting point (292 K). The uniaxial reorientation of the tert-butyl group is faster than the methyl group rotation in both solid phases. The confined TBCN solid exhibited a substantially different phase behaviour and molecular dynamics compared to the bulk material. The line shape and T-2 measurements revealed a two-phase system owing to a pore size dependent depression of the melting and transition points. The highly mobile narrow line component, exhibiting a remarkably high diffusion rate, was observed over a wide temperature range, even far below the transition point of the bulk material. (C) 1999 Elsevier Science B.V. All rights reserved.