NMR DEUTERON RELAXATION STUDY OF LOW-DIMENSIONAL EFFECTS IN MOLECULAR LIQUIDS IN RESTRICTED GEOMETRIES

The NMR study of the deuteron relaxation rates T1(-1) and T1p-1 of liquid nitrobenzene-d5 confined to porous silica glasses with small pore sizes reveals a logarithmic frequency dependence of the spectral densities for several decades of frequency, thus reflecting the two-dimensional character of molecular motions. In fact, the concept of reduced dimensionality allows one to explain the unique properties of the relaxation processes as well as their dependence on the pore size. The gradual transition from the predominantly two-dimensional logarithmic dependence of the relaxation rates on frequency to the three-dimensional bulk-like behavior with the increasing pore radius is observed. In addition, the experimental data both for nitrobenzene-d5 and cyclohexane-d12 confined to porous silica glasses and surface modified porous silica glasses allow us to discuss the relative role of geometrical confinement and the pore surface effects and, in particular, the applicability and limitations of the two-state fast-exchange model generally used to describe the dependence of the relaxation rate upon the pore radius.