ROTATIONAL MOTION OF A SOLUTE MOLECULE IN A HIGHLY VISCOUS-LIQUID STUDIED BY C-13 NMR - 1,3-DIBROMOADAMANTANE IN POLYMERIC CHLOROTRIFLUOROETHENE

The viscosity-dependent retarding effect of a polymeric solvent on the rotation of small solute molecules is investigated by C-13 NMR relaxa tion measurements. It is found that the relaxation data of 1,3-dibromo adamantane in highly viscous polymeric chlorotrifluoroethene can be ex plained neither by isotropic nor by realistic anisotropic tumbling in a single environment. The experimental data are rationalized in terms of fast exchange between at least two environments with correlation ti mes differing by up to two orders of magnitude. The study shows that a uniform retardation of molecular tumbling by a polymeric solvent, des irable for shifting the NMR observation window in studies of intramole cular mobility, is not always feasible. (C) 1998 Academic Press.