ANISOTROPIC MOTION OF TOLUENE ABOVE AND BELOW THE GLASS-TRANSITION STUDIED BY H-2 NMR

H-2 nuclear magnetic resonance spin-lattice relaxation experiments on two selectively deuterated toluene molecules have been performed over a wide temperature range, spanning liquid and glassy states, to examin e anisotropic molecular dynamics. In the liquid regime, the relaxation data are analyzed by the model of anisotropic rotational diffusion. A more phenomenological ansatz is used for the whole temperature regime to obtain information about anisotropic reorientation and its tempera ture dependence. We find that the anisotropy is reduced in the superco oled state and the motion becomes approximately isotropic below 140 K which is interpreted as the onset of cooperative reorientation on appr oaching the glass transition temperature T(g) = 117 K. Below T(g), spi n-lattice relaxation is caused by secondary relaxations which also sho w a pronounced anisotropy with increased librational amplitudes around the long molecular axis.