Anisotropic reorientational dynamics of toluene in neat liquid.: A 13C nuclear magnetic relaxation study

The reorientational motion of toluene in neat liquid was examined by using C-13 nuclear magnetic relaxation. The temperature-dependent dipolar spin-lattice relaxation rates and cross-correlation rates between the dipolar and the chemical-shift anisotropy relaxation mechanisms were measured for different C-13 nuclei in the molecule over a temperature range of 253 to 318 K. Assuming the molecular frame to show an anisotropic rotational motion, we found three different rotational diffusion constants about the molecular rotation axes. Reorientation velocities about the C-2 axis and the axis perpendicular to the molecular plane were of comparable magnitude, but change their ratio in the temperature range investigated in this study. The reorientation about the axis in the molecular plane and perpendicular to the C-2 axis was found to be approximately 2 to 3 times slower. The rotational diffusion constants were fitted to an Arrhenius equation, and activation energies from 3.5 to 9.1 kJ mol(-1) were found. Furthermore, correlation times for the reorientation of different C-13-H-1 bonds, spin-lattice relaxation rates for the chemical-shift anisotropy and spin-rotation mechanisms were also derived for the different C-13 nuclei.