Anisotropic reorientational dynamics of toluene in neat liquid. A C-13 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-la ttice relaxation rates and cross-correlation rates between the dipolar and the chemical-shift anisotropy relaxation mechanisms were measured for diffe rent 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 f ound three different rotational diffusion constants about the molecular rot ation axes. Reorientation velocities about the C-2 axis and the axis perpen dicular to the molecular plane were of comparable magnitude, but change the ir ratio in the temperature range investigated in this study. The reorienta tion about the axis in the molecular plane and perpendicular to the C-2 axi s was found to be approximately 2 to 3 times slower. The rotational diffusi on 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 t he reorientation of different C-13-H-1 bonds, spin-lattice relaxation rates for the chemical-shift anisotropy and spin-rotation mechanisms were also d erived for the different C-13 nuclei.