Molecular motion in liquid toluene and perfluorotoluene from carbon-13 spin-lattice relaxation measurements

Carbon-13 spin-lattice relaxation times and nuclear Overhauser enhancement factors are reported for neat toluene and octafluorotoluene at 301 K. A thorough analysis of the experimental data shows the overall reorientation of these molecules to be that of an axially symmetric ellipsoid, with diffusion coefficients D_⊥ = 6.45 and

and D_// = 14.20 and

respectively. The internal rotation of the attached methyl group is consistent with a 60° random jump model. The rotational barrier heights are of the same order of magnitude in both compounds : 5.1 and 6.1 kJ.mol⁻¹, thus showing that the decreased mobility of the perfluoromethyl group in perfluorotoluene with respect to that of the methyl substituent in toluene should be traced to inertial and not to structural factors. The dipolar relaxation time of the substituted carbon atom C1 of toluene can be computed from these data and is shown to coincide with the experimental value within 4 % (349 and 363 s).