Anisotropic rotational motion and internal rotation in liquid methylbenzenes

Abstract

The usual semiquantitative evaluations of temperature-dependent dipolar ¹³C spin-lattice relaxation times of liquid methylbenzenes (toluene, xylenes, tri- and tetramethylbenzenes), which assume isotropic molecular motion, do not allow any reliable statements to be made about the various hindered rotations of the methyl groups, and about preferred molecular rotation axes. The quantitative evaluation of the experimental data using a new formalism (complete anisotropic motion with arbitrary position of the internally rotating methyl groups) gave the following: physically meaningless results were obtained in all cases for the optimizations for one of the three possible axially symmetrical ellipsoidal motions, and in most cases for complete anisotropic motion. A quantitative evaluation of dipolar relaxation times which takes complete anisotropic motion into account is—at least for the methylbenzenes—only promising if the present 10 per cent level of experimental error in the dipolar relaxation times can be reduced to at least 1 per cent. However, the results do indicate clearly an anisotropic motion which is dependent on the molecular geometry. The degree of motional anisotropy is not only determined by effects of inertia: the activation parameters for the molecular motion do not increase continually with increasing molecular mass, as would be expected if the molecular motion is determined by inertial effects alone. At higher temperature, the suitability of the rotational diffusion model becomes increasingly dubious with decreasing molecular mass. An activation parameter was found from the calculated methyl jumping rates, which characterized the internal rotation barriers of the methyl groups. It became evident that the isotropic approximation of the overall molecular motion usually employed is insufficient, and can simulate hindered rotations for methyl groups which are not present according to experience. However, the values of the activation parameters for methyl groups which are known to show hindered rotation are independent of the model used for the molecular motion.