Molecular dynamics of benzene in neat liquid and a solution containing polystyrene.: 13C nuclear magnetic relaxation and molecular dynamics simulation results

The reorientational motion of benzene in the neat liquid and in a polystyrene/benzene solution was investigated by NMR relaxation as well as MD simulation methods. The temperatute-dependent C-13 dipolar spin-lattice relaxation rates and cross correlation rates between the dipolar relaxation mechanism and the relaxation by chemical-shift anisotropy were measured. From the NMR measurements and MD simulation results, the rotational diffusion constants for rotations about the C-6 axis and perpendicular to it were evaluated, and it was found that the values of the former were larger than those of the latter. The rotational anisotropy, which is the ratio of these values, decreases for the NMR data from 2.3 to 1.2 when increasing the temperature from 280 to 323 K. The activation energy for reorientation about the main symmetry axis was 3.4 kJ mol(-1) and about axes perpendicular to that 13.3 kJ mol(-1). The temperature effect was less pronounced for the MD results: the anisotropy changed from 1.9 to 1.3 between 280 and 360 K. The reorientational correlation functions showed a significant non-Debye behavior and were fitted with a Kohlrausch-Williams-Watts function. Furthermore, from the MD simulations the temperature dependence of the density and the translational diffusion coefficient were also determined. The NMR data for benzene rotational motions in a polystyrene matrix could not be explained by a simple rotational diffusion model. From the data, it was concluded that the rotations about the C-6 axis were much faster than about axes perpendicular to the C-6 axis. This finding is in accordance to a previous MD study by Muller-Plathe.