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

The reorientational motion of benzene in the neat liquid and in a polystyre ne/benzene solution was investigated by NMR relaxation as well as MD simula tion methods. The temperatute-dependent C-13 dipolar spin-lattice relaxatio n rates and cross correlation rates between the dipolar relaxation mechanis m and the relaxation by chemical-shift anisotropy were measured. From the N MR measurements and MD simulation results, the rotational diffusion constan ts 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 t he latter. The rotational anisotropy, which is the ratio of these values, d ecreases for the NMR data from 2.3 to 1.2 when increasing the temperature f rom 280 to 323 K. The activation energy for reorientation about the main sy mmetry 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: th e anisotropy changed from 1.9 to 1.3 between 280 and 360 K. The reorientati onal 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 translationa l diffusion coefficient were also determined. The NMR data for benzene rota tional motions in a polystyrene matrix could not be explained by a simple r otational diffusion model. From the data, it was concluded that the rotatio ns 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-P lathe.