Dynamics in molecular and molecular-ionic crystals: a combined experimental and molecular simulation study of reorientational motions in benzene, pyridinium iodide, and pyridinium nitrate.

Molecular dynamics (MD) simulations for crystalline benzene (C(6)H(6)), pyridinium iodide C(5)NH(6)I(-), and pyridinium nitrate C(5)NH(6)NO(3)(-) have been performed as a function of temperature and pressure. Despite the similar shape of the benzene molecule and the pyridinium cation, the experimental and simulated data have showed clear differences in their dynamics. Therefore, the rotational dynamics have been explored in detail by comparing thoroughly the existing experimental results together with new quasielastic neutron scattering (QENS) data obtained for (PyH)NO(3) and molecular dynamics simulations. The correlation times, activation energy, geometry of motion of benzene molecule and pyridinium cation, isothermal compressibility, and activation volume obtained from the simulations are compared with the experimental results obtained by nuclear magnetic resonance and QENS methods. MD simulations have also revealed that reorientation of the pyridinium cation in pyridinium nitrate between two inequivalent positions is strongly affected by the hydrogen bond N-H···O between the cation and the anion and the influence of temperature on strength of the hydrogen bond is much more important than that of the pressure.