Molecular dynamics simulations of quinoline in the liquid phase

Molecular dynamics simulations of liquid quinoline have been performed at experimental densities corresponding to the temperature range 276-320 K. The intermolecular potential is a simple effective two-body potential between rigid molecules having 17 atomic Lennard-Jones and electrostatic Coulomb interaction sites. The vaporization enthalpy is overestimated by 8-9% with respect to the experimental value. The translational diffusion coefficient exhibits a small non-Arrhenius behavior with a change in temperatures near 290 and 303 K. The rotational diffusion tensor is rotated around the z axis perpendicular to the molecular plane by an angle of 4-6° with respect to the frame of reference defined by the principal axes of inertia. The rotational diffusion tensor presents a significant anisotropy with D(rot,y)/D(rot,x) ≃ 0.6-0.5 and D(rot,z)/D(rot,x) ≃ 1.6-1.3 between 276 and 320 K when the x axis is defined as the long molecular axis and the y axis is situated nearly along the central C-C bond. The rotational diffusion coefficients, the reorientational correlation times of the C-H vectors, and the T1(13)C NMR relaxation times present a non-Arrhenius break around 288-290 K in agreement with several experimental results. In addition, a non-Arrhenius break can also be observed at 303 K for these properties. It has been found that the structure evolves smoothly in the studied temperature range. Center of mass-center of mass and atom-atom radial distribution functions show a monotonous evolution with temperature. Various types of first-neighbor dimers have been defined, and their population analysis has revealed a continuous monotonous evolution with temperature. Thus, the non-Arrhenius behavior observed for translational and rotational diffusion is correlated with the monotonous evolution of the population of first-neighbor dimers at a microscopic level and not with a sharp structural transition.