Molecular Dynamics Study of Anisotropic Translational and Rotational Diffusion in Liquid Benzene

Abstract

Equilibrium NPT and NVT molecular dynamics simulations were performed on liquid benzene over an extended range of temperature (from 260 to 360 K) using the COMPASS force field. Densities and enthalpies of vaporization (from cohesive energy densities) were within 1% of experiment at all temperatures. tumbling and spinning rotational diffusion coefficients, D(perpendicular) and D(parallel), computed as a function of temperature, agreed qualitatively with the results of earlier reported experimental and computational investigations. Generally, it was found that D(parallel)/D(perpendicular) approximately 1.4-2.5 and the activation energy for tumbling was significantly greater than for spinning about the C6 axis [Ea(D(perpendicular)) = 8.1 kJ mol(-1) and Ea(D(parallel)) = 4.5 kJ mol(-1)]. Calculated translational diffusion coefficients were found to be in quantitative agreement with experimental values at all temperatures [deviations were less than the scatter between different reported measurements]. In addition, translational diffusion coefficients were computed in the molecule-fixed frame to yield values for Dxy (diffusion in the plane of the molecule) and Dz (diffusion perpendicular to the plane). It was found that the ratio Dxy/Dz approximately 2.0, and that the two coefficients have roughly equal activation energies. This represents the first atomistic molecular dynamics study of translational diffusion in the molecular frame.

Molecular Dynamics Study of Translational and Rotational Diffusion in Liquid Ortho-terphenyl

“…It is satisfying to note that the diffusion coefficients computed from the molecular dynamics simulations exhibit the same non-Arrhenius temperature dependence found experimentally in OTP and other fragile glass-forming liquids. In contrast, in a recent investigation of the non-glass-forming liquid, benzene, translational diffusion coefficients were found to exhibit no deviations from Arrhenius behavior over the complete liquid range. In addition, the computed values are in very good semiquantitative agreement with experiment, with ratios D tr (molecular dynamics)/ D tr (exp) ranging from ∼0.65 (high T ) to ∼1.0 (low T ).…”

Diffusion of benzene through the beta zeolite phase

“…Rotation of the normal vector depends on the moment of inertia component in the plane of the molecule, I \ -= 1.46 Â 10 À45 kg m 2 which is smaller than the inertia component about the symmetry axis, I || = 2.93 Â 10 À45 kg m 2 . These results are qualitatively similar to the rotational anisotropy in liquid benzene reported in experimental studies [48][49][50][51][52][53] and MD simulations [48,[54][55][56][57][58]. We see from Fig.…”

Diffusion Tensors of Arbitrary-Shaped Nanoparticles in Fluid by Molecular Dynamics Simulation

“…And the asymmetrical shape and directionality of the particle at its original position may increase the effect. This process is called the ballistic regime 40,41 . After that the motion transform from ballistic to diffusive regime, MSD is proportional to time consistent with diffusion theory so the diagonal element of translational diffusion tensor can be calculated from the slope of fit.…”

“…To improve, researchers have alternatively used equivalent diffusion coefficients in specific directions (e.g. in-plane rotational diffusion coefficient) 39,40 , which benefits in obtaining general diffusion speed conveniently but with anisotropy neglected. Other methods via defining specific parameters such as rotation angle of rod at the major axis along displacement vector 37,41 , to which a cross comparison with others becomes difficult due to its customization.…”