Molecular dynamics of discotics in the isotropic phase

Abstract

Molecular dynamics (MD) simulation and multifield carbon-13 nuclear spin relaxation data are reported and used in an investigation of dynamic processes taking place in the isotropic phase, at 95 °C, of benzene-hexa-n-heptanoate (BHA7). The relaxation data are analyzed by using a dynamic model based on separation between a fast internal and a slow overall motion. The overall reorientations defined by the motion of the rigid aromatic core, occurs on nanosecond time scale, whereas the internal motion in the flexible, aliphatic side chains is much faster, with the correlation times in the picosecond range. Increased mobility and decreased orientational order along the chains—characterized by order parameters—is predicted by the experiments and also confirmed in the simulation. The translational diffusion constants are calculated in the MD simulation and measured using FT-PGSE technique. The molecular structure obtained from the MD simulation indicates that the aliphatic chains are situated alternatively up and down relative to the aromatic core. The radial distribution functions show that the molecules in the liquid are associated in pairs, with a parallel orientation of the planes defined by the aromatic core.