Atomistic Molecular Dynamics Simulation of Hexakis(pentyloxy)triphenylene: Structure and Translational Dynamics of Its Columnar State

Atomistic molecular dynamics computer simulations have been performed on the columnar state of the discogen hexakis(pentyloxy)triphenylene (HAT5). The parameters of the empirical model potential have been taken from the AMBER and OPLS force fields. A 10 ns simulation run has been carried out in Berendsen's ensemble at pressure P = 0.1 MPa and temperature T = 375 K to study bulk and molecular structure. The bulk structure has been characterized by obtaining structural parameters that compare favorably with those deduced from X-ray diffraction experiments. C-13 and H-2 NMR data suggest two completely different pictures of the position of the pendant chains with respect to the plane of the aromatic core: one, called diablo-like, with the chains in the triphenylene plane; the other, octopus-like, with the chains out of this plane and, alternatively, above and below it. Our simulated data are consistent with an intermediate situation. In agreement with all NMR data, we find that the chains become more and more disordered, moving away from the core. We have observed a decrease of the disorder of the lateral chains in going from gas phase to condensed bulk state. Information on translational dynamics has been obtained by performing a 1 ns simulation in the microcanonical ensemble; these calculations definitely assess the one-dimensional fluid character of the columnar state. We have found that the mean square displacement has a solidlike behavior in the plane perpendicular to the column axis. The translational dynamics along the direction parallel to it is dominated by the reciprocal, fluidlike sliding motion of the columns as a whole. If this collective dynamics is removed, also the parallel diffusion coefficient turns out to be solidlike.