Dynamics of alkane chains included in an organic matrix: Molecular dynamics simulation and comparison with neutron scattering experiment

摘要

An 800 ps molecular dynamics simulation is used to characterize a variety of 290 K rigid-molecule and intramolecular motions in an inclusion compound consisting of heptane guest molecules in channels formed by a matrix of TANO (C9H16NO2) molecules. The heptane molecules undergo highly anisotropic rigid-molecule motion, consisting of fast (characteristic time, tau=3ps) rotationally isotropic and slower translational diffusion. Internal conformational transitions in the heptane molecules are also present (tau=28 ps). The methyl rotational (tau=80 ps) and ring puckering (tau=160 ps) conformational transitions in the TANO molecules are examined in detail; the former reach equilibrium during the simulation whereas the latter do not. The vibrational density of states, dynamical structure factor, and elastic incoherent structure factors (EISFs) are computed from the simulation and found to be in good agreement with experiment, indicating that the small-amplitude vibrations, the characteristic times, and geometries of the large-amplitude diffusive motions are well represented. The main peaks in the density of states are assigned to methyl librations and rigid-molecule translational vibrations. The rigid-molecule diffusion of the heptane molecules is found to dominate the EISF decay. (C) 1998 American Institute of Physics. [References: 35]