Is there a coupling between rotational and translational motion of methane in silicalite-1 and AlPO4-5?

Methane, although often treated approximately as a spherical Lennard-Jones particle, has merely a tetrahedral shape. While moving in zeolites, the interaction of the molecule with the walls will cause a fluctuating angular momentum of this guest molecule. In zeolites containing channels, as it is the case with silicalite-1 or AlPO4-5, adsorbed guest molecules move along channels and change to other channel types at channel intersections. The question arises whether or not the corrugation of the zeolite channels leads to a coupling of rotational and translational motions. It shall be shown, however, that for methane in silicalite-1 and AlPO4-5 the two kinds of motion are uncoupled. The relaxation behavior of translational and angular momentum shows different patterns, although in both cases the relaxation time is on the same order of magnitude. (c) 2005 Elsevier B.V. All rights reserved.