  We have investigated the effect of molecular shape and charge asymmetry on
the translation dynamics of confined hydrocarbon molecules having different
shapes but similar kinetic diameters, inside ZSM-5 pores using molecular
dynamics simulations. The mean square displacement of propane, acetonitrile,
acetaldehyde, and acetone in ZSM-5 exhibit two different regimes - ballistic
and diffusive/sub-diffusive. All the molecules except propane exhibit
sub-diffusive motion at time scales greater than 1 ps. The intermediate
scattering functions reveal that there is a considerable rotational-
translational coupling in the motion of all the molecules, due to the strong
geometrical restriction imposed by ZSM-5. Overall the difference in shape and
asymmetry in charge imposes severe restriction inside the ZSM-5 channels for
all the molecules to different extents. Further, the behavior of molecules
confined in ZSM-5 in the present study, quantified wherever possible, is
compared to their behavior in bulk or in other porous media reported in
literature.
