Diffusion of isobutane in silicalite studied by transition path sampling

The diffusion process of isobutane in the zeolite silicalite has been investigated at 300 K using transition path sampling. At this temperature, isobutane is preferentially adsorbed at the intersections of silicalite. As the hopping from one intersection to another is a very infrequent event, conventional molecular dynamics techniques cannot be used to compute this hopping rate. Transition path sampling is not affected by this problem and, furthermore, does not require any preconceived notion of the transition mechanism. We use transition path sampling to compute hopping rates between stable states and to locate transition states. For isobutane, we found that not only the position but also the orientation is important in the characterization of the transition state. The Lennard-Jones size parameter sigma describing the alkane-zeolite interactions has a major influence on both adsorption and diffusion of isobutane in silicalite. (C) 2000 American Institute of Physics. [S0021-9606(00)50943-X].