Free energy profiles (Fig. and fig. ) reveal that C2H6 diffusion involves four transition states at ξ = -2.7, -0.7, 0.7, and 2.7, with the corresponding free energies summarized in table . The first transition state is associated to the crossing of the first carbon atom through the 8MR window, forming structure (ⅱ) with the methyl groups at opposite sides of the 8MR (snapshots in 10 fig. ). An additional barrier exists for the diffusion into the inter-ring space (structure (ⅲ)). The diffusion through the second 8MR window involves two similar transition states, forming structure (ⅳ).