Whereas the addition of methane results in similar 50 vibrational shifts (both in magnitude and direction) as compared with ethane, the effect is smaller or difficult to discern for butane (Table , Fig. ). We attribute this difference as evidence for only smaller hydrocarbons being able to migrate into the 55 HCN lattice to a significant degree. As a point of reference, butane itself also does not form a clathrate hydrate due to its molecular size being too large for the clathrate cages.