Moving atoms and small molecules out of open containers

Density functional theory with the M05-2X exchange/correlation functional is used to study the barriers for expulsion of atoms and small molecules (N2, CO, H2, Ar, Kr, Xe, H2O) out of open fullerenes (I20) and related molecular containers (C40H20, [5]beltene, cucurbit[5]uril). The reactions are examples where dispersion plays a critical role in determining the barrier heights. Calculations are compared with experimental kinetic data for N2@I20, CO@I20, and Xe@cucurbit[5]uril (Xe@CB[5]). Comparing the four molecular containers, the activation barriers for escape of an atom or small molecule correlate with the binding energies. A new open-fullerene model container C40H20 (C40) was constructed from C60 with a constriction at both ends formed by five methylene groups around the rim. The activation barriers for escape of N2 and CO from the model container are similar to those from the I20 open-cage fullerene. In the case of H2O@C40, charge analysis reveals an interesting charge transfer at the transition state as the escaping guest is "squeezed" out of the host container.