Energetics of intercavity diffusion in a simple model of a low-density p-tert-butylcalix[4]arene crystal.

Potentials of mean force calculated for the diffusion of small guest molecules (CO(2) and CH(4)) between adjacent host cavities in the beta(0) p-tert-butylcalix[4]arene polymorph reveal sufficiently high barriers to diffusion to justify characterizing the lattice voids in this crystal as independent host cavities rather than as extended volumes. In addition, the calculated barrier heights are consistent with a "relay" model for gas diffusion in this ostensibly nonporous organic solid involving the lateral sliding of the host bilayers.

Molecular dynamics of host-guest complexes of small gas molecules with calix[4]arenes.

The unexpected sorption of gases by a low-density p-tert-butylcalix[4]arene crystal polymorph raises fundamental questions about differential gas transport and sequestration in the organic solid state. To gain insight into the processes underlying these observations, we have used molecular dynamics simulations, augmented with calculations of potentials of mean force, to investigate the stability of isolated host-guest complexes and the relationship between the dynamics of these complexes and the dynamics of a solvated host molecule. Thermal fluctuations of the calixarenes themselves are found to be consistent with proposed mechanisms for gas entry into the host cavities, while relative host-guest stabilities correlate well with experimental absorption-desorption isotherms in some cases (CO2 and CH4) but not in others (C2H2). In these isolated systems, stable complexes characteristically form when the attractive interactions of the guest with the ring of negative charge density on the inner surface of the host cavity are not disrupted by thermal motion. The experimentally observed efficient uptake of gases such as C2H2 by the host crystals suggests, however, that stabilization of host-guest complexes in some systems may derive from dynamical constraints imposed by the crystal lattice.