Trapping of Small Molecules within Single or Double Cyclo[18]carbon Rings

The encapsulation of a set of small molecules, H₂, CO, CO₂, SO₂, and SO₃, by a circular C₁₈ ring is investigated by quantum calculations. These ligands lie near the center of the ring but, with the exception of H₂, are disposed roughly perpendicular to the ring plane. Their binding energies with the C₁₈ vary from 1.5 kcal/mol for H₂ up to 5.7 kcal/mol for SO₂, and the bonding is dominated by dispersive interactions spread over the entire ring. The binding of these ligands on the outside of the ring is weaker but allows the opportunity for each to bond covalently with the ring. A pair of C₁₈ units lie parallel to one another. This pair can bind each of these ligands in the area between them with only small perturbations of the double ring geometry. The binding energies of these ligands to this double ring configuration are amplified by some 50% compared to the single ring systems. The presented data concerning the trapping of small molecules may have larger implications regarding hydrogen storage or air pollution reduction.