Structures, energies, and electrostatics for methane coniplexed with alumina clusters

Ab initio calculations were used to investigate properties of complexes formed from the association of CH₄ with Al₂O₃, Al₄O₆, and Al₈O₁₂ alumina clusters. Methane attaches to a surface Al atom of the cluster to form a complex with an Al-C separation that varies between 2.2 and 2.5 Å. The rotational motion for methane in these complexes is highly fluxional. Extrapolated G2MP2 well depths for the CH₄- - -Al₂O₃, CH₄- - -Al₄O₆, and CH₄- - -A1₈O₁₂ complexes are 21, 14, and 17 kcal/mol, respectively. These different well depths are determined by the accessibility of the Al atom to which CH₄ binds and the size of the alumina cluster. The electrostatics of the three alumina clusters are very similar, with a charge on the surface Al atom of +2.2 to 2.3. The potential energy surface for a CH₄- - -Al_2nO_3n cluster is represented semiquantitatively by an analytic function consisting of two-body potentials. The results of this study suggest that the adsorption energy for alkane molecules binding to alumina materials depends very strongly on the structure of the binding site.