PM3 molecular orbital calculations on the complexation of α-cyclodextrin with acetophenone

PM3 calculations were performed on the inclusion complexation of α-cyclodextrin with acetophenone from a complete and unrestricted geometry optimization and conformational analysis. The complexation orientation in which the acetyl group is located near the secondary hydroxyl rim of the cyclodextrin cavity was found to be preferable, in contrast to the usual anti-parallel arrangement of host and guest dipoles in the molecular recognition of cyclodextrins. This abnormality was caused by the steric hindrance of the acetyl group in the guest, which led us to considerable van der Waals repulsion between the host and guest molecules upon complexation. It is suggested that steric effects play an important role in the energy and geometry of host–guest complexation.