Guest mobility increases hemicarceplex kinetic stability

When acetophenone is incarcerated within hemicarcerand 1, constructed with o-xylyl linker units, the guest resides exclusively along the long axis of the host. In contrast, the unsymmetric guests incarcerated within the more flexible hemicarcerand constructed with butylene linker units, 2, show clear evidence of exchange between the hemispheres. The barriers of rotation proceed as: acetophenone > quinoline > indole. Host 2 retains acetophenone more strongly within its interior than 1. The difference in stability is primarily due to an increased enthalpic component to the activation free energy for acetophenone egress from 2.