Acceleration of xenon kick-out exchange in a cryptophane host explained by ab initio simulation

The degenerate exchange involving xenon and anti-cryptophane-222-(OCH₂COOH)₆ in basic water is studied. The reaction consists of the intrusion of a xenon atom into a cavity hosting another xenon and the escape of the latter from the cage to reach the aqueous solution. A series of constrained ab initio molecular dynamics simulations were performed according to the Blue Moon ensemble method to reconstruct the free-energy profile for the degenerate exchange reaction at ambient temperature. In addition to the estimation of the free-energy barrier height of the reaction, analysis of the trajectories provides details on its mechanism as well as on the nature of its transition state, where both Xe atoms interact through strong dispersion forces owing to moderate confinement effects.