Rounding up

All of the isotopes of radon are radioactive, which means that its chemistry has not been studied as widely as its smaller and more stable cousin, xenon. In particular, the supramolecular chemistry of xenon has been the focus of much research, and it is known that individual xenon atoms can bind inside hollow cage-like molecules with relatively high affinities. Although radon is incorporated in some crystal hydrates, until now there have been no examples of discrete molecular species to which it has been found to bind. Based on their previous studies of xenon encapsulation by cryptophanes, a team led by Ivan Dmochowski at the University of Pennsylvania and co-workers at the National Institute of Standards and Technology have now shown that radon can also be bound to this type of host molecule. They first used calorimetry experiments to show that a new water-soluble cryptophane with aminofunctionalized side chains could bind to xenon with the highest reported association constant so far. Calorimetric measurements could not be used to study radon binding, however, because the quantity of radon available was limited (it is generated from the decay of radium chloride inside polymer capsules), so liquid scintillation counting had to be used instead. Radon was found to bind inside the cryptophane slightly more strongly than xenon and this is explained by radon’s larger size (it fills the cavity better than xenon) and more polarizable electron cloud. Both of these attributes are expected to increase the strength of van der Waals interactions between the noble-atom guest and cryptophane host. SC