Communication: Evidence of halogen bonds in clathrate cages.

We present a theoretical characterization of the interaction of Cl and Br in the 5 and 56 clathrate cages, respectively, based on energy partitioning analysis and a study of the electronic shifts associated with transitions to the main valence bands. Our analysis clearly shows that while Br@56 does not show halogen bonding interactions in its equilibrium geometry, Cl@5 presents all the characteristics expected for halogen bonding. This is accomplished by the interaction of the usual sigma-hole with the lone pair of the closest oxygen atom involved in hydrogen bonding within the cage framework, though breaking of the hydrogen bond is not required. This possibility, which had not been considered in previous analyses, opens up a new way of looking at the interactions of dihalogens with the nearest water molecules in the cage.