Halogen Bonding and Cooperative Effects in Chlorine Clathrate: Ab Initio Periodic Study

The possibility of forming halogen bonds inside the clathrate cages is an intriguing question. On the basis of X-ray measurements, a proposition of “multidirectional halogen bonding” was made, and subsequently, ab initio local correlation studies for dichlorine in a single dodecahedral cage showed evidence of its presence. In this work, the structure and energetics of dichlorine clathrate in its cubic structure I (sI) is analyzed by means of first-principles ab initio periodic calculations based on the density functional theory. We use functionals that are able to properly represent noncovalent interactions particularly halogen bonding. The structural characteristics of the crystal are very well reproduced. Compared with single cage calculations, one important effect of treating the whole crystal is a reduction of the regions with negative values of the electrostatic potential, which reflects in weaker inclusion energies for the crystal. Applying a variety of methods to characterize the electron density, we analyze the guest–host interactions in detail confirming the clear presence of halogen bonding in the dodecahedral cage. The case of the tetrakaidecahedral cage, the largest one, is more subtle, and we show that its quantitative treatment depends on allowing occupancy of several cages in the unit cell giving rise to a strengthening of the halogen bond through cooperative effects.