Does Confinement Force Marriage between Two Unwilling Partners?: A Case   Study of He2@BmNm (m=12, 16)
  TThe stability of Ngn@B12N12 and Ngn@B16N16 systems is assessed through a density functional study and ab initio simulation. Although they are found to be thermodynamically unstable with respect to the dissociation of individual Ng atoms and parent cage, ab initio simulation reveals that except Ne2@B12N12 they are kinetically stable to retain their structures intact throughout the simulation time (500 fs). The Ne2@B12N12 cage dissociates and the Ne atoms get separated as the simulation proceeds. The He-He unit undergoes translation, rotation and vibration inside the cavity of B12N12 and B16N16 cages. Electron density analysis (Atoms-in-Molecule (AIM)) shows that there is some degree of covalent character in He-He bond (Wc type) of He2@B12N12. In case of He2@B16N16, the He-He interaction is mostly of noncovalent type (Wn). In many cases, especially for the heavier Ng atoms the Ng-N/B bonds are found to be of covalent type or at least having some degree of covalent character. But Wiberg bond indices show zero bond order in He-He bond and very low bond order in cases of Ng-N/B bonds. Energy decomposition analysis (EDA) provides further insights into the bonding among the Ng atoms and the cage. The change in charge distribution, radius, hardness, electrophilicity and polarizability implies that they possess different properties and reactivity from their parent moieties. The HOMO energies of He2 units in He2@B12N12 and He2@B16N16 are significantly higher than that in free He atoms. Their kinetic stability and different calculated properties imply that the He-He interaction and Ng-N/B interaction may be considered as chemical bonds according to the IUPAC definition. 
Munmun Khatua, Sudip Pan and Pratim K. Chattaraj
physics.atm-clus