Theoretical study on prismatic (N₆)_n (n=16-35) molecules

As a continuation of our study on the stability of pure nitrogen molecules, twenty members of the (N₆)_n (n = 16-35) molecular sequence with D_3h or D_3d symmetry alternatively are studied in this work. The structures and energies are examined at the B3LYP/cc‒pvDZ computational level. "Natural Bond Orbital (NBO)" and "Atom In Molecule (AIM)" analyses are performed to investigate the bonding properties and the electronic topologies of the prismatic molecules. The van der Waals forces or the dispersion interactions are identified to be the dominant stabilizing factor for large prismatic nitrogen cages. The van der Waals forces interweave with covalent bonds to form a dense network which tightly bind nitrogen atoms at grid points, and consequently, to make the prismatic (N₆)_n molecules stable. The mechanism of generating van der Waals forces in the prismatic molecules is the focus of this paper. Geometrically, all the polygons on the prismatic surface are boat hexagons except the two caps. The van der Waals forces or dispersion interactions occur only between the nitrogen atoms at positions 1',4' of the boat hexagon on the surface. The atom‒atom proximity effect of nitrogen atoms at the positions 1',4' leads to the van der Waals interaction. Topologically, van der Waals forces are generated by partial overlap of sp³ hybrid orbitals of lone pairs of nitrogen atoms at positions 1',4'. These large prism‒shaped nitrogen cages may also be novel beeline nanotubes which is environmentally friendly. This work is expected to open up a bright prospect of the study and applications of nitrogen nanofibers.