Stabilization of Cylindrical N12 and N18 by Phosphorus Substitution

Molecules consisting entirely or predominantly of nitrogen are the subject of much research for their potential as high energy density materials (HEDM). The problem with many such HEDM candidates is their instability with respect to dissociation. For example, a low-energy dissociation path has been shown for a cylindrical cage isomer of N12. The instability is at least partially due to the ease of ring opening at triangles on either end of the molecule. In the current study, nitrogen cage molecules are examined to determine the stabilizing effect of substituting the triangle nitrogens with an element that more naturally forms triangles, namely phosphorus, which is valence isoelectronic with nitrogen. The cylindrical N12, and a larger analogue N18, form the structural basis for cage molecules of N6P6 and N12P6. Theoretical calculations using Hartree-Fock theory and perturbation theory (MP2 and MP4), along with the correlation-consistent basis sets of Dunning, have been carried out to determine dissociation energies along various pathways. The energies are discussed in terms of low-energy dissociation and the ability of the molecules to resist dissociation.