IS N8 CUBANE STABLE

The question of N8 cubane's stability is addressed via ab initio calculations. An Oh symmetry stationary point is found on the energy hypersurface for all three levels of theory used; these were restricted Hartree-Fock (RHF) self-consistent-field theory using STO-3G, 4-31G, and 4-31G* basis sets. Vibrational frequency calculations, at the same three levels of theory, all indicate that the cubic structure is stable. The effect of correlation on the stable structure energies is examined with Møller-Plesset perturbation theory up to fourth order; these post-Hartree-Fock calculations were performed at the RHF optimized geometries. The calculations indicate that, if it could be synthesized, N8 cubane would be a highly energetic material, metastable to dissociation to four N2 molecules; the energy release for this reaction is predicted to be 530 ± 50 kcal/mol. The energy barrier to dissociation is estimated via reaction coordinate calculations. The [4+4+4+4] symmetry-forbidden N8 → 4N2 concerted reaction has an activation barrier of about 162 kcal/mol at the RHF/4-31G* level. This dissociation could also take place as a sequence of four [2+2] symmetry-forbidden reactions. Each of these [2+2] reactions would probably have a reaction barrier of roughly 40 kcal/mol - this suggests that N8 cubane may be a reasonably stable structure. Estimates are given of the mass density of condensed-phase N8, along with predicted values of its Chapman-Jouguet detonation velocity and pressure. It appears that condensed-phase N8 would be a very powerful chemical explosive. © 1990 American Chemical Society.