Molecular electrostatic potentials and electron densities in nitroazacubanes

Successive introduction of nitrogen atoms in the cubyl corners instead of C-NO2 groups of octanitrocubane (CNO2)8, the most powerful explosives known to date, leads to a class of energy-rich compounds known as nitroazacubanes. In present work the ab initio Hartree-Fock and hybrid density functional calculations have been carried out on the possible conformers of (CNO2)(8-alpha)Nalpha (with alpha=0-8), nitroazacubanes. The charge distributions in these systems have been derived using the topography of the molecular electrostatic potential and electron density. Molecular electrostatic potential investigations reveal that of different nitroazacubane conformers, the electron-rich regions around nitro oxygens of the lowest energy conformer having face opposite nitrogen atoms within a cube are more delocalized. These conformers are predicted to have the largest difference of the energies of the highest occupied molecular orbital and lowest unoccupied molecular orbital relative to the other conformers. The dipole moments of nitroazacubanes are dependent on the nitrogen sites within a cube, caused by the resultant of C-N bond moments and nearly insensitive to position of the NO2 groups. The lowest frequency vibration (522 cm(-1)) suggests octa-azacubane having robust structure in the nitroazacubane series. Substitution of nitrogen atom instead of C-NO2 group leads to increase in electron density at the bond critical point of the X-N (X=C or N) bonds in a cube. The heats of formation of different nitroazacubanes were calculated by using the isodesmic reaction approach. The present calculation has shown that for the di- though hexanitroazacubanes the most destabilized conformer possess largest dipole moment and the heat of formation as well. A linear correlation of the electron density at the bond critical point of X-N bonds and the heat of formation has been obtained.