N-8 - A STRUCTURE ANALOGOUS TO PENTALENE, AND OTHER HIGH-ENERGY DENSITY MINIMA

Ab initio molecular electronic structure methods have been used to exa mine three isomers of Ns. In addition to the previously studied octaaz acubane structure (1), we have investigated a D-2d structure (2) analo gous to cyclooctatetraene, and a planar bicyclic form (3) analogous to pentalene. Using a double-zeta; plus polarization (DZP) basis set, we have optimized geometries with the Hartree-Fock self-consistent-field (SCF) method, with second-order Moller-Plesset perturbation theory (M P2), and with single and double excitation configuration interaction ( CISD) and coupled-cluster (CCSD) methods. Harmonic vibrational frequen cies and infrared intensities have been obtained at the SCF and MP2 le vels of theory. Although cubane and cyclooctatetraene are known experi mentally, unsubstituted pentalene has never been synthesized; neverthe less, our vibrational analysis indicates that all three nitrogen analo gues, including the pentalene analogue 3, represent potential energy m inima. The metastability of 3 is attributed to two additional pi elect rons present in 3 but not in pentalene, according to the rule of topol ogical charge stabilization. Further, since all three minima are very high-lying with respect to 4N(2), we have examined their potential app lication as high energy density materials (HEDMs). Our highest-quality results place the pentalene-like structure 3 approximately 225 kcal/m ol above 4N(2), 198 kcal/mol below 1, and 35 kcal/mol below 2.