``N8: A Structure Analogous to Pentalene, and Other High Energy Density Minima,'' M. L. Leininger, C. D. Sherrill and H. F. Schaefer, J. Phys. Chem. 99, 2324-2328 (1995).

Ab initio molecular electronic structure methods have been used to examine three isomers of N8. In addition to the previously-studied octaazacubane structure (1), we have investigated a D2d structure (2) analogous 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 (MP2), and with single and double excitation configuration interaction (CISD) and coupled-cluster (CCSD) methods. Harmonic vibrational frequencies and infrared intensities have been obtained at the SCF and MP2 levels of theory. Although cubane and cyclooctatetraene are known experimentally, unsubstituted pentalene has never been synthesized; nevertheless, our vibrational analysis indicates that all three nitrogen analogues, including the pentalene analogue (3) represent potential energy minima. The metastability of (3) is attributed to two additional pi electrons present in (3) but not in pentalene, according to the rule of topological charge stabilization. Further, since all three minima are very high-lying with respect to 4 N2, we have examined their potential application as high energy density materials (HEDM's). Our highest-quality results place the pentalene-like structure (3) approximately 225 kcal/mol above 4 N2, 198 kcal/mol below 1, and 35 kcal/mol below (2).