Protonated High Energy Density Materials: N₄ Tetrahedron and N₈ Octahedron

Abstract

Previous theoretical studies have demonstrated that tetrahedral N4 should be an extraordinarily effective high energy density material. But this species has thus far resisted laboratory efforts directed to its synthesis. Ab initio electronic structure methods have been used to examine the Td N4 and Oh N8 nitrogen clusters, including their protonated forms. We have optimized geometries using DZP, TZ2P, and TZ2P(f,d) basis sets with the Hartree−Fock self-consistent-field (SCF) method, second-order Møller−Plesset perturbation theory (MP2), single and double excitation configuration interaction (CISD), coupled-cluster (CCSD and CCSD(T)) methods, and three DFT/Hartree−Fock hybrid (B3LYP, B3P86, BHLYP) methods. Harmonic vibrational frequencies and infrared intensities have been obtained at the SCF, MP2, B3LYP, B3P86, and BHLYP levels of theory. The vertex protonated Oh N8 and Td N4 structures are found to represent minima on their respective potential energy surfaces. The bond protonated Td N4 molecule was determined to be a transition state leading to the vertex protonated Td N4 isomer. The predicted proton affinities of Td N4 and Oh N8 support the possibility that standard laboratory techniques for deprotonation may be used to yield these elusive high-energy density materials. The molecular properties determined at the DFT/Hartree−Fock hybrid level of theory are compared to large basis set coupled-cluster results and found to be in good agreement, even when relatively small basis sets are used for the former.