| Abstract
| - Previous theoretical studies have demonstrated that tetrahedralN4 should be an extraordinarily effective highenergy density material. But this species has thus far resistedlaboratory efforts directed to its synthesis.Abinitio electronic structure methods have been used to examine theTd N4 and OhN8 nitrogen clusters, includingtheir protonated forms. We have optimized geometries using DZP,TZ2P, and TZ2P(f,d) basis sets with theHartree−Fock self-consistent-field (SCF) method, second-orderMøller−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 frequenciesand infrared intensities have been obtained at the SCF, MP2, B3LYP,B3P86, and BHLYP levels of theory.The vertex protonated Oh N8 andTd N4 structures are found torepresent minima on their respective potentialenergy surfaces. The bond protonated TdN4 molecule was determined to be a transition state leadingto thevertex protonated Td N4 isomer.The predicted proton affinities of TdN4 and Oh N8 support thepossibilitythat standard laboratory techniques for deprotonation may be used toyield these elusive high-energy densitymaterials. The molecular properties determined at theDFT/Hartree−Fock hybrid level of theory are comparedto large basis set coupled-cluster results and found to be in goodagreement, even when relatively small basissets are used for the former.
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