Ab Initio Calculations on Al₂N₄ and AlN_n (n = 4 to 7):  Potential Precursors of High Energy Density Materials

Geometry optimization and harmonic vibrational frequency calculations have been carried out on various structures and low-lying, high- and low-spin electronic states of Al2N4 and AlNn clusters, where n = 4 to 7, at the B3LYP, MP2, and QCISD levels. The aim of these calculations was to search for states/structures that may be suitable candidates as precursors of high energy density materials. Well-bound charge-transfer states/structures with activated NN bonds were obtained. The exothermicities of the decomposition reactions of these states/structures to N2 molecules were computed at up to the RCCSD(T)/aug-cc-pVQZ(no g) level of theory. The most exothermic decomposition reaction considered is AlN6 Cs Al·NN2N2N (N6 ring), 4A‘ ‘, (a‘)1(a‘)1(a‘ ‘)1 → Al + 3N2. The calculated ΔH298 K is −226 kcal/mol, giving an energy release of over 75 kcal/mol per N2 molecule. We conclude that AlNn systems are potential precursors of high energy density materials. In addition, the HOMOs of these states/structures have been e...