Thermal decomposition of energetic materials 79: Thermal, vibrational, and X-ray structural characterization of metal salts of mono- and di-anionic 5-nitraminotetrazole

Abstract

The energetic alkali metal salts (Li +, Na +, K +, Rb +, Cs +) of the 5-nitraminotetrazole mono-anion(II) and di-anion(III) were synthesized and their properties compared with neutral 5-nitraminotetrazole(I). The decomposition temperatures by DSC follow the order (III)>(II)>(I), and for the salts of (III) Li +>Na +>K +>Rb +>Cs +. Both thermal stability trends are consistent with a major role played by the crystal lattice energy. Loss of H 2O is detected by TGA in cases where hydrates also form. The crystal and molecular structures of Cs(II) and Rb 3(III)(NCNNO 2)·H 2O were determined by X-ray diffraction. The latter compound is the first structural characterization of (III) and also contains the nitrocyanamide anion, NCNNO 2 −. The compound NCNNO 2 − appears to have formed by loss of HN 3 from (II) or N 3 − from (III) during one of the synthesis attempts of Rb(II). The major atom motions in the IR spectra of (I), (II), and (III) were assigned on the basis of density functional theory (DFT) using the B3LYP method. Both the IR spectral and crystal structure trends show that increasing the negative charge on the anion primarily shifts the partial double bond character from the CNNO 2 bond to the CNNO 2 bond.