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

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 H2O is detected by TGA in cases where hydrates also form. The crystal and molecular structures of Cs(II) and Rb3(III)(NCNNO2)·H2O were determined by X-ray diffraction. The latter compound is the first structural characterization of (III) and also contains the nitrocyanamide anion, NCNNO2−. The compound NCNNO2− appears to have formed by loss of HN3 from (II) or N3− 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NNO2 bond to the CNNO2 bond.