The Raman and IR vibration modes of metal pentazolate hydrates [Na(H₂O)(N₅)]·2H₂O and [Mg(H₂O)₆(N₅)₂]·4H₂O

The detailed illustrations of the structures, elastic properties, and Raman and IR vibration modes for [Na(H₂O)(N₅)]·2H₂O (a) and [Mg(H₂O)₆(N₅)₂]·4H₂O (b) have been presented in this investigation by using the first-principles method based on the density functional theory. Our results indicate that the active centers of both two types of the energetic metal pentazolate hydrates appear on the cyclo-N₅. The bonding character of N atoms in the cyclo-N₅ is shown to be covalent, and the cyclo-N₅ ring can be considered as an anion. Based on the analysis of elastic properties, we conclude that complex a is easier to deform than b, and both complexes are mechanically stable. From the calculated Raman and IR vibration modes, the vibration in the region of 960–1206 cm⁻¹ (for a) and 985–1208 cm⁻¹ (for b) is determined by basically mixing the cyclo-N₅ stretching and deformation modes. The vibrational modes of a and b in their highest frequency zones are both related to the stretching of the O–H bonds.