Density Functional Theory Studies on the Stability of Alkaline Metal Complexes of Pentazole and Oxopentazole Anions

We performed calculations on alkaline metal ion complexes of pentazole (N-5(-)) and oxopentazole anions ON5- using density functional theory methods to examine the stability of the complexes in the gas phase and in solution. The complexation energies and Gibbs energies were estimated, and the structures corresponding to local and global minima were also identified. In addition, the transition states along the decomposition pathways of the metal-complexed pentazole and oxopentazole were characterized to evaluate the kinetic stability of the complexes in the gas phase and in solution. The Gibbs energies for metal cation complexation were fairly negative in tetrahydrofuran (THF) solution. This reflected the good thermodynamic stability of the metal-complexed pentazole and oxopentazole. The kinetic stability of pentazole and oxopentazole was increased by several kilocalories per mole in the THF solution. Upon metal complexation, in the gas phase, the decomposition barrier of the pentazole was significantly reduced, while that of the oxopentazole was little changed. This showed that the metal cation preferentially bound to the oxygen atom rather than to one of the nitrogen atoms in the ring. In the THF solution, the decomposition barriers were rarely altered, when compared to the bare anions.