Crystal structure and thermal behaviors of the tetrapotassium salt of octahydroimidazo-[4,5-d]imidazol-1,3,4,6-tetrasulfonic acid (TACOS-K)

Abstract

The tetrahydrate of the tetrapotassium salt of octahydroimidazo-[4,5-d]imidazol-1,3,4,6-tetrasulfonic acid (TACOS-K), an new energetic material, was investigated in terms of its crystal structure, heat of combustion, thermal stability and decomposition kinetics. Its heat of combustion is -5487±96 J.g-1 from which a heat of formation of -3150 kJ.mol-1 was estimated. It has been found that this compound has a hexagonal crystal space group with a density of 2.026 g.mol-1/150 K. The organic anionic skeleton of the TACOS-K molecule is distinctly deformed. Six tetraanions interconnected by a coordination to the potassium cations and hydrogen bridges to water molecules form hydrophilic as well as hydrophobic cavities. As an intermediate to synthesize cis-1,3,4,6-tetranitro-octahydroimidazo-[4,5-d]imidazole (BCHMX), it decomposes at around 31°C with a first peak temperature range of 46.6-94.3 °C due to loss of water, depending on the heating rates. Hydrolysis of the N-S bonds might play an important role here. Crystalline water evaporation competes with this hydrolysis. TACOS-K has a residual mass of about 46% at 2°C.min-1, which increases with the heating rate. Peak of the exothermic process occurs at 235.6 °C at the same heating rate with an enthalpy change of 164 J.g-1. Dehydration occurs with an energy barrier of 36.7 kJ mol-1 followed by a shoulder mass loss process with a much higher activation energy 110.6 kJ mol-1, while the activation energy for the main exothermic reaction is about 136.2 kJ mol-1.