1,1-Diamino-2,2-dinitroethylene: The riddles of thermal decomposition and combustion

The thermal decomposition of deuterium derivatives α-FOX-7 and γ-FOX-7 have been studied using non-isothermal and isothermal kinetic methods. It was found that deuteration only affects the kinetics of the low-temperature decomposition step of α-FOX-7. A unique feature of the low-temperature stage of the decomposition of α-FOX-7 is that topochemical processes on crystal defects lead to intermolecular hydrogen transfer with the formation of its highly reactive aci-form. The decomposition of the aci-form of FOX-7 proceeds in an extremely unusual way via the molecular elimination of nitric acid and formation of nitrile N-oxide. The high-temperature decomposition stage involves the olefinic form of FOX-7 in a defect-free crystal lattice, and the decomposition is initiated by breaking the C–NO2 bond. In the gas phase, the FOX-7 molecule is also in its olefin form and exhibits high thermal stability. The combustion behavior and flame structure of α–FOX-7 and heat-treated substance (γ–FOX-7t) have been studied using a constant-pressure bomb and micro-thermocouple technique. Analysis of the thermocouple measurement data showed that the burning rate of both FOX-7 isomers is controlled by the rate of heat release in the condensed phase. This reaction is the decomposition of the substance in the melt. The higher burning rates of α-FOX-7 were due to the fact that the highly reactive aci-form of FOX-7 formed on the crystal defects remains in the melt and decomposes at the surface temperature. The break in the dependence of the burning rate on pressure observed for the α- and γ-polymorphs is associated with their complete isomerization into the internal aci-form of FOX-7 due to intramolecular hydrogen transfer. In this case, the surface temperature is determined by the vaporization of the internal aci-form of FOX-7, whose enthalpy of vaporization is higher than that of its olefin form.