Nitroderivatives of N-pyrazolyltetrazoles: Thermal decomposition and combustion

Thermal decomposition and combustion of a series of novel promising nitro and amino substituted energetic pyrazolyltetrazoles were studied using a number of complementary experimental (thermogravimetry, differential scanning calorimetry, manometry, microthermocouple measurements in a combustion wave, and HPLC) and theoretical (quantum chemical calculations) techniques. According to the DSC and manometry data, the N-(pyrazolyl)tetrazoles turned out to be less thermally stable than the C-(pyrazolyl)tetrazole: the activation energies of their melt-phase decomposition are 117–127 and 140 kJ mol⁻¹, respectively. The kinetics of decomposition in the solid phase is more complicated due to a strong influence of the effect of sub-melting and possible azido-tetrazole isomerization. The reliable CCSD(T)-F12 quantum chemical calculations provided mechanistic insight into the thermolysis of the compounds studied. All species decompose via the ring-opening reaction yielding a transient azide intermediate followed by the N₂ elimination. The effective activation barriers of the latter reactions are in reasonable agreement with the melt-phase experiments for N-pyrazolyltetrazoles. On the basis of the HPLC detected products, we also outlined the reactions leading to the stable decomposition products observed. All compounds studied showed a self-sustained fast-burning character. Their typical burning rates are 70−80 mm s⁻¹ at 10 MPa, which is as much as two times higher than the burning rate of a powerful explosive CL-20 under the same conditions. Using the pressure dependence of the surface temperature, we also determined the vapor pressures of (nitropyrazolyl)tetrazoles studied. The boiling points of the latter species are higher than the boiling point of hexogen. The melt-phase kinetic parameters also agree well with the condensed-phase-controlled burning rate model. The low activation energies of the leading combustion reactions are responsible for low pressure exponents of these compounds varying in the range of 0.5 – 0.75.