Thermal stability of energetic 6,8-dinitrotriazolo[1,5-a]pyridines: Interplay of thermal analysis and quantitative quantum chemical calculations

Abstract

Understanding the thermal stability of energetic materials is of great importance for storage and safety issues. In the present work, we report on the thermolysis kinetics and mechanism of the two fused energetic heterocycles, viz., 6,8-dinitrotriazolo[1,5-a]pyridine (1) and 2-amino-6,8-dinitro[1,2,4]triazolo[1,5-a]pyridine (2). The experimental kinetics obtained with the aid of advanced thermal analysis techniques was complemented by mechanistic insights from high-level quantum chemical calculations. More specifically, the simultaneous thermal analysis (STA) revealed that both compounds evaporate under atmospheric pressure. To shift the evaporation to higher temperatures and to study the sole decomposition kinetics, we employed the differential scanning calorimetry (DSC) under elevated external pressure (2.0 MPa). The analysis of DSC data was performed using the Kissinger, isoconversional, and model-fitting approaches. The thermolysis of 1 occurs in the melt and is described by the kinetic scheme comprised of two independent non-integer order autocatalytic reactions with the kinetic parameters Ea1=124.2±0.5 kJ mol−1, log(A1/s−1)=9.34±0.04 and Ea2=80.8±0.7 kJ mol−1, log(A1/s−1)= 4.86±0.07. At the same time, the obtained kinetic parameters for 2 are too large to be realistic, which is a consequence of the overlap of melting and decomposition processes near the melting point. To get a deeper insight into the decomposition mechanism, the experimental data were complemented by theoretical thermolysis pathways with the activation barriers calculated using the domain-based local pair natural orbital (DLPNO) modifications of coupled cluster techniques. The same primary decomposition channels, viz., the nitro-nitrite isomerization and CNO2 bond cleavage turned out to compete in the thermolysis of 1 and 2. Due to a lower activation barrier (∼240 kJ mol−1), the former reaction path dominates at lower temperatures, whereas in the experimental temperature ranges (>500 K) the CNO2 bond cleavage with a higher preexponential factor is the fastest elementary reaction (the activation barrier ∼280 kJ mol−1). The obtained experimental and theoretical Arrhenius parameters exhibit a kinetic compensation effect, that is, the rate constant values in solid, melt, and gas phases are close to each other.