Liquid-phase decomposition of RDX: Formation of Oxy-s-triazine and 1,3,4-oxadiazole

Cyclotrimethylene trinitramine (RDX) is widely used as an ingredient in solid propellants and explosives. As a result, RDX has been investigated extensively in many experimental and theoretical studies to elucidate its liquid-phase and gas-phase decomposition. The decomposition of liquid-phase RDX was studied using Fourier transform infrared (FTIR) spectroscopy of evolved gas-phase species from rapid thermolysis and of the condensate and residue formed from the decomposition, as well as time-of-flight mass spectrometry (ToFMS) of the evolved gas-phase species. Sub-milligram sample of RDX was heated at rates of about 2000K/s to a set temperature where decomposition occurred under isothermal conditions. Since RDX melts at around 205°C, decomposition studies were conducted at temperatures ranging from 265 to 305°C. Oxy-s-triazine (C₃N₃H₃O, m/z=97) and (1,3,4-oxadiazole (C₂N₂H₂O, m/z=70) were two of the compounds detected in the thermolysis tests. Chemical reaction mechanisms explaining the formation of oxy-s-triazine (OST) and 1,3,4-oxadiazole were formulated using quantum mechanics calculations guided by the acquired experimental data and thermochemical analysis. The quantum mechanics investigation is based on using density functional theory (DFT) at the B3LYP/6-311++G(d,p) level. The liquid-phase solvation effect is accounted via the Conductor-like Polarizable Continuum Model (CPCM) with water as solvent within the Gaussian program package.