Quantum mechanics investigation of initial reaction pathways and early ring-opening reactions in thermal decomposition of liquid-phase RDX

Cyclotrimethylene trinitramine (RDX) is a commonly used ingredient in solid propellants and explosives. As a result, RDX has been the subject of many experimental and theoretical investigations to elucidate its liquid-phase and gas-phase decomposition. In our experimental effort involving Fourier Transform Infrared (FTIR) spectroscopy and time-of-flight mass spectrometry (ToFMS) of fast thermolysis of RDX, the results indicate that ring-opening occurs very early due to the presence of carbon-containing species among the detected gas-phase species, such as HCN, H₂CO, CO and CO₂. Two existing pathways – 1) HONO elimination and 2) N[sbnd]NO₂homolysis – are re-examined for liquid-phase decomposition and early ring-opening reactions have been identified after a lengthy search using quantum mechanics. Three additional pathways – 1) reaction with NO and formation of ONDNTA, 2) prompt oxidation via HONO and ONNO₂addition, and 3) hydrogen abstraction via NO₂– are also identified along with early ring-opening reactions in each pathway. The quantum mechanics investigation is based on using density functional theory (DFT) at the B3LYP/6-311++G(d,p) level. The liquid-phase studies use the Conductor-like Polarizable Continuum Model (CPCM) for solvation with water as solvent within the Gaussian program package. Intrinsic reaction coordinate calculations have also been performed to verify that the reactants indeed are connected to the expected products. Results explain the mechanistic details of the formation of early carbon containing species, simultaneous formation of formaldehyde and N₂O, as well as the formation of minor species such as HNCO and HOCN. Proposed reactions can account for the experimentally observed autocatalytic behavior and can assist in the development of a detailed chemical kinetics mechanism of nitramine propellants containing RDX and possibly HMX.