(716h) Mechanism and Kinetics of Thermal Decomposition of Cl-20:

CL-20 (2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane) is a high-energy nitramine explosive. To improve atomistic understanding of its thermal decomposition in gas and solid phases, we performed a series of ab initio molecular dynamics simulations.

Unlike the case of the other nitramines (e.g. RDX/HMX), we found only one distinct initial reaction channel (homolysis of N–NO2 bond) during unimolecular decomposition. We did not observe any HONO elimination reaction under investigated conditions. However, we did find ring breaking followed by NO2 fission. Therefore, in spite of limited sampling that provided a mostly qualitative picture, we proposed a scheme of unimolecular decomposition of CL-20. The averaged products population over all trajectories was estimated at 4 HCN, 2-4 NO2, 2-4 NO, ~1 CO, and ~1 OH molecules per one CL-20.

These simulations provide a detailed description of the chemical processes in the initial stages of thermal decomposition of condensed CL-20. We elucidated key features of this process: composition of primary reaction products, reaction timing, Arrhenius behavior of the system, etc. They clearly indicate that the primary reactions leading to NO2, NO, N2O and N2 occur at very early stages. We also estimated the activation barrier for the formation of NO2 which essentially determines overall decomposition kinetics and effective rate constants for NO2 and N2. Calculated solid phase decomposition pathways correlate with available condensed phase experimental data. Unfortunately, a comparison of the predicted gas phase mechanism was not possible.