RESULTS AND DISCUSSIONS

Pyrolysis mechanisms of the CL-20 derivatives

CL-20 is a highly strained metastable compound and, therefore, it decomposes at high temperatures with a heat releasing. According to the previous studies, decomposition of CL-20 starts with the fission of nitro group that induces a skeleton destabilization and further decomposition.\cite{Isayev_2008} So, the NO2 fission determines the overall activation barrier.\cite{Isayev_2008} The N–NO2 bond dissociation is a barrierless process, and the corresponding bond dissociation energy needed for the NO2 fission is about 200 kJ/mol.\cite{Lin_lin_2018} In Ref. \cite{Tan_2014} the authors proposed a natural assumption that the same mechanism (NO2 or NF2 fission) is also relevant to the CL-20 derivatives. However, this assumption was not confirmed by our ab initio molecular dynamics simulations.
Observing the evolution of heated CL-20 derivatives, we found two competitive mechanisms of the initial decomposition step. The first mechanism is the NO2 or NF2 fission, and the second one is a migration of O or F atom with the formation of Si–O or Si–F covalent bond. Migration is possible for the Si-containing CL-20 derivatives because silicon possesses a larger covalent radius (1.11 Å) than the carbon (0.76 Å). Corresponding energetic diagrams are presented in Fig. 2. For the CSi5H6N12O12 system, the barrier for the NO2 fission is somewhat lower than that for the O migration. Therefore, the NO2 fission mechanism prevails at room temperature. However, at higher temperatures, one should consider thermal corrections. For example, at T = 3000 K (it is about the half of CL-20 explosive temperature) O migration mechanism becomes more feasible with regard to the lower Gibbs energy, as it is presented in Fig. 2a. Anyway, the energy barrier for the CSi5H6N12O12 decomposition is about 200 kJ/mol. This value is close to the corresponding value for the pristine CL-20.\cite{Lin_lin_2018} Therefore, it can be said that CSi5H6N12O12 is as kinetically stable as pristine CL-20.