Detonation Chemistry of High Explosives (HMX and RDX)
Debashis Chakraborty, Richard P. Muller, Siddharth Dasgupta, and William A. Goddard III*
Materials and Process Simulation Center, Beckman Institute (139-74)
Division of Chemistry and Chemical Engineering
California Institute of Technology, Pasadena, California 91125
The cyclic nitramines, hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX), and octahydro-1,3,5,7- tetranitro-1,3,5,7-tetrazocine (HMX), are important energetic ingredients, used in applications ranging from automobile air bags to rocket propellants and explosives, since they release a large amount of energy in bulk decomposition. Thermal decomposition of these energetic materials has been observed to form very simple final product molecules, such as, HCN, NO, N2O, NO2, CO, CO2, H2O, H2CO, etc. Understanding the underlying complex chemical processes is essential to obtain to an improved model for combustion or detonation of these energetic materials.
A number of experimental studies have been directed toward elucidating the mechanistic details of the thermal decomposition of RDX, and HMX and various plausible reaction pathways have been proposed. Most of these experiments dealt with bulk phase materials, including decomposition in the condensed phase (solid or liquid) and the gas phase flame structure near the burning surface. In order to determine the initial steps of decomposition for these condensed phase studies, we focused here on the mechanistic details of unimolecular decomposition of these energetic materials in gas phase using first principles quantum mechanics (ab initio density functional theory (DFT) with a modest 6-31G(d) basis set).
For the unimolecular decomposition of RDX and HMX several important decomposition channels have been identified. The final decomposition products identified in the present mechanism agree reasonably well with the observed mass fragments in the condensed phase and gas phase decomposition experiments. In addition, the RDX decomposition mechanism could semi-quantitatively account for majority of the mass fragments observed in the laser photolysis of isolated RDX in a molecular beam. Furthermore, a unified decomposition mechanism for both the nitramines has been proposed. Computed thermochemical quantities (Cv, H, S etc.) and rate constants for all the individual reaction in the unified mechanism were then used in the detailed modeling of nitramine detonation.
Figure 1. Unified decomposition mechanism of RDX and HMX