RDX Initial Decomposition Reactions

The energetic molecule that has received the most attention, due in large part to its practical importance (it certainly does not present easy kinetics problems ), is RDX. In spite of all the interest and attention it has received from both experimentalists and theorists, there is still uncertainty about the initial decomposition steps of RDX. 

A brief sketch of what we know about the kinetics of RDX decomposition is needed for context for the discussion of the simulation studies. In most experiments the data are taken with little experimental control, with the observations complicated by the rapid release of large amounts of energy, and with the liquid or solid undergoing phase transitions and chemical reactions to form small gaseous molecules such as N2O, H20, H2CO, HCN, NO, N02, CO, and C02. The reaction mechanisms involve many sequential, branched pathways, which are strongly dependent on the experimental conditions. It is not our purpose here to try to sort out the mechanisms for the various conditions, but we do need, for foundation, to discuss the experimental observations relevant to the elementary gas-phase reactions. 

The first experimental determination of the activation energy Ea was done in 1949 by Robertson [24], His studies of liquid RDX yielded a value of 47.5 kcal/mol for Ea. Subsequent experiments seem to confirm that the Ea value is in the range 47 to 48 kcal/mol. Cosgrove and Owen [25] concluded that the decomposition occurs in the vapor phase based on experiments carried out near the melting point. In 1969 Rauch and Fanelli [26] reported that the liquid- and gas-phase reactions yield different products. They found that the gas-phase reaction produced N02 and the liquid-phase reaction N20 and C02. It has been widely accepted that N-N bond fission to yield N02 is the initial step in the decomposition. However, it has been difficult to firmly establish that as fact because of the difficulties in making kinetics measurements and the unreliability of ab initio predictions for molecules of this size. 

In modeling studies of the decomposition it is usually assumed that RDX enters the vapor phase before dissociating [27-31]. These models usually assume a global reaction for the decomposition and thus are not dependent upon the details of the dissociation mechanism. Presumably the availability of such a mechanism could lead to refinement in the modeling. 

Recently, Long et al. [32] reported the results of a series of experiments in which they studied RDX decomposition in open and closed ( pierced ) containers to monitor the kinetics as a function of the extent of reaction. Heating RDX in a closed container causes decomposition to occur in the liquid phase for which they found Ea 47.8 kcal/mol, in accord with the accepted value for N-N bond fission. They also determined Ea 23.9 kcal/mol for evaporation, a value well below the energy for most chemical decomposition reactions. 

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