Methylene nitramine decomposition

It is likely that theoretical methods, both ab initio and MD simulations, will be needed to resolve the complicated chemical decomposition of energetic materials. There are species and steps in the branching, sequential reactions that cannot be studied by extant experimental techniques. Even when experiments can provide some information it is often inferred or incomplete. The fate of methylene nitramine, a primary product observed by Zhao et al. [33] in their IRMPD/molecular beam experiments on RDX, is a prime example. Rice et al. [99, 100] performed extensive classical dynamics simulations of the unimolecular decomposition of methylene nitramine in an effort to help clarify its role in the mechanism for the gas-phase decomposition of RDX. 

Zhao et al. [33] detected a product with mass 74 amu, which they identified as CH2NN02. There had been earlier predictions that it is formed in the dissociation of cyclic nitramines [101, 102], but this was the best 

Mowrey et al. [103] performed ab initio calculations to determine the structures, energies, and frequencies at the critical point of the possible dissociation pathways for methylene nitramine. There are three possible routes for H2CNN02 decomposition. The calculations predict that simple bond fission 

Rice et al. [99] developed a global potential energy surface based on the Mowrey et al. [103] results and performed extensive classical trajectory calculations to study the dynamics of the CH2NN02 dissociation reactions. They calculated rates for reactions (III) and (IV) with classical barriers of 35 and 37 kcal/mol, respectively. They found that N-N bond fission dominates at low energy but that HONO elimination is competitive. Chakraborty and Lin [104] predict the opposite on the basis of their ab initio barriers and RRKM theory calculations. The two dissociations channels are closely competitive and it is not clear that ab initio methods are sufficiently reliable to distinguish between two reactions that have such similar energy requirements. Also, the Zhao et al. results [33] are not in accord with the theoretical predictions. 

A strong motivation for the Rice et al. [99] simulations was to try to interpret the Zhao et al. [33] observations that the HONO elimination channel dominates while the N-N bond rupture reaction does not occur. A possible explanation is that the nascent CH2NN02 product of the RDX ring fission reaction is highly excited and has a nonstatistical distribution of energy. Sewell and Thompson [35] estimated that it may be formed with 55 to 65 kcal/mol of energy, which is well in excess of the predicted energy 

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A reaction-path based method is described to obtain information from ab initio quantum chemistry calculations about the dynamics of energy disposal in exothermic unimolecular reactions important in the initiation of detonation in energetic materials. Such detailed information at the microscopic level may be used directly or as input for molecular dynamics simulations to gain insight relevant for the macroscopic processes. The semiclassical method, whieh uses potential energy surface information in the broad vicinity of the steepest descent reaction path, treats a reaction coordinate classically and the vibrational motions perpendicular to the reaction path quantum mechanically. Solution of the time-dependent Schroedinger equation leads to detailed predictions about the energy disposal in exothermic chemical reactions. The method is described and applied to the unimolecular decomposition of methylene nitramine. 

As an illustrative example of the semiclassical reaction path dynamics method, we discuss the HONO elimination pathway for the unimolecular decomposition of methylene nitramine,... 

The conclusion drawn that the first two steps in the decomposition of TNAZ involve NO2 loss agrees with the observation by Brill and coworkers [5] that gaseous NO2 was the most abundant species in the initial phases of the thermal decomposition of bulk TNAZ. That the NO2 concentration decreases from its initially observed level in the bulk study is evidence that this species is already undergoing significant secondary reactions at the time of its initial appearance yet the surmisal that the NO2 is a primary product is correct. Additionally, the observation that no methylene nitramine formation occurs agrees with the same conclusion drawn from the bulk study where the N2O/H2CO pair was not present. However, the absence of NO as an initial product in the molecular beam experiment, shows that the NO observed in the bulk decomposition study is not due to gas phase unimolecular nitro-nitrite isomerization followed by NO loss. 

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