Molecular beams unimolecular 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. 

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. 

The important role of both intramolecular and intermolecular energy transfer in gas-phase chemical reactions has been stressed in the discussions of unimolecular decompositions and molecular beam studies. In this chapter, intermolecular energy transfer and energy partitioning in chemical reactions is considered more explicitly. The dissociation of homonuclear molecules is discussed since the details of the energy distribution appear to play a major role in the reaction mechanism. The reaction... 

We shall now first consider (for reactions similar to those mentioned above) molecular beam, chemiluminescence, and related techniques, in which the metastable molecules may be considered to be essentially isolated during their lifetime, and which allow for a detailed determination of the product quantum states after decomposition. We shall then shortly review bulk investigations on unimolecular systems with chranical activation and photoactivation, which continue to provide a wealth of information. 

A. General Remarks on Unimolecular Decompositions of Chemically Activated Radicals. A major portion of the molecular beam studies of fluorine atom chemistry (12-19,37) has been concerned with a class of reactions characterized by the formation of a transient species from bimolecular association of the reactants and whose lifetime is long compared to its rotational or vibrational periods. The formation of such a long lived complex implies that the reactants experience a net attraction and consequently the potential energy surface for the reaction possesses a deep well of course, the total energy of the system is greater than that required to dissociate the intermediate, either to reactants or products, so that in the absence of a third body or relatively improbable photon emission (radiative lifetime > 10 sec), the intermediate must decay prior to detection. Under certain favorable conditions where a large... 

Information of a different sort is obtained in a molecular beam experiment, although the means for producing the species undergoing unimolecular decomposition is also chemical activation. Whereas the conventional kinetic studies yield reaction rates for direct comparison with RRKM lifetimes, the beam technique yields product recoil energy distribution which, in principle, contain information regarding exit channel dynamics specifically ignored in RRKM. Comparison of experimental results with RRKM theory is indirect, requiring additional assumptions whose validity must be determined. Fortunately, however, statistical theories of a different sort exist which base their predictions on asymptotic (and therefore measureable) properties of the... 

---