Other energetic nitramines

Nitramine-nitrates of general structure (133) are known as NENAs and are conveniently prepared from the nitrative cleavage of A-alkylaziridines with dinitrogen pentoxide or from the direct nitration of the corresponding aminoalcohols. These compounds find use as energetic plastisizers in explosive and propellant formulations Bu-NENA (R = n-Bu) is a component of some LOVA (low vulnerability ammunition) propellants.  

Some compounds of general structures (137) and (138) have hydroxy or carboxy termini, making them potential monomers for the synthesis of energetic polymers (binders) and plasticizers for both explosive and propellant formulations.  

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In contrast with the AB system described above, RDX and most other energetic materials have long reaction times—fractions of a microsecond—and extended reaction zone lengths, on the order of a millimeter. Due to the size of the reaction zone and the complexity of the interatomic potentials necessary to describe real nitramines, steady-state NEMD simulations of detonation are beyond current and near-fixture capabilities, both in computation time and computer memory requirements. Keeping these limitations in mind, we use NEMD to study the initial chemical events in RDX under shock loading. In Section 5 we will describe equilibrium MD simulations to study phenomena at longer time-scales. 

A relatively broad variety of aquatic toxicity studies exists for nitro-substituted phenol, toluene, and benzene explosives and related compounds, but very little toxicological information is available for tetryl, cyclic nitramines, and the other energetic compounds discussed in this chapter. Several explosives, such as tetryl, are no longer manufactured and are, therefore, of diminishing environmental concern, although their persistence and the nature, stability, and toxicity of their breakdown products is not understood in sufficient detail and should be further investigated. A variety of other energetic compounds, for example, perchlorates, are used in military operations, and due to environmental concerns with their release, additional studies on their fate and effects in aquatic systems are recommended. 

Primary nitroaromatics are TNT the environmental breakdown products, including 1,3,5-trinitrobenzene (TNB), 1,3-dinitrobenzene (DNB), 2,4-and 2,6-dinitro-toluene (DNTs) and the primary reduction products 2-amino 4,6-dinitrotoluene and 4-amino 2,6-dinitrotoluene (ADNTs). Nitramines include RDX and octahydro-l,3,5,7-tetranitro-l,3,5,7-tetrazocine (HMX). Additional energetic compounds discussed in this chapter include nitroglycerin, white phosphorus, and ammonium perchlorate. Other energetic compounds are not discussed due to a lack of information regarding toxicity to wildlife species. 

Other properties of l,3-dinitro-l,3-diazacyclobutane (34) have been predicted in a series of publications by Zeman et al. [48-52]. By using data collected for a wide range of energetic nitramines and other polynitro com-... 

The replacement of amine and amide hydrogen with a nitro group via direct nitration is an important route to A-nitro functionality. However, the cleavage of other bonds is also important. In the case of C-N bond cleavage the process is known as nitrolysis and is an invaluable route to many energetic materials (Section 5.6). The nitrolysis of hexamine and the syntheses of the important explosives HMX and RDX are discussed in Section 5.15. This area of chemistry could easily demand a separate chapter of its own and is the most complex and diverse in the field of nitramine chemistry. 

Primary nitramines have acidic protons and are able to undergo condensation reactions to form functionalized nitramines. These reactions are discussed in Section 5.13 because the products have potential application as energetic polymer precursors or find use for the synthesis of other explosives. 

Primary nitramines contain an acidic proton which enables them to behave as nucleophiles and undergo addition and condensation reactions. These reactions are extremely useful in two respects. Firstly, these reactions convert primary nitramino functionality into secondary nitramino functionality, which is no longer acidic and much more chemically stable. Secondly, these addition and condensation reactions can be used to prepare functionalized derivatives of polynitramines which can be used to synthesize energetic polymers and other explosive compounds. 

In spite of the many years of research, there are limited possibilities to realize a substantial increase in performance from conventional C-H-N-0 explosives. Recent advances in energetics energy output have come in improved processing or inclusion of energetic binders to increase overall formulation energy, but limited success has been realized in the development of novel energetics. One reason for this is that conventional nitramine and nitroaromatic explosives such as TNT, RDX, HMX and other similar molecules share the same three limitations (Table 9.11) ... 

Despite this progress several areas require further development. For example, we have evidenced that only a limited number of force fields are presently available for treatment of ionic salts, It will be very beneficial that this gap will be filled and general, transferable sets of force fields for different classes of ionic systems will be available as is the case with other classes of energetic materials such as nitramines systems. We have also pointed out in this chapter that current classical force fields developed for ionic crystals are limited to description of nonreactive processes. Development of reactive force fields such as reactive empirical bond order potentials for the case of ionic systems will represent a major forward step for simulation of reactions and of combustion and denotation processes. 

Figure 6. Energetics of the reaction pathways for the conversion of nitramine, NH2NO2, to N2O and H2O. As a unimolecular four-centered process with no water molecule present (n = 0), the activation energies are high. On the other hand, as a six-centered bimolecular process involving another water molecule (n = 1), the activation energies are greatly lowered. Energies are in kcal-mol l.

In an analogous manner to the development of decomposition pathways for nitramines, estimates can be made for the decomposition pathways of other classes of energetic materials. For instance, nitrate esters will first break off an NO2 group. From Tables IV and V, one concludes that the resulting alkoxy radical will readily decompose to form an aldehyde, with the subsequent radical eliminating a nitro group, forming another aldehyde. 

Methyl nitrate and methyl nitramine have been chosen as models for two other important classes of energetic materials, nitrate esters and nitramines. The X-NO2 bond dissociation energy for the nitrate was calculated to be 38 kcal/mol and that of the nitramine 47 kcal/mol, consistent with the well known trends in stability and sensitivity for nitro, nitrate and nitramine compounds. Assuming that X-NO2 bond scission is rate determining, then neither nitrate nor nitramine decomposition is affected by the presence of ammonia. 

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