To avoid repetitions, we will include only two mechanisms that are most important for the initiation stage Equations 1 and 2. Reaction rates are displayed for the decomposition of PETN a in the gas phase and b on the surface. Extrapolating these findings to other nitrate esters, we can draw an important conclusion that the earliest stages of the initiation chemistry are defined by the relationship of the fast endothermic NO 2 loss reaction and the slow exothermic HONO elimination reaction. Notably, the HONO elimination exhibits the similar barriers in the molecule and the surfaces, meaning that this is an essentially intramolecular dissociation mechanism, which is barely affected by intermolecular interactions.
In addition, the HONO is a noticeably slower reaction than the N-NO 2 homolysis in both gaseous phase and on the surface Figure 5 a,b due to the lower pre-exponential factor and higher activation barrier Table 1. Thus, the changes of the molecular structure are ruled out as a possible reason for differences in sensitivities. The homolytic cleavage of the N-NO 2 bond Equation 1 remains the predominant decomposition pathway in both phases and requires similar energies, An electric field, generated by the molecular dipoles, induces a charge transfer to form positively and negatively charged surfaces Figure 6 c.
The N-NO 2 homolysis on the negatively charged surface requires Two important implications immediately follow from this knowledge. Details of synthesis [ ], structure [ ] and physico-chemical properties [ ] of DADNE Figure 8 were reported in when the material had a promise to combine two important properties, high performance and low sensitivity. Hence, it is not surprising that DADNE became an object of many experimental and theoretical studies. Despite the extensive effort of researchers, many questions regarding stability of DADNE remain unanswered.
We rigorously reevaluated conventional reactions and tested newly proposed mechanisms for DADNE decomposition. Our calculations confirm that the enamine-imine isomerization path III 1 , Figure 9 , leading to a formation of 1-aminoimino-2,2-dinitroethane AIDNE , has the lowest activation barrier The homolytic C-NO 2 bond cleavage path I , Figure 9 proceeding on the surface Figure 8 c requires slightly lower energy than that in the gas phase, whereas the CONO isomerization path II , Figure 9 on the surface has slightly higher activation barrier than that in the isolated molecule Table 1.
The strongest effect the surface makes on kinetics of fragmentation channels associated with the enamine-imine isomerization path III , Table 1. The activation barriers of the enamine-imine isomerization An increase of activation energies is also reflected in the reaction kinetics. As a result, the enamine-imine isomerization proceeds on the surface path III 1 , Table 1 at lower rates than the NO 2 loss Figure 10 b.
The thermal decomposition mechanism and kinetics of DADNE are strongly dependent upon the crystal structure. As a result DADNE will likely to decompose in the solid state via the homolytic NO 2 loss, though several kinetically more favorable fragmentation channels were proposed for gaseous phase.
This example with DADNE demonstrates that explicit modeling of the solid state decomposition processes is required for analyzing materials sensitivity and molecular calculations alone do not provide sufficient and reliable information. Results of some studies also suggest that an elimination of water may be the predominant channel for thermal decomposition of TATB [ ]. Surprisingly, however, a detailed theoretical study of thermal decomposition of TATB that analyses both molecular and crystalline processes based on quantum chemical methods is still lacking.
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Therefore, we simulated several viable fragmentation mechanisms of TATB in the gas phase and solid state aimed at obtaining kinetic data and performing a more comprehensive analysis of thermal sensitivity of this material than was available up to now. Finally, an elimination of the H 2 O molecule costs Elimination of the water molecule will unlikely compete with these two reactions due to the high overall activation barrier of the rate limiting stage associated with the rotation of the HONO fragment This leads us to exclude this mechanism from further consideration in simulations of thermal decomposition pathways of solid state TATB.
Reaction rates of TATB decomposition a in the gaseous phase and b on the surface. The decomposition of TATB on the surface obeys the same general trends as the process in the gas-phase. All reactions proceeding on the surface have slightly higher activation energies except the CONO isomerization Table 1.
Interestingly, our periodic calculations found that the activation barrier of the primary step, the intramolecular hydrogen transfer, and the formation of aci -isomer of TATB paths III 1 , Figure 12 , proceeding on surface is Despite this, the water loss reaction cannot be considered the primary initiation reaction of TATB decomposition. BNFF belongs to C-nitro compounds and is predicted to possess a low sensitivity due to the C-NO 2 bond, heterocyclic molecular structure, and densely packed molecular crystal arrangement Figure 14 a—c.
The idea here is to explore whether such an arrangement of the molecule and the crystal will gain additional stability against a thermal perturbation and thus improve i. The further investigation of alternative mechanisms revealed the reaction pathways, which can be activated with even lower energies than the C-NO 2 homolysis. For example, the CONO isomerization Another possible pathway for the opening of the outer heterocyclic rings path IV , Figure 15 proceeds via elimination of CN 2 O 2 and demands even lower energy Reaction rates of BNFF decomposition a in the gas phase and b on surface.
The elimination of CN 2 O 2 via concerted breaking of the outer oxadiazol ring remains the predominant decomposition channel on the BNFF surface. The calculated activation barriers of the outer ring cleavage of BNFF on the surface This review aimed at presenting a conceptual, first principles analysis of initiation of thermal decomposition reactions in several classes of organic nitro energetic crystals to instigate the development of the molecular theory of initiation of detonation. Despite existing classic macroscale theories [ 6 , , , , , ] and thousands of papers published in the field of energetic materials, the fundamental understanding of the initiation of chemistry phenomena is limited to micro- and nano-scale.
We reviewed relevant experimental and theoretical literature and augmented the available data by our own simulations to fill the gaps or to test notions recently suggested by researchers. Such a set, which combined conventional explosives in use PETN, HMX, and TATB with experimental promising energetic materials DADNE and BNFF , serves to reveal or confirm and enhance general trends in chemical decomposition, identify or supplement and refine meaningful correlations between structures and properties, and test some novel ideas in the field.
Our modeling of decomposition reactions in the gaseous phase and solid state showed that thermal degradation of explosives is a complex process that often involves several mechanisms triggered with similar activation barriers and progressing with close rates. It is rather unusual that only one reaction would initiate decomposition and continue to progress for a notable time, especially in crystals. One mechanism is typically dominating with another reaction or even two reactions starting somewhat later, which however can switch and become prevailing reactions at higher temperatures.
The decomposition initiation is comprehensively characterized by a set of descriptors: identified chemical mechanisms, activation barriers, reaction energies, pre-exponential factors, and reactions rates. Only together they reproduce the decomposition picture and if a parameter or two are missing, the lack of data may be misleading. This finding clearly illustrates how hopeless are the attempts to find a single descriptive parameter to measure the sensitivity of materials to initiation and even the thermal decomposition of materials, which represents only one component contributing in sensitivity.
Another factor that strongly affects the decomposition kinetics is a molecular and crystalline structure of the material of interest. The stability of BNFF, on the other hand, is mainly determined by the fragmentation mechanisms associated with the cleavage of oxadiazole rings, which require significantly lower activation energy than the C-NO 2 bond cleavage. Although the ring cleavage reaction is overpowered by the C-NO 2 loss upon a moderate increase of temperature, the sensitivity of BNFF is higher than what is expected from desired C-nitro materials.
This conclusion demonstrates that the chemical composition and the presence of functional groups are as important for defining the stabilization of molecules and materials as the details of the molecular structure and hence may serve as a useful feature for design of new materials with targeted properties and functions. The crystalline structure and intermolecular interactions impose additional limitations on kinetics and mechanisms of the decomposition of nitro compounds. Mechanisms that may dominate the gas-phase decomposition of single molecules may be restricted or even completely suppressed in the solid state.
For example, the recently proposed decomposition mechanism of DADNE via the enamino-imino isomerization followed by the elimination of either CH 2 N 2 or NO 2 requires, indeed, a low energy in the gas-phase, which makes this reaction a feasible initiation pathway. However, once the decomposing DADNE molecule is placed on the surface, the overall activation energy of this channel significantly increases due to Coulomb repulsion between neighbor molecules. Because energetic materials are densely packed, we predict that there are many other manifestations of the crystal arrangement governing the decomposition chemistry and hence the sensitivity.
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A variety of structural and electronic defects also play a role in changing decomposition chemistry in organic molecular crystals [ 19 , , , , , ]. Without a doubt, these factors have to be carefully analyzed in series of other energetic materials to confirm, refine, and generalize the trends and correlations. Advanced large-scale quantum chemical methods [ , , ] may need to be involved for such studies, as recently reviewed [ ], however they require serious adaptations to become useful for modeling of energetic materials.
We would like to emphasize here that partnerships between experimental and theoretical groups, combinations of available methods, and the development of new multi-scale multi-physics techniques are required for studies of the physical and chemical properties of the existing materials and assisting synthesis of novel energetic materials. Summing up, this review presents a conceptual assessment of thermal decomposition trends in energetic materials that should serve as a solid ground for building the molecular theory of detonation initiation from first principles.
By analyzing a large volume of experimental and theoretical studies we would like to emphasize several conclusions and observations.
However by no means conclusions made for gas-phase processes may be extrapolated to materials properties. They are insufficient due to the lack of intermolecular interactions in molecular models; ii The realistic quality simulations of materials should take into account the morphology of crystals and defects as they are imperative for reliable predictions and for comparison with experimental studies; iii In most energetic materials, the initiation chemistry is defined by the interplay of co-existing reactions that strongly depend upon the chemical composition of the material and its structural arrangement.
Any appearance of findings, conclusions, or recommendations expressed in this material are those of the author and do not necessarily reflect the views of NSF. National Center for Biotechnology Information , U. Journal List Molecules v.
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Published online Feb Roman V. Tsyshevsky , Onise Sharia , and Maija M. Roman Dembinski, Academic Editor. Author information Article notes Copyright and License information Disclaimer. Received Dec 15; Accepted Feb 6. Associated Data Supplementary Materials moleculess Abstract This review presents a concept, which assumes that thermal decomposition processes play a major role in defining the sensitivity of organic energetic materials to detonation initiation.
Introduction While detonation phenomena [ 1 , 2 , 3 , 4 , 5 , 6 ] were first observed a long time ago, a consistent microscale theory of detonation initiation does not yet exist [ 7 , 8 ]. Open in a separate window. Figure 1. Computational Details Our quantum-chemical calculations were performed using the approach [ 51 , 52 , 53 , 54 , 55 ] that links density functional theory DFT [ 56 , 57 ], variational transition state theory TST , and ab initio chemical kinetics with a realistic description of molecules and defect containing crystals.
Gas-Phase Calculations Modeling of gas-phase reactions is an essential and necessary part in all studies of thermally induced decomposition of nitro compounds. Results and Discussion We analyze multiple reaction mechanisms along with their kinetics and compare various aspects of the thermal decomposition processes to reveal similarities and differences between them.
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Figure 2. Figure 3. Figure 4. Figure 5. Figure 6. Figure 7. DADNE Details of synthesis [ ], structure [ ] and physico-chemical properties [ ] of DADNE Figure 8 were reported in when the material had a promise to combine two important properties, high performance and low sensitivity. Figure 8. Figure 9. Figure Conclusions This review aimed at presenting a conceptual, first principles analysis of initiation of thermal decomposition reactions in several classes of organic nitro energetic crystals to instigate the development of the molecular theory of initiation of detonation.
Supplementary Materials Click here for additional data file. Author Contributions All authors equally contributed to this review. Conflicts of Interest The authors declare no conflict of interest.
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