Elhanan Wurzberg

Bio: Elhanan Wurzberg is an academic researcher from Weizmann Institute of Science. The author has contributed to research in topics: Thermal decomposition & Homolysis. The author has an hindex of 1, co-authored 1 publications receiving 103 citations.

Mechanism of thermal unimolecular decomposition of TNT (2,4,6-trinitrotoluene): a DFT study.

Abstract: The widespread and long-term use of TNT has led to extensive study of its thermal and explosive properties. Although much research on the thermolysis of TNT and polynitro organic compounds has been undertaken, the kinetics and mechanism of the initiation and propagation reactions and their dependence on the temperature and pressure are unclear. Here, we report a comprehensive computational DFT investigation of the unimolecular adiabatic (thermal) decomposition of TNT. On the basis of previous experimental observations, we have postulated three possible pathways for TNT decomposition, keeping the aromatic ring intact, and calculated them at room temperature (298 K), 800, 900, 1500, 1700, and 2000 K and at the detonation temperature of 3500 K. Our calculations suggest that at relatively low temperatures, reaction of the methyl substituent on the ring (C-H alpha attack), leading to the formation of 2,4-dinitro-anthranil, is both kinetically and thermodynamically the most favorable pathway, while homolysis of the C-NO(2) bond is endergonic and kinetically less favorable. At approximately 1250-1500 K, the situation changes, and the C-NO(2) homolysis pathway dominates TNT decomposition. Rearrangement of the NO(2) moiety to ONO followed by O-NO homolysis is a thermodynamically more favorable pathway than the C-NO(2) homolysis pathway at room temperature and is the most exergonic pathway at high temperatures; however, at all temperatures, the C-NO(2) --> C-ONO rearrangement-homolysis pathway is kinetically unfavorable as compared to the other two pathways. The computational temperature analysis we have performed sheds light on the pathway that might lead to a TNT explosion and on the temperature in which it becomes exergonic. The results appear to correlate closely with the experimentally derived shock wave detonation time (100-200 fs) for which only the C-NO(2) homolysis pathway is kinetically accessible.

Decomposition of condensed phase energetic materials: interplay between uni- and bimolecular mechanisms.

Abstract: Activation energy for the decomposition of explosives is a crucial parameter of performance. The dramatic suppression of activation energy in condensed phase decomposition of nitroaromatic explosives has been an unresolved issue for over a decade. We rationalize the reduction in activation energy as a result of a mechanistic change from unimolecular decomposition in the gas phase to a series of radical bimolecular reactions in the condensed phase. This is in contrast to other classes of explosives, such as nitramines and nitrate esters, whose decomposition proceeds via unimolecular reactions both in the gas and in the condensed phase. The thermal decomposition of a model nitroaromatic explosive, 2,4,6-trinitrotoluene (TNT), is presented as a prime example. Electronic structure and reactive molecular dynamics (ReaxFF-lg) calculations enable to directly probe the condensed phase chemistry under extreme conditions of temperature and pressure, identifying the key bimolecular radical reactions responsible for ...

The co-crystal of TNT/CL-20 leads to decreased sensitivity toward thermal decomposition from first principles based reactive molecular dynamics

Abstract: To gain an atomistic-level understanding of the experimental observation that the cocrystal TNT/CL-20 leads to decreased sensitivity, we carried out reactive molecular dynamics (RMD) simulations using the ReaxFF reactive force field. We compared the thermal decomposition of the TNT/CL-20 cocrystal with that of pure crystals of TNT and CL-20 and with a simple physical mixture of TNT and CL-20. We find that cocrystal has a lower decomposition rate than CL-20 but higher than TNT, which is consistent with experimental observation. We find that the formation of carbon clusters arising from TNT, a carbon-rich molecule, plays an important role in the thermal decomposition process, explaining the decrease in sensitivity for the cocrystal. At low temperature and in the early stage of chemical reactions under high temperature, the cocrystal releases energy more slowly than the simple mixture of CL-20–TNT. These results confirm the expectation that co-crystallization is an effective way to decrease the sensitivity for energetic materials while retaining high performance.

Compressive Shear Reactive Molecular Dynamics Studies Indicating That Cocrystals of TNT/CL-20 Decrease Sensitivity

Abstract: To gain an atomistic-level understanding of how compounding the TNT and CL-20 energetic materials into a TNT/CL-20 cocrystal might affect the sensitivity, we carried out the compressive–shear reactive molecular dynamics (CS-RMD) simulations. Comparing with the pure crystal of CL-20, we find that the cocrystal is much less sensitive. We find that the molecular origin of the energy barrier for anisotropic shear results from steric hindrance toward shearing of adjacent slip planes during shear deformation, which is decreased for the cocrystal. To compare the sensitivity for different crystals, we chose the shear slip system with lowest energy barrier as the most plausible one under external stresses for each crystal. Then we used the temperature rise and molecule decomposition as effective measures to distinguish sensitivities. Considering the criterion as number NO2 fragments produced, we find that the cocrystal has lower shear-induced initiation sensitivity by ∼70% under atmospheric pressure and ∼46% under...

First-Principles-Based Reaction Kinetics for Decomposition of Hot, Dense Liquid TNT from ReaxFF Multiscale Reactive Dynamics Simulations

Abstract: The reaction kinetics of the thermal decomposition of hot, dense liquid TNT was studied from first-principles-based ReaxFF multiscale reactive dynamics simulation strategy. The decomposition process was followed starting from the initial liquid phase, decomposition to radicals, continuing through formation of carbon-clusters products, and finally to formation of the stable gaseous products. The activation energy of the initial endothermic decomposition rate and the subsequent exothermic reactions were determined as a function of density. Analysis of fragments production in different densities and temperatures is presented. We find that unimolecular C–N bond scission dominates at the lower densities (producing NO2), whereas dimer formation and decomposition to TNT derivatives and smaller gaseous fragments prevails at higher compressions. At higher densities, enhanced carbon-clustering is observed, while the initial gaseous fragments formation is suppressed. Increasing the temperature speeds up the producti...

Searching for Low-Sensitivity Cast-Melt High-Energy-Density Materials: Synthesis, Characterization, and Decomposition Kinetics of 3,4-Bis(4-nitro-1,2,5-oxadiazol-3-yl)-1,2,5-oxadiazole-2-oxide

Abstract: The most comprehensive approach to analyze and characterize energetic materials is suggested and applied to enable rational, rigorous design of novel materials and targeted improvements of existing materials to achieve desired properties. We report synthesis, characterization of the structure and sensitivity, and modeling of thermal and electronic stability of the energetic, heterocyclic compound, 3,4-bis(4-nitro-1,2,5-oxadiazol-3-yl)-1,2,5-oxadiazole-2-oxide (BNFF). The proposed novel, relatively simple synthesis of BNFF in excellent yields allows for an efficient scale up. Performing careful characterization indicates that these materials offer an unusual combination of properties and exhibit a relatively high energy density, high and controllable stability against decomposition, low melting temperature, and low sensitivity to initiation of detonation. First-principles calculations of activation barriers and reaction rate constants reveal the decomposition scenarios that govern the thermal stability and...