Reaction Mechanisms in the Thermal Decomposition of CL-20 Revealed by ReaxFF Molecular Dynamics Simulations

Abstract

The thermal decomposition of condensed CL-20 was investigated using reactive force field molecular dynamics (ReaxFF MD) simulations of a super cell containing 128 CL-20 molecules at 800-3000 K. The VARxMD code previously developed by our group is used for detailed reaction analysis. Various intermediates and comprehensive reaction pathways in the thermal decomposition of CL-20 were obtained. Nitrogen oxides are the major initial decomposition products, generated in a sequence of NO2, NO3, NO, and N2O. NO2 is the most abundant primary product and is gradually consumed in subsequent secondary reactions to form other nitrogen oxides. NO3 is the second most abundant intermediate in the early stages of CL-20 thermolysis. However, it is unstable and quickly decomposes at high temperatures, while other nitrogen oxides remain. At all temperatures, the unimolecular pathways of N-NO2 bond cleavage and ring-opening C-N bond scission dominate the initial decomposition of condensed CL-20. The cleavage of the N-NO2 bond is greatly enhanced at high temperatures, but scission of the C-N bond is not as favorable. A bimolecular pathway of oxygen-abstraction by NO2 to generate NO3 is observed in the initial decomposition steps of CL-20, which should be considered as one of the major pathways for CL-20 decomposition at low temperatures. After the initiation of CL-20 decomposition, fragments with different ring structures are formed from a series of bond-breaking reactions. Analysis of the ring structure evolution indicates that the pyrazine derivatives of fused tricycles and bicycles are early intermediates in the decomposition process, which further decompose to single ring pyrazine. Pyrazine is the most stable ring structure obtained in the simulations of CL-20 thermolysis, supporting the proposed existence of pyrazine in Py-GC/MS experiments. The single imidazole ring is unstable and decomposes quickly in the early stages of CL-20 thermolysis. Many C4 and C2 intermediates are observed after the initial fragmentation, but eventually convert into stable products. The distribution of the final products (N2, H2O, CO2, and H2) obtained in ReaxFF MD simulation of CL-20 thermolysis at 3000 K quantitatively agrees with the results of the CL-20 detonation experiment. The comprehensive understanding of CL-20 thermolysis obtained through this study suggests that ReaxFF MD simulation, combined with the reaction analysis capability of VARxMD, would be a promising method for obtaining deeper insight into the complex chemistry of energetic materials exposed to thermal stimuli.

The role of electric field on decomposition of CL‐20/HMX cocrystal: A reactive molecular dynamics study

“…We observed some bimolecular reactions, such as N 2 O 3 producing reactions, as well as intermolecular O transfer reactions (between two CL-20 molecules, or between HMX and NO 2 ), which were also reported in the decomposition of RDX. 50 These bimolecular reactions were not observed in large quantities during thermal decomposition of CL-20, [15][16][17][47][48][49]51 due to weak intermolecular interaction induced by the low stability of CL-20. 17 Thus, those bimolecular reactions are probably caused by large electric force of CEF.…”

A review of recent advances in tribology

“…In addition to the classical MD simulations, ab initio or first-principle molecular dynamics and reactive force field (ReaxFF) molecular dynamics [698], which enable to describe chemical reactions more precisely, have recently appeared as new members in the family of MD simulations. Using the ReaxFFMD simulations, for example, it is revealed that the formation and shear of interfacial bridge bonds lead to atom-by-atom removal of substrate materials, and the effect of tribochemical reactions caused by water molecules or OH groups has been investigated [699][700][701].…”

Molecular Simulations in Materials Science

“…Yin et al 2 parameterized a semiempirical density functional tight-binding (DFTB) model to study deposition of carbon on copper surface. At the atomic level, Ren et al 3 simulated the thermal decomposition of highenergy-density materials using reactive force field, and Wang et al 4 reported the use of a flexible and polarizable force field to calculate vibrational frequencies of molecules on liquid surface. Combining DFT calculations and atomistic force field, Liu et al 5 studied the effect of photoisomerization on binding energy and conformation of azobenzene-containing host-guest complex.…”

“…The articles presented in this special issue demonstrate the roles played by molecular simulation in the cutting edge of research and development. The topics covered include CO2 reduction 1 , molecular machinery 5 , graphene chemistry 10 , carbon deposition on metal surface 2 , complex reactions 3,9 , surface chemistry 4 , polymer morphology 6 , and self-assembly 8 . Although the collection of articles is far from complete in reflecting the research activities, we hope that it is helpful especially to those researchers who are new to the field to get a snapshot of what can be achieved by using molecular simulations in materials research.…”