CL-20 (2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane or HNIW) is a high-energy nitramine explosive. To improve atomistic understanding of the thermal decomposition of CL-20 gas and solid phases, we performed a series of ab initio molecular dynamics simulations. We found that during unimolecular decomposition, unlike other nitramines (e.g., RDX, HMX), CL-20 has only one distinct initial reaction channel—homolysis of the N−NO2 bond. We did not observe any HONO elimination reaction during unimolecular decomposition, whereas the ring-breaking reaction was followed by NO2 fission. Therefore, in spite of limited sampling, that provides a mostly qualitative picture, we proposed here a scheme of unimolecular decomposition of CL-20. The averaged product population over all trajectories was estimated at four HCN, two to four NO2, two to four NO, one CO, and one OH molecule per one CL-20 molecule. Our simulations provide a detailed description of the chemical processes in the initial stages of thermal d...

Ab initio molecular dynamics study on the initial chemical events in nitramines: thermal decomposition of CL-20.

Ultraviolet excitation (8-ns duration) is employed to study the decomposition of RDX (1,3,5-trinitro-1,3,5-triazacyclohexane) and HMX (1,3,5,7-tetranitro-1,3,5,7-tetrazacyclooctane) from their first excited electronic states. Isolated RDX and HMX are generated in the gas phase utilizing a combination of matrix-assisted laser desorption and supersonic jet expansion techniques. The NO molecule is observed as one of the initial dissociation products by both time-of-flight mass spectroscopy and laser-induced fluorescence spectroscopy. Four different vibronic transitions of NO are observed: AΣ2(v′=0)←XΠ2(v″=0,1,2,3). Simulations of the NO rovibronic intensities for the A←X transitions show that dissociated NO from RDX and HMX is rotationally cold (∼20K) and vibrationally hot (∼1800K). Another potential initial product of RDX and HMX excited state dissociation could be OH, generated along with NO, perhaps from a HONO intermediate species. The OH radical is not observed in fluorescence even though its transition intensity is calculated to be 1.5 times that found for NO per radical generated. The HONO intermediate is thereby found not to be an important pathway for the excited electronic state decomposition of these cyclic nitramines.

Decomposition of nitramine energetic materials in excited electronic states: RDX and HMX.

An increased necessity for energy independence and heightened concern about the effects of rising carbon dioxide levels have intensified the search for renewable fuels that could reduce our current consumption of petrol and diesel. One such fuel is biodiesel, which consists of the methyl esters of fatty acids. Methyl butanoate (MB) contains the essential chemical structure of the long-chain fatty acids and a shorter, but similar, alkyl chain. This paper reports on a detailed kinetic mechanism for MB that is assembled using theoretical approaches. Thirteen pathways that include fuel decomposition, isomerization, and propagation steps were computed using ab initio calculations [J. Org. Chem. 2008, 73, 94]. Rate constants from first principles for important reactions in CO2 formation, namely CH3OCOdCH3 + CO2 (R1) and CH3OCOdCH3O + CO (R2) reactions, are computed at high levels of theory and implemented in the mechanism. Using the G3B3 potential energy surface together with the B3LYP/6-31G(d) gradient, Hessian and geometries, the rate constants for reactions R1 and R2 are calculated using the Rice-Ramsperger-Kassel-Marcus theory with corrections from treatments for tunneling, hindered rotation, and variational effects. The calculated rate constants of reaction R1 differ from the data present in the literature by at most 20%, while those of reaction R2 are about a factor of 4 lower than the available values. The new kinetic model derived from ab initio simulations is combined with the kinetic mechanism presented by Fisher et al. [Proc. Combust. Inst. 2000, 28, 1579] together with the addition of the newly found six-centered unimolecular elimination reaction that yields ethylene and methyl acetate, MB ) C2H4 + CH3COOCH3. This latter pathway requires the inclusion of the CH3COOCH3 decomposition model suggested by Westbrook et al. [Proc. Combust. Inst. 2008, accepted]. The newly composed kinetic mechanism for MB is used to study the CO2 formation during the pyrolysis of MB as well as to investigate the autoignition of MB in a shock tube reactor at different temperatures and pressures. The computed results agree very well with experimental data present in the literature. Sensitivity and flux (rateof-production) analyses are carried out for the CO2 formation with the new MB mechanism, together with available reaction mechanisms, to assess the importance of various kinetic pathways for each regime. With the new mechanism, the flux analyses for the formation of C2H species, one of the most important species for ignition delay time, are also presented at different conditions. In addition to giving a better chemical insight of the pyrolysis/oxidation of MB, the results suggest ways to improve the mechanism’s capability to predict CO2 formation and ignition delay times in pyrolysis and oxidation conditions.

Kinetic modeling of methyl butanoate in shock tube.

Computational study of imidazole derivative as high energetic materials.

Energetic cocrystallization, by combining existing molecules together, is thought to be new strategy for creating energetic materials. Nevertheless, the underlying mechanism of its influences on properties and performances in comparison with their pure components remains unclear. The present work reveals the cocrystallization influence of a typical energetic cocrystal of CL-20/HMX on thermal stability, by ReaxFF molecular reactive dynamic simulations and kinetics calculations on the pure and cocrystals. As a result, we find that the cocrystal mediates the thermal stability of pure crystalsand this is in agreement with experimental observations. The initial decay steps in pure crystals remain still in the cocrystal, that is, the independent and intramolecular reactions of N–N bond cleavage governing the initial decay of the pure CL-20 and HMX crystals also dominate in the cocrystal of CL-20/HMX. Meanwhile, during the thermal decomposition of the cocrystal, CL-20 releases heat faster than HMX, thus the heat...

Initial Decay Mechanism of the Heated CL-20/HMX Cocrystal: A Case of the Cocrystal Mediating the Thermal Stability of the Two Pure Components

The dynamics of the initial thermal decomposition step of gas-phase α-HMX is investigated using the master equation method. Both the NO2 fission and HONO elimination channels were considered. The structures, energies, and Hessian information along the minimum energy paths (MEP) of these two channels were calculated at the B3LYP/cc-pVDZ level of theory. Thermal rate constants at the high-pressure limit were calculated using the canonical variational transition state theory (CVT), microcanonical variational transition state theory (μVT). The pressure-dependent multichannel rate constants and the branching ratio were calculated using the master equation method. Quantum tunneling effects in the HONO elimination are included in the dynamical calculations and found to be important at low temperatures. At the high-pressure limit, the NO2 fission channel is found to be dominant in the temperature range (500−1500 K). Both channels exhibit strong pressure dependence at high temperatures. Both reach the high-pressur...

Branching Ratio and Pressure Dependent Rate Constants of Multichannel Unimolecular Decomposition of Gas-Phase α-HMX: An Ab Initio Dynamics Study†

The NO2 fission reaction of gas phase α-HMX has been studied using a direct ab initio method within the framework of microcanonical variational transition state theory (μVT). The potential energy calculations were calculated using the hybrid nonlocal B3LYP density functional theory with the cc-pVDZ basis set. The calculated results show that the potential energy of breaking the axial NO2 groups is lower than that of breaking the equatorial NO2 groups. No traditional transition state was found along the reaction path. Microcanonical rate constants calculation shows the variational transition state varies from 2.0 to 3.5 A of the breaking N−N bond length as a function of the excess energy. The μVT method was used for thermal rate constants calculation over a temperature range from 250 to 2000 K. The fitted Arrhenius expression from the calculated data is k(T) = 1.66 × 1015 exp(−18748K/T) s-1, which is in good agreement with the experimental data at low temperatures.

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Shaowen Zhang and

Papers

Branching Ratio and Pressure Dependent Rate Constants of Multichannel Unimolecular Decomposition of Gas-Phase α-HMX: An Ab Initio Dynamics Study†

Thermal Rate Constants of the NO2 Fission Reaction of Gas Phase α-HMX: A Direct ab Initio Dynamics Study