The decomposition mechanism of hot liquid nitromethane at various compressions was studied using reactive force field (ReaxFF) molecular dynamics simulations. A competition between two different initial thermal decomposition schemes is observed, depending on compression. At low densities, unimolecular C–N bond cleavage is the dominant route, producing CH3 and NO2 fragments. As density and pressure rise approaching the Chapman–Jouget detonation conditions (∼30% compression, >2500 K) the dominant mechanism switches to the formation of the CH3NO fragment via H-transfer and/or N–O bond rupture. The change in the decomposition mechanism of hot liquid NM leads to a different kinetic and energetic behavior, as well as products distribution. The calculated density dependence of the enthalpy change correlates with the change in initial decomposition reaction mechanism. It can be used as a convenient and useful global parameter for the detection of reaction dynamics. Atomic averaged local diffusion coefficients are shown to be sensitive to the reactions dynamics, and can be used to distinguish between time periods where chemical reactions occur and diffusion-dominated, nonreactive time periods.

Figure S1: Time evolution of main species for 1800K and 3500K at 0.7V0 Scheme S1: Hydrogen transfer routes between TNT and TNT-H and the cleavage of C-NO 2 from TNT molecule missing a ring bound hydrogen. Units: kcal/mol. S4 Scheme S2: Reactions of TNT radical missing a NO 2 group (TNT-NO 2). Units: kcal/mol. S5 Scheme S3. Total energies for ortho hydrogen transfer (top part) and further decomposition routes (bottom part). Units: kcal/mol.

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 production of both clusters and gaseous products. The activation energy for the initial decomposition stage of ambient liquid TNT is ∼36 kcal/mol, close to the measured value (∼40 kcal/mol). This value is ∼25 kcal/mol lower than the corresponding gas phase C–N bond scission. Finally, we suggest a simple linear growth kinetic model for describing the clustering process, which provides very good agreement with simulation results.

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