Reaction Rates in Nitromethane under High Pressure from Density Functional Tight Binding Molecular Dynamics Simulations.

We use density functional tight binding (DFTB) molecular dynamics (MD) simulations to determine the reaction rates of nitromethane CH3NO2 (NM) under high pressure (P = 14-28 GPa), and temperature (T = 1450-1850 K). DFTB-MD simulations performed with the same initial conditions (P0, T0) reveal a stochastic behavior, both in terms of reaction times and chemical paths. By running series of MD simulations, we are able to obtain average reaction times with quantified errors and devise a simple two-step model for NM explosion: ignition/explosion. While our model bypasses the chemical complexity due to the numerous reaction paths and intermediates observed during reactions, the chemistry is accounted for via the accurate parameterization of the DFTB model, and our results suggest a single main reaction pathway for the pressure range considered here, dominated in the earlier stages by the formation of the aci-ion, CH2NOO-. By fitting our data to a Frank-Kamenetskii model, we extract prefactors and pressure-independent activation energies and volumes for the ignition and explosion stages. A two-step model is then built and compared to experimental observations. Single and two-step Arrhenius models are also provided for comparison with literature data. This work presents an efficient way of investigating the reactivity of high explosives by performing electronic structure-based MD simulations and provides reaction rates for simplified models that can be implemented into hydrocodes.