Thermal decomposition of condensed-phase nitromethane from molecular dynamics from reaxff reactive dynamics

We studied the thermal decomposition and subsequent reaction of the energetic material nitromethane (CH₃NO₂) using molecular dynamics with ReaxFF, a first principles-based reactive force field. We characterize the chemistry of liquid and solid nitromethane at high temperatures (2000-3000 K) and density 1.97 g/cm3 for times up to 200 ps. At T = 3000 K the first reaction in the decomposition of nitromethane is an intermolecular proton transfer leading to CH₃NOOH and CH₂NO₂. For lower temperatures (T = 2500 and 2000 K) the first reaction during decomposition is often an isomerization reaction involving the scission of the C-N bond the formation of a C-O bond to form methyl nitrate (CH₃ONO). Also at very early times we observe intramolecular proton transfer events. The main product of these reactions isH₂Owhich starts forming following those initiation steps. The appearance ofH₂Omarks the beginning of the exothermic chemistry. Recent quantum-mechanics-based molecular dynamics simulations on the chemical reactions and time scales for decomposition of a crystalline sample heated to T = 3000 K for a few picoseconds are in excellent agreement with our results, providing an important, direct validation of ReaxFF.