AB INITIO MOLECULAR DYNAMICS SIMULATIONS OF MULTIMOLECULAR COLLISIONS OF NITROMETHANE AND COMPRESSED LIQUID NITROMETHANE

Our current interest focuses on the atomic level details of the decomposition mechanism of liquid NM under shock and highly compressed conditions, using Car-Parrinello ab initio molecular dynamics (CPMD) simulations. Multimolecular collision simulations indicate that neighboring molecules act as a trap to confine the recoiling fragments produced during the initial collision, thereby enabling them to recombine to form intact NM molecules. This leads to higher threshold collision velocities than found previously in bimolecular collision simulations. The threshold collision velocities determined from the multimolecular and bimolecular collision simulations are higher than the average atomic velocities expected at the detonation shock front, suggesting that molecular dissociation most likely occurs with thermalization after the shock front passes. To probe the pressure induced dissociation mechanism of liquid NM, CPMD simulations were performed under conditions of high pressure compression at low temperatures to avoid strong thermal influence. Compression of liquid NM by factors of 2.0 and 2.5 at 300 K showed no pressure induced molecular dissociation. A proton transfer reaction between two closely spaced NM molecules aligned in an anti-parallel orientation, leading to CH2NO2 and CH3NO(OH), was observed in the CPMD simulation of liquid NM compressed by a factor of 3.0 at 150 K. BP86/DZVP quantum chemical calculations indicate that this reaction is highly endothermic and proceeds with an activation barrier of about 25 kcal/mol