Ab initio molecular dynamics simulations of nitromethane under shock initiation conditions

摘要

Multimolecular collisions were employed as a simplified model to study shock-induced dissocia-tion in homogeneous liquid nitromethane using first principles Car-Parrinello molecular dynamics simulations within the Kohn-Sham scheme at the density functional theory level. Simulations were carried out for a variety of collision orientations and collision velocities. The critical velocity for successful dissociation was found to be 11-12 km/s under the impact of a single molecule on multiple molecules and between 8 and 10 km/s under the impact of multiple molecules on multiple molecules. The neighbouring molecules in the current multimolecular collision simulations act as a trap to confine the recoiling fragments produced during the initial impact, thereby enabling them to recombine to form intact nitromethane molecules. This leads to higher threshold collision velocities than found previously in bimolecular collision simulations (i.e., 7.0-10.5 km/s for collision orientations investigated). Although our rationale for extending the bimolecular collision simulations to multimolecular collision simulations was to account for potential collision-induced reactions involving multiple molecules, the current CPMD simulations indicate that this only occurs at very high incident collision velocities. In accord with previous bimolecular collision simulations, C-N bond cleavage is the primary mechanism of nitromethane dissociation in the multimolecular collision simulations at the threshold collision velocities. An alternative C-H bond scission fragmentation mechanism was observed only in high collision velocity simulations. The threshold collision velocities determined from the bimolecular and multimolecular collision simulations are much higher than the average atomic velocities expected at the detonation shock front of nitromethane, suggesting that molecular dissociation is likely effected with thermalization after the shock front passes. However, to draw a firm conclusion, more complex multimolecular collision scenarios should be investigated in an effort to more accurately model the stochastic nature of shock front propagation through the bulk liquid.