ReaxFF reactive molecular dynamics on silicon pentaerythritol tetranitrate crystal validates the mechanism for the colossal sensitivity

Recently quantum mechanical (QM) calculations on a single Si-PETN (silicon-pentaerythritol tetranitrate) molecule were used to explain its colossal sensitivity observed experimentally in terms of a unique Liu carbon-silyl nitro-ester rearrangement (R₃Si-CH₂-O-R₂→ R₃Si-O-CH₂-R₂). In this paper we expanded the study of Si-PETN from a single molecule to a bulk system by extending the ReaxFF reactive force field to describe similar Si-C-H-O-N systems with parameters optimized to reproduce QM results. The reaction mechanisms and kinetics of thermal decomposition of solid Si-PETN were investigated using ReaxFF reactive molecular dynamics (ReaxFF-RMD) simulations at various temperatures to explore the origin of the high sensitivity. We find that at lower temperatures, the decomposition of Si-PETN is initiated by the Liu carbon-silyl nitro-ester rearrangement forming Si-O bonds which is not observed in PETN. As the reaction proceeds, the exothermicity of Si-O bond formation promotes the onset of NO₂ formation from N-OC bond cleavage which does not occur in PETN. At higher temperatures PETN starts to react by the usual mechanisms of NO₂ dissociation and HONO elimination; however, Si-PETN remains far more reactive. These results validate the predictions from QM that the significantly increased sensitivity of Si-PETN arises from a unimolecular process involving the unusual Liu rearrangement but not from multi-molecular collisions. It is the very low energy barrier and the high exothermicity of the Si-O bond formation providing energy early in the decomposition process that is responsible.