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

Abstract

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 (R_3Si–CH_2–O–R_2 → R_3Si–O–CH_2–R_2). 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 NO_2 formation from N–OC bond cleavage which does not occur in PETN. At higher temperatures PETN starts to react by the usual mechanisms of NO_2 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.

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