Reaction Mechanisms and Sensitivity for Silicon Nitrocarbamate and Related Systems from Quantum Mechanics Reaction Dynamics

Abstract

Temperature induced instability is an important issue in developing new molecules and materials, but there is no clear understanding about how molecular structure and crystal packing control sensitivity. This is particularly the case for energetic materials (EM) important in propulsion and detonation. We propose here using the quantum mechanics molecular dynamics (QM-MD) based tempereature programmed reaction dynamics for predicting the relative sensitivity of various materials while simultaneously obtaining the reaction mechanisms underlying to provide guidance in improving materials. We illustrate this for four closely related molecules, pentaerythritol tetranitrate, pentaerythritol tetranitrocarbamate, and their silicon analogs, that have minor intramolecular differences but exhibit different sensitivities experimentally. Our study finds dramatic differences in reaction mechanisms and energy variation under heating that suggest explanations for the different sensitivities. Important here are both the initial decomposition and the secondary reactions between products. The higher sensitivity of the Si analogs originates from the highly exothermic Si–O bond formation as a paramount initial reaction that promotes other reactions, leading to the generations of various intermediates and final products, thus accelerating the decomposition process and energy release. The nitrocarbamates have low sensitivity because their large complex branching impedes the exothermic Si/C–O bond formation and triggers multiple initial endothermic reaction pathways with higher reaction barrier, delaying secondary exothermic reactions and energy release. We find two computational measures that correlate well with sensitivity: the temperatures at which the energy changes from endothermic to exothermic and the total absorbed energy. This study provides mechanistic insight on the molecular and structural determinants controlling the sensitivity of EMs and provides a practical way to predict the relative sensitivity in advance of experimental synthesis and characterization, benefiting the design of novel EMs.

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