Thermal Decomposition Mechanisms of Nitroesters: Ab Initio Modeling of Pentaerythritol Tetranitrate

The lack of fundamental understanding of the intricate interplay between large amounts of chemical energy stored in energetic molecular materials and their sensitivity to detonation initiation represents a stubborn outstanding challenge. This study sheds much needed light on the atomic decomposition mechanisms of nitroesters with an example of pentaerythritol tetranitrate (C5H8N4O12, also known as PETN), used as an explosive and a medication. The performed study is based on density functional theory combined with variational transition-state theory and large-scale massively parallel periodic supercell calculations and presents a detailed analysis of thermal initiation of chemical reactions and their kinetics in the gas-phase PETN molecules, the ideal bulk crystals, and on two low energy surfaces, (101) and (110). The obtained activation barriers, reaction energies, and reaction rates of simulated bond dissociation mechanisms reveal that the overall decomposition of condensed nitroesters is defined by the interplay of two major reactions, the fast endothermic O-NO2 homolysis and the slow exothermic HONO elimination. While the O-N cleavage has a low activation barrier and essentially initiates the degradation of the material, the HONO elimination pathway serves to accelerate the global process by generating heat to support the reaction. This trend is suggested to be general for a large class of traditional and novel nitroesters.