Computational Investigation of Energetic Materials: Influence of Electronic and Steric Properties on Sensitivity and Decomposition Mechanisms

Developing novel high energy density materials (HEDMs) requires knowledge of the causes and mechanisms of detonation. These chemical events are almost instantaneous and involve the release of a large amount of energy, which limits the experimental studies that can be performed on them. Computational methods including density functional theory (DFT) and molecular dynamics (MD) simulations have been used to investigate trigger bonds, those which break to initiate detonation. These bonds are commonly found within explosophores, substituents that increase the explosive potential of a molecule. The Wiberg bond index (WBI) is an estimation of orbital overlap and bond strength between two atoms. For azo-based HEDMs, the trigger bonds are strengthened by intermolecular hydrogen bonding which favors planar molecular conformations of the azo bridge and increased π bonding. Large substituents can force the R-groups to twist and activate the bond. When nitrobased explosophores are present, they are assigned as including the trigger bond due to the weaker nature of their C-NO2 and N-NO2 bonds. Many single molecule computational methods have been employed to predict impact sensitivity of HEDMs. Oxygen balance, which uses the chemical formula to determine if enough oxygen present to completely oxidize the backbone, correlates reasonably well with impact sensitivity. However, quantum-chemical methods using the electron density to predict the trigger bond do not perform as well as oxygen balance. The lack of correlation with experimental impact sensitivities for these single-molecule, gas-phase calculations could be attributed to missing contributions from the crystal structure. Solid-state MD calculations create a more realistic environment to determine the trigger bond and impact sensitivity. Ammonium nitrate has been studied for its thermal decomposition mechanism. Here, pressure is applied to supercells of this HEDM to simulate the response of the compound to impact. MD simulations show that decomposition can be observed at elevated pressures consistent with experimental impact sensitivities. These condensed-phase calculations can be continued with HEDMs containing different explosophores to provide insight into how each respond to impact pressure. This information could be used to develop novel HEDMs and safer store and transport procedures for these materials.