Computational Investigation of Energetic Materials: Influence of Intramolecular and Intermolecular Interactions on Sensitivity

Abstract

The development of novel high energy density materials (HEDMs) with superior energetic properties depends on characterizing how and why detonation occurs. Detonation is highly energetic and a nearly instantaneous process, making experimental studies challenging; thus, computational modeling through density functional theory (DFT) and molecular dynamics (MD) can be used to propose weakened, or activated, bonds that break to initiate explosive decomposition, termed trigger bonds. Bond activation is characterized by the Wiberg bond index (WBI), a measure of interatomic electron density. Trigger bonds in HEDMs are commonly found in explosophores, functional groups that contribute to energetic potential such as X-NO2 (X=N,C,O) and N-N2 linkages. Comparison of WBIs of potential trigger bonds to the same bond type in reference molecules provides a relative scale for bond activation (%ΔWBIs) which could be used to screen novel HEDMs for trigger bonds and potentially guide development of new materials. %ΔWBIs of nitroaromatic energetic materials indicate that intramolecular hydrogen bonding deactivates C-NO2 bonds through resonance, while steric effects activate trigger bonds by increasing C-NO2 dihedral angles. In aromatic azide-based and azole-based energetic materials, %ΔWBIs and activation energies predict that that the N-N2 bond of the azide and N-NO2 are more activated than C-NO2. An ortho nitro group to an azide yields a lower activation energy for N2 extrusion from the azide. Thus, %ΔWBIs only provide a clue into the influence of intramolecular interactions on the sensitivity of trigger bonds. Detonation is unique to the solid state, making condensed-phase calculations necessary to characterize the effect of intermolecular interactions on trigger bond sensitivity. In models of ammonium nitrate, increased pressure compresses the unit cell and hydrogen bonding between ions becomes stronger. In molecular dynamics simulations at high pressure, hydrogen transfer from ammonium to nitrate producing ammonia and nitric acid, the initiation step for explosive decomposition, is observed around 40 GPa. These condensed-phase calculations can be extended to characterize the effect of pressure on intramolecular and intermolecular interactions to provide information that can be used to guide the synthesis of novel energetic materials.