Predicting Trigger Bonds in Explosive Materials through Wiberg Bond Index Analysis

Abstract

Understanding the explosive decomposition pathways of high-energy-density materials (HEDMs) is important for developing compounds with improved properties. Rapid reaction rates make the detonation mechanisms of HEDMs difficult to understand, so computational tools are used to predict trigger bonds-weak bonds that break, leading to detonation. Wiberg bond indices (WBIs) have been used to compare bond densities in HEDMs to reference molecules to provide a relative scale for the bond strength to predict the activated bonds most likely to break to trigger an explosion. This analysis confirms that X-NO2 (X=N,C,O) bonds are trigger linkages in common HEDMs such as TNT, RDX and PETN, consistent with previous experimental and theoretical studies. Calculations on a small test set of substituted tetrazoles show that the assignment of the trigger bond depends upon the functionality of the material and that the relative weakening of the bond correlates with experimental impact sensitivities.

Trigger bond analysis of nitroaromatic energetic materials using wiberg bond indices

“…For example, o CNO 2 and p CNO 2 bonds in TNT were compared to the C( sp 2 )NO 2 bond in reference molecule nitrobenzene (NB) for %ΔWBIs of −1.11% and −0.33%, respectively (Fig. ) . The trigger bond is assigned as the more activated o CNO 2 bond, in agreement with the results of previous computational and experimental studies .…”

“…Explosophores such as nitro (NO 2 ) and azide (N 3 ) functional groups are generally needed to sensitize energetic materials. X‐NO 2 (X = N,C,O) bonds substituted on energetic materials have been proposed to form “trigger bonds,” activated (i.e., more readily cleaved) bonds that break to initiate an explosive reaction . Initiation steps proposed based on mass spectrometry and DFT for common secondary explosives (1,3,5‐trinitro‐1,3,5‐triazacyclohexane (RDX), 1,3,5,7‐tetranitro‐1,3,5,7‐tetrazacyclooctane (HMX), 2,4,6‐trinitrotoluene (TNT), trinitrobenzene (TNB) and pentaerythritol tetranitrate (PETN)) generally include homolytic bond cleavage, interatomic rearrangements, molecular eliminations and/or ring fission.…”

“…WBIs on an optimized closed‐shell molecular structure have a potential advantage over BDEs which require multiple calculations on open‐shell molecular fragments. Interpreting the factors that activate trigger bonds in novel HEDMs through WBI analysis can guide determination of decomposition initiation and may allow prediction of energetic properties (i.e., impact sensitivity and electrical spark sensitivity) …”

“…The strength of possible trigger bonds in HEDMs will be affected by steric effects, electron withdrawing groups, and ring strain . Activated bonds are expected to be longer and have lower electron density relative to reference molecules with the same bond type, hybridization and explosophore.…”

“…A relative scale for the activation of each potential trigger bond can be determined by comparing the difference of the WBI for specific bond types in the HEDMs and the reference molecules (%ΔWBI) [eq. ] …”