The effect of nitro groups on N2 extrusion from aromatic azide-based energetic materials

Abstract

ortho nitroaromatic azides extrude N₂ through cyclization to a benzofuroxan derivative. This process has been proposed to initiate explosive decomposition in related energetic materials such as 1-azido-2,4,6-trinitrobenzene (AzTNB) and 1,3,5-triazido-2,4,6-trinitrobenzene (TAzTNB). Density functional theory calculations show that extrusion occurs through a lower activation barrier than in the case of the absence of an ortho nitro group (i.e., azidobenzene, m-nitroazidobenzene, p-nitroazidobenzene) where the product is a triplet nitrene. In ortho nitroaromatic azides, Wiberg bond index (WBI) analysis indicates that the N–N₂ bond is more weakened, or activated for cleavage, than the C–NO₂ bond. Therefore, the N–N₂ bond breaking is a key factor in initiation of explosive decomposition in energetic materials containing both nitro and azide groups. However, cyclization requires that the activation barrier also be dependent upon the formation of an N–O bond from an adjacent nitro group. ortho and para nitro substitution can increase N–N₂ bond activation through electron withdrawing, but this effect can be counterbalanced by steric interactions that twist the azide out of the plane of the ring or increase the N⋯O distance in the reactant. For example, steric factors decrease the sensitivity of energetic material TAzTNB relative to AzTNB consistent with experimental trends in impact sensitivity.