Acid-base interactions in energetic materials: 1. The hard and soft acids and bases (HSAB) principle - Insights to reactivity and sensitivity of energetic materials

The chemistry of energetic materials can be described applying acid-base reaction formalism. Addressing the HSAB concept, the number of electrons transferred, Delta N, in an acid-base reaction, allows for description and prediction of properties of composite and homogeneous materials. At first Delta N helps in estimating the rate of reaction of binary systems with either given fuel or oxidizer. Nevertheless Delta N is only a relative number thus the range of comparability remains narrow. At second Delta N can be used as a measure for the sensitivity of homogeneous explosives. The increased reactivity of hypothetical fragments to recombine in a reaction such as R3Ccenter dot+(NO2)-N-center dot = R3C - NO2 given by Delta N correlates very well with experimentally determined reduced impact sensitivity of 1,3,5-trinitrobenzene compounds. On the contrary the rising impact sensitivity of metal azides correlates with rising values of Delta N of Mn+/N-3(-) reaction because increased reactivity, that is increased electron transfer from the azide anion to the metal cation triggers formation of the azide radical (N-center dot(3)). The latter then decomposes rapidly to give dinitrogen. This increased reactivity/sensitivity of metral azides coincides with covalent bonding whereas ionic azides are relatively insensitive.