N-N Bond Lengths and Initiation Reactivity of Nitramines

For the 16 selected nitramines, it is shown that an increase in the energy content of these molecules (represented by enthalpies of formation) is connected with an increase in the lengths of the longest N-N bonds in the molecules. These lengths are directly proportional to the activation energies of the low-temperature thermal decomposition of the pure nitramines in all states of matter for this reaction. Raising the energy content also leads to reductions in the rate constants of thermal decomposition. Both of these facts are in contrast to expectations and also with similar published findings concerning thermal decomposition of nitramines in solution, which can be explained by the solvation effect and termination of the emerging aza-radicals in solutions. The calculated dissociation energies of the weakest N-N bonds yielded a relatively good reciprocal conformity with the lengths of the longest N-N bonds of the nitramines studied, especially when using the UB3LYP/6-31G* method. The relationship between the impact sensitivity of these nitramines and the lengths of their longest N-N bond is not completely clear. Such lengths cannot be a measure of impact reactivity, because the longest N-N bond might be stabilized in some cases by suitable intermolecular interactions with adjacent molecules in the crystal lattice.