The synthesis of explosives, propellants, and pyrotechnics has come a long way since Alfred Nobel’s work with dynamite, but chemists are still modifying energetic materials for better payloads. The challenge now is to make more powerful explosives, pyrotechnics with purer light emission, and materials that produce greener byproducts (yes, that last one's a bit ironic). Popular explosives in current use like TNT and RDX (1,3,5-trinitro-1,3,5-triazinane) are mainly fueled by the oxidation of the carbon atoms in their molecular backbone. Modern techniques involve designing molecules with high heats of formation (meaning there's more energy to burn) and added strain in their molecular structures (meaning the energy's released more quickly) for more efficient explosions and safer handling.
With those techniques in mind, Thomas Klapötke’s research group at the Ludwig-Maximilian University of Munich created a new type of energetic material based on the anionic (negatively charged) tetrazole, which is paired with nitrogen-rich energetic cations (positively charged ions) in explosives. Tetrazoles come with several beneficial properties: high heats of formation, ring strain, high density, and thermal stability. One thing that tetrazoles lack is a good oxygen balance.
Oxygen balance is important for explosives, as it represents the percentage of available oxygen atoms in a molecule that can react with the other atoms (oxidation is a major energy source). Tetrazoles have a low oxygen balance, meaning that they don’t have enough oxygen atoms in its structure. Thus, when it is paired with high-energy, nitrogen-rich cations, it fails to oxidize those nitrogens, which cuts down on the explosive punch.
Klapötke’s group dealt with this problem by oxidizing an anionic tetrazole to give it one more oxygen atom. They took nitrotetrazolate and subjected it to oxone (a popular oxidizing agent) to create the nitrotetrazolate oxide anion, which can be paired with a variety of high-energy cations to form explosive salts. They are the first to report success with this type of reaction.
In terms of thermal stability, the nitrotetrazole oxide anion salts decompose between 153 and 211°C, depending on what cation it is paired with. A couple of the salts had good liquid ranges, with at least a 70°C difference between their melting and decomposition temperatures, so they would be ideal for making melt-cast explosives. As for heats of formation, most of the nitrotetrazolate oxide salts actually showed lower heats of formation than the unmodified nitrotetrazolate salts, but that disadvantage did not seem to have a negative impact on their calculated detonation properties.
All eight of the nitrotetrazolate oxides that the authors made showed significantly improved detonation parameters compared to the original nitrotetrazolates. The detonation velocities increased by as much as 1000 meters per second in one case, and even in the worst performing oxide salt, the velocity increased by 300 meter per second. The majority of the oxide salts also had relatively low sensitivities, making them reasonably safe to handle.
The authors demonstrated a simple method for creating a new class of energetic materials. Their computational calculations of detonation parameters show that these oxide salts have potential for field use. Obviously, actual experimental work is necessary to determine if these salts are truly viable explosives. In addition, we need to know what byproducts these nitrotetrazole oxide salts produce after detonation, especially if they are intended to be green improvements on current explosives. However, they are probably greener than RDX and TNT, as tetrazoles generally produce non-toxic reaction products.
J. Am. Chem. Soc., 2010. DOI: 10.1021/ja106892a