Our recently admired millennium fireworks were largely based on gunpowder, a mixture of potassium nitrate, sulfur, and charcoal that was already being used in eighth century China. In contrast, explosives of an entirely different magnitude have to be deployed when large-scale blasting is involved, such as in tunnel or mountain road construction.
A new explosive - octanitrocubane - has been synthesized by chemists Philip E. Eaton and Mao-Xi Zhang at the University of Chicago and its structure proven by Richard Gilardi of the Naval Research Laboratory in Washington D.C. According to calculations, octanitrocubane could be more powerful than the best non-nuclear explosives.
How does the explosive effect come about? Oxygen atoms contained in the molecules of the explosive oxidize the other "combustible" parts of the molecule, in most cases carbon and hydrogen. This causes heat and hot gases to be released very quickly - leading to an explosion. In a detonation, the most violent form of explosion, speeds up to 10,000 m/s, temperatures up to 6,000 °C, and pressures up to 300,000 bar can be achieved in the shock wave.
Molecules that contain nitro groups (NO2) are good candidates for explosives. The nitro group provides the essential oxygen for the combustion, and furthermore, the nitrogen atoms are converted to dinitrogen (N2), increasing the volume of liberated gas. The widely-used explosive trinitrotoluene (TNT) is already a "classic": TNT was discovered in 1863. More powerful than TNT is HMX (octogen), an eight-membered ring made of carbon and nitrogen atoms, which supports four nitro groups.
Synthesis of the new octanitrocubane explosive was tricky, because it is based on cubane, whose molecular "framework" consists of eight carbon atoms held rigidly at the corners of a cube. Such a frame is under high strain and can very easily burst, giving off energy. The researchers had to very cautiously attach a nitro group to each of the corners of the cube in order to make the desired product.
Octanitrocubane is one of the most dense compounds built only of carbon, nitrogen and oxygen. Surprisingly, however, in its present crystal form it is significantly less dense than predicted by theory. High density is crucial to the strength of an explosive, because the pressure of the detonation increases tremendously with increasing density of the explosive. "We are now looking for this denser crystalline form of octanitrocubane, which is in better agreement with predictions," says Eaton.