The electronic look of explosives.

The electronic look of explosives

Chemical explosives with similar molecular structures often show great differences in their sensitivity to shock waves used to detonate them. One such group is the nitroaromatic explosives, which includes compounds such as trinitrotoluene (TNT) and triaminotrinitrobenzene (TATB). These explosives have a molecular structure consisting of an aromatic or benzene ring of six carbon atoms to which nitro (NO2) and sometimes other chemical groups are attached. Depending on the specific molecular arrangement, the pressure needed to initiate an explosion varies by as much as a factor of five.

Recently, a team of scientists at the Sandia National Laboratories in Albuquerque, N.M., led by J. William Rogers Jr., used a sophisticated technique known as X-ray-excited Auger electron spectroscopy to investigate in detail the arrangement of electrons in molecules of these explosives. The "soft' X-rays, unlike electron beams, are gentle enough to excite electrons within the material's atoms without causing the explosive to detonate. The technique allows the researchers to look at the energy levels associated with bonds between carbon atoms.

Their analysis confirms theoretical predictions that the addition of nitro groups weakens the carbon-carbon links in the aromatic ring. This makes molecules with nitro groups easier to break apart and hence more sensitive to shock waves. If amino (NH2) groups are also present, the compounds are more shock resistant. The amino groups seem to strengthen carbon-carbon bonds by adding to their electron density. Moreover, this redistribution of charge makes amino groups slightly positive and any nitro groups present slightly negative. As a result, molecules having both nitro and amino groups can attract one another to form a network. This network absorbs some of the energy carried by a shock wave, reducing the amount of energy reaching the ring itself.

Overall, the Sandia studies show that the stability of the carbon ring is at least partly responsible for shock sensitivity. However, macroscopic properties such as particle size and shape and the material's density also play important roles. A full understanding of how explosions are initiated is likely to come only after the interplay between microscopic and macroscopic effects is studied.