Sila-Explosive's Sensitivity Explained

Why is the silicon-based explosive silicon pentaerythritol tetranitrate (Si-PETN) so sensitive? Except for the central atom, which is silicon instead of carbon, Si-PETN is identical to the commercial explosive pentaerythritol tetranitrate (PETN). Yet Si-PETN's propensity for exploding at the slightest touch makes it too dangerous for practical use. Caltech's William A. Goddard III, Thomas M. Klapötke of Ludwig Maximilian University, in Munich, Germany, and colleagues now think they've identified the source of Si-PETN's unique reactivity (J. Am. Chem. Soc., DOI: 10.1021/ja809725p). Using density functional theory, the group studied the energetics of a number of different decomposition pathways for PETN and Si-PETN. Although most of these pathways were energetically similar for the two compounds, one pathway's energetics differed dramatically. Both compounds can undergo a rearrangement in which a methylene group and the adjacent oxygen from one of the molecules' four CH₂ONO₂ branches switch places—the oxygen atom ends up bonded to the central atom. This process has a much lower energy barrier and is more exothermic for Si-PETN than PETN. The researchers say this is because the Si–O bond is much stronger than the C–O bond, and also because silicon is much larger than carbon, making the rearrangement transition state more stable.