Correlation of Structure and Sensitivity in Inorganic Azides III. A Mechanistic Interpretation of Impact Sensitivity Dependency on Non‐bonded Nitrogen to Nitrogen Distance

Abstract

AbstractPreviously established correlations between impact sensitivity and minimum, non‐bonded, nitrogen to nitrogen distances in inorganic azides, imply that there must be a mechanism in operation, which can predict the non reaction of alkali metal azides and the violent decomposition of copper, silver, and lead azides. This paper examines the molecular orbitals used for bonding in the azides. The orbital energy level diagram indicates that the highest occupied molecular orbital, HOMO, are two, π type, non‐bonded orbitals, each occupied by an electron pair. The electron density lobes for these π type orbitals protrude into the space beyond both ends of the azide ion. These orbitals can overlap with ‘p’ and ‘d’ type orbitals on the metal cation, facilitating the transfer of the electron back to the metal cation; an integral part of the decomposition reaction. If an exciton is generated on the azide ion, the electron can migrate, via the extended three center MO, to the metal cation, leaving the positive hole on the terminal nitrogen atom. A similar hole on an adjacent azide, would allow the non‐bonding orbitals on each azide to interact. As the distance between neighboring azide ions decreases, it is postulated that these, non‐bonding, π type orbitals start to overlap and become bonding orbitals between adjacent azide ions. This process forms an unstable N6 moiety, which leads to the formation of three nitrogen molecules from two original azide ions. Thus, a feasible mechanism for the reaction can explain the observation that azides with non‐bonded nitrogen to nitrogen distances of >300 nm do not show impact sensitivity but, as this distance decreases below 300 nm, the sensitivity increases. The non impact sensitive azides could respond to thermal stimulus, which increases the thermal motion, thus reducing the critical nitrogen to nitrogen non‐bonded distance and reducing the energy for exciton production. Further work is required on the energy changes for such a reaction.