We have investigated the possibility of a link between the impact sensitivities of energetic compounds and the space available to their molecules in their crystal lattices. As a measure of this space, we use Delta V=V(eff)-V(0.002), where V(eff) is the effective molecular volume obtained from the crystal density and V(0.002) is that enclosed by the 0.002 au contour of the molecule's gas phase electronic density, determined computationally. When experimental impact sensitivity was plotted against Delta V for a series of 20 compounds, the nitramines formed a separate group showing little dependence upon Delta V. Their impact sensitivities correlate well with an anomalous imbalance in the electrostatic potentials on their molecular surfaces, which is characteristic of energetic compounds in general. The imbalance is symptomatic of the weakness of the N-NO(2) bonds, caused by depletion of electronic charge. The impact sensitivities of non-nitramines, on the other hand, depend much more strongly upon Delta V, and can be quite effectively related to it if an electrostatically-based correction term is included.

Invoking the known link between impact sensitivity and compressibility, we have expanded upon an earlier preliminary study of the significance of the available free space per molecule in the unit cell, ΔV. We express ΔV as V(eff) - V(int), where V(eff) corresponds to zero free space, V(eff) = molecular mass/density. V(int) is the intrinsic gas phase molecular volume. We demonstrate that V(int) can be appropriately defined as the volume enclosed by the 0.003 au contour of the molecule's electronic density; this produces packing coefficients that have the range and average value found crystallographically. Measured impact sensitivities show an overall tendency to increase as ΔV becomes larger. For nitramines, the dependence upon ΔV is rather weak; we interpret this as indicating that a single overriding factor dominates their initiation mechanism, e.g., N-NO(2) rupture. (An analogous situation appears to hold for many organic azides.) In addition to the conceptual significance of identifying ΔV as a factor in impact sensitivity, the present results allow rough estimates of relative sensitivities that are not known.

Layered materials provide a two-dimensional interlayer space suitable for accommodating molecules with a designed functionality. In this study, inorganic−organic hybrids were prepared by intercalation of anionic porphyrin sensitizers into the host structure of layered zinc hydroxide salts. The inorganic host offers stabilization and protection, whereas the guest species provide the photofunction. The properties and arrangement of porphyrin molecules in the interlayer space were studied by a combination of experimental techniques and molecular simulations. Intercalation of porphyrins led to a gallery height that is comparable with the size of porphyrin molecules. Molecular simulations showed that the interlayer space is filled with disordered porphyrin units. The porphyrin sulfonate groups interact with the brucite-like layers via dominant electrostatic interactions similarly to layered double hydroxides. The photophysical experiments proved that intercalated anionic Pd porphyrins produce singlet oxygen, O2(1Δg), with long effective lifetimes, suggesting that layered zinc hydroxide salts are good carriers of porphyrin sensitizers.