Sensitivity and the available free space per molecule in the unit cell

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, Delta V. We express Delta V 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 Delta V becomes larger. For nitramines, the dependence upon Delta V is rather weak; we interpret this as indicating that a single overriding factor dominates their initiation mechanism, e.g., N-NO2 rupture. (An analogous situation appears to hold for many organic azides.) In addition to the conceptual significance of identifying Delta V as a factor in impact sensitivity, the present results allow rough estimates of relative sensitivities that are not known.