As initiation of an energetic material requires rupture of a covalent bond, and therefore population of antibonding electronic states, consideration of the electronic band gap has dominated initiation mechanisms for solid state materials. Most prominent are models based on metallisation, where static mechanical perturbation leads to closing of the electronic band gap. This work explores an alternative mechanism for the dynamic metallisation of a model energetic material, where vibrational excitation resulting from mechanical impact is found to induce transient metallisation of α-NaN3. The normal coordinates associated with bending the azido anion close the electronic band gap, facilitating the formation of highly reactive species important for initiation of energetic materials. The DFT simulated vibrational spectrum of α-NaN3 exhibits excellent reproduction of the experimental low-temperature inelastic neutron scattering spectrum (INS).