3.6 Conclusions
105
3.6 Conclusions
Azides represent a broad class of energetic compounds, covering a wide range of
structural chemistry and impact sensitivity. To understand the initiation of a chemical
explosion, it is necessary to understand the decomposition of the explosophoric
moiety. For the simple azides, this is rupture of an N-N covalent bond. The electronic
structure of the N
−
3 molecule was therefore investigated. Bond dissociation was
found to be possible via an athermal mechanism, provided sufficient excitation of the
bending vibrational mode of N
−
3 . At sufficient perturbation of the nuclear geometry,
a conical intersection (CI) was observed between the S 0 and T 1 electronic states. The
potential energy surface of the latter readily facilitates N-N bond dissociation. The
existence of this CI was verified in the solid state.
Noting that mechanical impact leads to rapid excitation of lattice vibrations,
the up-pumping model was explored as a mechanism for impact initiation. Based
on the electronic structure calculations, the bending motion was selected as the
target vibration into which vibrational energy must up-convert to initiate an explosion. Considering both overtone and combination up-conversion pathways, it was
found that the relative rate of up-pumping into this target vibration led to excellent
correlation with experimental impact sensitivities. Thus, the work presented in this
chapter demonstrates the first fully ab initio approach to the prediction of the relative
impact sensitivities of energetic azide materials, without the need for any empirical
correlations.
The rate of up-conversion was found to be largely dependent on two key vibrational
frequencies: (1) the maximum phonon frequency, Ω max , and (2) the frequency of the
N
−
3 bending mode, ω T . This therefore offers targets and rationales for the design of
novel materials:
1. max . This value depends on the nature of the external lattice modes and crystal
packing. As such, a model based on vibrational energy transfer includes potential
for understanding the different sensitivities of polymorphic and multi-component
materials (co-crystals and salts). Stronger bonding of the N
−
3 anion within the
lattice (i.e. polymeric or molecular structures) was found to correlate with higher
max . It also follows that more compressible materials will exhibit a higher max
when subject to a mechanical perturbation. As such, materials based on weaker
non-covalent interactions between energetic molecules may be more sensitive
2. ω T . This value depends on the bonding nature of the N
−
3 anion within the crystal
lattice. Higher covalent character leads to a decrease in ω T , and thus enhanced
sensitivity. The increased covalent character between the N
−
3 molecule and a
cation also appear to weaken the N-N bond. Initiation may therefore be easier.
A particular strength of the model presented in this chapter is the fact that it
encompasses many aspects of earlier models reported in the literature. For example,
within the framework of this chapter, there are clear rationales for a correlation
of band gap and bond dissociation energy with impact sensitivity. Further, effects
such as packing density and crystal packing can all find a physical basis within this
105
3.6 Conclusions
Azides represent a broad class of energetic compounds, covering a wide range of
structural chemistry and impact sensitivity. To understand the initiation of a chemical
explosion, it is necessary to understand the decomposition of the explosophoric
moiety. For the simple azides, this is rupture of an N-N covalent bond. The electronic
structure of the N
−
3 molecule was therefore investigated. Bond dissociation was
found to be possible via an athermal mechanism, provided sufficient excitation of the
bending vibrational mode of N
−
3 . At sufficient perturbation of the nuclear geometry,
a conical intersection (CI) was observed between the S 0 and T 1 electronic states. The
potential energy surface of the latter readily facilitates N-N bond dissociation. The
existence of this CI was verified in the solid state.
Noting that mechanical impact leads to rapid excitation of lattice vibrations,
the up-pumping model was explored as a mechanism for impact initiation. Based
on the electronic structure calculations, the bending motion was selected as the
target vibration into which vibrational energy must up-convert to initiate an explosion. Considering both overtone and combination up-conversion pathways, it was
found that the relative rate of up-pumping into this target vibration led to excellent
correlation with experimental impact sensitivities. Thus, the work presented in this
chapter demonstrates the first fully ab initio approach to the prediction of the relative
impact sensitivities of energetic azide materials, without the need for any empirical
correlations.
The rate of up-conversion was found to be largely dependent on two key vibrational
frequencies: (1) the maximum phonon frequency, Ω max , and (2) the frequency of the
N
−
3 bending mode, ω T . This therefore offers targets and rationales for the design of
novel materials:
1. max . This value depends on the nature of the external lattice modes and crystal
packing. As such, a model based on vibrational energy transfer includes potential
for understanding the different sensitivities of polymorphic and multi-component
materials (co-crystals and salts). Stronger bonding of the N
−
3 anion within the
lattice (i.e. polymeric or molecular structures) was found to correlate with higher
max . It also follows that more compressible materials will exhibit a higher max
when subject to a mechanical perturbation. As such, materials based on weaker
non-covalent interactions between energetic molecules may be more sensitive
2. ω T . This value depends on the bonding nature of the N
−
3 anion within the crystal
lattice. Higher covalent character leads to a decrease in ω T , and thus enhanced
sensitivity. The increased covalent character between the N
−
3 molecule and a
cation also appear to weaken the N-N bond. Initiation may therefore be easier.
A particular strength of the model presented in this chapter is the fact that it
encompasses many aspects of earlier models reported in the literature. For example,
within the framework of this chapter, there are clear rationales for a correlation
of band gap and bond dissociation energy with impact sensitivity. Further, effects
such as packing density and crystal packing can all find a physical basis within this