Chapter 6
General Conclusions and Future
Directions
6.1 General Conclusions
The work in this thesis has explored the development and application of a model to
predict the relative impact sensitivity of a range of EMs. This model is based on the
concept of vibrational up-pumping, which was developed to rationalise the localisation (and hence intensification) of energy resulting from mechanical perturbation of
a solid. In contrast to previous attempts, the model in this thesis is based purely on
ab initio input. Hence, this work provides a new approach to predict relative impact
sensitivities of EMs.
A model was first constructed for a series of nine azide-based EMs, selected
for investigation on the basis of their diverse structural types and range of experimental impact sensitivities. Based on literature reports, the relative sensitivities of
these compounds should follow the order: NaN 3 ≈ TAGZ (triaminoguanidinium
azide) ≈ NH 4 N 3 < LiN 3 < Ba(N 3 ) 2 < AgN 3 < Sn(N 3 ) 2 , with the exact position of
HN 3 and Zn(N 3 ) 2 within this order being unknown, except that they are sensitive
to impact. Due to the simplicity of the N
−
3 explosophore, it was possible to investigate these systems within the framework of a ‘direct’ up-pumping mechanism.
Hence, the vibrational normal coordinates of the explosophore were followed and
its electronic structure was monitored. It was found that the bending modes (δθ NNN )
led to crossing of the ground-state (S 0 ) and first triplet-state (T 1 ) potential energy
surfaces (PES). Dissociation of the N–N bond of N
−
3 is favourable for the T 1 PES,
and hence δθ NNN is suggested as the target vibrational mode. This was confirmed
in the solid state by monitoring the evolution of the electronic band structure as
a function of the normal coordinates associated with crystalline NaN 3 . Based on
ab initio phonon dispersion curves, the vibrational up-pumping into the target mode
within each system was considered. In line with previous consideration of the uppumping model, the overtone and combination pathway contributions were isolated.
Using only the overtone contributions (which is the method proposed in previous
work [1, 2]), the sensitivity ordering was not well reproduced. While the prediction
does generally place the insensitive compounds at lower sensitivity than the sensitive
© Springer Nature Switzerland AG 2020
A. A. L. Michalchuk, Mechanochemical Processes in Energetic Materials,
Springer Theses, https://doi.org/10.1007/978-3-030-56966-2_6
179
General Conclusions and Future
Directions
6.1 General Conclusions
The work in this thesis has explored the development and application of a model to
predict the relative impact sensitivity of a range of EMs. This model is based on the
concept of vibrational up-pumping, which was developed to rationalise the localisation (and hence intensification) of energy resulting from mechanical perturbation of
a solid. In contrast to previous attempts, the model in this thesis is based purely on
ab initio input. Hence, this work provides a new approach to predict relative impact
sensitivities of EMs.
A model was first constructed for a series of nine azide-based EMs, selected
for investigation on the basis of their diverse structural types and range of experimental impact sensitivities. Based on literature reports, the relative sensitivities of
these compounds should follow the order: NaN 3 ≈ TAGZ (triaminoguanidinium
azide) ≈ NH 4 N 3 < LiN 3 < Ba(N 3 ) 2 < AgN 3 < Sn(N 3 ) 2 , with the exact position of
HN 3 and Zn(N 3 ) 2 within this order being unknown, except that they are sensitive
to impact. Due to the simplicity of the N
−
3 explosophore, it was possible to investigate these systems within the framework of a ‘direct’ up-pumping mechanism.
Hence, the vibrational normal coordinates of the explosophore were followed and
its electronic structure was monitored. It was found that the bending modes (δθ NNN )
led to crossing of the ground-state (S 0 ) and first triplet-state (T 1 ) potential energy
surfaces (PES). Dissociation of the N–N bond of N
−
3 is favourable for the T 1 PES,
and hence δθ NNN is suggested as the target vibrational mode. This was confirmed
in the solid state by monitoring the evolution of the electronic band structure as
a function of the normal coordinates associated with crystalline NaN 3 . Based on
ab initio phonon dispersion curves, the vibrational up-pumping into the target mode
within each system was considered. In line with previous consideration of the uppumping model, the overtone and combination pathway contributions were isolated.
Using only the overtone contributions (which is the method proposed in previous
work [1, 2]), the sensitivity ordering was not well reproduced. While the prediction
does generally place the insensitive compounds at lower sensitivity than the sensitive
© Springer Nature Switzerland AG 2020
A. A. L. Michalchuk, Mechanochemical Processes in Energetic Materials,
Springer Theses, https://doi.org/10.1007/978-3-030-56966-2_6
179