1.3 Prediction and Rationalisation of Energetic Material Sensitivity
13
similar approach was also based on 61 non-quantum mechanical descriptors [59].
Other authors have employed considerably smaller numbers of descriptors. Badder
et al. [60], for example, built a QSPR model for 10 nitro compounds based on
eight quantum-mechanical descriptors, with Shu [61] employing as few as two (nitro
group charge and oxygen balance) descriptors. In both cases, reasonable models were
obtained for the small test set of nitro-based compounds studied.
While these empirical methods are promising as screening tools, they offer no
physical insight into sensitivity properties.
1.3.1.2 Oxygen Balance
Based on the assumption that structurally related compounds should undergo similar
decomposition pathways, Kamlet [62] and subsequently Kamlet and Adolph [63]
proposed a comparison of the impact sensitivity of a compound against its oxygen
balance (OB). The OB was suggested to be relevant as it describes the ability of
a molecule to oxidise itself. That is, compounds that contain sufficient oxygen to
convert all nitrogen to NO 2 , all carbon to CO 2 and all H to H 2 O. For C-H-N-O
molecules, this is defined as [63]
O B 100 =
100(2n o − n h − 2n c − 2n coo )
M w
(1.4)
Across a series of over 70 compounds [62], a logarithmic correlation between the
50% impact heights (h 50 ) and OB. However, the compounds were found to follow two
different trends, depending on the structural features. For example, compounds with
the same number and relative position of nitro groups followed trends, or compounds
with/without α-C-H linkage followed their own trends (see Fig. 1.8). This method
has since been applied numerous times in the literature [64], often with very good
results. However, these methods are largely restricted to correlations within structural
types, and cannot establish correlations between these series—even for structurally
similar compounds like 2,4,6-trinitrobenzaldehyde and picric acid. If trend lines are
mixed, 2,4,6-trinitrobenzaldehyde is predicted to be much less sensitive than picric
acid, Fig. 1.8.
1.3.1.3 NMR Chemical Shift
The chemical shifts obtained in NMR are strongly dependent on the electronic structure of the molecule. It was therefore proposed that these chemical shifts should
reflect the relative bond strength of a structural moiety to the molecule backbone,
and thus give an indication of the stability of the structure. Correlations were initially
made by Owens [65] between the
1 H NMR chemical shifts and the impact sensitivity of trinitroarene compounds. A similar model was subsequently extended based
on
15 N and
13 C NMR chemical shifts by Zeman [66–68] It has also been extended
13
similar approach was also based on 61 non-quantum mechanical descriptors [59].
Other authors have employed considerably smaller numbers of descriptors. Badder
et al. [60], for example, built a QSPR model for 10 nitro compounds based on
eight quantum-mechanical descriptors, with Shu [61] employing as few as two (nitro
group charge and oxygen balance) descriptors. In both cases, reasonable models were
obtained for the small test set of nitro-based compounds studied.
While these empirical methods are promising as screening tools, they offer no
physical insight into sensitivity properties.
1.3.1.2 Oxygen Balance
Based on the assumption that structurally related compounds should undergo similar
decomposition pathways, Kamlet [62] and subsequently Kamlet and Adolph [63]
proposed a comparison of the impact sensitivity of a compound against its oxygen
balance (OB). The OB was suggested to be relevant as it describes the ability of
a molecule to oxidise itself. That is, compounds that contain sufficient oxygen to
convert all nitrogen to NO 2 , all carbon to CO 2 and all H to H 2 O. For C-H-N-O
molecules, this is defined as [63]
O B 100 =
100(2n o − n h − 2n c − 2n coo )
M w
(1.4)
Across a series of over 70 compounds [62], a logarithmic correlation between the
50% impact heights (h 50 ) and OB. However, the compounds were found to follow two
different trends, depending on the structural features. For example, compounds with
the same number and relative position of nitro groups followed trends, or compounds
with/without α-C-H linkage followed their own trends (see Fig. 1.8). This method
has since been applied numerous times in the literature [64], often with very good
results. However, these methods are largely restricted to correlations within structural
types, and cannot establish correlations between these series—even for structurally
similar compounds like 2,4,6-trinitrobenzaldehyde and picric acid. If trend lines are
mixed, 2,4,6-trinitrobenzaldehyde is predicted to be much less sensitive than picric
acid, Fig. 1.8.
1.3.1.3 NMR Chemical Shift
The chemical shifts obtained in NMR are strongly dependent on the electronic structure of the molecule. It was therefore proposed that these chemical shifts should
reflect the relative bond strength of a structural moiety to the molecule backbone,
and thus give an indication of the stability of the structure. Correlations were initially
made by Owens [65] between the
1 H NMR chemical shifts and the impact sensitivity of trinitroarene compounds. A similar model was subsequently extended based
on
15 N and
13 C NMR chemical shifts by Zeman [66–68] It has also been extended