12
1 Introduction
dubbed h 50 . For the purpose of the following, discussion will be limited to the main
models that are aimed at predicting sensitivity to mechanical stimulation.
1.3.1 Isolated Molecule Methods
Much of the work that has attempted to rationalise the impact sensitivity of EMs is
based on the study of isolated molecules. Initially, this was a consequence of limited
access to crystal structures of these compounds as well as limitations on computational methods. Numerous models were developed, and the relative simplicity of
models based on isolated molecules keeps their development an ongoing area of
research still today. Broadly, they can be classified into methods based on structural
parameters and those based on quantum-mechanical properties.
1.3.1.1 Empirical Fitting of Molecular Descriptors
Many attempts have been made to develop fully empirical fits between molecular
descriptors and sensitivities. These methods have been particularly popular for largescale screening programmes, and indeed amongst the most powerful, although they
offer no physical mechanism for their success. For example, Keshavarz [53] proposed
a general equation based on chemical composition that was able to fit the impact
sensitivities of EM compounds with formula C a’ H b’ N c’ O d’ ,
log(h 50 ) = c 1 a
+ c 2 b
+ c 3 c
+ c 4 d
(1.3)
where c 1 –c 4 are adjustable fitting parameters. It was found that by adjusting the
relative coefficients, a broad range of structural types could be analysed. However,
this fitting remains limited to families of compounds. A variety of similar equations
have also been proposed [54–56] and have generally offered an excellent and rapid
means to assess impact sensitivity of large libraries of materials.
In a similar fashion, empirical models based on molecular descriptors have become
a popular approach, known as the Quantitative Structure-Property Relation (QSPR)
methods. In these methods, a large number of molecular descriptors are chosen,
including ionisation potentials, electrostatic potentials, oxygen balance, molecular
orbital energies, bond lengths, and many others. Regression models are subsequently
established on large databases and empirical equations established. Early QSPRbased models employed structural descriptors, with Fayet [57, 58] being amongst
the first to employ quantum-mechanical descriptors for EMs.
Based on a series of 300 quantum-mechanical molecular descriptors, Fayet [58]
investigated the impact sensitivity of a total of 161 nitro compounds, separated
into three structural classes, using the QSPR approach. For each class of materials,
this proved very promising, with R
2 > 0.8 in each case. The inability to produce
stronger correlations was largely ascribed to poor experimental data [58, 59]. A
1 Introduction
dubbed h 50 . For the purpose of the following, discussion will be limited to the main
models that are aimed at predicting sensitivity to mechanical stimulation.
1.3.1 Isolated Molecule Methods
Much of the work that has attempted to rationalise the impact sensitivity of EMs is
based on the study of isolated molecules. Initially, this was a consequence of limited
access to crystal structures of these compounds as well as limitations on computational methods. Numerous models were developed, and the relative simplicity of
models based on isolated molecules keeps their development an ongoing area of
research still today. Broadly, they can be classified into methods based on structural
parameters and those based on quantum-mechanical properties.
1.3.1.1 Empirical Fitting of Molecular Descriptors
Many attempts have been made to develop fully empirical fits between molecular
descriptors and sensitivities. These methods have been particularly popular for largescale screening programmes, and indeed amongst the most powerful, although they
offer no physical mechanism for their success. For example, Keshavarz [53] proposed
a general equation based on chemical composition that was able to fit the impact
sensitivities of EM compounds with formula C a’ H b’ N c’ O d’ ,
log(h 50 ) = c 1 a
+ c 2 b
+ c 3 c
+ c 4 d
(1.3)
where c 1 –c 4 are adjustable fitting parameters. It was found that by adjusting the
relative coefficients, a broad range of structural types could be analysed. However,
this fitting remains limited to families of compounds. A variety of similar equations
have also been proposed [54–56] and have generally offered an excellent and rapid
means to assess impact sensitivity of large libraries of materials.
In a similar fashion, empirical models based on molecular descriptors have become
a popular approach, known as the Quantitative Structure-Property Relation (QSPR)
methods. In these methods, a large number of molecular descriptors are chosen,
including ionisation potentials, electrostatic potentials, oxygen balance, molecular
orbital energies, bond lengths, and many others. Regression models are subsequently
established on large databases and empirical equations established. Early QSPRbased models employed structural descriptors, with Fayet [57, 58] being amongst
the first to employ quantum-mechanical descriptors for EMs.
Based on a series of 300 quantum-mechanical molecular descriptors, Fayet [58]
investigated the impact sensitivity of a total of 161 nitro compounds, separated
into three structural classes, using the QSPR approach. For each class of materials,
this proved very promising, with R
2 > 0.8 in each case. The inability to produce
stronger correlations was largely ascribed to poor experimental data [58, 59]. A