Sensitivities of High Energy Compounds

Convenient Preparation, Thermal Properties and X-ray Structure Determination of 2,3-Dihydro-5,6,7,8-tetranitro-1,4-benzodioxine (TNBD): A Promising High-Energy-Density Material

2,3-dihydro-5,6,7,8-tetranitro-1,4-benzodioxine (TNBD), molecular formula = C8H4N4O10, is a completely nitrated aromatic ring 1,4-benzodioxane derivative. The convenient method of TNBD synthesis was developed (yield = 81%). The detailed structure of this compound was investigated by X-ray crystallography. The results of the thermal analysis (TG) obtained with twice re-crystallized material revealed the onset at 240 °C (partial sublimation started) and melting at 286 °C. The investigated material degraded completely at 290-329 °C. The experimental density of 1.85 g/cm3 of TNBD was determined by X-ray crystallography. The spectral properties of TNBD (NMR, FT-IR and Raman) were explored. The detonation properties of TNBD calculated by the EXPLO 5 code were slightly superior in comparison to standard high-energy material-tetryl (detonation velocity of TNBD-7727 m/s; detonation pressure-278 kbar; and tetryl-7570 m/s and 226.4 kbar at 1.614 g/cm3, or 260 kbar at higher density at 1.71 g/cm3. The obtained preliminary results might suggest TNBD can be a potential thermostable high-energy and -density material (HEDM).

Exploring the thermal decomposition and detonation mechanisms of 2,4-dinitroanisole by TG-FTIR-MS and molecular simulations

2,4-dinitroanisole (DNAN), an insensitive explosive, has replaced trinitrotoluene (TNT) in many melt-cast explosives to improve the safety of ammunition and becomes a promising material to desensitize novel explosives of high sensitivity. Here, we combine thermogravimetric-Fourier transform infrared spectrometry-Mass spectrometry (TG-FTIR-MS), density functional theory (DFT), and ReaxFF molecular dynamics (MD) to investigate its thermal decomposition and detonation mechanisms. As revealed by TG-FTIR-MS, the thermal decomposition of DNAN starts at ca. 453 K when highly active NO2 is produced and quickly converted to NO resulting in the formation of a large amount of Ph(OH)(OH2)OCH3+. DFT calculations show that the activation energy of DNAN is higher than that of TNT due to the lack of α-H. Further steps in both thermal decomposition and detonation reactions of the DNAN are dominated by bimolecular O-transfers. ReaxFF MD indicates that DNAN has a lower heat of explosion than TNT, in accordance with the observation that the activation energies of polynitroaromatic explosives are inversely proportional to their heat of explosion. The inactive -OCH3 group and less nitro groups also render DNAN higher thermal stability than TNT.

The effect of {O,N}=X⋯M={Ti,Zr,Hf} interactions on the sensitivity of CNO2 trigger bonds in FOX-7: Approach based on the QTAIM/EDA-NOCV analysis

The local chemical reactivity of FOX-7 (1,1-diamino-2,2-nitroethylene, also known as DADNE from DiAminoDiNitroEthylene) was elucidated through a quantitative study of the electrostatic potential on the molecular surface, topological analysis based on Bader's theory, and the EDA-NOCV method. Unlike (O2N)2CC(NH2)H2N⋯Cp2MCH3+ complexes, which exhibit both σ-donor and π-acceptor features, the situation is different concerning the (H2N)2CC(NO2)(O)NO⋯Cp2MCH3+ complexes, where both charge transfers correspond to the σ-donation. The two charge transfers reinforce each other, resulting in increased stability for (H2N)2CC(NO2)(O)NO⋯Cp2MCH3+. This seems to strengthen the (H2N)2CC(NO2)(O)NO⋯M={Ti,Zr,Hf} bond, which may explain the high stability of (H2N)2CC(NO2)(O)NO⋯Cp2MCH3+ compared to (O2N)2CC(NH2)-H2N⋯Cp2MCH3+. Results from topological analysis revealed that the decreased sensitivity to decomposition of CNO2 bonds depends on the chemical nature of the interacting metal, and the best achievements are obtained for the Hf-based complex. Our results demonstrate that the interaction of M={Ti,Zr,Hf} with CNO2 is more favourable than that with CNH2, this specific action on the trigger bond may support the use of Metallocene Methyl Cations (MMC) as possible neutralisers.

Thermal stability of emerging N6-type energetic materials: kinetic modeling of simultaneous thermal analysis data to explain sensitivity trends

A number of new high-performing energetic materials possess explosophoric functionalities, high nitrogen content, and fused heterocyclic blocks. Two representatives of these materials have been synthesized recently, namely, 1,2,9,10-tetranitrodipyrazolo[1,5-d:5',1'-f][1,2,3,4]-tetrazine (1) and 2,9-dinitrobis([1,2,4]triazolo)[1,5-d:5',1'-f][1,2,3,4]tetrazine (2). The thermal stability of these energetic materials bearing the N-N-N = N-N-N fragment and three closely related compounds has been investigated for the first time. The thermal decomposition process of analyzed compounds was complicated by the appearance of the liquid phase, sublimation of the material, and autocatalysis by reaction products. In contrast to the traditional approach to the kinetic modeling based on data from either TGA or DSC, we use both signals' data measured at the same time and perform the joint kinetic analysis using the model-fitting technique to obtain the pertinent kinetic description of the process. Of the analyzed materials, 1 and 2 show the lowest thermal stability in melt with a characteristic rate constant of 2.6 × 10^-3 s^-1 at 250 °C. The kinetic parameters and calculated detonation performance data were used in the model to describe the mechanical sensitivity. The model output and the experimental friction sensitivity data show a respectable agreement, but more data are required to draw firm conclusions. In general, the provided thermal stability and kinetic data can be used for thermal response and storage modeling of these new N6-type energetic materials. The developed thermokinetic approach, joint model-fitting of several thermal analysis signals, can be applied to other complex thermally induced processes to increase the value and credibility of the kinetic findings.

Surprising Chemistry of 6-Azidotetrazolo[5,1-a]phthalazine: What a Purported Natural Product Reveals about the Polymorphism of Explosives

6-Azidotetrazolo[5,1-a]phthalazine (ATPH) is a nitrogen-rich compound of surprisingly broad interest. It is purported to be a natural product, yet it is closely related to substances developed as explosives and is highly polymorphic despite having a nearly planar structure with little flexibility. Seven solid forms of ATPH have been characterized by single-crystal X-ray diffraction. The structures show diverse patterns of molecular organization, including both stacked sheets and herringbone packing. In all cases, N···N and C-H···N interactions play key roles in ensuring molecular cohesion. The high polymorphism of ATPH appears to arise in part from the ability of virtually every atom of nitrogen and hydrogen in the molecule to take part in close N···N and C-H···N contacts. As a result, adjacent molecules can adopt many different relative orientations that are energetically similar, thereby generating a polymorphic landscape with an unusually high density of potential structures. This landscape has been explored in detail by the computational prediction of crystal structures. Studying ATPH has provided insights into the field of energetic materials, where access to multiple polymorphs can be used to improve performance and clarify how it depends on molecular packing. In addition, our work with ATPH shows how valuable insights into molecular crystallization, often gleaned from statistical analyses of structural databases, can also come from in-depth empirical and theoretical studies of single compounds that show distinctive behavior.

Theoretical investigation into the solvent effect on the thermal decomposition of RDX in tetrahydrofuran, acetone, toluene, and benzene

In order to clarify the solvent effect on the thermal decomposition of explosive, the N-NO₂ trigger-bond strengths and ring strains of RDX (cyclotrimethylenetrinitramine) in its H-bonded complexes with solvent molecules (i.e., tetrahydrofuran, acetone, toluene, and benzene), and the activation energies of the intermolecular hydrogen exchanges between the solvent molecules and C₃H₈O₂N₄ or CH₄O₂N₂, as the model molecule of RDX, were investigated by the BHandHLYP, B3LYP, MP2(full), and M06-2X methods with the 6-311 +  + G(2df,2p) basis set, accompanied by a comparison with the calculations by the integral equation formalism polarized continuum model. The solvent effects ignore the ring strain while strengthening the N-NO₂ bond, leading to a possible decreased sensitivity, as is opposite to the experimental results. However, the activation energies are in the order of C₃H₈O₂N₄/CH₄O₂N₂∙∙∙acetone < C₃H₈O₂N₄/CH₄O₂N₂∙∙∙THF < C₃H₈O₂N₄/CH₄O₂N₂∙∙∙toluene < C₃H₈O₂N₄/CH₄O₂N₂∙∙∙benzene < C₃H₈O₂N₄/CH₄O₂N₂, suggesting that the order of the critical explosion temperatures might be RDX∙∙∙acetone < RDX∙∙∙THF < RDX∙∙∙toluene < RDX∙∙∙benzene < RDX, as is roughly consistent with the experimental results. Therefore, the intermolecular hydrogen exchange with the HONO elimination is a possible mechanism of the solvent effect on the initial thermal decomposition of RDX. The solvent effect on the sensitivity is analyzed by the surface electrostatic potentials.

Nature of the Trigger Linkage in Explosive Materials Is a Charge-Shift Bond

Explosion begins by rupture of a specific bond, in the explosive, called a trigger linkage. We characterize this bond in nitro-containing explosives. Valence-bond (VB) investigations of X-NO₂ linkages in alkyl nitrates, nitramines, and nitro esters establish the existence of Pauli repulsion that destabilizes the covalent structure of these bonds. The trigger linkages are mainly stabilized by covalent-ionic resonance and are therefore charge-shift bonds (CSBs). The source of Pauli repulsion in nitro explosives is unique. It is traced to the hyperconjugative interaction from the oxygen lone pairs of NO₂ into the σ_(X-N)^* orbital, which thereby weakens the X-NO₂ bond, and depletes its electron density as X becomes more electronegative. Weaker trigger bonds have higher CSB characters. In turn, weaker bonds increase the sensitivity of the explosive to impacts/shocks which lead to detonation. Application of the analysis to realistic explosives supports the CSB character of their X-NO₂ bonds by independent criteria. Furthermore, other families of explosives also involve CSBs as trigger linkages (O-O, N-O, Cl-O, N-I, etc. bonds). In all of these, detonation is initiated selectively at the CSB of the molecule. A connection is made between the CSB bond-weakening and the surface-electrostatic potential diagnosis in the trigger bonds.

Design and exploration of 5-nitro-3-trinitromethyl-1H-1,2,4-triazole and its derivatives as energetic materials

According to the fact that 5-nitro-3-trinitromethyl-1H-1,2,4 triazole (NTNMT) is a successful, good explosive, energetic groups such as -CH3, -NH2, -NHNO2, -NO2, -ONO2, -NF2, -CN, -NC, -N3 groups were introduced into NTNMT and their oxygen balance was at about zero. The energetic properties, detonation performance, and sensitivity were studied at the B3LYP/6-31G** level of density functional theory to seek for possible high energy density compounds. The effects of substituent groups on heat of formation (HOF), density ρ, detonation velocity D, detonation pressure P, detonation energy Q, and sensitivity (evaluated using oxygen balance OB, the nitro group charges -QNO2, and bond dissociation energies BDE were studied and discussed. The order of contribution of the substituent groups to ρ, D, and P was -NF2 > -ONO2 > -NO2 > -NHNO2 > -N3 > -NH2 > -NC > -CN > -CH3; while to HOF is -N3 > -NC > -CN > -NO2 > -NF2 > -ONO2 > -NH2 > -NHNO2 > -CH3. The trigger bonds in the pyrolysis process for NTNMT derivatives may be N-NO2, N-NH2, N-NHNO2, C-NO2, or O-NO2 varying with the attachment of different substituents. Results show that NTNMT-NHNO2, -NH2, -CN, and -NC derivatives have high detonation performance and good stability. In a word, the oxygen balance at about zero strategy in this work offers new routes for the improvement in properties and stabilities of energetic materials. In the present paper, several 5-nitro-3-trinitromethyl-1H-1,2,4 triazole (NTNMT) derivatives were designed. Their energetic properties, detonation performance, and sensitivity were studied at the B3LYP/6-31G** level of density functional theory (DFT) to seek for possible high energy density compounds (HEDCs). The different substituents have some changes in the influence on heat of formation (HOF), density ρ, detonation velocity D, detonation pressure P, detonation energy Q, and sensitivity. In a word, the oxygen balance at about zero strategy in this work offers new routes for the improvement in properties and stabilities of energetic materials.

Are HOMO-LUMO gaps reliable indicators of explosive impact sensitivity?

A high priority in designing and evaluating proposed explosives is to minimize sensitivity, i.e., vulnerability to unintended detonation due to an accidental stimulus, such as impact. In order to establish a capability for predicting impact sensitivity, there have been numerous attempts to correlate it with some molecular or crystal property or properties. One common approach has been to relate impact sensitivity to the difference between the energies of the highest-occupied and lowest-unoccupied molecular orbitals of the explosive molecule, the "HOMO-LUMO gap." In the present study, we tested this approach for a series of twelve explosive nitroaromatics, using four different computational methods. We found that the HOMO-LUMO gap does not appear to be a reliable indicator of relative impact sensitivity. Since detonation initiation involves a series of steps, all of which influence sensitivity; it seems more realistic to try to identify fundamental factors and general trends related to sensitivity ‒ an approach that has already had some success ‒ rather than to seek correlations with one or two specific properties.

Quantifying bond strengths via a Coulombic force model: application to the impact sensitivity of nitrobenzene, nitrogen-rich nitroazole, and non-aromatic nitramine molecules

The quantification of bond strengths is a useful and general concept in chemistry. In this work, a Coulombic force model based on atomic electric charges computed using the accurate distributed multipole analysis (DMA) partition of the molecular charge density was employed to quantify the weakest N-NO₂ and C-NO₂ bond strengths of 19 nitrobenzene, 11 nitroazole, and 10 nitramine molecules. These bonds are known as trigger linkages because they are usually related to the initiation of an explosive. The three families of explosives combine different types of molecular properties and structures ranging from essentially aromatic molecules (nitrobenzenes) to others with moderate aromaticity (nitroazoles) and non-aromatic molecules with cyclic and acyclic skeletons (nitramines). We used the results to investigate the impact sensitivity of the corresponding explosives employing the trigger linkage concept. For this purpose, the computed Coulombic bond strength of the trigger linkages was used to build four sensitivity models that lead to an overall good agreement between the predicted values and available experimental sensitivity values even when the model included the three chemical families simultaneously. We discussed the role of the trigger linkages for determining the sensitivity of the explosives and rationalized eventual discrepancies in the models by examining alternative decomposition mechanisms and features of the molecular structures.

Review of the molecular and crystal correlations on sensitivities of energetic materials

Highly efficient design on the levels of molecule and crystal, as well as formulation, is highly desired for accelerating the development of energetic materials (EMs). Sensitivity is one of the most important characteristics of EMs and should be compulsorily considered in the design. However, owing to multiple factors responsible for the sensitivity, it usually undergoes a low predictability. Thus, it becomes urgent to clarify which factors govern the sensitivity and what is the importance of these factors. The present article focuses upon the progress of the molecular and crystal correlations on the sensitivity, and the molecule-based numerical models for sensitivity prediction in the past decades. On the molecular level, composition, geometric structure, electronic structure, energy and reactivity can be correlated with the sensitivity; while the sensitivity can be also related with molecular packing pattern, intermolecular interaction, crystal morphology, crystal size and distribution, crystal surface/interface and crystal defect on the crystal level. And most of these factors, in particle on the crystal level, have been employed as variables in numerical models for predicting sensitivity of categorized EMs. Besides, we stress that more attention should be paid to the sensitivity correlations on the inherent structures of EMs, molecule and crystal packing, because they can be readily dealt by molecular simulations nowadays, facilitating to reveal the physical nature of sensitivity.

5,6-Fused bicyclic tetrazolo-pyridazine energetic materials

Two 5,6-fused tetrazolo-pyridazine compounds were synthesized and characterized, which exhibited high thermal stability, excellent energetic properties and low mechanical sensitivity.

Correlation between molecular charge densities and sensitivity of nitrogen-rich heterocyclic nitroazole derivative explosives

Nitroazole derivatives are nitrogen-rich heterocyclic ring molecules with potential application as energetic materials. Thirty-three of them-nitroimidazoles, nitrotriazoles, and nitropyrazoles-were investigated. Computed density functional theory molecular charge densities were partitioned employing the accurate distributed multipole analysis (DMA) method. Based on the magnitude of the DMA atom-centered electric multipoles (monopole, dipole, and quadrupole values), mathematical models were developed to compute the impact sensitivity of the explosives composed of these molecules. Charge localization and delocalization of the ring nitrogen atoms as well as charges of the atoms of the nitro group affect the sensitivity of explosives composed of nitroazole derivatives. The sensitivity is strongly dependent on the ring position of the nitrogen atoms and the bonding site of the substituent groups. The N/C ratio and the repulsion of the non-bonding electron pairs of the vicinal nitrogen atoms of the ring also play an important role in the stability of nitroazoles. The influence of the withdrawing group (NO₂) and the electron injector groups (NH₂ and CH₃) including their bonding position on the ring could be quantified.

Applying machine learning techniques to predict the properties of energetic materials

We present a proof of concept that machine learning techniques can be used to predict the properties of CNOHF energetic molecules from their molecular structures. We focus on a small but diverse dataset consisting of 109 molecular structures spread across ten compound classes. Up until now, candidate molecules for energetic materials have been screened using predictions from expensive quantum simulations and thermochemical codes. We present a comprehensive comparison of machine learning models and several molecular featurization methods - sum over bonds, custom descriptors, Coulomb matrices, Bag of Bonds, and fingerprints. The best featurization was sum over bonds (bond counting), and the best model was kernel ridge regression. Despite having a small data set, we obtain acceptable errors and Pearson correlations for the prediction of detonation pressure, detonation velocity, explosive energy, heat of formation, density, and other properties out of sample. By including another dataset with ≈300 additional molecules in our training we show how the error can be pushed lower, although the convergence with number of molecules is slow. Our work paves the way for future applications of machine learning in this domain, including automated lead generation and interpreting machine learning models to obtain novel chemical insights.

Impact sensitivity and the maximum heat of detonation

We demonstrate that a large heat of detonation is undesirable from the standpoint of the impact sensitivity of an explosive and also unnecessary from the standpoints of its detonation velocity and detonation pressure. High values of the latter properties can be achieved even with a moderate heat of detonation, and this in turn enhances the likelihood of relatively low sensitivity.

Some molecular/crystalline factors that affect the sensitivities of energetic materials: molecular surface electrostatic potentials, lattice free space and maximum heat of detonation per unit volume

We discuss three molecular/crystalline properties that we believe to be among the factors that influence the impact/shock sensitivities of energetic materials (i.e., their vulnerabilities to unintended detonation due to impact or shock). These properties are (a) the anomalously strong positive electrostatic potentials in the central regions of their molecular surfaces, (b) the free space per molecule in their crystal lattices, and (c) their maximum heats of detonation per unit volume. Overall, sensitivity tends to become greater as these properties increase; however these are general trends, not correlations. Nitramines are exceptions in that their sensitivities show little or no variation with free space in the lattice and heat of detonation per unit volume. We outline some of the events involved in detonation initiation and show how the three properties are related to different ones of these events.

Predicting impact sensitivities of nitro compounds on the basis of a semi-empirical rate constant

A physically motivated model is put forward to estimate impact sensitivity of nitro compounds on the basis of the relationship h(50) ∝ k(pr)(-4) between drop weight impact test data h(50) and rate constant k(pr) for the propagation of the decomposition. An approximate expression involving two adjustable parameters is introduced to estimate k(pr) from molecular structure. As a result, using only a hand-held calculator, ln(h(50)) values are estimated with a good reliability (R(2) ≃ 0.8) compared to previous schemes. These results support the underlying assumption that sensitivity primarily depends on the ability of reacting species to propagate the decomposition before the released energy dissipates away.

A multireference configuration interaction study of the photodynamics of nitroethylene

Extended multireference configuration interaction with singles and doubles (MR-CISD) calculations of nitroethylene (H₂C=CHNO₂) were carried out to investigate the photodynamical deactivation paths to the ground state. The ground (S₀) and the first five valence excited electronic states (S₁–S₅) were investigated. In the first step, vertical excitations and potential energy curves for CH₂ and NO₂ torsions and CH₂ out-of-plane bending starting from the ground state geometry were computed. Afterward, five conical intersections, one between each pair of adjacent states, were located. The vertical calculations mostly confirm the previous assignment of experimental spectrum and theoretical results using lower-level calculations. The conical intersections have as main features the torsion of the CH₂ moiety, different distortions of the NO₂ group and CC, CN, and NO bond stretchings. In these conical intersections, the NO₂ group plays an important role, also seen in excited state investigations of other nitro molecules. Based on the conical intersections found, a photochemical nonradiative deactivation process after a π–π* excitation to the bright S₅ state is proposed. In particular, the possibility of NO₂ release in the ground state, an important property in nitro explosives, was found to be possible.

Impact sensitivity and crystal lattice compressibility/free space

There is considerable evidence, which we discuss, indicating that compressibility and available free space in the crystal lattice are among the factors that govern the sensitivity of an explosive compound. Expanding and extending earlier work, we demonstrate, for 25 explosives, that there is an overall general tendency for greater impact sensitivity as the estimated free space per molecule increases. More specific relationships can be discerned by looking at subgroups of the compounds. The nitramine sensitivities, most of which are quite high, increase nearly linearly but only very gradually with free space. The nitroaromatics cover a wide range of sensitivities but all have an approximately similar intermediate level of free space. The remaining types of compounds show a reasonable sensitivity-free space relationship with one outlier: FOX-7 (1,1-diamino-2,2-dinitroethylene).

Theoretical studies on vibrational spectra, thermodynamic properties, and detonation properties for 1,2,4,5-tetrazine derivatives

A density functional theory calculation was performed to study the molecular structures, heats of formation (HOFs), infrared spectra, detonation properties, and thermodynamic properties for five 1,2,4,5-tetrazine derivatives. Based on the full optimized molecular structures at the B3LYP/6-311++G** level, the assigned infrared spectra of the studied compounds were obtained. The isodesmic reaction method was employed to calculate the HOFs of the derivatives. The detonation velocities and pressures were also evaluated by using Kamlet−Jacobs equations with the calculated densities and condensed HOFs. The result shows that 3,6-diazido-1,2,4,5- tetrazine may be a potential candidate of high-energy density materials (HEDMs). Natural bond orbital analysis indicated that the title compounds all have higher bond dissociation energies when compared with 1,3,5,7-tetranitro-1,3,5,7-tetrazocane and 1,3,5-trinitro-1,3,5-triazinane. The results may provide basis information for the molecular design of new HEDMs.

Safer energetic materials by a nanotechnological approach

Energetic materials - explosives, thermites, populsive powders - are used in a variety of military and civilian applications. Their mechanical and electrostatic sensitivity is high in many cases, which can lead to accidents during handling and transport. These considerations limit the practical use of some energetic materials despite their good performance. For industrial applications, safety is one of the main criteria for selecting energetic materials. The sensitivity has been regarded as an intrinsic property of a substance for a long time. However, in recent years, several approaches to lower the sensitivity of a given substance, using nanotechnology and materials engineering, have been described. This feature article gives an overview over ways to prepare energetic (nano-)materials with a lower sensitivity.

Topological analysis of the molecular charge density and impact sensitivy models of energetic molecules

Important explosives of practical use are composed of nitroaromatic molecules. In this work, we optimized geometries and calculated the electron density of 17 nitroaromatic molecules using the Density Functional Theory (DFT) method. From the DFT one-electron density matrix, we computed the molecular charge densities, thus the electron densities, which were then decomposed into electric multipoles located at the atomic sites of the molecules using the distributed multipole analysis (DMA). The multipoles, which have a direct chemical interpretation, were then used to analyze in details the ground state charge structure of the molecules and to seek for correlations between charge properties and sensitivity of the corresponding energetic material. The DMA multipole moments do not present large variations when the size of the Gaussian basis set is changed; the largest variations occurred in the range 10-15% for the dipole and quadrupole moments of oxygen atoms. The charges on the carbon atoms of the aromatic ring of each molecule become more positive when the number of nitro groups increases and saturate when there are five and six nitro groups. The magnitude and the direction of the dipole moments of the carbon atoms, indicators of site polarization, also depend on the nature of adjacent groups, with the largest dipole value being for C-H bonds. The total magnitude of the quadrupole moment of the aromatic ring carbon atoms indicates a decrease in the delocalized electron density due to an electron-withdrawing effect. Three models for sensitivity of the materials based on the DMA multipoles were proposed. Explosives with large delocalized electron densities in the aromatic ring of the component molecule, expressed by large quadrupole values on the ring carbon atoms, correspond to more insensitive materials. Furthermore, the charges on the nitro groups also influence the impact sensitivity.

Computational design and structure-property relationship studies on heptazines

This study aimed to design novel nitrogen-rich heptazine derivatives as high energy density materials (HEDM) by exploiting systematic structure-property relationships. Molecular structures with diverse energetic substituents at varying positions in the basic heptazine ring were designed. Density functional techniques were used for prediction of gas phase heat of formation by employing an isodesmic approach, while crystal density was assessed by packing calculations. The results reveal that nitro derivatives of heptazine possess a high heat of formation and further enhancement was achieved by the substitution of nitro heterocycles. The crystal packing density of the designed compounds varied from 1.8 to 2 g cm(-3), and hence, of all the designed molecules, nitro derivatives of heptazine exhibit better energetic performance characteristics in terms of detonation velocity and pressure. The calculated band gap of the designed molecules was analyzed to establish sensitivity correlations, and the results reveal that, in general, amino derivatives possess better insensitivity characteristics. The overall performance of the designed compounds was moderate, and such compounds may find potential applications in gas generators and smoke-free pyrotechnic fuels as they are rich in nitrogen content.

Sensitivity and the available free space per molecule in the unit cell

Invoking the known link between impact sensitivity and compressibility, we have expanded upon an earlier preliminary study of the significance of the available free space per molecule in the unit cell, ΔV. We express ΔV as V(eff) - V(int), where V(eff) corresponds to zero free space, V(eff) = molecular mass/density. V(int) is the intrinsic gas phase molecular volume. We demonstrate that V(int) can be appropriately defined as the volume enclosed by the 0.003 au contour of the molecule's electronic density; this produces packing coefficients that have the range and average value found crystallographically. Measured impact sensitivities show an overall tendency to increase as ΔV becomes larger. For nitramines, the dependence upon ΔV is rather weak; we interpret this as indicating that a single overriding factor dominates their initiation mechanism, e.g., N-NO(2) rupture. (An analogous situation appears to hold for many organic azides.) In addition to the conceptual significance of identifying ΔV as a factor in impact sensitivity, the present results allow rough estimates of relative sensitivities that are not known.