What makes nitro groups explosive

Our final look at the infrared spectra of organic nitrogen functional groups will be of the nitro, or NO 2 , group. This group has two strong and characteristic infrared features, making it easy to identify.

However, attaching a NO 2 group to a molecule scrambles its electronic structure, which is why these compounds are explosive, and why nitro groups make some of the infrared interpretation rules we learned irrelevant. Over the several years that I have been writing these articles, there have been large sections of material devoted to the spectroscopy of functional groups that contain specific chemical bonds.

When I began discussing organics containing nitrogen, I stated that the stretching and bending of N-H bonds would be important in determining whether or not there was nitrogen present in a molecule 1. We subsequently used N-H stretching and bending peaks to analyze the spectra of amines 2,3 , amides 4,5 , imides 6 , and urethanes 7. The nitro group is going to be the odd man out in this discussion. This functional group consists of a nitrogen atom with no hydrogens, but with two oxygens and a carbon attached, as seen in Figure 1.

Note that the nitrogen in the NO 2 group is called the nitro nitrogen , and that the carbon atom singly bonded to the nitro nitrogen is called the alpha carbon. Depending on whether the alpha carbon is saturated, or part of an aromatic ring, nitro molecules can be divided into saturated and aromatic nitro compounds.

The chemical bonding of the NO 2 group is unusual. Normally, oxygen atoms form two chemical bonds 8. However, there is also a C-N bond in the nitro group, as seen in Figure 1. Given that nitrogen normally forms three bonds 8 , how do we apportion the electrons in a nitro group to keep these compounds from falling apart? The dashed lines in Figure 1 represent the half bonds. These NO bonds are similar to the carboxylate C-O bonds and a half discussed in a previous column 9.

Neither oxygen atom has its full complement of two full chemical bonds, making the nitro groups unstable. Saturated nitro compounds such as nitroalkanes, otherwise known as rocket fuel 8 , are rarely analyzed by infrared spectroscopy because they are liable to detonate during analysis. We will not study them further. However, nitro groups attached to benzene rings can be relatively stable, assuming not too many nitro groups are attached.

The chemical structure of trinitrotoluene, a commonly used explosive known as TNT , is shown in Figure 2. Di-nitrotoluene and mono-nitrotoluene are stable. We will restrict our discussion to non-explosive aromatic nitro compounds. The infrared spectrum of an aromatic nitro compound, meta-nitrotoluene, is seen in Figure 3. Oxygen is more electronegative than nitrogen; therefore, the N-O bonds in the nitro group are relatively polar, and as a result, their asymmetric and symmetric stretching peaks are unusually large.

The details of these vibrations are shown in Figure 4. The asymmetric NO 2 stretch typically falls from to cm -1 , and is seen in Figure 3 labeled A at cm -1 assume all peak positions are in cm -1 units, even if not labeled as such.

The symmetric stretch is seen in Figure 3 labeled B at cm -1 , and, in general, this peak appears from cm -1 to cm Note how peaks A and B in Figure 3 are the two most intense peaks in the spectrum, and they stick down like eye teeth in the middle of the spectrum. One of the most important thermodynamic properties that determine the performance of these explosives and propellants is the heat of explosion HE.

The HE is the quantity of heat liberated when an HEM undergoes detonation as a high explosive or deflagration as a low explosive. The processes of detonation and deflagration occur even in the absence of external oxygen or air because HEM contain oxygen themselves.

The HE can be theoretically calculated [ 1 ] and experimentally determined [ 2 ]. The calculated value is determined by the difference between the energies of formation of the explosive components and the energies of formation of the explosion products. Experimentally, the HE is determined using a calorimetric bomb. The sample quantity is usually chosen to obtain a loading density of 0. Thus, the HE of the sample powder can be calculated from the mixture. The calorimetric values used in this work are based on liquid H 2 O as a reaction product.

Predicting the performance and thermochemical properties of energetic materials from a given molecular structure with or without using experimental measurements is critical in the research and development of explosives. Several relationships have been found that relate explosive characteristics with measured and predicted molecular properties.

The relationship between thermal [ 3 ], impact [ 4 , 5 ], and electric spark [ 6 ] sensitivities and molecular structure of nitrocompounds and nitrates [ 7 ] has been the subject of many investigations. In this context, the NO 2 nitro functional group corresponds to the reaction center of the HEM and should be correlated with the explosive characteristics of individual energetic materials.

The electron configuration and the steric conditions within the reaction center of the molecule can be represented by NMR chemical shifts of the key atoms of the center. The shifts of these atoms should correlate with characteristics of initiation reactivity of individual energetic materials [ 6 ]. The dominant factor in the initiation by shock, electric spark, and in low-temperature thermolysis should be the electronic structure and close chemical ambient of the nitrogen atom of the primary leaving nitro group.

Recently, we established a quantitative model for predicting and calculating the heat of detonation and heat of explosion for a series of nitro paraffins, nitramines [ 9 ], and nitrate esters [ 10 ] by utilizing natural bond orbital NBO charge analysis and the 15 N NMR chemical shifts of the nitro groups.

In addition, the calculated 15 N Nitro NMR chemical shifts for the nitramines were obtained using a protocol established in our recent work [ 11 ], which uses the continuous set of gauge transformations CSGT method [ 12 ]. The ab initio calculations of the NMR chemical shifts for different nuclei have proven to be useful structural parameters in interpreting experimental data in several applications [ 13 , 14 ]. There are many both empirical and theoretical efforts to predict detonation parameters from a given molecular structure.

Rice and Hare [ 15 ] introduced a computational methodology that uses only quantum mechanical QM information about isolated molecules to predict the heats of detonation for pure and explosive formulations. In a series of studies, Keshavarz has established simple methods for predicting heats of detonation and other explosive properties [ 16 — 19 ]. In the application of these methods, there is no need to use any experimental and computed data of explosives.

A simple approximation is introduced for calculating heats of detonation using chemical composition of explosives and their gas phase heat of formation that can be calculated by a group additivity rule [ 16 ]. Recently, a simple method to predict heats of detonation of important classes of energetic compounds including nitroaromatics, nitramines, nitroaliphatics, and nitrate esters was introduced [ 17 ]. The methodology is based on the ratios of oxygen to carbon and hydrogen to oxygen atoms as well as the contribution of some specific functional groups or structural parameters.

The root mean squared rms deviation between predicted and experimentally determined heats of detonation was 0. Also, the heats of detonation for aromatic energetic materials were estimated assuming that the heat of detonation of an explosive compound of composition C a H b N c O d can be approximated as the difference between the heat of formation of all H 2 O-CO 2 arbitrary H 2 O, CO 2 , N 2 detonation products and that of the explosive, divided by the formula weight of the explosive [ 19 ].

The rms deviation for this method was 0. Also, an empirical approach to calculate the heats of formation in condensed phase for different energetic compounds has been developed by Keshavarz [ 20 ]. It should be mentioned that the heat of explosion is not a constant of a high explosive and depends on the expansion ratio of the detonations products.

The heat of explosion is reasonably included in the energetic characterization of the working capacity of commercial high explosives, but it has no relation to the power of brisance, which is determined by the propellant capacity. Even the detonation velocity is not a measure of the power of high explosives [ 21 ].

The purpose of this work was to establish a quantitative model to calculate and predict the heats of explosion of 35 nitroaromatic compounds from two structural parameters, nitro charges and 15 N NMR chemical shifts, obtained by theoretical methods.

The predictive ability of the model was assessed by the leave-one-out LOO cross-validation method [ 22 ].

To show the reliability of the calculated heat of explosion using the quantitative model, the results for eight nitroaromatic compounds that were not used to build the model were compared with their experimental values. The model was applied to calculate heats of explosion of twenty energetic compounds and compared against results predicted using other empirical and quantum mechanical methods and against experimental values available.

In addition, the model was employed to calculate the heat of explosion of fourteen nitroaromatic compounds whose experimental values were unavailable. Importantly, this work provides a simple and rapid method for estimating heats of explosion of HEM without experimental data for a systematic set of nitroaromatic compounds and should be of help in synthesizing and developing new high explosives.

Gaussian 09 [ 23 ] software was used for all theoretical calculations. The absolute energies and C—N Nitro bond lengths for the thirty-five nitroaromatic compounds are shown in Tables 1 , 2 , and 3. The effect of the solvent on the theoretical NMR parameters was included using a default integral equation formalism polarized continuum model IEF-PCM [ 27 ] provided by the Gaussian 09 software suite. The experimental values of the heat of explosion used in the present paper were obtained from the literature [ 2 ].

The optimized C—N Nitro bond lengths and nitro charges, with the corresponding total energies, for thirty-five nitroaromatic compounds are listed in Tables 1 , 2 and 3. These results revealed that the compounds with higher stability lower total energy had higher nitro group charge values. Lower more negative nitro group charges were consistent with a decrease in the C—N Nitro bond length.

Therefore, the value of the charge on the nitro group reflects the strength of the corresponding bond and stability in these compounds. Our calculations showed that the introduction of amino groups into multinitrobenzene decreased the NBO nitro group charges and shortened the C—N Nitro bond length.

Thus, a sensitive to impact explosive is expected to be more easily convertible to the metal upon compression, to possess a spherical crystal habit and to have a greater no. Consequently, an insensitive explosive has the inverse characterization.

Theory Comput. A method is developed for automatic generation of intermol. It is also possible to substitute SAPT interaction energies by values computed using sufficiently high-level supermol.

The long-range component of the potential is obtained from a rigorous asymptotic expansion with ab initio computed coeffs. An accompanying software package has been developed and tested successfully on eight systems ranging in size from the Cl--H2O dimer to the cyclotrimethylene trinitramine dimer contg.

The potentials have a typical fit errors of about 0. The accuracy may be further improved by including off-at. All aspects of potential development were designed to work reliably on a broad range of systems with no human intervention. Report on the sixth blind test of organic crystal structure prediction methods. Acta Crystallogr. B: Struct. Reilly, Anthony M. International Union of Crystallography. The sixth blind test of org.

This blind test has seen substantial growth in the no. Significant progress has been seen in treating flexible mols. Despite many remaining challenges, it is clear that CSP methods are becoming more applicable to a wider range of real systems, including salts, hydrates and larger flexible mols.

The results also highlight the potential for CSP calcns. Study of thermal instability of HMX crystalline polymorphs with and without molecular vacancies using reactive force field molecular dynamics. Royal Society of Chemistry. It has become common to ref. Here, we review exptl. We therefore propose that these terms are misleading and should no longer be used. Even without these terms, electrostatic considerations relating to polarized pi systems, as described by Hunter and Sanders, have provided a good qual.

Atomic charge densities generated using an iterative stockholder procedure. American Institute of Physics. A simple computational technique is introduced for generating at. It is proven that the procedure is always convergent and leads to unique at.

The resulting at. Redefining the atom: atomic charge densities produced by an iterative stockholder approach. However, the real-space algorithm is known to converge very slowly, if at all.

Here, we present a robust, basis-space algorithm of the ISA method and demonstrate its applicability on a variety of systems. We show that this algorithm exhibits rapid convergence taking around iterations with the no. Further, we show that the multipole moments calcd. This can have significant consequences in the development of intermol. Polymorphism of 1,3,5-trinitrobenzene induced by a trisindane additive.

Thallapally, Praveen K. The most stable is not necessarily the most easy to obtain!. Two new stable crystal forms of the yr old compd. These elusive forms were obtained using an additive, trisindane, that can act as a mimic in the crystn. Crystal and molecular structure of hexanitrobenzene. Polymorphism in Trinitrotoluene. Growth Des. Vrcelj, Ranko M. Of these two forms, the most stable is the monoclinic and the less stable is the orthorhombic form.

These two polymorphs are orientational, rather than configurational in character. Due to their restricted mol. The two structures are closely related and an anal. Calorimetric studies show that the two polymorphs are monotropic and that the enthalpy of transformation is very low, concurring with the similarity shown by the diffraction data and calcd.

The thermal expansion coeffs. Redetermination of cyclo-trimethylenetrinitramine. E: Struct. Online , 64 , o DOI: The redetd. It is of interest with respect to energetic compds. The structure was originally studied through x-ray diffraction by Hultgren.

A single-crystal neutron diffraction study was performed by Choi and Prince to ascertain the H-atom positions, which had not been defined by the earlier x-ray diffraction studies. All previous studies were performed at or near room temp. The ring atoms are arranged in the chair conformation with 2 nitro groups occupying pseudo-equatorial positions and the remaining nitro group is axial.

The crystal packing is stabilized by close intramol. Davidson, Alistair J. Millar, David I. The high-pressure, high-temp. Applications of the Cambridge Structural Database in organic chemistry and crystal chemistry. Blackwell Munksgaard. Relevant refs. This database was used to review research applications of the CSD in org. The review concs. Applications of CSD information in studies of crystal structure precision, the detn. The Cambridge Structural Database: A quarter of a million crystal structures and rising.

The information content of the CSD, together with methods for data acquisition, processing and validation, are summarized, with particular emphasis on the chem. A statistical survey of the CSD is also presented and projections concerning future accession rates indicate that the CSD will contain at least crystal structures by the year Mercury CSD 2. Macrae, Clare F. The program Mercury, developed by the Cambridge Crystallog. Data Center, is designed primarily as a crystal structure visualization tool.

A new module of functionality has been produced, called the Materials Module, which allows highly customizable searching of structural databases for intermol. This new module also includes the ability to perform packing similarity calcns.

In addn. Mercury: visualization and analysis of crystal structures. Blackwell Publishing Ltd. Since its original release, the popular crystal structure visualization program Mercury has undergone continuous further development.

Comparisons between crystal structures are facilitated by the ability to display multiple structures simultaneously and to overlay them. Improvements were made to many aspects of the visual display, including the addn.

Textual and numeric data assocd. Some limited mol. Data Center. Sensitivity Relationships in Energetic Materials. A review of energetic materials synthesis.

Acta , , — , DOI: Pagoria, Philip F. Elsevier Science B. A review of advances in the synthesis of heterocycles as energetic materials and complement to the review of recent advances in energetic materials by J. Agrawal Energetic materials explosives, propellants and pyrotechnics are used extensively for both civilian and military applications.

There are ongoing research programs worldwide to develop pyrotechnics with reduced smoke and new explosives and propellants with higher performance or enhanced insensitivity to thermal or shock insults. In recent years, the synthesis of energetic, heterocyclic compds. Heterocycles generally have a higher heat of formation, d. Polynitroaromatic Explosives. Polynitroaromatic explosives. The impact sensitivities of org. For classes of explosives with similar decompn. These results are consistent with the finding that products of thermal decompn.

Dependence of particle morphology and size on the mechanical sensitivity and thermal stability of octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine. Elsevier B. By changing the tech. All as-prepd. Taking advantage of mech. Results indicated that particle size played a significant role in the safety of HMX, and that morphol.

However, the trends of these changes exhibit much variance if the microstructure of the HMX particles is altered. Consequently, the difference in safety for these kinds of samples has to do with their specific morphol. A relationship between the impact sensitivity and the electronic-structures for the unique N-N bond in HMX Polymorphs.

Flame , 96 , — , DOI: CL performance exceeds that of HMX and its sensitivity is moderate. Simpson, R. This makes it the most powerful explosive ever tested at small vol. Some Perspectives on Sensitivity to Initiation of Detonation.

Earlier work has shown a link-not necessarily a causal relationship-between the impact sensitivities of energetic compds. The latter are anomalous in that the pos. Thus, insofar as these are trigger linkages, the rupture of which is a key step in detonation initiation, an approx. A group of eight nitramines is used to demonstrate this. Work is in progress to elucidate the basis for the surface potential-sensitivity link when a non-trigger-linkage mechanism is operative.

The intermol. AIM anal. The variation tendency of entropy and enthalpy shows that the formation of the cocrystal is an exothermic process and low temp. The calcd. Finally, first-principles calcns. Theoretical insight into the binding energy and detonation performance of epsilon-, gamma-, beta-CL cocrystals with beta-HMX, FOX-7, and DMF in different molar ratios, as well as electrostatic potential. Some general guidelines and specific mol. A key point is that a large detonation heat release should be avoided; it is undesirable with respect to sensitivity and it is not required in terms of performance.

Virtual cocrystal screening. Musumeci, Daniele; Hunter, Christopher A. The approach assumes that all of the interactions that can be made in the solid are made and that the details of three-dimensional structure and crystal packing are of secondary importance. Tests on an exptl. For systems that are exptl. An advantage of this approach is that it is sufficiently fast to be used as a high throughput virtual screening tool.

Toward reliable density functional methods without adjustable parameters: The PBE0 model. We present an anal. The results obtained for structural, thermodn.

The way in which the functional is derived and the lack of empirical parameters fitted to specific properties make the PBE0 model a widely applicable method for both quantum chem. Rationale for mixing exact exchange with density functional approximations. We argue that the optimum integer n is approx. We also propose a continuous generalization of n as an index of correlation strength, and a possible mixing of second-order perturbation theory with the generalized gradient approxn.

Generalized gradient approximation made simple. American Physical Society. Generalized gradient approxns. GGA's for the exchange-correlation energy improve upon the local spin d.

LSD description of atoms, mols. We present a simple derivation of a simple GGA, in which all parameters other than those in LSD are fundamental consts. Improvements over PW91 include an accurate description of the linear response of the uniform electron gas, correct behavior under uniform scaling, and a smoother potential.

The atoms boron through neon and hydrogen. Guided by the calcns. As in the oxygen atom calcns. This leads to the concept of correlation-consistent basis sets, i. Correlation-consistent sets are given for all of the atoms considered. The most accurate sets detd. It is estd. Gaussian 09 ; Gaussian, Inc. Tables of bond lengths determined by x-ray and neutron diffraction. Part 1. Bond lengths in Organic compounds. Bond lengths in organic compounds. Allen, Frank H. Guy; Taylor, Robin.

The av. CamCASP: a program for studying intermolecular interactions and for the calculation of molecular properties in distributed form ; University of Cambridge. We aim to use conformational information from the crystal structures in the Cambridge Structural Database CSD to facilitate this task.

This method is tested for five pharmaceutical-like mols. The CSD informatics-derived set of crystal structure searches generates almost all the low-energy crystal structures previously found, including all exptl. The workflow effectively combines information on individual torsion angles and then eliminates the combinations that are too high in energy to be found in the solid state, reducing the resources needed to cover the solid-state conformational space of a mol.

This provides insights into how the low-energy solid-state and isolated-mol. In most simulations of of flexible mols. The electrostatic models for large mols. Both assumptions neglect polarization of the atoms by the rest of the mol. The accuracy of these assumptions for peptide systems are tested. The electrostatic fields are calcd. DMA of the wavefunction. Model DMAs can be constructed by transforming the at. The accuracy of the transferability assumptions are then tested by comparing the electrostatic potential around the mol.

The results show that large errors in the electrostatic potential outside the mol. However, the different models agree well on the positions although not the relative energies of the probable water binding sites. This implies that there is some limited utility in the crude approxn. However, model charge distributions for I transferred from the DMAs of blocked single peptides with the same torsion angles are much more successful, and provide a promising route forward to accurate electrostatic models for polypeptides.

Properties of Atoms in Molecules.