Isoconversional kinetic analysis of decomposition of nitroimidazoles: Friedman method vs Flynn-Wall-Ozawa method.

We have investigated the decomposition kinetics of imidazole, 2-nitroimidazole, and 4-nitroimidazole using TG-DTA technique under nitrogen atmosphere. Isoconversional methods were used for the evaluation of kinetic parameters from the kinetic data of different heating temperatures. The Friedman method provided comparably higher values of activation energy than the Flynn-Wall-Ozawa method. Imidazole, 2-nitroimidazole, and 4-nitroimidazole were decomposed by the multistep reaction mechanism evident from the nonlinear relationship of activation energy and the conversion rate. The NO2 elimination and nitro-nitrite isomerization are expected to be competitive reactions in the decomposition of 2-nitroimidazole and 4-nitroimidazole. The present study may be helpful in understanding how the position of NO2 group affects the decomposition kinetics of substituted imidazoles.

Theoretical studies on the structure, thermochemical and detonation properties of amino and nitroso substituted 1,2,4-triazol-5-one-N-oxides.

DFT calculations at the B3LYP/aug-cc-pVDZ level have been carried out to explore the structure, stability, electron density, heat of formation, detonation velocity and detonation pressure of substituted amino- and nitroso-1,2,4-triazol-5-one-N-oxides. Heats of formation of substituted triazol-5-one-N-oxides have been computed at the B3LYP/aug-cc-pVDZ level via isodesmic reaction procedure. Materials Studio 4.1 package was used to predict the crystal density of model compounds. Kamlet-Jacob equations were used to calculate detonation properties based on the calculated heat of explosion and crystal density. The designed compounds 4, 6, 7 and 8 have shown higher performance compared with those of 2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane and octanitrocubane. Atoms-in-molecule (AIM) analyses have also been carried out to understand the nature of intramolecular interactions in the designed molecules.

Dynamical and statistical behavior of discrete combustion waves: a theoretical and numerical study.

We present a detailed theoretical and numerical study of combustion waves in a discrete one-dimensional disordered system. The distances between neighboring reaction cells were modeled with a gamma distribution. The results show that the random structure of the microheterogeneous system plays a crucial role in the dynamical and statistical behavior of the system. This is a consequence of the nonlinear interaction of the random structure of the system with the thermal wave. An analysis of the experimental data on the combustion of a gasless system (Ti + xSi) and a wide range of thermite systems was performed in view of the developed model. We have shown that the burning rate of the powder system sensitively depends on its internal structure. The present model allows for reproducing theoretically the experimental data for a wide range of pyrotechnic mixtures. We show that Arrhenius' macrokinetics at combustion of disperse systems can take place even in the absence of Arrhenius' microkinetics; it can have a purely thermal nature and be related to their heterogeneity and to the existence of threshold temperature. It is also observed that the combustion of disperse systems always occurs in the microheterogeneous mode according to the relay-race mechanism.

A DFT study of tautomers of 3-amino-1-nitroso-4-nitrotriazol-5-one-2-oxide.

We report herein the structure and explosive properties of the possible isomers of 3-amino-1-nitroso-4-nitrotriazol-5-one-2-oxide computed from the B3LYP/aug-cc-pVDZ level. The optimized structures, vibrational frequencies and thermodynamic values for triazol-5-one-N-oxides were obtained in the ground state. Several designed compounds have densities varying from 2.103 to 2.177 g/cm(3). The detonation properties were evaluated by the Kamlet-Jacob equations based on the predicted density and the calculated heat of explosion. The detonation properties of triazol-5-one-N-oxides (D 9.87 to 10.11 km s(-1) and P 48.95 to 50.61 GPa) appear to be promising compared with those of 2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane (D 9.20 km s(-1), P 42.0 Gpa) and octanitrocubane (D 9.90 km s(-1), P 48.45 GPa). The substitution of secondary amino hydrogen of the triazole ring by amino group shows better impact sensitivity/or stability however the model compounds seem to be highly sensitive.

Synthesis of amino, azido, nitro, and nitrogen-rich azole-substituted derivatives of 1H-benzotriazole for high-energy materials applications.

The amino, azido, nitro, and nitrogen-rich azole substituted derivatives of 1H-benzotriazole have been synthesized for energetic material applications. The synthesized compounds were fully characterized by (1)H and (13)C NMR spectroscopy, IR, MS, and elemental analysis. 5-Chloro-4-nitro-1H-benzo[1,2,3]triazole (2) and 5-azido-4,6-dinitro-1H-benzo[1,2,3]triazole (7) crystallize in the Pca2(1) (orthorhombic) and P2(1)/c (monoclinic) space group, respectively, as determined by single-crystal X-ray diffraction. Their densities are 1.71 and 1.77 g cm(-3), respectively. The calculated densities of the other compounds range between 1.61 and 1.98 g cm(-3). The detonation velocity (D) values calculated for these synthesized compounds range from 5.45 to 8.06 km s(-1), and the detonation pressure (P) ranges from 12.35 to 28 GPa.

Incorporation of support vector machines in the LIBS toolbox for sensitive and robust classification amidst unexpected sample and system variability.

Despite the intrinsic elemental analysis capability and lack of sample preparation requirements, laser-induced breakdown spectroscopy (LIBS) has not been extensively used for real-world applications, e.g., quality assurance and process monitoring. Specifically, variability in sample, system, and experimental parameters in LIBS studies present a substantive hurdle for robust classification, even when standard multivariate chemometric techniques are used for analysis. Considering pharmaceutical sample investigation as an example, we propose the use of support vector machines (SVM) as a nonlinear classification method over conventional linear techniques such as soft independent modeling of class analogy (SIMCA) and partial least-squares discriminant analysis (PLS-DA) for discrimination based on LIBS measurements. Using over-the-counter pharmaceutical samples, we demonstrate that the application of SVM enables statistically significant improvements in prospective classification accuracy (sensitivity), because of its ability to address variability in LIBS sample ablation and plasma self-absorption behavior. Furthermore, our results reveal that SVM provides nearly 10% improvement in correct allocation rate and a concomitant reduction in misclassification rates of 75% (cf. PLS-DA) and 80% (cf. SIMCA)-when measurements from samples not included in the training set are incorporated in the test data-highlighting its robustness. While further studies on a wider matrix of sample types performed using different LIBS systems is needed to fully characterize the capability of SVM to provide superior predictions, we anticipate that the improved sensitivity and robustness observed here will facilitate application of the proposed LIBS-SVM toolbox for screening drugs and detecting counterfeit samples, as well as in related areas of forensic and biological sample analysis.

Density functional study of electronic, bonding, and vibrational properties of Ca(NH2BH3)2.

We present structural, electronic, bonding and vibrational properties of new type hydrogen storage material calcium amidoborane Ca(NH(2)BH(3))(2) by first principles density functional theory using plane wave pseudopotential method. The calculated ground state properties are in good agreement with experiments. The computed Bulk modulus of Ca(NH(2)BH(3))(2) is found to be 28.7 GPa which is slightly higher than that of NH(3)BH(3) indicating that the material is hard over NH(3)BH(3). From the band structure calculations, the compound is found to be a direct band gap insulator with a band gap of 3.27 eV at the Γ point. The calculated band structure shows that the top of the valance band is from the p states of N and the bottom of the conduction band is from d states of Ca. The Mulliken bond populations, Born effective charges and charge density distributions are used to analyze the bonding nature of the compound. It is found that the N-H and B-H bonds are covalent in nature. Further we also compared the phonon density of states and vibrational frequencies of Ca(NH(2)BH(3))(2) with NH(3)BH(3). The study reveals that in both the cases the heavier mass atoms Ca, N, B are involved in the low frequency vibrations whereas the higher frequency vibrations are from H atoms. It is also observed that the vibrational frequencies of B-H bonds are soft in Ca(NH(2)BH(3))(2) when compared to NH(3)BH(3) and thereby concluded that Ca(NH(2)BH(3))(2) is a potential hydrogen storage material for fuel cell applications when compared to NH(3)BH(3).

A DFT study of aminonitroimidazoles.

Density functional theory (DFT) calculations at the B3LYP/aug-cc-pVDZ level were performed to explore the geometric and electronic structures, band gaps, thermodynamic properties, densities and performances of aminonitroimidazoles. The calculated performance properties, stabilities and sensitivities of the model compounds appear to be promising compared with those of the known explosives 2,4-dinitro-1H-imidazole (2,4-DNI), 1-methyl-2,4,5-trinitroimidazole (MTNI), hexahydro-1,3,5-trinitro-1,3,5-triazinane (RDX), and octahydro-1,3,5,7-tetranitro-1,3,5,7-tetraazocane (HMX). The position of the NH(2) or the number of NO(2) groups on the diazole presumably determines the structure, heat of formation, stability, sensitivity, density and performance of the compound.

Molecular design of aminopolynitroazole-based high-energy materials.

The density functional theory (DFT) was employed to calculate the energetic properties of several aminopolynitroazoles. The calculations were performed to study the effect of amino and nitro substituents on the heats of formation, densities, detonation performances, thermal stabilities, and sensitivity characteristics of azoles. DFT-B3LYP, DFT-B3PW91, and MP2 methods utilizing the basis sets 6-31 G* and 6-311 G (2df, 3p) were adopted to predict HOFs via designed isodesmic reactions. All of the designed aminopolynitroazoles had heats of formation of >220 kJ mol(-1). The crystal densities of the aminopolynitroazoles were predicted with the cvff force field. All of the energetic azoles had densities of >1.83 g/cm(3). The detonation velocities and pressures were evaluated using the Kamlet-Jacobs equations, utilizing the predicted densities and heats of formation. It was found that aminopolynitroazoles have a detonation velocity of about 9.1 km/s and detonation pressure of 36 GPa. The bond dissociation energies for the C-NO(2) and N-NO(2) bonds were analyzed to investigate the stabilities of the designed molecules. The charge on the nitro group was used to assess impact sensitivity in the present study. The results obtained imply that the designed molecules are stable and are expected to be candidates for high-energy materials (HEMs).

Laser-induced breakdown spectroscopy-based investigation and classification of pharmaceutical tablets using multivariate chemometric analysis.

We report the effectiveness of laser-induced breakdown spectroscopy (LIBS) in probing the content of pharmaceutical tablets and also investigate its feasibility for routine classification. This method is particularly beneficial in applications where its exquisite chemical specificity and suitability for remote and on site characterization significantly improves the speed and accuracy of quality control and assurance process. Our experiments reveal that in addition to the presence of carbon, hydrogen, nitrogen and oxygen, which can be primarily attributed to the active pharmaceutical ingredients, specific inorganic atoms were also present in all the tablets. Initial attempts at classification by a ratiometric approach using oxygen (∼777 nm) to nitrogen (742.36 nm, 744.23 nm and 746.83 nm) compositional values yielded an optimal value at 746.83 nm with the least relative standard deviation but nevertheless failed to provide an acceptable classification. To overcome this bottleneck in the detection process, two chemometric algorithms, i.e. principal component analysis (PCA) and soft independent modeling of class analogy (SIMCA), were implemented to exploit the multivariate nature of the LIBS data demonstrating that LIBS has the potential to differentiate and discriminate among pharmaceutical tablets. We report excellent prospective classification accuracy using supervised classification via the SIMCA algorithm, demonstrating its potential for future applications in process analytical technology, especially for fast on-line process control monitoring applications in the pharmaceutical industry.

High-pressure study of lithium azide from density-functional calculations.

The structural, electronic, optical, and vibrational properties of LiN(3) under high pressure have been studied using plane wave pseudopotentials within the generalized gradient approximation for the exchange and correlation functional. The calculated lattice parameters agree quite well with experiments. The calculated bulk modulus value is found to be 23.23 GPa, which is in good agreement with the experimental value of 20.5 GPa. Our calculations reproduce well the trends in high-pressure behavior of the structural parameters. The present results show that the compressibility of LiN(3) crystal is anisotropic and the crystallographic b-axis is more compressible when compared to a- and c-axes, which is also consistent with experiment. Elastic constants are predicted, which still awaits experimental confirmation. The computed elastic constants clearly show that LiN(3) is a mechanically stable system and the calculated elastic constants follow the order C(33) > C(11) > C(22), implying that the LiN(3) lattice is stiffer along the c-axis and relatively weaker along the b-axis. Under the application of pressure the magnitude of the electronic band gap value decreases, indicating that the system has the tendency to become semiconductor at high pressures. The optical properties such as refractive index, absorption spectra, and photoconductivity along the three crystallographic directions have been calculated at ambient as well as at high pressures. The calculated refractive index shows that the system is optically anisotropic and the anisotropy increases with an increase in pressure. The observed peaks in the absorption and photoconductivity spectra are found to shift toward the higher energy region as pressure increases, which implies that in LiN(3) decomposition is favored under pressure with the action of light. The vibrational frequencies for the internal and lattice modes of LiN(3) at ambient conditions as well as at high pressures are calculated from which we predict that the response of the lattice modes toward pressure is relatively high when compared to the internal modes of the azide ion.

Structural, electronic, bonding, and elastic properties of NH3BH3: a density functional study.

The structural, electronic, bonding, and elastic properties of the low-temperature orthorhombic phase of NH(3)BH(3) are studied by means of first-principles total energy calculations based on the pseudopotential method. The calculated structural parameters of NH(3)BH(3) are found to be in good agreement with the experimental values. From the band structure calculations, the compound is found to be an indirect bandgap insulator with the bandgap of 5.65 eV (5.90 eV) with LDA(GGA) along the Γ-Z direction. The Mulliken bond population and the charge density distributions are used to analyze the chemical bonding in NH(3)BH(3) . The study reveals that B-H bonds are more covalent than N-H bonds. The elastic constants are predicted for ambient as well as pressures up to 6 GPa, from which theoretical values of all the related mechanical properties such as bulk modulus, shear modulus, Young's modulus, Poisson's ratio, and anisotropy factors are calculated. It is found that NH(3)BH(3) is mechanically stable at ambient and also external pressures up to 6 GPa. As pressure increases all the calculated elastic moduli of NH(3)BH(3) increase, indicating that the compound becomes more stiffer and hard under pressure. From the ratio of shear modulus to bulk modulus (G/B), we conclude NH(3)BH(3) to be ductile in nature, and the ductility increases with pressure. The present results confirm the experimentally observed less plastic nature of the low-temperature phase of NH(3)BH(3) .

Theoretical studies on nitrogen rich energetic azoles.

Different nitro azole isomers based on five membered heterocyclics were designed and investigated using computational techniques in order to find out the comprehensive relationships between structure and performances of these high nitrogen compounds. Electronic structure of the molecules have been calculated using density functional theory (DFT) and the heat of formation has been calculated using the isodesmic reaction approach at B3LYP/6-31G* level. All designed compounds show high positive heat of formation due to the high nitrogen content and energetic nitro groups. The crystal densities of these energetic azoles have been predicted with different force fields. All the energetic azoles show densities higher than 1.87 g/cm(3). Detonation properties of energetic azoles are evaluated by using Kamlet-Jacobs equation based on the calculated densities and heat of formations. It is found that energetic azoles show detonation velocity about 9.0 km/s, and detonation pressure of 40GPa. Stability of the designed compounds has been predicted by evaluating the bond dissociation energy of the weakest C-NO(2) bond. The aromaticity using nucleus independent chemical shift (NICS) is also explored to predict the stability via delocalization of the π-electrons. Charge on the nitro group is used to assess the impact sensitivity in the present study. Overall, the study implies that all energetic azoles are found to be stable and expected to be the novel candidates of high energy density materials (HEDMs).

Theoretical studies on the structure and detonation properties of amino-, methyl-, and nitro-substituted 3,4,5-trinitro-1H-pyrazoles.

In this study, 3,4,5-trinitro-1H-pyrazole (R20), 3,4,5-trinitro-1H-pyrazol-1-amine (R21), 1-methyl-3,4,5-trinitro-1H-pyrazole (R22), and 1,3,4,5-tetranitro-1H-pyrazole (R23) have been considered as potential candidates for high-energy density materials by quantum chemical treatment. The geometric and electronic structures, band gap, thermodynamic properties, crystal density and detonation properties were studied using density functional theory at the B3LYP/aug-cc-pVDZ level. The calculated energy of explosion, density, and detonation performance of model compounds are comparable to 1,3,5-trinitro-1,3,5-triazinane (RDX), and 1,3,5,7-tetranitro-1,3,5,7-tetrazocane (HMX). Atoms-in-molecules (AIM) analyses have also been carried to understand the nature of intramolecular interactions and the strength of trigger bonds.