NO2 Bond Cleavage Research Articles

Thermochemistry, Tautomerism, and Thermal Stability of 5,7-Dinitrobenzotriazoles

Nitro derivatives of benzotriazoles are safe energetic materials with remarkable thermal stability. In the present study, we report on the kinetics and mechanism of thermal decomposition for 5,7-dinitrobenzotriazole (DBT) and 4-amino-5,7-dinitrobenzotriazole (ADBT). The pressure differential scanning calorimetry was employed to study the decomposition kinetics of DBT experimentally because the measurements under atmospheric pressure are disturbed by competing evaporation. The thermolysis of DBT in the melt is described by a kinetic scheme with two global reactions. The first stage is a strong autocatalytic process that includes the first-order reaction (Ea1I = 173.9 ± 0.9 kJ mol-1, log(A1I/s-1) = 12.82 ± 0.09) and the catalytic reaction of the second order with Ea2I = 136.5 ± 0.8 kJ mol-1, log(A2I/s-1) = 11.04 ± 0.07. The experimental study was complemented by predictive quantum chemical calculations (DLPNO-CCSD(T)). The calculations reveal that the 1H tautomer is the most energetically preferable form for both DBT and ADBT. Theory suggests the same decomposition mechanisms for DBT and ADBT, with the most favorable channels being nitro-nitrite isomerization and C-NO2 bond cleavage. The former channel has lower activation barriers (267 and 276 kJ mol-1 for DBT and ADBT, respectively) and dominates at lower temperatures. At the same time, due to the higher preexponential factor, the radical bond cleavage, with reaction enthalpies of 298 and 320 kJ mol-1, dominates in the experimental temperature range for both DBT and ADBT. In line with the theoretical predictions of C-NO2 bond energies, ADBT is more thermally stable than DBT. We also determined a reliable and mutually consistent set of thermochemical values for DBT and ADBT by combining the theoretically calculated (W1-F12 multilevel procedure) gas-phase enthalpies of formation and experimentally measured sublimation enthalpies.

Temperature and Pressure Effects on HMX/Graphene via ReaxFF Molecular Dynamics Simulations

Studying the thermal decomposition of energetic materials at high temperatures can provide detailed reaction and mechanistic information, which is critical for understanding the reactivity of energetic materials, designing mixed explosives, and achieving improved safety. In this work, the effects of temperature and pressure on graphene (Gr)-based HMX crystals were investigated using ReaxFF molecular dynamics simulations. The thermal decomposition processes of perfect HMX crystals, HMX crystals with (001), (010), or (100) crystal planes, and HMX/Gr mixed systems were studied at high temperatures and pressures. In the mixed systems, different configurations of HMX molecules adsorbed on the Gr surface were confirmed by theoretical calculation methods. With the pressure ranging from atmospheric pressure to 31 GPa, 3, 5, and 3 configurations of HMX adsorbed on the Gr surface were identified for the (001)/Gr, (010)/Gr, and (100)/Gr systems, respectively. The time-dependent curves for the evolution of fragments, intermediates, and pyrolysis products were analyzed. The rate constant for the thermal decomposition of HMX was found to be significantly affected by the addition of Gr. In particular, the thermal decomposition reaction was strongly inhibited in the (010)/Gr system. This result indicates that Gr promotes an anisotropic thermal effect, resulting from the steric hindrance of the NO2 functional groups and the interaction between Gr and HMX molecules. Gr also affected the initial reaction pathway of homolytic N–NO2 bond cleavage, with C=O, C–OH, and C–OC bonds on the Gr surface participating in the formation of nitro radicals and HONO.

Denitrative functionalization of nitroarenes

Denitrative functionalization of nitroaromatic compounds has become a very intriguing challenge in synthetic chemistry. Herein, various methodologies for $${\text{C}}_{{sp^{2} }}$$ –NO2 bond cleavage of nitroaromatic compounds have been reviewed. We survey the literature for the reactions in which nitroarenes have been used as the electrophilic coupling partners for the formation of $${\text{C}}_{{sp^{2} }}$$ –O, $${\text{C}}_{{sp^{2} }}$$ –S, $${\text{C}}_{{sp^{2} }}$$ –N, $${\text{C}}_{{sp^{2} }}$$ –H and $${\text{C}}_{{sp^{2} }}$$ –C bonds via transition metal-catalyzed cross-coupling reactions. A brief discussion about the mechanism of the reactions has also been presented.

Effects of different types of defects on ignition mechanisms in shocked β-cyclotetramethylene tetranitramine crystals: A molecular dynamics study based on ReaxFF-lg force field

Microscopic defects affect shock sensitivity remarkably. We exhibit a molecular dynamics study of defective β-cyclotetramethylene tetranitramine (HMX) crystals with void (VH), entrained oxygen (OH), and entrained amorphous carbon (CH), as well as a perfect HMX crystal (PH) for contrast. The crystals were shocked with 9 km/s shock velocity perpendicular to the (010) plane for 50 ps using a ReaxFF-lg force field. The results demonstrate that the shock sensitivity of HMX crystals with different defects is enhanced to different degrees. OH has the highest shock sensitivity, which is slightly higher than that in VH; both OH and VH crystals have much higher shock sensitivity than that in CH. Obvious local high temperature areas are found in defective systems, which are not found in PH. The initial reaction is N–NO2 bond cleavage in PH and is N–NO2 and N–O bond cleavages in defective systems. More products are found in defective HMX crystals during shock simulations, while fewer immediate products and no final product are found in PH. The average temperature, pressure, and decrement of potential energy during simulation are much higher in defective systems compared to the perfect ones.

Ab initio molecular dynamics simulations study on initial decompositions of β‐HMX at high temperature coupled with high pressures

We performed ab initio molecular dynamics simulations to investigate initial decomposition mechanisms and subsequent chemical processes of β‐HMX (cyclotetramethylene tetranitramine) (octahydro‐1,3,5,7‐tetranitro‐1,3,5,7‐tetrazocine) crystals at high temperature coupled with high pressures. It was found that the initial decomposition step is the simultaneous C–H and N–NO2 bond cleavage at 3,500 K. When the pressure (1–10 GPa) is applied, the first reaction steps are primarily the C–N and C–H bond fission at 3,500 K. The C–H bond cleavage is a triggering decomposition step of the HMX crystals at 3,500 K coupled with 16 GPa. This indicates that the C–H bonds are much easier to be broken and the hydrogen radicals are much more active. The applied pressures (1–10 GPa) accelerate the decompositions of HMX at 3,500 K. The decomposition pathways and time evolution of the main chemical species demonstrate that the temperature is the foremost factor that affects the decomposition at high pressures (1–10 GPa). However, the decomposition of HMX is dependent on both the temperature (3,500 K) and the pressure (16 GPa). This work will enrich the knowledge of the decompositions of condensed energetic materials under extreme conditions.

How To Perform Suzuki–Miyaura Reactions of Nitroarene or Nitrations of Bromoarene Using a Pd0 Phosphine Complex: Theoretical Insight and Prediction

Pd0(BrettPhos) (1) was experimentally applied to Suzuki–Miyaura reactions of nitroarenes which occur through oxidative addition of nitroarene (ArNO2) to Pd0. On the other hand, Pd0(tBu-BrettPhos) (2) was experimentally applied to nitrations of bromoarenes which occur through reductive elimination of nitroarene. DFT calculations disclosed that oxidative addition of 4-nitroanisole to 1 was exergonic but that to 2 was endergonic, indicating that 1 is useful for oxidative addition of an Ar–NO2 bond but 2 is useful for reductive elimination of an Ar–NO2 bond. This difference in reactivity between 1 and 2 is explained in that bulky tBu groups on tBu-BrettPhos destabilize the PdII complex PdII(Ar)(NO2)(tBu-BrettPhos) due to large steric repulsion between Ar and tBu-BrettPhos but less bulky cyclohexyl groups on BrettPhos do not. It is theoretically predicted here that NMe2-substituted BrettPhos is better for Ar–NO2 bond cleavage than BrettPhos but xyl-BrettPhos is good for Ar–NO2 formation, as is tBu-BrettPhos, w...

Early Events When Heating 1,1-Diamino-2,2-dinitroethylene: Self-Consistent Charge Density-Functional Tight-Binding Molecular Dynamics Simulations

A self-consistent charge density-functional tight-binding method combined with molecular dynamics (MD) simulations and static density functional theory (DFT) calculations is employed to reveal the initial steps responsible for the thermal decay of the low sensitive and high energetic material (EM) 1,1-diamino-2,2-dinitroethylene (FOX-7). Constant temperature heating and temperature-programmed heating are accounted in our MD simulations to resemble the practice. We find that the heating style has an impact on the initial decomposition mechanism of FOX-7. The N–O bond rupture to produce an O radical is observed for the first time in the temperature-programmed heating simulation from 300 to 3000 K, together with the C–NO2 bond cleavage. We also first capture the intra- and intermolecular hydrogen transferred structures in our constant temperature heating simulation at 3000 K, and in this case, the NO2 partition serves as another kind of primary step for thermally decaying FOX-7 too. Static DFT calculations s...

Kinetic and Mechanistic Study of the Thermal Decomposition of Ethyl Nitrate

Thermal decomposition of ethyl nitrate (ENT; CH3CH2ONO2) has been studied in a low-pressure flow reactor combined with a quadrupole mass spectrometer. The rate constant of the nitrate decomposition was measured as a function of pressure (1–12.5 Torr of helium) and temperature in the range 464–673 K using two different approaches: from kinetics of ENT loss and those of the formation of the reaction product (CH3 radical). The fit of the observed falloff curves with two-parameter expression provided the following low- and high-pressure limits for the rate constant of ENT decomposition: k0 = 1.0 × 10−4 exp(−16,400/T) cm3 molecule−1 s−1 and k∞ = 1.08 × 1016 exp(−19,860/T) s−1, respectively, which allow to reproduce (via above expression and with 20% uncertainty) all the experimental data obtained for k1 in the temperature and pressure range of the study. It was observed that the initial step of the thermal decomposition of ethyl nitrate is O–NO2 bond cleavage leading to formation of NO2 and CH3CH2O radical, which rapidly decomposes to form CH3 and formaldehyde as final products. The yields of NO2, CH3, and formaldehyde upon decomposition of ethyl nitrate were measured to be near unity. In addition, the kinetic data were used to determine the O–NO2 bond dissociation energy in ENT: 38.3 ± 2.0 kcal mol−1.

Pressure-induced structure and properties of crystalline β-FOX-7 by LDA and GGA calculations

The structural, electronic, and absorption properties of crystalline β-FOX-7 were studied by density functional theory–generalized gradient approximation (GGA) and local density approximation (LDA) calculations. The results indicate that there are irregular changes occurred at 9, 17, 32, and 37 GPa for the GGA results and at 2, 7, 22, and 32 GPa for the LDA ones, in agreement with previous reports that the LDA generally underestimated, while the GGA overestimated the structure and properties of the crystals. The compression ratios of the lattice constants suggest that the structure of β-FOX-7 is much stiffer in a- and c-directions than in b-direction. The total decrease ratio of C–NO2 band lengths is larger than that of other bond lengths, indicating that the C–NO2 bond cleavage may trigger the initiation decomposition of β-FOX-7. An analysis of density of states suggests that the interactions between electrons are strengthened under the influence of pressure. The absorption spectra indicate that the bands in the fundamental absorption region have relatively high optical activity at high pressure.

Theoretical study of substituents effect on C–NO2 bond strength in mono substituted nitrobenzenes

Enthalpies of the C−N bond dissociation of nitrobenzene and twenty-seven mono substituted nitrobenzenes along with their formation enthalpies were calculated using different multilevel (G2, G3, G3B3, CBS-QB3) and density functional theory-based B3LYP and wB97XD techniques. The gas phase enthalpies of formation of mono substituted phenyl radicals were also calculated using these methods. The calculated values of reaction enthalpies were compared with available experimental data. We have concluded based on results of present and recent article, that B3LYP level of theory with 6-31G(d′f,p′) basis sets should proposed as the accurate method for calculating formation enthalpies of mono substituted nitrobenzenes as well as enthalpies of C–NO2 bond scission. Based on satisfactory correlation between experimental and calculated data we proposed revision of experimental activation energy of C–NO2 bond rupture in o-chloronitrobenzene, o- and m-nitrotoluene as wells as p-nitroaniline. Enthalpies of formation of some substituted phenyl radicals were estimated using experimental enthalpies of C–NO2 bond cleavage together with formation enthalpies of mono substituted nitrobenzenes and NO2 moiety.

Molecular design of trinitromethyl-substituted nitrogen-rich heterocycle derivatives with good oxygen balance as high-energy density compounds

Density functional theory method was used to study the heats of formation (HOFs), electronic structure, energetic properties, and pyrolysis mechanism of a series of trinitromethyl-substituted heterocycle (including triazole, tetrazole, furazan, tetrazine, and fused heterocycles) derivatives. It is found that the fused ring, tetrazine, and tetrazole are effective structural units for increasing the HOFs of the derivatives. The substitution of the combination of nitro and trinitromethyl is very useful for improving their HOFs. The calculated energetic properties indicate that the combination of the nitro and trinitromethyl is very helpful for improving their detonation properties and oxygen balances (OB). Most of the title compounds have a good OB over zero. The OB of six compounds are very high and over 22. An analysis of the bond dissociation energies for several relatively weak bonds suggests that the N–O bond in the ring is a trigger bond for BIII-1, CI-3, and CI-4, and the ring–NO2 and (NO2)2C–NO2 bond cleavage is likely to happen in thermal decomposition for the remaining compounds. Considering the detonation performance and thermal stability, seven compounds could be regarded as potential candidates for high-energy compounds. Four compounds may be used as the novel high-energy oxidizers.

Nuclear fukui functions and the deformed atoms in molecules representation of the electron density: Application to gas‐Phase RDX (hexahydro‐1,3,5‐trinitro‐1,3,5‐ triazine) electronic structure and decomposition

AbstractThe electronic structure and the onset of decomposition processes of the four conformers of the RDX molecule were studied through the density functional theory (DFT) along with the B3LYP functional and the 6‐311+G(2d,p) Gaussian basis set. The computed DFT electron density was decomposed into atomic contributions using the deformed atoms in molecules (DAM) method, which allowed us to identify regions of electron accumulation and electron depletion of each conformer. The nuclear Fukui functions nuclear stiffness and nuclear reactivity index were then calculated. Both functions are atomic vectors, intrinsic molecular properties, which provide information for the onset of the fragmentation process, were further discussed through the analysis of the DAM electron density. The computed, decomposed electronic structures indicate that equatorial (E) NO2 (nitro) groups are less bulky than axial (A) groups, with the former contributing more than the latter to the increase of the ring delocalized electrons and, therefore, to the lower stability of conformers with more E groups. Concerning the decomposition, the RDX NNO2 bond cleavage is the most probable process for the AAA, AEE, and EEE conformers, while for the AAE conformer, HONO elimination is favored in partial agreement with previous study. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2010