The thermal decomposition of azidopyridines

The thermal decomposition of new heteroaromatic polyazides 2,6-diazido-3,5-dicyanopyridine, 2,4,6-triazido-3,5-dicyanopyridine, and 2,3,4,5-tetraazido-6-cyanopyridine was studied by thermogravimetry, volumetry, mass-spectrometry, and IR spectroscopy. Reaction kinetic parameters were determined. The only gaseous product of the thermal decomposition of all the azides studied was nitrogen, its degree of purity was 99.0–99.8 vol %. 2,6-Diazido-3,5-dicyanopyridine and 2,4,6-triazido-3,5-dicyanopyridine had thermal stability and thermal decomposition parameters close to those of the majority of aromatic azides. The mechanism of thermal decomposition of these azides includes the splitting off of the nitrogen molecule at the initial limiting process stage. Subsequent intermolecular reactions with the participation of nitrenes result in the formation of an amorphous substance containing polyconjugated fragments with sp ² hybridization, which form planar two-dimensional networks. 2,3,4,5-Tetraazido-6-cyanopyridine has very low thermal stability; the rate of nitrogen release during its decomposition is almost 1000 times higher than with 2,6-diazido-3,5-dicyanopyridine and 2,4,6-triazido-3,5-dicyanopyridine at comparable temperatures. This was explained by the presence of the ortho azido group (there is no ortho arrangement of azido groups in 2,6-diazido-3,5-dicyanopyridine and 2,4,6-triazido-3,5-dicyanopyridine).     

Electron-impact total cross sections for inelastic processes for furan,tetrahydrofuran and 2,5-dimethylfuran

We report total inelastic, total ionisation and summed total excitation cross sections for electron scattering on furan, tetrahydrofuran (THF) and 2,5-dimethylfuran at energies between the ionisation threshold and 5 keV. We have employed the spherical complex optical potential formalism (SCOP) to calculate the total inelastic cross sections (Q_inel) and have used complex scattering potential-ionisation contribution (CSP-ic) method to derive total ionisation cross sections (Q_ion) and summed total excitation cross sections (∑Q_exc) from the calculated Q_inel. We have also computed Q_ion for these molecules using binary-encounter-Bethe (BEB) approach. We have compared our total cross sections (TCS) with available experimental as well as previous theoretical results and have found good agreement. The results are presented graphically as well as numerically.     

Effects of blending 2,5-dimethylfuran on the laminar burning velocity and ignition delay time of isooctane/air mixture

The prospects of 2,5-dimethylfuran (DMF) as a bio-derived fuel that can be blended with gasoline are believed to be impressive. However, the effects of blending DMF on the key combustion parameters like the laminar burning velocity and ignition delay time of gasoline/air mixture need to be studied extensively for the successful implementation of the fuel mixture in spark ignition engines. Therefore, a skeletal chemical kinetic mechanism, comprising of 999 reactions among 218 species, has been developed in the present work for this purpose. The proposed chemical kinetic model has been validated against a wide range of experimental data for the laminar burning velocity and ignition delay time of isooctane (representing gasoline), DMF and their blends. It has been found from the present study that the thermal diffusivity of the unburnt gas mixture changes by a very small amount from the corresponding value for the pure isooctane/air mixture when DMF is added. Unlike isooctane, the DMF molecule does not consume H radicals during its primary breakup. Therefore, the maximum laminar burning velocity increases marginally when 50% DMF is blended with isooctane due to the increased presence of H radicals in the flame. The negative temperature coefficient behaviour in the ignition delay time of the isooctane fuel vanishes when 30% DMF (v/v) is blended to it.     

Dynamic modeling of cyclotetracubyl thermal decomposition

Cyclotetracubyl (C₈H₆)₄ is the smallest possible two-dimensional oligomer constructed from cubylene units. An original quantum-chemical method for molecular modeling is for the first time used to determine its thermokinetic parameters (activation energy and frequency factor). The temperature dependence of the lifetime of cyclotetracubyl over the temperature range 700–1600 K is examined. The results are compared with the data for individual cubane and linear tetracubyl in order to analyze the influence of the dimension of oligomers on their thermal stability and decomposition mechanisms. Modeling of heavier oligomers, C₇₂H₄₈, C₁₂₈H₈₀, and C₂₀₀H₁₂₀, made it possible to obtain, by extrapolation, the binding energy per cubylene fragment in an infinite two-dimensional layer (E _b = 8.62 eV/mer) and to determine the dependence of the HOMO-LUMO gap on the characteristic size of the oligomer.     

Chemiluminescence in the thermal decomposition of dicumyl peroxide

Chemiluminescence accompanying the dicumyl peroxide decomposition in cumene and tetraline with different content of tetralyl hydroperoxide has been studied under nitrogen at 140°C. Simultaneously, the main reaction products of dicumyl peroxide decomposition have been determined chromatographically. Chemiluminescence intensity is proportional to the square root of dicumyl peroxide concentration in the system. Chemiluminescence intensity increases with increasing concentration of tetraline in cumene and also with increasing concentration of tetralyl hydroperoxide. The amount of acetophenone formed decreases. The various elementary processes responsible for the chemiluminescence are suggested.     

Predictive kinetics for the thermal decomposition of RDX

The application of Variable Reaction Coordinate Transition State Theory for an energetic material is presented. The homolysis of the N–N bond in RDX is characterized using an embedding methodology in which key atoms in the bond-dissociation process are computed using CASPT2(10e,7o)/jun-cc-pVTZ, while the rest of the molecule is computed using M06-2X/jun-cc-pVTZ. Microcanonical rate theory is used to quantify the temperature and pressure dependent rate constants. The cleavage of the N–N bond is by far the dominant channel, with HONO elimination a distant second. The predicted rate constants are in excellent agreement with the experimental data. The computational approach can be used to provide accurate models for the combustion properties of novel energetic materials.     

高压下固相硝基甲烷分解的分子动力学计算

基于ReaxFF, 采用NVT系综和Berendsen方法对0–7 GPa时和2500 K时固相硝基甲烷的 分解过程进行分子动力学计算, 通过分析硝基甲烷发生分解反应生成的碎片数量随时间的变化, 对不同压强下硝基甲烷的分解机理进行研究. 计算结果表明在0–3 GPa时, 初始分解路径为C–N键断裂和硝基甲烷的异构化; 在4–7 GPa 时, 初始分解路径为分子间质子转移和C–N, N–O键的断裂; 在硝基甲烷的第二阶段反应中存在H₂O, NO, NO₂, HONO, 硝基甲烷分子自身的催化反应. 硝基甲烷在高温高压下发生热分解反应生成碳团簇, 且团簇中碳原子的数量和碳团簇的空间构型随着压强的变化而变化. 关键词： ReaxFF 分子动力学 热分解 压强效应 碳团簇     

The thermal decomposition of mono-, di-, and tripotassium salts of trinitrophloroglucinol

The kinetics of the thermal decomposition of mono-, di-, and tripotassium salts of trinitrophloroglucinol (TNPG) was studied in the solid phase by the manometric method at m/V from 1 × 10^?4 to 15 × 10^?4 g/cm³ over the temperature ranges 125–150°C (K₁TNPG), 200–230°C (K₂ TNPG), and 200–250°C (K₃ TNPG). All the compounds decomposed according to the topochemical mechanism: there was an induction period, after which the rate of gas release was maximum. This rate then gradually decreased. The second decomposition stage was observed for K₁TNPG as the temperature increased to 200–250°C. The special features of changes in the rate of the process during transformation and the influence of the degree of vessel filling with a substance, particle size, and temperature on the kinetics of decomposition were studied. The kinetic results and composition of gaseous products and some condensed decomposition products lead to certain conclusions concerning the mechanism of the chemical transformations.     

The decisive role of pericyclic reactions in the thermal decomposition of organophosphorus compounds

The understanding of the thermal decomposition chemistry of chemical warfare nerve agents is largely limited by the scarcity of kinetic data. Because of the high toxicity of these molecules, experimental determination of their chemical properties is very difficult. In the present work, a comprehensive detailed kinetic model for the decomposition of sarin and some simulants, i.e. di-isopropyl methyl phosphonate (DIMP), diethyl methylphosphonate (DEMP), and triethyl phosphate (TEP) were developed, containing possible molecular and radical pathways. The importance of unimolecular pericyclic decomposition led to evaluate precisely the rate constants of these reactions with high level theoretical calculations. The QCISD(T)/cc-PV∞QZ//B2PLYPD3/6–311+G(2d,d,p) level of theory was selected after a benchmark. The contribution of hindered rotors was included with the 1D-HR-U approach. Tunneling was taken into account for H-atom transfer. Transition state theory was used to calculate high-pressure limit rate constants and pressure dependent rate constants were calculated using Master Equation modeling. The model was validated against experimental pyrolysis and oxidation experimental data available in literature. Flux analyses showed that whatever the conditions are, the first step of decomposition of the studied phosphorus compounds are pericyclic eliminations leading to successive decompositions, whereas bond-breaking or H-atom abstraction remain negligible, even at high temperature.     

The effect of pressure on the thermal decomposition of nitroalkanes catalyzed by amines

Abstract

Solutions of 1° and 2° nitroalkanes were thermolyzed in the presence of amines at hydrostatic pressures up to 1.5 GPa, and their rates of decomposition were measured. The probable mechanism of base catalyzed decomposition involves a bimolecular reaction of the aci-nitroalkane with the nitronate ion to yield a carbonyl compound, its corresponding oxime, and nitrite ion as the early rate determining steps. In the case of nitroethane, the products react further to give 3,4,5-trimethylisoxazole as a major product above 0.4 GPa. The effects of pressure on the rates of decomposition of nonacidic nitroalkanes, 2,2-dinitropropane and 2-methyl-2-nitropropane, were measured and used to infer the rate-controlling steps. 2,2-Dini-tropropane appears to undergo rearrangement to a nitrite, followed by homolysis to give acetone, NO and NO₂. 2-Methyl-2-nitropropane gives mostly isobutylene and HONO, probably via a 5-membered cyclic transition state.     

Kinetics and mechanism of the thermal decomposition of keto-RDX

The thermal decomposition of keto-RDX occurs through the homolysis of the N-NO₂ bond in one of the nitramide groups. This bond is substantially longer (1.438 Å) than the analogous bond in the RDX molecule (1.382 Å). The activation parameters of the decomposition, E (kcal/mol) and logA [s^?1] were found to be 36.2 and 14.8 in benzene and 35.4 and 12.84 in the solid phase. In the latter case, the reaction proceeds through the dislocation mechanism. The crushing of large crystals produces no effect on the decomposition rate, which, however, depends on the regime of crystallization. The rearrangement into unstable diazoxy esters, a process typical of linear nitramides, does not occur in keto-RDX because of steric hindrances.     

Theoretical study of the gas-phase thermal decomposition of urea

Urea is used in Selective Catalytic and Non Catalytic Reduction (SCR and SNCR) methods for NOx abatement in post-combustion processes. The decomposition of urea-water solution in exhaust systems leads to the formation of NH₃, which is the NOx reducing agent. However, the decomposition of urea can also lead to the formation of unwanted pyrolysis by-products, particularly in region where the temperature and pressure conditions are not optimal or via wall-spray interactions. In this study, a gas phase kinetic model for urea pyrolysis is proposed to explain the growth of pyrolysis by-products of urea, and in particular, the route of formation of the major product: isocyanuric acid. Systematic theoretical calculations, using electronic structure calculations and transition state theory, were performed to explore all the possible unimolecular and bimolecular reactions of the initial species. Kinetic modeling was used to select the relevant reaction pathways at each growing step, and their rate constants were refined using CCSD(T)/CBS//B2PLYP-D3/cc-pvTZ calculations. Quantum calculations showed that the postulated growing schemes of the literature, based on the successive urea + HNCO ? biuret, biuret + HNCO ? triuret followed by the cyclization of triuret into isocyanuric acid are not energetically favored. It is observed that the most favored reaction route to isocyanuric acid involves carbamimidic acid, the urea tautomer, that first yields biuret in a reaction with HNCO and then is involved in the formation of triuret by reacting with a decomposition product of biuret. Isocyanuric acid is produced from the reaction between HNCO and the same decomposition product of biuret. This new mechanism of formation of isocyanuric acid is tested at different conditions to explore its most favored conditions of formation in the gas phase. Exploratory simulations of a pseudo condensed phase are also performed to qualitatively simulate condensed phase experiments and explain the observed pyrolysis yields.     

On the primary thermal decomposition pathways of hydroxycinnamic acids

Valorization of pyrolytic lignin to fuels and chemicals is still poorly understood due to its ill-defined structure and the complexity of the decomposition chemistry. To shed some light on the dominant reaction pathways of lignin thermolysis, novel experimental and first-principles based calculations of its building blocks have been carried out. Pyrolysis chemistry of hydroxycinnamic acids is investigated in this work using a unique Py-GC × GC-FID/TOF-MS coupled with a customized GC to detect water and gases, to gain an understanding of the role of the branching ratios in lignin and its linkages with hemicellulose. Mean residence times of cinnamic and ferulic acids were estimated to be 12 and 21 s at 573 K, based on time-resolved experiments. Cinnamic acid undergoes a CO₂ elimination reaction at temperatures higher than 873 K without an intermediate liquid phase. At temperatures as low as 573 K, –OH and –OCH₃ substituted cinnamic acids underwent decarboxylation despite bearing similar BDEs for Cβ–Cγ scission. At these temperatures, p-coumaric and ferulic acids were converted into 4-vinylphenol and 4-vinylguaiacol by 40 wt% and 30 wt%, respectively. On the other hand, sinapinic acid converted nearly by 80 wt% at temperatures below its boiling point of 676 K. In conjunction with novel quantum chemical calculations, it could be ruled out that decarboxylation was not occurring via concerted unimolecular reactions at low temperatures. Instead, water-catalyzed reactions of hydroxycinnamic acids seem to be the primary cause for the CO₂ elimination in the intermediate liquid phase via a 6-centered transition state.     

Cage effect in the thermal decomposition of solid-state azobisisobutyronitrile

The method of cross-recombination of isotopically labeled NC(CH₃)₂C and NC(CD₃)₂C radicals with equal free-valence reactivity is applied to measure the cage effect, F = k _rec/(k _rec + k _dif), in the thermal decomposition of azobis(isobutironitrile) in isomorphic crystals consisting by half of fully deuterated molecules of the initial substance. It is shown that the decomposition occurs at crystal lattice defects. The products are analyzed by chromatography-mass spectrometry at the degree of conversion of 1% at 70°C. The CE effect and diffusion coefficient at 70°C have been found to be F = 0.87 and D = 7.7 · 10^?7 cm²/s. These results are indicative of a high mobility of molecules on the inner surface of the crystal.     

Characterization of Prussian blue and its thermal decomposition products

The mixed-valence state of Prussian blue and its thermal decomposition products has been studied by Mössbauer and infrared spectra, and X-ray powder diffraction and conductivity measurements. The⁵⁷Fe Mössbauer spectra have revealed that the coordination environment of Prussian blue is not changed by the heat-treatment at lower than 200°C while the flipping of the cyano ligands takes place when the heat-treatment temperature exceeds 250°C. The electrical conductivity of the Prussian blue samples heat-treated in vacuo at 300 and 350°C is higher than that of the samples heat-treated at lower temperatures. All the spectral measurements have demonstrated that a new mixed-valence state is produced in Prussian blue by thermally flipping the cyano ligand and quenching the flipped cyano ligand to liquid nitrogen temperature.     

Raman Spectroscopy of PVA with metal compounds thermal decomposition

The products of polyvinyl alcohol (PVA) films with metal compounds pyrolysis were investigated with Raman spectroscopy. The results of heating in vacuum are conjugated bonds, metallization and other consequences. There is a great difference in physicochemical properties and Raman spectra of products for PVA with alkaline metal additives on the one hand and PVA with noble metal additives on the other hand.     

Magnesium oxide nanocrystals via thermal decomposition of magnesium oxalate

The present work reports the synthesis of magnesium oxide (MgO) nanocrystals via a thermal decomposition route and the study of physicochemical properties of products. The MgO nanocrystals were prepared from magnesium oxalate powders as precursor. Transmission electron microscopy (TEM) analysis demonstrated MgO nanocrystals with an average diameter of about 20−25 nm. The products were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), selected area electronic diffraction (SAED), and Fourier transform infrared (FT-IR) spectroscopy. Optical absorption and photoluminescence emission properties of MgO nanocrystals were investigated.     

Accelerated thermal decomposition of AlH3 for hydrogen-fueled vehicles

The potential for using aluminum hydride, AlH₃, for vehicular hydrogen storage is explored. It is shown that particle-size control and doping of AlH₃ with small levels of alkali-metal hydrides (e.g. LiH) results in accelerated desorption rates. For AlH₃–20 mol% LiH, 100 °C desorption kinetics are nearly high enough to supply vehicles. It is highly likely that 2010 gravimetric and volumetric vehicular system targets (6 wt% H₂ and 0.045 kg/L) can be met with onboard AlH₃. However, a new, low-cost method of off-board regeneration of spent Al back to AlH₃ is needed. PACS 68.43.Vx; 68.43.Mn; 83.80.Fg; 84.60.Ve; 51.30.+i; 61.10.Nz     

Laser schlieren study of the thermal decomposition of 2-ethylhexyl-nitrate

The decomposition kinetics of 2-ethylhexyl nitrate behind incident shock waves in a diaphragmless shock tube facility were studied with a laser schlieren densitometry diagnostic at temperatures from 670 to 940 K and pressures of 35, 59, and 118 Torr. Measured density gradients informed improvements to the decomposition mechanism of 2-ethylhexyl nitrate. In particular, the analysis revealed important 3-heptyl radical chemistry. The measured and predicted rate constants are near the high-pressure limit.