An experimental and kinetic modeling study of a premixed nitromethane flame at low pressure

A premixed nitromethane/oxygen/argon flame at low pressure (4.67 kPa) has been investigated using tunable vacuum ultraviolet (VUV) photoionization and molecular-beam mass spectrometry. About 30 flame species including hydrocarbons, oxygenated and nitrogenous intermediates have been identified by measurements of photoionization efficiency spectra. Mole fraction profiles of the flame species have been determined by scanning burner position at some selected photon energies. The results indicate that N₂ and NO are the major nitrogenous products in the nitromethane flame. Compared with previous studies on nitromethane combustion, a number of unreported intermediates, including C₃H₄, C₄H₆, C₄H₈, C₂H₂O, C₂H₄O, CH₃CN, H₂CNHO, C₃H₃N and C₃H₇N, are observed in this work. Based on our experimental results and previous modeling studies, a detailed oxidation mechanism including 69 species and 314 reactions has been developed to simulate the flame structure. Despite some small discrepancies, the predictions by the modeling study are in reasonable agreement with the experimental results.     

Application of the overtone CO laser to atmosphere gas analysis

In this paper, we consider the applicability of the overtone CO laser to atmosphere gas analysis. The atmosphere transmittance was calculated for lasing lines at the overtone frequencies of the CO laser. The results of the search for informative wavelengths for sensing the minor gas components of the atmosphere N₂O, NO₂, H₂CO, CH₄, C₂H₂, HCl, HBr, and HCN are presented.     

On the connection between the symmetry of internal dynamics of a nonrigid molecule and the geometry of its equilibrium configurations

For the example of nonrigid nitromethane and toluene molecules CH₃NO₂ and CH₃C₆H₅ in the ground electronic states, the possibility of using the observed symmetry properties of internal dynamics of a nonrigid molecule for the correct choice of its equilibrium configurations is considered in the case where different possible geometries of such configurations correspond to the same point group.     

Optical properties of clusters and molecules from real-time time-dependent density functional theory using a self-consistent field

We present a detailed study of optical absorption spectra of finite-size structures, using a method based on time-dependent density-functional theory (TDDFT), which involves a self-consistent field for the propagation of the Kohn–Sham wavefunctions in real-time. Although our approach does not provide a straightforward assignment of absorption features to corresponding transitions between Kohn–Sham orbitals, as is the case in frequency-domain TDDFT methods, it allows the use of larger timesteps while conserving total energy and maintaining stable dipole moment oscillations. These features enable us to study larger systems more efficiently. We demonstrate the efficiency of our method by applying it to a hydrogen-terminated silicon cluster consisting of 364 atoms, with and without P impurities. For cases where direct comparison to experiment can be made, we reproduce the absorption features of fifteen small molecules [N₂, O₂, O₃, NO₂, N₂O, NH₃, H₂O, H₂CO, H₂CO₃, CO₂, CH₄, C₂H₂, C₂H₄, C₂H₆, C₆H₆] and find generally good agreement with experimental measurements. Our results are useful for the detection and the determination of orientation of these molecules.     

AP/(N2 + C2H2 + C2H4) gaseous fuel diffusion flame studies

Counterflow diffusion flame experiments and modeling results are presented for a fuel mixture consisting of N₂, C₂H₂, and C₂H₄ flowing against decomposition products from a solid AP pellet. The flame zone simulates the diffusion flame structure that is expected to exist between reaction products from AP crystals and a hydrocarbon binder. Quantitative species and temperature profiles have been measured for one strain rate, given by a separation of 5 mm, between the fuel exit and the AP surface. Species measured include C₂H₂, C₂H₄, N₂, CN, NH, OH, CH, C₂, NO, NO₂, O₂, CO₂, H₂, CO, HCl, H₂O, and soot volume fraction. Temperature was measured using a combination of a thermocouple at the fuel exit and other selected locations, spontaneous Raman scattering measurements throughout the flame, NO vibrational populations, and OH rotational population distributions. The burning rate of the AP was also measured for this flame’s strain rate. The measured eighteen scalars are compared with predictions from a detailed gas-phase kinetics model consisting of 105 species and 660 reactions. Model predictions are found to be in good agreement with experiment and illustrate the type of kinetic features that may be expected to occur in propellants when AP particles burn with the decomposition products of a polymeric binder.     

Inelastic atom scattering from layers of atoms and molecules on Cu(110)

The vibrational properties of adsorbed layers have been studied using inelastic He atom scattering. The substrate was Cu(110) viewed along the [001] and [11̄0] azimuth. The adsorbates included, Kr, Xe, CH₄, CD₄, C₂H₆, CO, CO₂ and O₂ and covered the region of both physisorption and chemisorption. Despite a range of binding energies and mass ratios the vibrational frequencies showed no dependence on the momentum change, i.e. scattering was dispersionless, although intensity changes occurred in the inelastic peaks. There seemed to be no dependence of the adsorbate spectra on coverage below a monolayer. The peaks of CH₄ and C₂H₆ appeared broader than those of Kr and Xe suggesting molecular motions. Further, CH₄ and C₂H₆ gave very similar spectra. Both energy gain and loss events were observed in the inelastic spectrum. For those adsorbates which gave multilayer adsorption (Xe, C₂H₆) changes in the spectra were observed as the second layer developed. In the latter category also, mixed layers (Xe on C₂H₆, Xe on CO and C₂H₆ on Xe) were studied.     

Relative intensities in x-ray photoelectron spectra. Part VI. The spectra of He(I), He(II), Y Mζ and Zr Mζ

The 2s- and 2p-electron photoionization cross-sections at photon energies up to 190 eV have been calculated, using the RPAE method for averaged configurations of the C, N, O and Ne atoms. The RPAE method ensures a more accurate relation between the cross-sections, 2s/2p, than that obtained using the Hartree—Fock method. Within the framework of the Gelius—Siegbahn model, but with the use of theoretical atomic cross-sections, we have calculated the photoionization cross-sections for He(I), He(II), Y Mζ, Zr Mζ for CH₄, C₂H₆, C₃H₈, C₂H₄, C₂H₂, NH₃, H₂O, CN^?, N₂, CO, CO₂, N₂O and NO₂^? molecules. For CO, N₂, CO₂, N₂O and H₂O molecules, a comparison is made between the theoretical and experimental cross-sections for hν < 60 eV. The calculated absolute and relative values of the molecular-orbital cross-sections are in reasonable agreement with experiment, especially at hν ? 40 eV. The calculations correctly reproduce the change in intensities under the transition He(I) → He(II). We have shown that our calculations have a significant advantage over those performed using the PW and OPW approximations. It is shown for NO, N₂, CO, H₂O, CH₄, NH₃ and N₂O molecules that the total photoionization cross-section calculated taking into account the real structure of the molecular orbitals is in better agreement with the experimental photoabsorption cross-section than is the sum of the cross-sections for the atoms in a molecule.     

Investigation of the kinetics of OH∗ and CH∗ chemiluminescence in hydrocarbon oxidation behind reflected shock waves

The temporal variation of chemiluminescence emission from OH^?(A² Σ ⁺) and CH^?(A² Δ) in reacting Ar-diluted H₂/O₂/CH₄, C₂H₂/O₂ and C₂H₂/N₂O mixtures was studied in a shock tube for a wide temperature range at atmospheric pressures and various equivalence ratios. Time-resolved emission measurements were used to evaluate the relative importance of different reaction pathways. The main formation channel for OH^? in hydrocarbon combustion was studied with CH₄ as benchmark fuel. Three reaction pathways leading to CH^? were studied with C₂H₂ as fuel. Based on well-validated ground-state chemistry models from literature, sub-mechanisms for OH^? and CH^? were developed. For the main OH^?-forming reaction CH+O₂=OH^?+CO, a rate coefficient of k ₂=(8.0±2.6)×10¹⁰ cm³?mol^?1?s^?1 was determined. For CH^? formation, best agreement was achieved when incorporating reactions C₂+OH=CH^?+CO (k ₅=2.0×10¹⁴ cm³?mol^?1?s^?1) and C₂H+O=CH^?+CO (k ₆=3.6×10¹²exp(?10.9 kJ?mol^?1/RT) cm³?mol^?1?s^?1) and neglecting the C₂H+O₂=CH^?+CO₂ reaction.     

On the mechanism of inhibition of a hydrogen flame by propane

Numerical simulations demonstrated that small additives of propane to rich hydrogen-air mixtures suppress the formation of HO₂ and OH in the low-temperature region of the flame zone, thereby causing a substantial decrease in the laminar flame speed. In the low-and high-temperature regions, propane interacts predominantly with OH and H, respectively. In the flame zone, propane is completely converted to CO, CO₂, CH₄, C₂H₂, H₂, and H₂O, being oxidized concurrently with hydrogen at that.     

Simultaneous detection of trace gases using multiplexed tunable diode lasers and a photoacoustic cell containing a cantilever microphone

We report what we believe to be a novel demonstration of simultaneous detection of multiple trace gases by near-IR tunable diode laser photoacoustic spectroscopy using a cell containing a cantilever microphone. Simultaneous detection of carbon monoxide (CO), ethyne (C₂H₂), methane (CH₄) and combined carbon monoxide/carbon dioxide (CO+CO₂) in nitrogen-based gas mixtures was achieved by modulation frequency division multiplexing the outputs of four near-IR tunable diode lasers. Normalized noise-equivalent absorption coefficients of 3.4×10^?9, 3.6×10^?9 and 1.4×10^?9 cm^?1?W?Hz^?1/2 were obtained for the simultaneous detection of CO, C₂H₂ and CH₄ at atmospheric pressure. These corresponded to noise-equivalent detection limits of 249.6 ppmv (CO), 1.5 ppmv (C₂H₂) and 293.7 ppmv (CH₄) respectively over a measurement period of 2.6 s at the relevant laser power. The performance of the system was not influenced by the number of lasers deployed, the main source of noise arising from ambient acoustic effects. The results confirm that small-volume photoacoustic cells can be used with low optical power tunable diode lasers for rapid simultaneous detection of trace gases with high sensitivity and specificity.     

Catalytic effect of water,water dimer,water trimer,HCOOH, H2SO4, CH3CH2COOH and HN(NO2)2 on the isomerisation of HN(NO2)2 to O2NNN(O)OH: a mechanism study

In this article, the isomerisation mechanisms of HN(NO₂)₂ to O₂NNN(O)OH without and with catalyst X (X = H₂O, (H₂O)₂, (H₂O)₃, HCOOH, H₂SO₄, CH₃CH₂COOH and HN(NO₂)₂) have been investigated theoretically at the CBS-QB3 level of theory. Our results show that the catalyst X (X = H₂O, (H₂O)₂, (H₂O)₃, HCOOH, H₂SO₄ and CH₃CH₂COOH) shows different positive catalytic effects on reducing the apparent activation energy of the isomerisation reaction processes. Such different catalytic effects are mainly related to the number of hydrogen bonds and the size of the ring structure in X (X = H₂O, (H₂O)₂ and (H₂O)₃)-assisted transition states, as well as different values of pK_a for H₂SO₄, HCOOH and CH₃CH₂COOH. Very interesting is also the fact that H₂SO₄-assisted reaction is the most favourable for the hydrogen transfer from HN(NO₂)₂ to O₂NNN(O)OH, due to the smallest pK_a (?3.0) value of H₂SO₄ than H₂O, HCOOH, H₂SO₄ and CH₃CH₂COOH, and also because of the largest ∠X???H???Y (the angle between the hydrogen bond donor and acceptor) involved in H₂SO₄-assisted transition state. Compared to the self-catalysis of the isomerisation mechanisms of HN(NO₂)₂ to O₂NNN(O)OH, the apparent activation energy of H₂SO₄-assisted channel also reduces by 9.6 kcal?mol^?1, indicating that H₂SO₄ can affect the isomerisation of HN(NO₂)₂ to O₂NNN(O)OH, most obvious among all the catalysts H₂O, (H₂O)₂, (H₂O)₃, HCOOH, H₂SO₄, CH₃CH₂COOH and HN(NO₂)₂.     

A statistical model for the product energy distribution in reactions leading to prompt dissociation

Direct dynamics calculations have been performed for three reactions: C₃H₈ + H → i-C₃H₇ + H₂, C₃H₈ + H → n-C₃H₇ + H₂, and C₂H₃ + O₂ → HCO + CH₂O. The fraction of the population for the radical products that promptly dissociates is computed. The results for C₃H₈ + H are qualitatively similar to previous results for C₃H₈ + OH, but the new results exhibit a slightly higher branching fraction for prompt dissociation products, owing to the fact that a greater fraction of the internal energy in the transition state ends up in the radical. For C₂H₃ + O₂ → HCO + CH₂O, the fraction of HCO that promptly dissociates is in excess of 99%. Consequently, the main product for C₂H₃ + O₂ at lower temperatures should be written as H + CO + CH₂O and not HCO + CH₂O. These results are then compared with four previous systems: CH₂O + H → HCO + H₂, CH₂O + OH → HCO + H₂O, C₃H₈ + OH → i-C₃H₇ + H₂O, and C₃H₈ + OH → n-C₃H₇ + H₂O. Based upon these seven system, several statistical models are presented. The goal of these statistical models is to predict the fraction of the transition state energy that ends up in the rovibrationally excited radical. On average, these statistical models provide an excellent prediction of product energy distribution. Consequently, these models can be used instead of costly trajectory simulations for predicting prompt radical dissociation for larger species.     

XeCl laser ablation of polyetheretherketone

Ablation of polyetheretherketone (PEEK), a high temperature thermoplastic, by XeCl laser radiation occurs at fluences in excess of 0.07±0.01 J cm^–2. The volatile products of ablation are CO and C₂H₂ with smaller quantities of CH₄, C₄H₂, C₆H₆ and other C₃ and C₄ hydrocarbons. At fluences close to the threshold ablation produces involatile material of relatively high molecular weight but at high fluences extensive disruption of the PEEK structure occurs with conversion of all of the oxygen in the polymer to carbon monoxide.     

Mechanism for sonochemical reduction of Au(III) in aqueous butanol solution under Ar based on the analysis of gaseous and water-soluble products

When an aqueous Au(III) solution containing 1-butanol was sonicated under Ar, Au(III) was reduced to Au(0) to form Au particles. This is because various reducing species are formed during sonication, but the reactivity of these species has not yet been evaluated in detail. Therefore, in this study, we analyzed the effects of Au(III) on the rates of the formation of gaseous and water-soluble compounds (CH₄, C₂H₆, C₂H₄, C₂H₂, CO, CO₂, H₂, H₂O₂, and aldehydes), and the rate of Au(III) reduction as a function of 1-butanol concentration. The following facts were recognized: 1) for Au(III) reduction, the contribution of the radicals formed by the pyrolysis of 1-butanol was higher than that of the secondary radicals formed by the abstraction reactions of 1-butanol with ·OH, 2) ·CH₃ and CO acted as reductants, 3) the contribution of ·H to Au(III) reduction was small in the presence of 1-butanol, 4) aldehydes and H₂ did not act as reductants, and 5) the types of species that reduced Au(III) changed with 1-butanol concentration.     

Experimental and kinetic modeling study of C2H4 oxidation at high pressure

A detailed chemical kinetic model for oxidation of C₂H₄ in the intermediate temperature range and high pressure has been developed and validated experimentally. New ab initio calculations and RRKM analysis of the important C₂H₃ + O₂ reaction was used to obtain rate coefficients over a wide range of conditions (0.003-100 bar, 200-3000 K). The results indicate that at 60 bar and medium temperatures vinyl peroxide, rather than CH₂O and HCO, is the dominant product. The experiments, involving C₂H₄/O₂ mixtures diluted in N₂, were carried out in a high pressure flow reactor at 600-900 K and 60 bar, varying the reaction stoichiometry from very lean to fuel-rich conditions. Model predictions are generally satisfactory. The governing reaction mechanisms are outlined based on calculations with the kinetic model. Under the investigated conditions the oxidation pathways for C₂H₄ are more complex than those prevailing at higher temperatures and lower pressures. The major differences are the importance of the hydroxyethyl (CH₂CH₂OH) and 2-hydroperoxyethyl (CH₂CH₂OOH) radicals, formed from addition of OH and HO₂ to C₂H₄, and vinyl peroxide, formed from C₂H₃ + O₂. Hydroxyethyl is oxidized through the peroxide HOCH₂CH₂OO (lean conditions) or through ethenol (low O₂ concentration), while 2-hydroperoxyethyl is converted through oxirane.     

Study on CH4 dry reforming process by high power inductively coupled plasma torch at atmospheric pressure

CO₂ dissociation and CH₄ dry reforming, by high power inductively coupled plasma (ICP) torch at atmospheric pressure, have been studied. At a frequency of 400 kHz and power of 30 kW, the ICP torch source dissociates CO₂ gas directly, causing CH₄ dry reforming. The resulting products are composed of syngas, C₂H₆, C₂H₄, and C₂H₂. The results show conversion efficiencies (CE) for both CO₂ and CH₄ of 95%. At an input power of 22 kW, a CO₂ flow rate of 30 SLM, and a CH₄ flow rate of 36 SLM, the energy conversion efficiency (ECE) is 57%. As input CH₄ flow rate increases, the selectivity of CO and H₂ decreases and that of C₂ hydrocarbons increases. In this condition, ECE increases. As a result, the high CE of CO₂ and CH₄, the large amount of products, and the high selectivity of C₂ hydrocarbons can be seen as important factors for achieving higher energy conversion efficiency in the CH₄ dry reforming process. The associated chemical reactions are simulated using CHEMKIN-PRO tool, and the results illustrate the tendency of CE to vary with variations in selected parameters, and syngas and C₂ hydrocarbons production trends achieved in the simulation agree with experimental results.     

The absorption cross sections of N2, O2, CO,NO, CO2, N2O,CH4, C2H4, C2H6 and C4H10 from 180 to 700Å

The absorption cross sections of N₂, O₂, CO, NO, CO₂, N₂O, CH₄, C₂H₄, C₂H₆, C₄H₁₀ have been measured photoelectrically in the 180–700 Å region using synchrotron radiation. The absorption cross sections in the region λ ≥ 500 Å was found to be structureless and to increase monotonically with wavelength for all gases. The positions of the structure observed in the 520–720 Å region for N₂, O₂, CO₂ and N₂O are consistent with the various Rydberg series reported by previous authors.     

Flame structure of laminar premixed anisole flames investigated by photoionization mass spectrometry and photoelectron spectroscopy

Two laminar, premixed, fuel-rich flames fueled by anisole-oxygen-argon mixtures with the same cold gas velocity and pressure were investigated by molecular-beam mass spectrometry at two synchrotron sources where tunable vacuum-ultraviolet radiation enables isomer-resolved photoionization. Decomposition of the very weak O–CH₃ bond in anisole (C₆H₅OCH₃) by unimolecular decomposition yields the resonantly-stabilized phenoxy radical (C₆H₅O). This key intermediate species opens reaction routes to five-membered ring species, such as cyclopentadiene (C₅H₆) and cyclopentadienyl radicals (C₅H₅). Anisole is often discussed as model compound for lignin to study the phenolic-carbon structure in this natural polymer. Measured temperature profiles and mole fractions of many combustion intermediates give detailed information on the flame structure. A very comprehensive reaction mechanism from the literature which includes a sub-scheme for anisole combustion is used for species modeling. Species with the highest measured mole fractions (on the order of 10^?3–10^?2) are CH₃, CH₄, C₂H₂, C₂H₄, C₂H₆, CH₂O, C₅H₅ (cyclopentadienyl radical), C₅H₆ (cyclopentadiene), C₆H₆ (benzene), C₆H₅OH (phenol), and C₆H₅CHO (benzaldehyde). Some are formed in the first destruction steps of anisole, e.g., phenol and benzaldehyde, and their formation will be discussed and with regard to the modeling results. There are three major routes for the fuel destruction: (1) formation of benzaldehyde (C₆H₅CHO), (2) formation of phenol (C₆H₅OH), and (3) unimolecular decomposition of anisole to phenoxy (C₆H₅O) and CH₃ radicals. In the experiment, the phenoxy radical could be measured directly. The phenoxy radical decomposes via a bicyclic structure into the soot precursor C₅H₅ and CO. Formation of larger oxygenated species was observed in both flames. One of them is guaiacol (2-methoxyphenol), which decomposes into fulvenone. The presented speciation data, which contain more than 60 species mole fraction profiles of each flame, give insights into the combustion kinetics of anisole.     

13C-NMR Study of Some Substituted 9-Acridanone Derivatives

¹³C-NMR spectra of several 9-acridanones with different substituents both on the ring (R₁ = CH₃, OCH₃, NH₂, N(CH₃)₂, NO₂) and at the nitrogen atom (R₂ = H, CH₃ C₂H₅, CH₂-C₆H₅, C[tbnd]C-CH₃, (CH₂)₂N(C₂H₂)₂, CH=C=CH₂) have been recorded. The C-NMR chemical shifts are discussed as a function of the nature of the substituent, the importance of peri steric interactions and the electronic structure of the acridanone ring. There is a good linear relationship between the total electronic density and the chemical shifts.     

A pyrolysis study on C4–C8 symmetric ethers

Pyrolysis of diethyl (C₄), di-n-propyl (C₆), di-isopropyl (C₆) and di-n-butyl (C₈) ethers were studied in a jet-stirred reactor between 720 and 1140 K, at 10 atm with an initial ether mole fraction of 0.1%. Major common pyrolysis products were observed to be CO, CH₄, H₂, and C₂H₄. All ethers produced the n/2 alcohol and olefin as products of molecular reaction to a small extent. Under pyrolysis conditions at 10 atm, hydrogen abstraction reactions by H atoms and CH₃ radicals were found to be important. Acetylene and benzene were formed for all ethers when T > 1000 K. A kinetic mechanism is used to represent these results. This study shows that there is need of systematic studies in determining site specific rate constants of important fuel related reactions of ethers.