Theoretical performance of an HCl chemical cw laser

A quantitative analysis of an H₂ + Cl₂ cw chemical laser is presented that includes the influence of stimulated emission on the reacting medium. Numerically-determined solutions encompass one-dimensional fluid mechanics, chemical kinetics, radiative de-excitation, and their mutual interaction. The excited HCl(v) is produced vy a chain reaction, with only the H + Cl₂ reaction providing v ?1. Because of the large amount of HCl(0) produced by the Cl + H₂ reaction, lasing occurs primarily on the 3→2 and 2→1 vibrational bands. A chemical efficiency of 7 per cent is predicted with a cavity pressure of 0.1 atm, no diluent, and a substantial excess of H₂. A comprehensive parametric study examines the effects due to changed initial cavity conditions, mixture ratios, and output coupling.     

Quantum stereodynamics study for the reaction F + HD

This paper studies the quantum stereodynamics of the F + HD(υ = 0, j = 0) → HD + F/HF + D reaction at the collision energies of 0.52 and 0.87~kcal/mol. The quantum scattering calculations, based on Stark-Werner potential energy surfaces, show that the differential cross sections for the HF(υ' = 2) + D and DF(υ' = 3) + H channels are consistent with the recent theoretical results. Furthermore, the product rotational angular momentum orientation and alignment have been determined for some selected rovibrational states of the HF + D and DF + H channels.     

Analyses of the semi-classical wavepacket approach to chemical reactions: the F + H2 → HF + H reaction

Mixed quantum-classical calculations using propagation of 3D wavepackets in hyperspherical coordinates have been performed for the F + H₂ → HF + H reaction using two different potential energy surfaces. Vibrationally resolved cross-sections show good agreement with those obtained from accurate quantum mechanical calculations.     

H2F2 chain reaction rate investigation

Experimental performance of chemical lasers pumped by the H₂ + F₂ chain reaction has consistently fallen below expectations, although the “hot”, H+F₂→HF(ν)+F, pumping reaction produces greater vibrational excitation than the “cold”, F+H₂→HF(ν)+H, reaction used in most HF cw chemical lasers. The reasons for this discrepancy are examined by measuring spatially-resolved HF(ν) number density and the temperature profiles in a laminar, parallel flow, hydrogen-fluorine mixing layer and comparing the results with theoretical computations. By dissociating either the hydrogen or fluorine molecules with arc heaters, kinetics of the hot and cold reaction systems are separately investigated. From a comparison of the experimental vibrational populations and the theoretical predictions, it is concluded that: (1) previously-used pumping and deactivation rates associated with the cold reaction are approximately correct, (2) the deactivation of high vibrational levels populated by the hot reaction is much faster than previously stated in Ref. (1), and (3) the inclusion of multiquantum HF(ν) V-T (or V-R) deactivation reactions, which sharply decreases the number density of the upper vibrational levels, greatly improves the agreement between theory and experiment.     

A Hamilton–Jacobi type equation for computing minimum potential energy paths

A new method for computing minimum-energy reaction paths is presented. Unlike existing approaches (e.g. intrinsic reaction coordinate methods), our approach works for any reactant configuration: the structure of the transition state, reactive intermediates and product will be determined by the algorithm, and so need not be known beforehand. The method we have developed is based on solving a Hamilton–Jacobi type equation. Specifically, we introduce a speed function so that the ‘first arrival times’ from the Hamilton–Jacobi equation correspond to least-potentials. Then, adopting a back-tracing method, we can use the first arrival times to determine the minimum-energy path between any classically allowed molecular conformation and the initial (reactant) conformation. The method is illustrated by applying it to six different systems: (1) a model system with four different minima in the potential energy surface, (2) a model Muller–Brown potential, (3) the isomerization reaction of malonaldehyde using a fitting potential energy surface, (4) a model Minyaev–Quapp potential representative of con- and dis-rotations of two BH₂ groups in the BH₂–CH₂–BH₂ molecule, (5) the F?+?H₂→FH?+?H reaction and (6) the H?+?FH?→?HF?+?H reaction. Our results demonstrate that the proposed method represents a robust alternative to existing techniques for finding chemical reaction paths.     

O~+ + H_2及其同位素取代反应的立体动力学研究

运用准经典轨线方法,基于RODRIGO势能面,对碰撞能为20kcal/mol时,O++H2及其同位素取代反应的立体动力学性质进行了理论研究,对k-j′两矢量相关和k-k′-j′三矢量相关的分布函数、极化微分反应截面,以及产物转动取向参数进行了详细的讨论.结果表明,O++H2→OH++H,O++DH→OD++H和O++TH→OT++H反应的立体动力学性质对体系的质量因数非常敏感.     

Chemical pumping of the far infrared HCN laser

This paper reports the observation of 337 μm and 311 μm stimulated emission from HCN in which the (11¹0)?(04⁰0) inversion has been established by photopumping of HCN and by chemical pumping with reactions between CN and H₂ or saturated hydrogen-rich organic compounds. Similarities in output pulse behavior between the discharge and chemical versions of the HCN laser are suggestive that the pumping mechanism in the discharge is the chemical reaction, CN + H₂ → HCN² + H. The well-known inefficiency of this laser is then due to the fact that the reaction is a slow one and its exothermicity does not match the energy of the upper lasing level, but depends for inversion on (randomizing) relaxation into (11¹0) among others. Substantial improvements in the power of the HCN laser cannot be made from this route.     

Influence of water on HFO-1234yf oxidation pyrolysis via ReaxFF molecular dynamics simulation

ABSTRACT

The effect of water molecules on HFO-1234yf oxidation pyrolysis was investigated by ReaxFF-molecular dynamics simulation from 1900 to 4200?K. The initial pyrolysis of HFO-1234yf starts around 2500?K and the water molecules participate in chemical reactions at 2800?K when the reactants pyrolysis reached the highest reaction rate. The primary products including HF, COF₂ and CO₂ are observed at 2600, 2700 and 2900?K, respectively. The influence of water molecules on products is mainly reflected in the promotion activity on the conversion from COF₂ to CO₂ and the generation of HF molecules. Four formation pathways are observed and calculated to further elucidate the procedure of pyrolysis. The main conversion process from H₂O to HF is the ?F?+?H₂O?=?HF+?OH reaction, and the paths from H₂O to ?OH radical and COF₂ to ?CFO radical which are promoted by ?F and ?H radical, respectively, have relatively low energy barriers of 10.44 and 40.29?kJ/mol, and both reaction processes released HF molecules.     

Initial performance of a CW chemical laser

The performance of a continuous HF chemical laser is presented in this paper. Population inversion was obtained by diffusion of H₂ into a supersonic jet containing F atoms [H₂+F HF(v)+H₁ H=–31.7 kcal/mole]. A peak power of 630 W was obtained with an F atom flow rate of 0.040 mole/sec, and the efficiency of conversion of chemical energy to laser energy was 12%. The performance of a corresponding DF laser is also given. Major laser output is from 2-1 and 1-0 transitions for both lasers. Radiation is at 2.6 to 2.9m and 3.6 to 4.0m for the HF and DF lasers, respectively. The ratio of DF to HF laser power is 0.7 under similar flow conditions.Work conducted under US Air Force Contract F04701-69-C-0066.     

Dynamics on reactive potential energy surfaces: hyperspherical view and signatures of ‘quantum chaos’

Adiabatic energy levels for two prototypical reactions, F + H₂ → HF + H and He + H⁺ ₂ → HeH⁺ + H, are analysed by means of statistical tests. These levels result from quantum mechanical calculations of dynamics based on the hyperspherical approach, and are given as a function of the total inertia of the system measured by the hyperradius ρ The nearest neighbour level spacing distributions of Brody and of Berry and Robnik, the spectral rigidity δ₃ of Dyson and Mehta and the correlation coefficient are reported, together with other properties, such as variance, skewness and kurtosis of the distributions. Trends are studied as a function of ρ, proposed as a natural control variable. For low ρ, which correspond to the transition state, evidence is found of Wigner-like behaviour, which is interpreted as the signature of quantum chaos. On the passage of the systems through intermediate ρ a mixture of Wigner- and Poisson-like behaviour emerges. The situation for high ρ where reactants and products of the reactions are well separated, is characterized by a tendency towards regular Poisson-like behaviour. A comparison between the two investigated systems shows that the chaotic regime in the transition state region is more pronounced for the He reaction, which proceeds through a deep well and whose dynamics are characterized by a rich resonance pattern.     

Direct ab initio molecular dynamics study of F atom reaction with methane

The dynamics of the F + CH₄ → HF + CH₃ and F + CD₄ → DF + CD₃ reactions have been investigated using classical trajectory calculations at the MP2/cc-pvdz level of theory. The trajectories were calculated directly from electronic structure computations, and a Hessian based method with updating was used to integrate the trajectories. Using this method, product rovibrational populations and internal energy distributions were obtained for the F + CH₄ and F + CD₄ reactions. The theoretical results were compared with the available experimental data and previous calculations results. The state distributions of the reaction F + CH₄ in these calculations are in reasonable agreement with the experimental results, which indicates that the experimental behavior of the reaction could be well reproduced by the direct classical trajectory calculations at MP2/cc-pvdz level. As such, the product rovibrational populations and internal energy distributions for the reaction F + CD₄ were predicted. The same degree of agreement between theory and experiment as the F + CH₄ reaction is expected.     

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.     

Study of the Characteristics of Elementary Processes in a Chain Hydrogen Burning Reaction in Oxygen

The characteristics of possible chain explosive hydrogen burning reactions in an oxidizing medium are calculated on the potential energy surface. Specifically, reactions H₂ + O₂ → H₂O + O, H₂ + O₂ → HO₂ + H, and H₂ + O₂ → OH + OH are considered. Special attention is devoted to the production of a pair of fast highly reactive OH radicals. Because of the high activation threshold, this reaction is often excluded from the known kinetic scheme of hydrogen burning. However, a spread in estimates of kinetic characteristics and a disagreement between theoretical predictions with experimental results suggest that the kinetic scheme should be refined.     

Dilution effects analysis of opposed-jet H2/CO syngas diffusion flames

This paper reported the analysis of dilution effects on the opposed-jet H₂/CO syngas diffusion flames. A computational model, OPPDIF coupled with narrowband radiation calculation, was used to study one-dimensional counterflow syngas diffusion flames with fuel side dilution from CO₂, H₂O and N₂. To distinguish the contributing effects from inert, thermal/diffusion, chemical, and radiation effects, five artificial and chemically inert species XH₂, XCO, XCO₂, XH₂O and XN₂ with the same physical properties as their counterparts were assumed. By comparing the realistic and hypothetical flames, the individual dilution effects on the syngas flames were revealed. Results show, for equal-molar syngas (H₂/CO = 1) at strain rate of 10 s^?1, the maximum flame temperature decreases the most by CO₂ dilution, followed by H₂O and N₂. The inert effect, which reduces the chemical reaction rates by behaving as the inert part of mixtures, drops flame temperature the most. The thermal/diffusion effect of N₂ and the chemical effect of H₂O actually contribute the increase of flame temperature. However, the chemical effect of CO₂ and the radiation effect always decreases flame temperature. For flame extinction by adding diluents, CO₂ dilution favours flame extinction from all contributing effects, while thermal/diffusion effects of H₂O and N₂ extend the flammability. Therefore, extinction dilution percentage is the least for CO₂. The dilution effects on chemical kinetics are also examined. Due to the inert effect, the reaction rate of R84 (OH+H₂ = H+H₂O) is decreasing greatly with increasing dilution percentage while R99 (CO+OH→CO₂+H) is less affected. When the diluents participate chemically, reaction R99 is promoted and R84 is inhibited with H₂O addition, but the trend reverses with CO₂ dilution. Besides, the main chain-branching reaction of R38 (H+O₂→O+OH) is enhanced by the chemical effect of H₂O dilution, but suppressed by CO₂ dilution. Relatively, the influences of thermal/diffusion and radiation effects on the reaction kinetics are then small.     

Nitric oxide reduction by CO ON Rh(111): Temperature programmed reaction

The reaction of NO with CO on Rh(111) has been studied with temperature programmed reaction (TPR). Comparisons are made with the reaction of O₂ with CO and the reaction of NO with H₂. The rate-determining step for both CO oxidation reactions is CO(a) + O(a) → CO₂(g). Repulsive interactions between adsorbed CO and adsorbed nitrogen atoms lead to desorption of CO in a peak at 415 K which is in the temperature range where the reaction between CO(a) and O(a) produces CO₂(g). Thus the extent of reaction of CO(a) with NO(a) is less than that between CO(a) and O(a) due to the lower coverage of CO caused by adsorbed N atoms and NO. A similar repulsive interaction between NO(a) and H(a) suppresses the NO + H₂ reaction. CO + NO reaction behavior on Rh(111) is compared to that observed on Pt(111).     

Dynamics of reaction of O with H2 and its isotopic variants in different rotational excited states

Scalar properties and vector correlations of the reactions O+H₂ →OH+H, O+HD→OH+D, O+DH→OD+H, and O+D₂ →OD+D at collision energies of 25 and 34.6 kcal/mole have been studied via quasi-classical-trajectory (QCT) method based on a BMS1 potential energy surface (PES). Generalized polarization-dependent differential cross section and the distributions of the dihedral angle at the collision energy of 34.6 kacl/mole are presented. The calculated results indicate that both reagent rotational angular momentum and the mass factor have a significant influence on the scalar properties and vector correlations of the title reactions.     

Manifestation of nonuniformity in the electron charge distribution in H<Stack><Subscript>2</Subscript><Superscript>+</Superscript></Stack> molecular ions colliding with each other

The anomalous character of threshold properties in the ion-molecule reactions H ₂ ⁺ + H ₂ ⁺ → H ₃ ⁺ + p and H ₂ ⁺ + H ₂ ⁺ → H + p + H + p has been theoretically analyzed. It has been shown that these reactions proceed through the formation of the intermediate H ₄ ⁺⁺ complex. Molecules H ₂ ⁺ in the collision process are described by a chemical model, where the positive charge is concentrated in one of the nuclei. The calculations of the reaction cross sections are in good agreement with the experimental data. It has been shown that the chemical model of the H ₂ ⁺ molecule can be consistently explained only in terms of dynamic interactions, i.e., polarization forces and van der Waals forces.     

The Hammond postulate: A quantitative interpretation

The Hammond postulate is considered in terms of the model of intersecting parabolas. It is shown that, in radical detachment reactions of the type X _f ^· + HX _i → X _f H + X _i ^· with a symmetrical reaction center X _i …H…X _f , the H atom in the transition state is equidistant from the X _i and X _f atoms if the enthalpy of the reaction ΔH = 0. The X _i …H distance increases and the X _f …H distance decreases linearly as ΔH grows. The dependence remains linear over the range ΔH _min ≤ ΔH ≤ H _max. The same result was obtained in quantum-chemical calculations for reactions of the type R _f ^· + R _i H. In reactions of the type X^· + HY → XH + Y^· with an asymmetric reaction center X…H…Y, the X…H and Y…H interatomic distances in the transition state at ΔH = 0 depend on the force constants and lengths of the X-H and Y-H bonds. The Y…H distance increases and the X…H distance decreases linearly as ΔH grows. A similar picture is observed in the model of intersecting Morse curves, where the dependence of interatomic distances on ΔH in the transition state is nonlinear. Equations describing interatomic distances in the transition state as functions of the enthalpy of the reaction are presented.     

Calculation of accurate imaginary frequencies and tunnelling coefficients for hydrogen abstraction reactions using IRCmax

The effect of level of theory on the imaginary frequency and corresponding tunnelling coefficients has been studied for a test set of hydrogen abstraction reactions: ?CH₂X + CH₃Y → CH₃X + ?CH₂Y for (X,Y) = (H,H), (H,CN), (H,F), (H,Li) and (F,Li). It is found that the imaginary frequency is very sensitive to the level of theory used, with Hartree-Fock (HF) methods severely overestimating the imaginary frequency compared with high-level CCSD(T)/6-311G(d,p) calculations. The errors for the other methods are smaller but nonetheless significant, with MP2 methods overestimating the imaginary frequency and density functional theory (DFT) methods underestimating it. In the case of the HF methods, this leads to errors in the tunnelling coefficient of several orders of magnitude, while for the better DFT and MP2 methods errors of a factor of 2–3 are observed. To address this problem, an IRCmax procedure for estimating the imaginary frequency has been developed and it is found that IRCmax imaginary frequencies calculated with CCSD(T)/6-311G(d,p) single points along a low-level HF/6-31G(d) minimum energy path provide excellent approximations to the high-level values, at a fraction of the computational cost.     

A one-step procedure to prepare LiAsF6 and other allied lithium-based fluoro compounds used as electrolyte in lithium cells

A low-temperature all-solid-state thermal method has been developed to synthesize electrolytes like LiAsF₆, LiPF₆ and allied lithium-based fluoro-chemicals useful for lithium secondary cells. This developed procedure is simple and environmentally friendly compared with the existing procedures, which are poisonous and hazardous to health due to the use of obnoxious gases or liquids like F₂, AsF₃, PF₃, BF₃, AsF₃, or AsF₅, which are difficult to handle. In this proposed procedure, all chemicals are taken as analytical grade. Lithium salts and all other required salts are mixed well in required proportions and heated in between 300–400 °C to get the products examined by X-ray. The chemical reactions proposed for the preparation are given below.

1. 2LiOH + As₂O₅ + 12NH₄F → 2LiAsF₆ + 12NH₃ + 7H₂O
2. LiOH + (NH₄)₂HPO₄ + 6NH₄F → LiPF₆ + 8NH₃ + 5H₂O
3. 2LiOH + B₂O₃ + 8NH₄F → 2LiBF₄ + 8NH₃ + 5H₂O