Close-coupling time-dependent quantum dynamics study of the H+HCl reaction

The paper presents a theoretical study of the dynamics of the H + HCl system on the potential energy surface (PES) of Bian and Werner (Bian, W.; Werner, H. -J., J. Chem. Phys. 2000, 112, 220). A time-dependent wave packet approach was employed to calculate state-to-state reaction probabilities for the exchanged and abstraction channels. The most recent PES for the system has been used in the calculations. Reaction probabilities have also been calculated for several values of the total angular momentum J > 0. Those have then been used to estimate cross sections and rate constants for both channels. The calculated cross sections can be compared with the results of previous quasiclassical trajectory calculations and reaction dynamics experimental on the abstraction channel. In addition, the calculated rate constants are in the reasonably good agreement with experimental measurement.

Theoretical study on the mechanism of the reaction of CH4+MgO

The reactions of (1) CH4 + MgO --> MgOH. + CH3. and (2) CH4 + MgO --> Mg + CH3OH have been studied on the singlet spin state potential energy surface at the MP2/6-311+G(2d,2p) level. These two reaction channels, both involving intermediates and transition states, have been rationalized by the structures of the species involved, natural bond orbital (NBO), and vibrational frequency analysis. We have considered two initial interacting models between CH4 and MgO: a collinear C-H approach to the O end of the MgO forming the MgOCH4 complex with C-3nu symmetry and three hydrogen atoms of the methane point to the Mg end of the MgO forming the OMgCH4 complex with C-1 symmetry. The calculations predict that reactions 1 and 2 are exothermic by 39.8 and 86.5 kJ mol(-1), respectively. Also, the former reaction proceeds more easily than the latter, and the complex HOMgCH3 is energetically preferred in the reaction of MgO + CH4.

An alternative approach to ascertain the rate-determining steps of TiO2 photoelectrocatalytic reaction by electrochemical impedance spectroscopy

In an attempt to ascertain the rate-determining steps (RDS) of TiO2 photoelectrocatalytic (PEC) reaction, the PEC oxidation of sulfosalicylic acid (SSA) solution in a TiO2-coated electrode reactor system was monitored by applying the electrochemical impedance spectroscopy (EIS) method. In the meantime, an EIS mathematical model was first established to theoretically simulate the PEC reaction. Based on the EIS model, the theoretical simulation indicates three typical reactions in a PEC oxidation process, which include the charge-transfer-dominated reaction, both the charge-transfer- and adsorption-dominated reaction, and the adsorption-dominated reaction. The experimental results of EIS measurement showed that there was only one arc/semicircle on the EIS plane display when the external bias applied was below 200 mV (vs SCE) in the SSA PEC degradation whereas there were two arcs/semicircles when the externally applied bias exceeded 200 mV (vs SCE). The experimental results have a good agreement with the model simulation. The EIS method in this study provides an easier way to determine the RDS in a PEC oxidation process, which would be helpful to better control the reaction in practice.

Accurate quantum mechanical calculation for the N+OH reaction

Accurate three-dimensional time-dependent quantum wave packet calculations for the N+OH reaction on the (3)A' potential energy surface [Guadagnini, Schatz, and Walch, J. Chem. Phys. 102, 774 (1995)] have been carried out. The calculations show for the first time that the initial state-selected reaction probabilities are dominated by resonance structures, and the lifetime of the resonance is generally in the subpicosecond time scale. The calculated reaction cross sections indicate that they are a decreasing function of the translational energy, which is in agreement qualitatively with the quasiclassical trajectory calculations. The rate constants obtained from the quantum mechanical calculations are consistent with the quasiclassical trajectory results and the experimental measurements. (C) 2003 American Institute of Physics.

Theoretical studies of product polarization and state distributions of the H+HCl reaction

The angular momentum polarization and rotational state distributions of the H-2 and HCl products from the H + HCl reaction are calculated at a relative translational energy of 1.6 eV by using quasiclassical trajectories on two potential energy surfaces, one from G3 surface [T.C. Allison et al., J. Phys. Chem. 100 (1996) 13575], and the other from BW2 surface [W. Bian, H.-J. Werner, J. Chem. Phys. 112 (2000) 220]. Product rotational distributions obtained on the G3 potential energy surface (PES) are much closer to the experimental results (P.M. Aker et al., J. Chem. Phys. 90 (1989) 4795; J. Chem. Phys. 90 (1989) 4809) than the distributions calculated on the BW2 PES. The distributions of P(phi(r)) for the H-2 and HCl products obtained on the G3 PES are similar, whereas the rotational alignment effect of the H-2 product is stronger than that of the HCl product. In contrast to the polarization distributions obtained on the G3 PES, the rotational alignment effect of the two products calculated on the BW2 PES is similar. However, the abstraction reaction is dominated by out-of-plane mechanisms, while the exchange reaction is dominated by in-plane mechanisms. The significant difference of the product rotational polarization obtained on the G3 and BW2 PESs implies that the studies of the dynamical stereochemistry can provide a sensitive test for the accuracy of the PES. (C) 2002 Elsevier Science B.V. All rights reserved.

Theoretical studies on the reaction mechanism of palladium(0)-catalyzed addition of thiocyanates to alkynes

The reaction mechanism of the Pd(0)-catalyzed alkyne cyanothiolation reaction is investigated by MP2, CCSD(T) and the density functional method B3LYP. The overall reaction mechanism is examined. The B3LYP results are consistent with the results of CCSD(T) and MP2 methods for the isomerization, acetylene insertion and reductive elimination steps, but not for the oxidative addition step. For the oxidative addition, the bisphosphine and monophosphine pathways are competitive in B3LYP, while the bisphosphine one is preferred for CCSD(T) and MP2 methods.

Theoretical Studies on the Reaction Mechanism of Platinum-Catalyzed Diboration of Allenes

The reaction mechanism of Pt(0)-catalyzed diboration reaction of allenes is investigated by the density functional method B3LYP. The overall reaction mechanism is examined. The electronic mechanisms of the allene insertion into the Pt-B bond are discussed in terms of the electron donation, back-donation, and d-pi interaction. During allene insertion into the Pt-B bond, the internal carbon atom of allene is preferred over the terminal one due to the stronger electron back-donation and stronger charge transfer in the former case than that in the latter one.

Theoretical studies on the reaction mechanism of oxidation of primary alcohols by Zn/Cu(II)-phenoxyl radical catalyst

The catalytic mechanism for the oxidation of primary alcohols catalyzed by the two functional models of galactose oxidase (GOase), M-II L (M = Cu, Zn; L = N,N'-bis(3,5-di-tert-butyl-2-hydroxyphenyl)1-2-diiminoquinone)), has been studied by use of the density functional method B3LYP The catalytic cycle of Cu- and Zn-catalysts consists of two parts, namely, substrate oxidation (primary alcohol oxidation) and O-2 reduction (catalyst regeneration). The catalytic mechanisms have been studied for the two reaction pathways (route 1 and route 2). The calculations indicate that the hydrogen atom transfer within the substrate oxidation part is the rate-determining step for both catalysts, in agreement with the experimental observation.

Phenol hydroxylation by iron(II) phenanthroline: The reaction mechanism

Phenol hydroxylation catalyzed by iron(II)-1,10-phenanthroline is investigated through kinetics, ESR, W-Vis as well as cyclic voltammogram studies. The optimum reaction conditions are obtained for diphenols production. Radical substitution mechanism is first proposed to explain the effects of pH, reaction medium and other factors on the phenol hydroxylation with H2O2 as oxidant, and found that the coexisting of iron(II)-1,10-phenanthroline and iron(III)-1,10-phenanthroline is the key for phenol hydroxylation to occur with H2O2 as oxygen donor.

THEORETICAL-STUDIES ON THE CYCLIZATION PROCESS OF A NEW ORGANOSULFUR REACTION

The cyclization process of a new organosulfur reaction was studied by the MNDO (UHF) method. The first reaction path was assumed to be via the organosulfur radical intermediate, the second via the ionic (cationic and anionic) intermediates. The dehydroxylation process was assumed to occur with the synergistic cyclization. The results obtained indicate that the potential energy barrier of the first reaction path was about 102 kcal mol(-1), and although the formation of the ionic intermediate is comparatively difficult, the potential energy barrier of the second path is comparable to the first. The sequential reaction path via the radical intermediate, i.e. first cyclization, then dehydroxylation, was investigated for comparison. The cyclization reaction was found to be the thermodynamically favored process, while the ensuing dehydroxylation process was found to have a potential energy barrier of about 62 kcal mol(-1).

Methane aromatization in the absence of an added oxidant and the bench scale reaction test

A bench scale reaction test for methane aromatization in the absence of an added oxidant was performed and its reaction result evaluated based on the carbon balance of the system. The result was compared with those obtained from the micro-reaction test to ensure the accuracy of the internal standard analyzing method employed in this paper. The catalytic performances of modified Mo/HZSM-5 catalysts were examined. It was found that pre-treatment by steam on HZSM-5 weakened the serious deposition of coke, and pre-impregnation of n-ethyl silicate on HZSM-5 could improve the conversion of CH4, but had little effect on coke formation. A low temperature activation procedure including pre-reduction of the catalyst with methane prevents the zeolite lattice from being seriously destroyed by high valence state Mo species when the Mo loading is high. It was suggested that Mo2C species detected by XRD spectra was the active phase for CH4 aromatization.

Effect of ambient pressure and radiation reabsorption of atmosphere on the flame spreading over thermally thin combustibles in microgravity

For the flame spread over thermally thin combustibles in an atmosphere, if the atmosphere cannot emit and absorb the thermal radiation (e.g. for atmosphere Of O-2-N-2), the conductive heat transfer from the flame to the fuel surface dominates the flame spread at lower ambient atmosphere. As the ambient pressure increases, the flame spread rate increases, and the radiant heat transfer from the flame to the fuel surface gradually becomes the dominant driving force for the flame spread. In contrast, if the atmosphere is able to emit and absorb the thermal radiation (e.g. for atmosphere Of O-2-CO2), at lower pressure, the heat transfer from flame to the fuel surface is enhanced by the radiation reabsorption of the atmosphere at the leading edge of the flame, and both conduction and thermal radiation play important roles in the mechanism of flame spread. With the increase in ambient pressure, the oxygen diffuses more quickly from ambient atmosphere into the flame, the chemical reaction in the flame is enhanced, and the flame spread rate increases. When the ambient pressure is greater than a critical value, the thermal radiation from the flame to the solid surface is hampered by the radiation reabsorption of ambient atmosphere with the further increase in ambient pressure. As a result, with the increase in ambient pressure, the flame spread rate decreases and the heat conduction gradually dominates the flame spread over the fuel surface.

On nonequilibrium of pyrolysis process in the manufacture of ethene

Nonequilibrium process for cracking ethane and n-buthane in the manufacture of ethene has been analytically and numerically investigated in a Heavi-side function temperature field and through a normal shock wave. The results demonstrate that, while the reaction temperature increases, the maximum value of ethene yield is increased, and the optimal reaction duration is sharply shortened. For the identical initial reaction temperature, the maximum value of ethene yield through a stationary normal shock wave is less than that in a Heavi-side function temperature field. However, the ethene consumption after the maximum value in the former case is less than that in the latter. Higher ethene yield will be obtained by using the gasdynamic heating method than by using the current methods.

Studies of the Effect of a Coal Concentrator on NO Formation in Swirling Coal Combustion

To develop low-pollution burners, the effect of a coal concentrator on NO formation in swirling coal combustion is studied using both numerical simulation and experiments. The isothermal gas-particle two-phase velocities and particle concentration in a cold model of swirl burners with and without coal concentrators were measured using the phase Doppler particle anemometer (PDPA). A full two-fluid model of reacting gas-particle flows and coal combustion with an algebraic unified second-order moment (AUSM) turbulence-chemistry model for the turbulent reaction rate of NO formation are used to simulate swirling coal combustion and NO formation with different coal concentrators. The results give the turbulent kinetic energy, particle concentration, temperature and NO concentration in cases of with and without coal concentrators. The predicted results for cold two-phase flows are in good agreement with the PDPA measurement results, showing that the coal concentrator increases the turbulence and particle concentration in the recirculation zone. The combustion modeling results indicate that although the coal concentrator increases the turbulence and combustion temperature, but still can remarkably reduce the NO formation due to creating high coal concentration in the recirculation zone.

Reconsideration On The Role Of The Specific Heat Ratio In Arrhenius Law Applications

Arrhenius law implicates that only those molecules which possess the internal energy greater than the activation energy E-a can react. However, the internal energy will not be proportional to the gas temperature if the specific heat ratio gamma and the gas constant R vary during chemical reaction processes. The varying gamma may affect significantly the chemical reaction rate calculated with the Arrhenius law under the constant gamma assumption, which has been widely accepted in detonation and combustion simulations for many years. In this paper, the roles of variable gamma and R in Arrhenius law applications are reconsidered, and their effects on the chemical reaction rate are demonstrated by simulating one-dimensional C-J and two-dimensional cellular detonations. A new overall one-step detonation model with variable gamma and R is proposed to improve the Arrhenius law. Numerical experiments demonstrate that this improved Arrhenius law works well in predicting detonation phenomena with the numerical results being in good agreement with experimental data.