Quantum dynamics calculation of reaction probability for H+Cl_2→HC1+Cl

We present in this paper a time-dependent quantum wave packet calculation of the initial state selected reaction probability for H + CI2 based on the GHNS potential energy surface with total angular momentum J= 0. The effects of the translational, vibrational and rotational excitation of CI2 on the reaction probability have been investigated. In a broad region of the translational energy, the rotational excitation enhances the reaction probability while the vibrational excitation depresses the reaction probability. The theoretical results agree well with the fact that it is an early down-hill reaction.     

A Study of the Reaction Dynamics for the H+BrCH_3 System Using Quasi-classical Trajectory Theory

In the present paper, scattering probabilities and rate constants of different channels for the H + BrCH_3 reaction system have been calculated by means of quasiclassical trajectory (QCT) method. Several important kinetic effects such as vibrational enhancement, channel competition, vibrational adiabaticity, mass combination, coupling of angular momenta and the relation between the kinetic effects and the feature of the potential energy surface have been discussed. Based on these analyses, a direct-type rebonded mechanism for this reaction has been inferred and used to explain the nonsymmetric angular distribution of the products crossed-molecular beam experiment. The agreement of calculation with experimental results is satisfactory.     

Exact quantum and TST-CEQ study of the collinear reaction O+HCl

Exact quantum calculations of reaction probabilities have been carried out using hyperspherical coordinates for the collinearr reaction O+HCl(v <1) -OH(v'<1)+Cl . A generalized LEPS potential energy surface with a barrier height of 8.12 kcal/mol has been used in the calculations. According to the calculated results we found that (1) the reaction probability oscillates with energy, (2) the reaction probability shows vibrational adiabaticity, although it is poorer than that for symmetric reaction Cl + HC1. The analysis of resonance has also been done. The reaction rate constants and average cross sections have been calculated by TST-CEQ method. The rate constants are in agreement with that by QCT and smaller than the experimental one. Finally, the threshold has been estimated and is in good agreement with that of the literature.     

Quasi-classical Trajectory Study of F-I-H20→HF-I-OH Reaction： Influence of Barrier Height,Reactant Rotational Excitation,and Isotopic Substitution

The reaction dynamics of the F＋H20/D20→HF/DF＋OH/OD are investigated on an accurate potential energy surface （PES） using a quasi-classical trajectory method. For both isotopomers, the hydrogen/deuterium abstraction reaction is dominated by a direct rebound mechanism over a very low ＂reactant-like＂ barrier, which leads to a vibrationally hot HF/DF product with an internally cold OH/OD companion. It is shown that the lowered reaction barrier on this PES, as suggested by high-level ab initio calculations, leads to a much better agreement with the experimental reaction cross section, but has little impact on the product state distributions and mode selectivity. Our results further indicate that rotational excitation of the H20 reactant leads to significant enhancement of the reactivity, suggesting a strong coupling with the reaction coordinate.     

Quasi-Classical Trajectory Study of the Chemical Reaction Ca+CH3I

The Ca＋CH3I→CaI＋CH3 reaction system has been studied with the quasi-classical trajectory method on the extended Lond-Eyring-Polanyi-Sato（LEPS） potential energy surface. At collision energy Ecol=10.78 kJ/mol, the calculated results show that the CaI vibrational population peaks are located at v=2. The calculated cross section decreases slowly with the collision energy increasing. The angle product distributions tend toward backward scattering. The calculated （P2（J^1·K）） values deviate slightly from-0.5 and decrease with increasing collision energy. The Quasiclassical trajectory calculation（QCT） results are in reasonable agreement with experimental data. Moreover, the dynamics of the reaction has been discussed.     

Theoretical study of the mechanism for C-H bond activation in spin-forbidden reaction between Ti~+ and C_2H_4

The mechanism of the spin-forbidden reaction Ti+(4F, 3d24s1) + C2H4 → TiC2H2+ (2A2) + H2 on both doublet and quartet potential energy surfaces has been investigated at the B3LYP level of theory. Crossing points between the potential energy surfaces and the possible spin inversion process are discussed by means of spin-orbit coupling (SOC) calculations. The strength of the SOC between the low-lying quartet state and the doublet state is 59.3 cm-1 in the intermediate complex IM1-4B2. Thus, the changes of its ...     

THEORETICAL STUDY ON THE 1,4-CYCLO-ADDITION REACTION BETWEEN SINGLET OXYGEN (~(1)O_(2)) AND IMIDAZOLE

Quantum chemical ab initio method has been used to study the cycloaddition reaction between singlet oxygen and imidazole theoretically. The intermediate product, an endoper-oxide, of which the structure has not yet been found experimentally was optimized by using energy gradient method. The transition state (TS) structure of this reaction was also located on the potential energy surface and characterized. From the structural features of the TS, e.g. the transition vector of the imaginary vibration mode, as well as the interaction of orbitals, it is realized that this cycloaddition is a synchronous concerted reaction. The activation barriers calculated are 69.5 kJ·mol (forward reaction) and 140.4 kJ·mol-1 (backward reaction) respectively (6-31G results).     

Theoretical Studies on the Reaction Mechanism of AsCl3 with H2 in the Vapor Phase Epitaxy of GaAs

The reaction mechanism of AsCl3 with H2 has been studied by using the method of BHandHLYP in Density Functional Theory (DFT) at the 6-311G** basis set. The transition state of each reaction is verified via the analysis of vibration mode and Intrinsic Reaction Coordinate (IRC). Meanwhile,single-point energy has been calculated at the QCISD(T)/6-311G** level,and the zero-point energy correction has been made to the total energy and reaction energy barrier. It shows that AsCl3 reacts with H2 to first result in AsHCl2 which may incline to self-decompose and finally afford the product As2,or continue to react with H2 to provide the product AsH3. The computing result demonstrates that the former is the main reaction channel.     

THEORETICAL APPROACHES TO PROTEIN STRUCTURE-FUNCTION ANALYSIS (Ⅰ)——DIRECTED PERTURBATION CONFORMATIONAL ANALYSIS

A new computational technique called directed perturbation conformational analysis has been developed for use in protein model building and structure-function studies. Designed to perform an efficient local search of a macromolecular potential energy surface, the algorithm can be used to locate multiple energy minimum conformers via low energy transition state structures from a single starting or trial structure. The algorithm contains developments to stabilize transition state optimizations for systems described by many degrees of freedom displaying anharmonic potential energy surfaces. It has been found to be efficient in the generation of alternative equilibrium structures from a given trial structure when compared with those generated from a standard molecular dynamics simulation of N-acetyl, N'-methyl-deca-L-alaninamide.     

Scattering Resonance States and Partial Potential Energy Surface of Reaction I ＋ HI （ v = 0 ） → IH （ v＇ = 0 ） ＋ I

The partial potential energy surface of the I ＋ HI →IH ＋ I reaction involving the translational and vibrational motions has been constructed at the QCISD（ T ）//MP4SDQ level with the pseudo potential method that is helpful to interpreting the scattering resonance states. The lifetimes of the scattering resonance states in the title reaction obtained from the partial potential energy surface are about 90-120 fs, which agrees with the result of high-resolved threshold photodetachment spectroscopy of anion IHI^- measured by Neumark.     

Theoretical study of the reaction Ne + H+2 → NEH+ + H in the 2A′ ground state

A three-dimensional potential energy surface for the ²A′ ground state of the system (Ne? H₂)⁺ (²Σ⁺ in collinear geometry) has been calculated at SCF and CEPA levels. This surface describes the abstraction reaction which is endoergic by 0.57 eV (ΔH⁰₀) and has been studied recently by different experimental groups at low collision energies. Our CEPA calculations yield an endoergicity of 0.55 eV (ΔH⁰₀). The ²A′ surface has a minimum at collinear geometry with R_Ne—H = 2.29 a₀ and R_{H? H} = 2.08 a₀ and a well depth of 0.49 eV relative to Ne + H⁺₂. The effects of electron correlation on the shape of the surface and on the well depth are discussed. An analytic fit of the collinear part of the surface has been constructed based on Simon's proposal of using polynomials in the coordinates (R? R_e)/R instead of (R? R_e). The fitted potential is used for quantum mechanical scattering calculations with the finite element method (FEM ). Preliminary results for reaction probabilities for H⁺₂ in different vibrationally excited states are given and compared to the experimental results.     

Molecular beam scattering studies of the reaction D + H2 (v = 0) and D + H2 (v = 1) → HD + H

The reaction D + H₂ → HD + H has been investigated in two molecular beam scattering experiments. Angular and time-of-flight distributions have been measured for the initial vibrational ground state (v = 0) at a most probable collision energy of E_cm = 1.5 eV and for the first vibrational excited state (v = 1) at E_cm = 0.28 eV with the same apparatus. Results for the ground-state experiment are compared with quasiclassical trajectory calculations(QCT) on the LSTH-hypersurface transformed into the laboratory system and averaged over the apparatus distributions. The agreement isquite satisfactory. At this high collision energy the HD products are no longer scattered in a backward direction but in a wide angular region concentrated about θ = 90° in the center-of-mass system. The absolute reactive cross section has been determined and the agreement with the theoretical value from QCT calculations is within the experimental error. The high sensitivity of the experiment to different properties of the doubly differential cross section has also been demonstrated. A preliminary evaluation of the experiment with initial vibrational excitation (v = 1) shows that the HD-product molecules are preferably backward scattered and the change of internal energy is small supporting the concept of a reaction which is adiabatic with respect to the internal degrees of freedom.     

Energy resonance and inverse population of the product vibrational states for the reaction F + H2(v = 0) → HF(v′) + H,LCAC‐SW theoretical quantum scattering study

LCAC‐SW (linear combination of arrangement channel‐scattering wavefunction) method was used to calculate collinear state‐to‐state reaction probabilities for the reaction F + H₂(v = 0) → HF(v′) + H on the 6SEC potential energy surface. The results show that reaction probabilities P₀₂ and P₀₃ [i. e., v′ = 2,3 for reaction F + H₂ (v = 0) + HF(v′) + H] are primary, the population of product vibrational states is inverse and the reaction probabilities are oscillatory with collision energies, i.e., there is energy resonance in this reaction, which agrees with a new experiment.     

Thermal and state-selected rate constant calculations for O(3p) + H2 → OH + H and isotopic analogs

We present a new parametrization (based on ab initio calculations) of the bending potentials for the two lowest potential energy surfaces of the reaction O(³P) + H₂, and we use it for rate constant calculations by variational transition-state theory with multidimensional semiclassical tunneling corrections. We present results for the temperature range 250–2400 K for both the rate constants and the intermolecular kinetic isotope effects for the reactions of O(³P) with D₂ and HD. In general, the calculated rate constants for the thermal reactions are in excellent agreement with available experiments. We also calculate the enhancement effect for exciting H₂ to the first excited vibrational state. The calculations also provide information on which aspects of the potential energy surfaces are important for determining the predicted rate constants.     

Theoretical study on the partial potential energy surface and formation mechanism of the reactive resonance state of HO + CH4 → H2O + CH3 system

In the course of an extensive investigation aimed at understanding the detailed mechanism of a prototypical polyatomic reaction, several remarkable observations were uncovered. To interpret these findings, we surmise the existence of a reactive resonance in this polyatomic reaction. The concerned system is HO + CH₄ → H₂O + CH₃, of which the partial potential energy surface is constructed by the coupling between vibrational models and reactive coordinates. Then we explain the formation mechanism of the reactive resonance state by the partial potential energy surface. Finally, we estimated the lifetime of the resonance state, and it is about 45fs. The study of the reactive resonance in a polyatomic reaction is more than just an extension from a typical atom + diatom reaction. As shown here, it holds great promise to disentangle the elusive intramolecular vibrational dynamics of the transient collision complex in the critical transition‐state region. © 2007 Wiley Periodicals, Inc. Int J Quantum Chem, 2008     

Dissection of the Multichannel Reaction O(3P) + C2H2: Differential Cross-Sections and Product Energy Distributions

The O(³P) + C₂H₂ reaction plays an important role in hydrocarbon combustion. It has two primary competing channels: H + HCCO (ketenyl) and CO + CH₂ (triplet methylene). To further understand the microscopic dynamic mechanism of this reaction, we report here a detailed quasi-classical trajectory study of the O(³P) + C₂H₂ reaction on the recently developed full-dimensional potential energy surface (PES). The entrance barrier TS1 is the rate-limiting barrier in the reaction. The translation of reactants can greatly promote reactivity, due to strong coupling with the reaction coordinate at TS1. The O(³P) + C₂H₂ reaction progress through a complex-forming mechanism, in which the intermediate HCCHO lives at least through the duration of a rotational period. The energy redistribution takes place during the creation of the long-lived high vibrationally (and rotationally) excited HCCHO in the reaction. The product energy partitioning of the two channels and CO vibrational distributions agree with experimental data, and the vibrational state distributions of all modes of products present a Boltzmann-like distribution.     

Ab initio study on the reaction of Y+ + NH3 → Y+ NH + H2

The reaction of Y⁺ + NH₃ → Y⁺ NH + H₂ was theoretically investigated by ab initio MO methods. Two possible pathways (1–1 H₂ loss and 1–2 H₂ loss) on the singlet potential energy surface and reaction mechanism were examined and discussed. The singlet and triplet PESs of this reaction system were compared to confirm the correctness of spin conservation concepts. © 1996 John Wiley & Sons, Inc.     

Time‐dependent quantum study of H(2S) + FO(2Π) → OH(2Π) + F(2P) reaction on the 13A′ and 13A″ states

The dynamics of the H(²S) + FO(²Π) → OH(²Π) + F(²P) reaction on the adiabatic potential energy surface of the 1³A′ and 1³A″ states is investigated. The initial state selected reaction probabilities for total angular momentum J = 0 have been calculated by using the quantum mechanical real wave packet method. The integral cross sections and initial state selected reaction rate constants have been obtained from the corresponding J = 0 reaction probabilities by means of the simple J‐Shifting technique. The initial state‐selected reaction probabilities and reaction cross section do not manifest any sharp oscillations and the initial state selected reaction rate constants are sensitive to the temperature. © 2010 Wiley Periodicals, Inc. J Comput Chem 2010     

Reaction rates for O + HD → OH + D and O + HD → OD + H

We have calculated reaction rates for the reactions O + HD → OH + D and O + DH → OD + H using improved canonical variational transition state theory and least-action ground-state transmission coefficients with an ab initio potential energy surface. The kinetic isotope effects are in good agreement with experiment. The optimized tunneling paths and properties of the variational transition states and the rate enhancement for vibrationally excited reactants are also presented and compared with those for the isotopically unsubstituted reaction O + H₂ → OH + H. The thermal reactions at low and room temperature are predicted to occur by tunneling at extended configurations, i.e., to initiate early on the reaction path and to avoid the saddle point regions. Tunneling also dominates the low and room temperature reactions for excited vibrational states, but in these cases the results are not as sensitive to the nature of the tunneling path. Overbarrier mechanisms dominate for both thermal and excited-vibrational state reactions for T > 600 K. For the excited-state reaction (with initial vibrational quantum number n > 0) a transition state switch occurs for T > 1000 K for the O + HD(n = 1) → OD + H case and for T > 1500 K for the O + DH(n = 1) → OD + H reaction, and this may be a general phenomenon for excited-state reactions at higher temperature. In the present case the switch occurs from an early variational transition state where the vibrationally adiabatic approximation is expected to be valid to a tighter variational transition state where nonadiabatic effects are probably important and should be included.     

An ab initio study of the potential surface for the reaction H2COH → HCO + H2

The potential surface for the reaction H₂CO⁺H → HCO⁺ + H₂ has been studied by ab initio SCF calculations, using gaussian-type basis functions. A saddle point on the surface has been found, and a reaction path is proposed to explain the observed release of kinetic energy. The energy of activation and ΔE for the reaction have been estimated.