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The ring polymer molecular dynamics (RPMD) calculations are performed to calculate rate constants for the title reaction on the recently constructed potential energy surface based on permutation invariant polynomial (PIP) neural-network (NN) fitting [J. Li et al., J. Chem. Phys. 142, 204302 (2015)]. By inspecting convergence, 16 beads are used in computing free-energy barriers at 300 K ≤ T ≤ 1000 K, while different numbers of beads are used for transmission coefficients. The present RPMD rates are in excellent agreement with quantum rates computed on the same potential energy surface, as well as with the experimental measurements, demonstrating further that the RPMDis capable of producing accurate rates for polyatomic chemical reactions even at rather low temperatures. [ABSTRACT FROM AUTHOR]

We report a chemically accurate global potential energy surface for the HOCO system based on high-level ab initio calculations at ∼35 000 points. The potential energy surface is shown to reproduce important stationary points and minimum energy paths. Quasi-classical trajectory calculations indicated a good agreement with experimental data. [ABSTRACT FROM AUTHOR]

A new analytical potential energy surface is presented for the reaction of hydrogen abstraction from methane by a hydrogen atom. It is based on an analytical expression proposed by Jordan and Gilbert [J. Chem. Phys. 102, 5669 (1995)], and its fittable parameters were obtained by a multibeginning optimization procedure to reproduce high-level ab initio electronic structure calculations obtained at the CCSD(T)/cc-pVTZ level. The ab initio information employed in the fit includes properties (equilibrium geometries, relative energies, and vibrational frequencies) of the reactants, products, saddle point, points on the reaction path, and points on the reaction swath. No experimental information is used. By comparison with the reference results we show that the resulting surface reproduces well not only the ab initio data used in the fitting but also other thermochemical and kinetic results computed at the same ab initio level, such as equilibrium constants, rate constants, and kinetic isotope effects, which were not used in the fit. In this way we show that the new potential energy surface is correctly fitted and almost as accurate as the CCSD(T)/cc-pVTZ method in describing the kinetics of the reaction. We analyze the limitations of the functional form and the fitting method employed, and suggest some solutions to their drawbacks. In a forthcoming communication, we test the quality of the new surface by comparing its results with experimental values. [ABSTRACT FROM AUTHOR]

Extensive ab initio calculations have been performed on the reaction N+3(P)+H21(Σ+g)NH+ (X 2Π)+H(2S), using complete-active-space self-consistent-field and multireference contracted configuration interaction methods. The 752 calculated energy points belonging to the 1 3A‘ surface were fitted to an analytical potential energy surface, with an average least-squares deviation of only 0.026 eV (2.5 kJ/mol). A key to this accurate fit of the surface, which does include minima as deep as 6 eV, is the use of R-n polynomials in the asymptotic region, and Rn polynomials in the deep minima regions. The calculated reaction endothermicity is 0.03 eV, after correcting for the differences in zero point vibrational energies. This is in agreement with the essentially thermoneutral reaction enthalpy, obtained in recent experiments. An important feature of the potential energy surface is the anisotropy in the reactant channel, arising from the ion–quadrupole interaction. [ABSTRACT FROM AUTHOR]

We report ‘‘exact’’ quantum Monte Carlo calculations of the potential energy surface for the reaction H+H2→H2+H. The method used is free of systematic error. The statistical or sampling error was reduced to ±0.10 kcal/mol for several hundred points distributed across the surface, to ±0.02 kcal/mol for the minimum energy approach of H to H2, to ±0.02 kcal/mol near the saddle point, and to ±0.01 kcal/mol at the saddle point. The upper and lower surfaces in the region of the Jahn–Teller cusp were determined with a statistical error of ±0.2 kcal/mol. [ABSTRACT FROM AUTHOR]

Three different functional forms are fit to a calculated coupled electron pair approach potential energy surface for the reaction Ne+H2+→NeH++H. Minimum energy pathways and stationary points of the various fits are discussed. [ABSTRACT FROM AUTHOR]

The potential energy surface of the HO2+NO reaction has been investigated at second order Moller–Plesset perturbation (MP2) and density functional (DFT) methods with the 6-311++G** basis set and at complete active space [CAS(8,8)] self-consistent field level using the 6-31G** basis set. The reaction is shown to give three different groups of products, viz., HO–NO2, NO2+OH, and HNO+O2. The thermodynamically stable HO–NO2 can be formed from the energized ONOOH adduct by the 1,2 migration of the OH group via a loose transition state (referred to as TS2) with a relatively higher barrier height compared to O–O bond fission. The other exothermic product, NO2+OH, arises from a direct O–O dissociation of ONOOH and is expected to be the most favorable process on account of its low barrier height. HNO+O2 can be formed by two different channels: (i) the direct hydrogen abstraction and/or (ii) the barrierless association of the reactants to form the peroxynitrous acid, ONOOH, which then undergoes 1,3 hydrogen migration, giving rise to the HN(O)OO biradical followed by N–O dissociation. Of the two channels, channel (i) has been found to be dominant. Owing to their higher barrier heights, HNO formation is expected only at high temperatures. NOH+O2 and HONO+O are not expected to compete in the kinetics of the HO2+NO system. The energetic of the key reactions, namely HO2+NO→HO–NO2 and HO2+NO→NO2+OH, has also been obtained at the QCISD/6-311++G(2df,2pd)//MP2/6-311++G** level. © 1997 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We report large-basis-set CASSCF/MR-CISD/SEC (complete active space self-consistent-field orbitals used for multireference configuration interaction with all single and double excitations and scaled external correlation) and MP4 (Mo\ller–Plesset fourth order perturbation theory) calculations of the FH2 potential energy surface for collinear and bent geometries in the vicinity of the F---H--H saddle point. These calculations indicate that higher order correlation effects become much more important as the generalized transition states are bent, and that the unrestricted saddle point for this reaction is noncollinear. This means that the sterically allowed cone of reactive configurations is much broader than either previously available ab initio calculations or the present lower-order ones would predict. [ABSTRACT FROM AUTHOR]

Accurate 3D quantum reaction probabilities for the F+H2→HF+H reaction are presented for the new 5SEC potential energy surface (PES) of Truhlar and co-workers. This PES was found to be more reactive, on the average, than the closely related 5A PES of Truhlar and co-workers. The resonance structure in the probabilities also differs between the two PESs. [ABSTRACT FROM AUTHOR]

A detailed quasi-classical trajectory study of the H + CO2 → HO + CO reaction is reported on an accurate potential energy surface based on ab initio data. The influence of the vibrational and rotational excitations of CO2 was investigated up to the collision energy of 2.35 eV. It was found that the total reaction integral cross section increases monotonically with the collision energy, consistent with experimental results. The excitation of the CO2 bending vibration enhances the reaction, while the excitation in its asymmetric stretching vibration inhibits the reaction. The calculated thermal rate constants are in excellent agreement with experiment. At the state-to-state level, the rotational state distributions of the HO product are in good agreement with experimental results, while those for the CO product are much hotter than measurements. The calculated differential cross sections are dominated by forward scattering, suggesting that the lifetime of the HOCO intermediate may not be sufficiently long to render the reaction completely statistical. [ABSTRACT FROM AUTHOR]

The Combined-Hyperbolic-Inverse-Power-Representation method, which treats evenly both short- and long-range interactions, is used to fit an extensive set of ab initio points for HO2 previously utilized [Xu et al., J. Chem. Phys. 122, 244305 (2005)] to develop a spline interpolant. The novel form is shown to perform accurately when compared with others, while quasiclassical trajectory calculations of the O + OH reaction clearly pinpoint the role of long-range forces at low temperatures. [ABSTRACT FROM AUTHOR]

We report ab initio calculations of the potential-energy surface and variational transition-state-theory calculations of the temperature dependence of the reaction rate for the chemical reaction H+HNO→H2+NO. This reaction is very exothermic and proceeds nearly without activation. Steepest-descent reaction pathways on the potential-energy surface for the abstraction reaction are presented both at the five-electron, five-orbital complete-active-space self-consistent-field level, and at the single- and double-substitution configuration-interaction level. Multireference configuration-interaction calculations are reported for several points along each of the reaction pathways. All electronic structure calculations employ a correlation consistent polarized valence double-zeta basis set. Canonical variational transition-state-theory calculations using the calculated potential-energy surface information predict a rate constant for this reaction that is an order of magnitude greater than that predicted previously by flame experiments. Our calculated rate constant is well represented by the three-parameter expression, k(T)=7.406×10-13T0.720 exp(-0.655/0.001 99T) cm3 molecules-1 s-1 over the range 200–3000 K. [ABSTRACT FROM AUTHOR]

Rotationally resolved reaction probabilities, integral cross sections, and rate constant for the H+ + H2 (v = 0, j = 0 or 1) → H2 (v' = 0, j') + H + reaction are calculated using a timeindependent quantum mechanical method and the potential energy surface of Kamisaka et al. [J. Chem. Phys. 116,654 (2002)] (say KBNN PES). All partial wave contributions of the total angular momentum, J, are included to obtain converged cross sections at low collision energies and rate constants at low temperatures. In order to test the accuracy of the KBNN PES, the results obtained here are compared with those obtained in our earlier work [P. Honvault et al., Phys. Rev. Lett. 107,023201 (2011)] using the accurate potential energy surface of Velilla et al. [J. Chem. Phys. 129, 084307 (2008)]. Integral cross sections and rate constants obtained on the two potential energy surfaces considered here show remarkable differences in terms of magnitude and dependence on collision energy (or temperature) which can be attributed to the differences observed in the topography of the surfaces near to the entrance channel. This clearly shows the inadequacy of the KBNN PES for calculations at low collision energies. [ABSTRACT FROM AUTHOR]

We show in this paper a time-dependent (TD) quantum wave packet calculation for the combustion reaction H+O2 using the DMBE IV (double many-body expansion) potential energy surface which has a deep well and supports long-lived resonances. The reaction probabilities from the initial states of H+O2(3Σ-g) (v=0–3, j=1) for total angular momentum J=0 are obtained for scattering energies from threshold up to 2.5 eV, which show numerous resonance features. Our results show that, by carrying out the wave packet propagation to several picoseconds, one can resolve essentially all the resonance features for this reaction. The present TD results are in good agreement with other time-independent calculations. A particular advantage of the time-dependent approach to this reaction is that resonance structures—strong energy dependence of the reaction probability—can be mapped out in a single wave packet propagation without having to repeat scattering calculations for hundreds of energies. We also report calculations of some low-lying vibrational energies of the hydroperoxyl radical HO2(2A‘) and their spectroscopic assignments. The vibrational frequencies of HO2(2A‘) on the DMBE IV potential energy surface are lower than experimental values, indicating the need to further improve the accuracy of the potential energy surface. [ABSTRACT FROM AUTHOR]

The cumulative reaction probability (CRP) (the Boltzmann average of which is the thermal rate constant) has been calculated for the reaction H2+OHH2O+H in its full (six) dimensionality for total angular momentum J=0. The calculation, which should be the (numerically) exact result for the assumed potential energy surface, was carried out by a direct procedure that avoids having to solve the complete state-to-state reactive scattering problem. Higher angular momenta (J>=0) were taken into account approximately to obtain the thermal rate constant k(T) over the range 300

A quasi-classical study of the endoergic Au+(1S) + H2(X1Σg+) → AuH+ (2Σ+) + H(2S) reaction, and isotopic variants, is performed to compare with recent experimental results [F. Li, C. S. Hinton, M. Citir, F. Liu, and P. B. Armentrout, J. Chem. Phys. 134, 024310 (2011)]. For this purpose, a new global potential energy surface has been developed based on multi-reference configuration interaction ab initio calculations. The quasi-classical trajectory results show a very good agreement with the experiments, showing the same trends for the different isotopic variants of the hydrogen molecule. It is also found that the total dissociation into three fragments, Au++H+H, is the dominant reaction channel for energies above the H2 dissociation energy. This results from a well in the entrance channel of the potential energy surface, which enhances the probability of H-Au-H insertion. [ABSTRACT FROM AUTHOR]

Seven-dimensional time-dependent wave packet calculations were carried out to study the title reaction on the potential energy surface constructed recently by the group [Y. Zhou, B. Fu, C. Wang, M. A. Collins, and D. H. Zhang, J. Chem. Phys. 134, 064323 (2011)]. Total reaction probabilities and integral cross sections were calculated for a number of initial vibration states. It is found that the fundamental C-H stretching excitation can promote the reaction dramatically. At E = 1.53 eV, it enhances the integral cross section by a factor of 13.2, which is in good agreement with the quasiclassical trajectory result on the ZBB1 potential energy surface, but larger than that on the the EG-2002 potential energy surface. The thermal rate constants were obtained for the title reaction by taking into account the contributions from all relevant initial vibration states. It is found that the ground initial state has a dominant contribution to the thermal rate constant at low temperature region. As the temperature increases, the relative contribution to the thermal rate constant from the ground initial state decreases quickly, and those from the stretching and bending excited states increases substantially. It is estimated that the C-H stretching mode contributes about 40% of the thermal rate constant at T = 1000 K despite the fact that the overall population for stretching excited states is tiny. [ABSTRACT FROM AUTHOR]

We extend to full dimensionality a recently developed wave packet method [M. T. Cvitasˇ and S. C. Althorpe, J. Phys. Chem. A 113, 4557 (2009)] for computing the state-to-state quantum dynamics of AB + CD → ABC + D reactions and also increase the computational efficiency of the method. This is done by introducing a new set of product coordinates, by applying the Crank-Nicholson approximation to the angular kinetic energy part of the split-operator propagator and by using a symmetry-adapted basis-to-grid transformation to evaluate integrals over the potential energy surface. The newly extended method is tested on the benchmark OH + H2 → H2O + H reaction, where it allows us to obtain accurately converged state-to-state reaction probabilities (on the Wu-Schatz-Fang-Lendvay-Harding potential energy surface) with modest computational effort. These methodological advances will make possible efficient calculations of state-to-state differential cross sections on this system in the near future. [ABSTRACT FROM AUTHOR]

The energetics of the reaction of HOCO with HO2 have been studied using the quadratic configuration interaction with single and double excitations (QCISD(T)) method and a large basis set on the singlet and triplet potential energy surfaces of the system. The results show that the ground-state O2+HOC(O)H products can be produced by a direct hydrogen abstraction via a transition state with a small barrier (1.66 kcal/mol) on the lowest triplet surface. A similar hydrogen abstraction can occur on the singlet electronic surface, but it leads to the singlet O2(a1Δ) and HOC(O)H. On the singlet surface, a new stable intermediate, HOC(O)OOH, hydroperoxyformic acid, has been found. This intermediate is formed by the direct addition of the terminal oxygen atom in HO2 onto the carbon atom in HOCO in a barrierless reaction. The HOC(O)OOH intermediate may dissociate into either the CO2+H2O2 or CO3+H2O products through elimination reactions with four-center transition states, or into HOC(O)O+OH through an O–O bond cleavage. The heat of formation of HOC(O)OOH is predicted to be -118.9±1.0 kcal/mol. In addition, the dynamics of the HO2+HOCO reaction have been investigated using a scaling-all correlation couple cluster method with single and double excitation terms (CCSD) on the singlet potential energy surface. Reaction mechanisms have been studied in detail. It was found that the direct and addition reaction mechanisms coexist. For the addition mechanism, the lifetime of the HOC(O)OOH intermediate is predicted to be 880±27 fs. At room temperature, the calculated thermal rate coefficient is (6.52±0.44)×10-11 cm3 molecule-1 s-1 with the product branching fractions: 0.77 (CO2+H2O2), 0.15 (HOC(O)O+OH), 0.056 (CO3+H2O), 0.019 (O2(a1Δ)+HOC(O)H), and 0.01 (O2(X 3Σ)+HOC(O)H). [ABSTRACT FROM AUTHOR]

The CH+H→C+H[sub 2] reaction is studied with quantum-mechanical wave packet calculations and quasiclassical trajectory calculations using a CH[sub 2] ground-state potential energy surface. Although quantum tunneling is important for direct hydrogen abstraction, the dominance of the complex formation mechanism ensures the reliability of quasiclassical calculations. Most collisions (≈80%) are nonreactive, because of a too-weak excitation of the CH vibration after a H-CH collision with H approaching CH with HCH angles larger than 60 deg. In this aspect the reaction differs from reactions such as the well-studied O(¹D) + H[sub 2] reaction, where the H-H vibration in the triatomic complex is strongly excited. Also presented is the rate constant for a temperature range between 50 and 2000 K, obtained from quasiclassical cross-section results for collision energies between 0.0005 and 0.3 eV. The role of the excited triplet and singlet states of CH[sub 2] on the reaction dynamics is discussed. [ABSTRACT FROM AUTHOR]

In order to better characterize the transition-state region for the I + HI, Br + HI, and Br + HBr reactions, the photoelectron spectra of IHI-, IDI-, BrHI-, BrHBr-, and BrDBr- have been simulated using a three-dimensional adiabatic approach. This method of simulation uses a Born–Oppenheimer separation in time scales between the fast hydrogen-atom motion and the slow halogen-atom motion to greatly simplify the computation of the photoelectron spectrum. The resulting simulations are compared to the experimental photoelectron and threshold photodetachment spectra of these anions, and to ‘‘exact’’ simulations of the IHI- and IDI- spectra. The comparison with the exact simulations shows that the adiabatic method is reasonably accurate, and is a considerable improvement over previous approximate simulation schemes. Potential-energy surfaces for the I + HI and Br + HI reactions are evaluated based on a comparison between the simulated and experimental spectra. A three-dimensional surface for the Br + HBr reaction that reproduces the experimental photoelectron spectrum is constructed by extending a fitted collinear surface to three dimensions. [ABSTRACT FROM AUTHOR]

We report high level ab initio calculations that characterize the quartet potential energy surfaces 4A‘ and 4A’ for the O+NH2 direct hydrogen abstraction reaction. Minimum energy reaction pathways have been computed on both surfaces at the full-valence complete active space self-consistent field level using a correlation consistent polarized valence double zeta basis set. Energies along these reaction pathways have been further refined by multireference configuration interaction calculations with a correlation-consistent polarized valence triple zeta basis set. Canonical variational transition state theory calculations using the ab initio potential energy surface information as input and incorporating tunneling through the ground state vibrationally adiabatic potential energy curves indicate that below about 2000 K, the O+NH2 reaction is dominated by addition/(isomerization)/dissociation pathways, and for temperatures below about 1000 K, even the OH+NH product channel is dominated by the addition/isomerization/dissociation route. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Ammonia generation through reaction of H2 with neutral cobalt nitride clusters in a fast flow reactor is investigated both experimentally and theoretically. Single photon ionization at 193 nm is used to detect neutral cluster distributions through time-of-flight mass spectrometry. ComNn clusters are generated through laser ablation of Co foil into N2/He expansion gas. Mass peaks ComNH2 (m = 6, 10) and ComNH3 (m = 7, 8, 9) are observed for reactions of H2 with the ComNn clusters. Observation of these products indicates that clusters ComN (m = 7, 8, 9) have high reactivity with H2 for ammonia generation. Density functional theory (DFT) calculations are performed to explore the potential energy surface for the reaction Co7N + 3/2H2 → Co7NH3, and a barrierless, thermodynamically favorable pathway is obtained. An odd number of hydrogen atoms in ComNH3 (m = 7, 8, 9) probably come from the hydrogen molecule dissociation on two active cobalt nitride clusters based on the DFT calculations. Both experimental observations and theoretical calculations suggest that hydrogen dissociation on two active cobalt nitride clusters is the key step to form NH3 in a gas phase reaction. A catalytic cycle for ammonia generation from N2 and H2 on a cobalt metal catalyst surface is proposed based on our experimental and theoretical investigations. [ABSTRACT FROM AUTHOR]

Despite its relative simplicity, the role of tunneling in the reaction OH + CO → H + CO2 has eluded the quantitative predictive powers of theoretical reaction dynamics. In this study a one-dimensional effective barrier to the formation of H + CO2 from the HOCO intermediate is directly extracted from dissociative photodetachment experiments on HOCO and DOCO. Comparison of this barrier to a computed minimum-energy barrier shows that tunneling deviates significantly from the calculated minimum-energy pathway, predicting product internal energy distributions that match those found in the experiment and tunneling lifetimes short enough to contribute significantly to the overall reaction. This barrier can be of direct use in kinetic and statistical models and aid in the further refinement of the potential energy surface and reaction dynamics calculations for this system. [ABSTRACT FROM AUTHOR]

The low-lying triplet and singlet potential energy surfaces of the O(3P)+CH3CN reaction have been studied at the G3(MP2)//B3LYP/6-311+G(d,p) level. On the triplet surface, six kinds of pathways are revealed, namely, direct hydrogen abstraction, C-addition/elimination, N-addition/elimination, substitution, insertion, and H-migration. Multichannel Rice–Ramsperger–Kassel–Marcus theory and transition-state theory are employed to calculate the overall and individual rate constants over a wide range of temperatures and pressures. It is predicted that the direct hydrogen abstraction and C-addition/elimination on triplet potential energy surface are dominant pathways. Major predicted end products include CH3+NCO and CH2CN+OH. At atmospheric pressure with Ar and N2 as bath gases, CH3C(O)N (IM1) formed by collisional stabilization is dominated at T<700 K, whereas CH3 and NCO produced by C-addition/elimination pathway are the major products at the temperatures between 800 and 1500 K; the direct hydrogen abstraction leading to CH2CN+OH plays an important role at higher temperatures in hydrocarbon combustion chemistry and flames, with estimated contribution of 64% at 2000 K. Furthermore, the calculated rate constants are in good agreement with available experimental data over the temperature range 300–600 K. The kinetic isotope effect has also been calculated for the triplet O(3P)+CH3CN reaction. On the singlet surface, the atomic oxygen can easily insert into C–H or C–C bonds of CH3CN, forming the insertion intermediates s-IM8(HOCH2CN) and s-IM5(CH3OCN) or add to the carbon atom of CN group in CH3CN, forming the addition intermediate s-IM1(CH3C(O)N); both approaches were found to be barrierless. It is indicated that the singlet reaction exhibits a marked difference from the triplet reaction. This calculation is useful to simulate experimental investigations of the O(3P)+CH3CN reaction in the singlet state surface. [ABSTRACT FROM AUTHOR]

On a new potential energy surface (PES-2008) developed by our group (preceding paper), we performed an extensive kinetics study using variational transition-state theory with semiclassical transmission coefficients over a wide temperature range of 250–2000 K and a dynamics study using quasiclassical trajectory (QCT) and quantum-mechanical (QM) calculations at collision energies between 0.7 and 2.0 eV for the title reaction and isotopically substituted versions. Kinetically, the H+CH4 forward and reverse thermal rate constants reproduce the available experimental data, with a small curvature of the Arrhenius plot indicating the role of tunneling in this hydrogen abstraction reaction. Five sets of kinetic isotope effects are also calculated. In general, they reproduce the experimental information. Dynamically, we focused on the H+CD4 reaction because there are more experimental studies for comparison. Most of the available energy appears as product translational energy (55%–68%), with the HD product being vibrationally cold (v′=0,1) in agreement with experiment, although rotationally hotter than experiment. The reaction cross section is practically negligible at 0.7 eV and still small at 1.5 eV, reproducing the experimental evidence, although our values are smaller. The product angular distribution is analyzed using QCT and QM methods. While at low energies (0.7 eV) both the QCT and the QM calculations yield forward scattered CD3 product, i.e., a rebound mechanism, at high energy (1.2 eV) only the QM calculations reproduce the experiment. The agreement with this wide variety of kinetic and dynamic experimental data (always qualitative and in some cases quantitative) shows the capacity of the PES-2008 surface to describe the reaction system. [ABSTRACT FROM AUTHOR]

The H+LiF(X 1Σ+,υ=0-2,j=0)→HF(X 1Σ+,υ′,j′)+Li(2S) bimolecular process is investigated by means of quantum scattering calculations on the chemically accurate X 2A′ LiHF potential energy surface of Aguado et al. [A. Aguado, M. Paniagua, C. Sanz, and J. Roncero, J. Chem. Phys. 119, 10088 (2003)]. Calculations have been performed for zero total angular momentum for translational energies from 10-7 to 10-1 eV. Initial-state selected reaction probabilities and cross sections are characterized by resonances originating from the decay of metastable states of the H...F-Li and Li...F-H van der Waals complexes. Extensive assignment of the resonances has been carried out by performing quasibound states calculations in the entrance and exit channel wells. Chemical reactivity is found to be significantly enhanced by vibrational excitation at low temperatures, although reactivity appears much less favorable than nonreactive processes due to the inefficient tunneling of the relatively heavy fluorine atom strongly bound in van der Waals complexes. [ABSTRACT FROM AUTHOR]

A recently proposed approach, called “string method,” allows us to find minimum energy pathways connecting two metastable states of a system [W. E et al., Phys. Rev. B 66, 052301 (2002)]. So far this approach has been only used with empirical force field parametrizations of the atomic potential energy surface or in the context of macroscopic continuum models. Here we show that the string method can be efficiently combined with first-principles molecular dynamics to provide an accurate description of chemical reaction pathways and barriers. We illustrate the first-principles string molecular dynamics by applying it to the study of a surface chemical reaction, for which extensive experimental and theoretical works are available, namely, the adsorption of H2 on the reconstructed Si(100) surface. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The gas-phase radical–radical reaction dynamics of O(3P)+C3H5→H(2S)+C3H4O was studied at an average collision energy of 6.4 kcal/mol in a crossed beam configuration. The ground-state atomic oxygen [O(3P)] and allyl radicals (C3H5) were generated by the photolysis of NO2 and the supersonic flash pyrolysis of allyl iodide, respectively. Nascent hydrogen atom products were probed by the vacuum-ultraviolet–laser induced fluorescence spectroscopy in the Lyman-α region centered at 121.6 nm. With the aid of the CBS–QB3 level of ab initio theory, it has been found that the barrierless addition of O(3P) to C3H5 forms the energy-rich addition complexes on the lowest doublet potential energy surface, which are predicted to undergo a subsequent direct decomposition step leading to the reaction products H+C3H4O. The major counterpart C3H4O of the probed hydrogen atom is calculated to be acrolein after taking into account the factors of barrier height, reaction enthalpy, and the number of intermediates involved along the reaction pathway. The nascent H-atom Doppler profile analysis shows that the average center-of-mass translational energy of the H+C3H4O products and the fraction of the total available energy released as the translational energy were determined to be 3.83 kcal/mol and 0.054, respectively. On the basis of comparison with statistical calculations, the reaction proceeds through the formation of short-lived addition complexes rather than statistical, long-lived intermediates, and the polyatomic acrolein product is significantly internally excited at the moment of the decomposition. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Six-dimensional quantum dynamical calculations are presented for the reaction of (v,j) H[SUB2] on Cu(100), at normal incidence, for v=0-1 and j=0-5. The dynamical calculations employed a potential energy surface computed with density functional theory, using the generalized gradient approximation and a slab representation for the adsorbate-substrate system. The aim of the calculations was to establish signatures from which experiments could determine the dominant reaction site of H[SUB2] on the surface and the dependence of the reaction site on the initial rovibrational state of H[SUB2]. Two types of signatures were found. First, we predict that, at energies near threshold, the reaction of (v = 1)H[SUB2] is rotationally enhanced, because it takes place at the top site, which has an especially late barrier and a reaction path with a high curvature. On the other hand, we predict the reaction to be almost independent of j for (v = 0) H[SUB2], which reacts at the bridge site. Second, we predict that, at collision energies slightly above threshold for which the reaction probabilities of the (v = 0) and (v = 1 ) states are comparable, the rotational quadrupole alignment of (v = 1 ) reacting molecules should be larger than that of (v =0) reacting molecules, for j= 1, 4, and 5. For (j = 2) H[SUB2], the opposite should be true, and for (j = 3) H[SUB2], the rotational quadrupole alignment should be approximately equal for (v = 1) and (v = 0)H[SUB2]. These differences can all be explained by the difference in the predicted reaction site for (v = 1) and (v=0)H[SUB2] (top and bridge) and by the differences in the anisotropy of the potential at the reaction barrier geometries associated with these sites. Our predictions can be tested in associative desorption experiments, using currently available experimental techniques. [ABSTRACT FROM AUTHOR]

Quantum mechanical (QM) and quasi-classical trajectory (QCT) calculations have been performed for the Cl+H[sub 2], Cl+D[sub 2], Cl+HD→ HCl(DCl)+D(H) reactions in order to determine integral cross sections as a function of collision energy and for different reagent rotational quantum numbers using the recent ab initio BW2 potential energy surface (PES) by Bian and Werner [J. Chem. Phys. 112, 220 (2000)]. The results are compared with experimental data obtained by using the Doppler-selected time-of-flight technique. It has been found theoretically by both the QM and QCT methods that reagent rotation enhances reactivity in agreement with experiment. The QM results are found to be in quantitative agreement with the experimental excitation functions for the Cl+p-H[sub 2] and Cl+n-H[sub 2] reactions, whereas those obtained quasi-classically fail to reproduce the experimental data. These results are in strong contrast with those reported on the previous G3 PES, in which QM and QCT calculations predicted that reactivity decreases with reagent rotation. The intermolecular isotope effect, i.e., the ratio between the cross sections of the Cl+n-H[sub 2] and Cl+n-D[sub 2] reactions, Γ[sub inter](Cl+n-H[sub 2]/Cl+n-D[sub 2]), predicted by QM calculations on the BW2 surface is notably larger than that obtained experimentally. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

A reduced dimensionality, time-dependent wave-packet calculation is reported for the H+CH[sub 4]→H[sub 2]+CH[sub 3] reaction in six degrees of freedom and for zero total angular momentum, employing the Jordan–Gilbert potential energy surface. Reaction probabilities for seven initial vibrational states of nonrotating “CH[sub 4],” and for the three lowest energy vibrational states and numerous initial rotational states are presented. Excitation of the C–H stretch, and the bending of H–CH[sub 3], enhances the reaction probability more than excitation of the umbrella mode. The six-degree-of-freedom cumulative reaction probability (CRP) for zero total angular momentum is obtained by direct summation over initial state-resolved reaction probabilities. An approximate full-dimensional CRP for zero total angular momentum is obtained using the energy-shift approximation to account for the contribution of degrees of freedom missing in the reduced dimensionality calculations. Then J–K shifting is applied to this CRP to obtain the thermal rate constant which is compared with previous calculations. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The room temperature rate constants for quenching of the fluorescence of H2, HD, and D2 B1Σ+u by 4He have been measured as a function of the initially excited rotational and vibrational levels of the hydrogen molecule. The effective quenching cross sections increase with increasing vibrational energy from about 1 Å2 up to a maximum of about 6 Å2. The effective cross sections for D2 (B, v’ = 0) were independent of the rotational level excited for 0 < J’ ≤ 7, and the cross sections for (v’ = 0, J’ = 0) were about 80% of the values for (v’ = 0, J’ >= 0) for all three isotopes studied. Quenching occurs via formation of an electronically excited (H2He)* collision complex followed by crossing to the repulsive H2(X)–He potential energy surface. The vibrational state dependence of the quenching cross sections fits a vibrationally adiabatic model for complex formation. From the vibrational state dependence of the quenching cross section, the barrier height for the quenching reaction is found to be 250±40 cm-1, and the difference in the H–H stretching frequencies between H2(B) and the H2–He complex at the barrier to reaction is 140±80 cm-1. Both values are substantially smaller than results from ab initio calculations. The rotational state dependence of the quenching cross sections suggests that quenching occurs with H2 rotating in a plane perpendicular to the relative velocity vector, in qualitative agreement with the rotational anisotropy of the H2(B)–He ab initio electronic potential energy surface. [ABSTRACT FROM AUTHOR]

We present the results of quasiclassical trajectory (QCT) and quantum centrifugal sudden hyperspherical (CSH) scattering calculations for the Cl+HCl→ClH+Cl reaction using a semiempirical potential energy surface. In particular, we report state-to-state integral and differential cross sections in the vicinity of a transition state resonance that occurs at a total energy E of 0.642 eV. This resonance, which is labeled by the transition state quantum numbers (0,0,2), strongly perturbs the cross sections for the initial rovibrational state HCl(v=1, j=5), which was therefore considered in all our calculations. For E≥0.680 eV, which is well removed from the resonance energy, the QCT and CSH results are in good agreement, but for E near the resonance energy, important quantum effects are found in the integral cross sections, product state distributions, and differential cross sections. The CSH integral cross sections show smooth steplike increases for E≊0.642 eV, which are not seen in the QCT results. Associated with these steps are increased branching to the v’=0 product HCl vibrational state, and a strong propensity for the production of rotational states with j’=15 and 16 for v’=0. These features of the product energy partitioning are not present in the QCT results, although the correct rotational distributions are approximately recovered if the final vibrational action is constrained to match its quantum value. The CSH differential cross sections show a sudden shift from backward to sideward scattering between 0.642 and 0.660 eV, while the QCT cross sections remain backward peaked. An analysis of the ‘‘number of atom–diatom encounters,’’ during the course of a reactive collision, shows that there are chattering trajectories. These are associated with sideward scattering, but their probability is low and as a result they do not produce distinct features in the angular distributions. However... [ABSTRACT FROM AUTHOR]

The potential energy surface for H(1 2S)+SO2 has been investigated computationally in order to study the catalytic removal of atomic hydrogen in flames by sulfur dioxide. HF/3–21G(*) and MP2/3–21G(*) levels of theory were employed to locate stationary points, which were then characterized by calculation of the vibrational frequencies. Some geometries were also optimized with the 6–31G* basis set. Two adducts HOSO and HSO2, with H bonded to O or S, respectively, were studied. Energies were estimated at the optimized geometries using spin-projected MP4/6–31G* calculations, which show that planar cis HOSO is more stable than Cs HSO2. An H–OSO bond energy of 109 kJ mol-1 is predicted. By contrast HSO2 is predicted to be 25 kJ mol-1 endothermic with respect to H+SO2, and is insufficiently stable to be significant in combustion chemistry. Transition states were located and the information used to derive the kinetics of H+SO2+Ar⇄HOSO+Ar from 298 to 2000 K. An unusually large energy barrier to recombination, of about 95 kJ mol-1 relative to H+SO2, is proposed. The results are compared with available kinetic measurements. Other potential decomposition channels for HOSO, to SO+OH and isomerization to HSO2, were also analyzed. [ABSTRACT FROM AUTHOR]

We present a quasiclassical trajectory study of the OH+CO reaction using a potential surface that has been derived from ab initio calculations. Among quantities that have been studied are cross sections for reaction and for HOCO complex formation, cross sections associated with reaction from excited vibrational and rotational states, product energy partitioning and CO2 vibrational-state distributions, HOCO lifetime distributions, and thermal and state-resolved rate constants. We also present the results of Rice–Ramsberger–Kassel–Marcus (RRKM) calculations, using the same potential-energy surface, of HOCO lifetimes and of reactive and complex formation rate constants. The trajectory results indicate that the dominant mechanism for reaction involves complex formation at low energies. However, a direct reaction mechanism is responsible for half the reactive cross section at higher energies. This leads to a rate constant that is weakly temperature dependent at low temperatures, and becomes strongly temperature dependent at high temperature. Our trajectory results agree with measured rates over a wide range of temperatures, but the trajectory results at low temperatures are dominated by classical ‘‘leak’’ through zero-point barriers, so this agreement may be somewhat fortuitous.Rate constants for nonreactive processes such as OH(v=1) deactivation by CO that are controlled by the HOCO formation step are well above experiment (factor of 6), while rate constants for processes such as CO(v=1) reaction with OH that are controlled by decay of HOCO into H+CO2 are much closer (factor of 2). This suggests that the entrance channel barrier on our surface is too loose while the exit barrier is accurate. The error in the entrance channel barrier is studied using RRKM, and it is found to be due to an incorrect out-of-plane bend potential in the analytical surface used. Modifying the potential so that it is more consistent with ab... [ABSTRACT FROM AUTHOR]

We develop a quantal model for studying four-center reactions, A2+B2→2AB, and collision induced dissociation A2+B2→A+B2+A. The method involves using hyperspherical coordinates to describe vibrations of the A2 and B2 bonds and a global vibration and rotation of the exchange products. Application to the H4 system is presented, using a realistic potential energy surface. The reaction goes through a four-center linear transition state located just above the dissociation threshold. In the energy range studied (5–5.5 eV), collision induced dissociation competes with the four-center reaction and is the dominant process. It is found that vibrational energy, originally deposited in one of the diatomic partners, is much more efficient than translational energy in promoting reaction. Vibrational and rotational final distributions show that the products are internally hot. This simple quantal model, yet very demanding computationally, illustrates in detail many features of the H4 dynamics above the dissociation threshold, and could serve to study other four center reactions with trapezoidal or linear transition states. © 1996 American Institute of Physics. [ABSTRACT FROM AUTHOR]

A time-dependent (TD) quantum wavepacket calculation of reaction probabilities is reported for the reaction HO + CO → H + CO2 for total angular momentum J=0. The dynamics calculation employs the potential-averaged five-dimensional model (PA5D) and is made possible by using a normalized angular quadrature scheme to minimize the requirement for computer memory. Reaction probabilities are obtained from the ground state as well as rotationally excited state in either one of the reactant diatoms. Strong resonances are found in the present study and calculated reaction probabilities are dominated by many narrow and overlapping resonances. These features are in qualitative agreement with several lower dimensional quantum dynamics studies. However, quantitative comparison of the present result with previously reported quantum calculations, including a recent planar four-dimensional (4D) calculation of Goldfield et al., shows that our calculated reaction probabilities are much smaller than those found in reduced dimensionality calculations. We also found reaction probability to be more sensitive to the rotational motion of CO than of HO. In addition to reaction probabilities, the bound state calculation for the stable intermediate complex HOCO has also been carried out and energies of several low-lying vibrational states are obtained. The potential energy surface (PES) of Schatz–Fitzcharles–Harding (SFH) is used in all the calculates presented in this paper. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The dynamics of a direct reaction between a gas phase H atom and an adsorbed H atom, often referred to as an Eley–Rideal mechanism, is explored using a fully three-dimensional flat surface model for Cu(111). The model is based on a flat-surface approximation for a single electronically adiabatic potential energy surface (PES). This reduces the inherently six-dimensional reactive scattering problem (for a rigid surface) to a three-dimensional problem by introducing three constants of motion. The resulting scattering problem is treated quantum mechanically by solving the time-dependent Schrödinger equation, and also by quasiclassical trajectory calculations. We have considered four different model PESs which are all more or less compatible with available knowledge about the interactions between hydrogen atoms and molecules and Cu(111). We have studied the dependence of the reactive cross section and product translational energy and rovibrational state distributions on the kinetic energy and angle of the incident H atom, and the vibrational state of the adsorbed H atom. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

A detailed comparison of quasiclassical trajectory (QCT) and quantum mechanical (QM) reaction probabilities and differential cross sections for the H+D2→HD+D reaction at the collision energies of 0.54 and 1.29 eV has been carried out using the same potential energy surface. The theoretical simulation of the recently published experimental results is also reported. The comparisons made here demonstrate the level of agreement between QCT and QM approaches, as well as between theory and experiment for this reaction. [ABSTRACT FROM AUTHOR]

The Photoloc technique has been employed to measure the state-resolved differential cross sections of the HD(v′, j′) product in the reaction H + D2 over a wide range of collision energies and internal states. The experimental results were compared with fully dimensional, time-dependent quantum mechanical calculations on the refined Boothroyd-Keogh-Martin-Peterson potential energy surface. We find nearly perfect agreement between theory and experiment for HD(v′, j′) product states with low to medium rotational excitation, e.g., HD(v′ = 1, j′ = 3) at a collision energy, Ecoll, of 1.72 eV, HD(v′ = 1, j′ = 3, 5) at Ecoll = 1.97 eV, and HD(v′ = 3, j′ = 3) at Ecoll = 1.97 eV. As the rotational angular momentum, j′, of HD(v′, j′) increases, the agreement between theoretical predictions and experimental measurements worsens but not in a simple fashion. A moderate disagreement between theory and experiment has been found for HD(v′ = 0, j′ = 12) at Ecoll = 1.76 eV and increased monotonically for HD(v′ = 0, j′ = 13) at Ecoll = 1.74 eV, HD(v′ = 0, j′ = 14) at Ecoll = 1.72 eV, and HD(v′ = 0, j′ = 15) at Ecoll = 1.70 eV. Disagreement was not limited to vibrationless HD(v′, j′) product states: HD(v′ = 1, j′ = 12) at Ecoll = 1.60 eV and HD(v′ = 3, j′ = 8, 10) at Ecoll = 1.97 eV followed a similar trend. Theoretical calculations suggest more sideways/forward scattering than has been observed experimentally for high j′ HD(v′, j′) states. The source of this discrepancy is presently unknown but might be the result of inaccuracy in the potential energy surface. [ABSTRACT FROM AUTHOR]

The gas-phase radical–radical reaction dynamics of O(3P)+C3H5→H(2S)+C3H4O was studied at an average collision energy of 6.4 kcal/mol in a crossed beam configuration. The ground-state atomic oxygen [O(3P)] and allyl radicals (C3H5) were generated by the photolysis of NO2 and the supersonic flash pyrolysis of allyl iodide, respectively. Nascent hydrogen atom products were probed by the vacuum-ultraviolet–laser induced fluorescence spectroscopy in the Lyman-α region centered at 121.6 nm. With the aid of the CBS–QB3 level of ab initio theory, it has been found that the barrierless addition of O(3P) to C3H5 forms the energy-rich addition complexes on the lowest doublet potential energy surface, which are predicted to undergo a subsequent direct decomposition step leading to the reaction products H+C3H4O. The major counterpart C3H4O of the probed hydrogen atom is calculated to be acrolein after taking into account the factors of barrier height, reaction enthalpy, and the number of intermediates involved along the reaction pathway. The nascent H-atom Doppler profile analysis shows that the average center-of-mass translational energy of the H+C3H4O products and the fraction of the total available energy released as the translational energy were determined to be 3.83 kcal/mol and 0.054, respectively. On the basis of comparison with statistical calculations, the reaction proceeds through the formation of short-lived addition complexes rather than statistical, long-lived intermediates, and the polyatomic acrolein product is significantly internally excited at the moment of the decomposition. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The adiabatically adjusting principal-axis hyperspherical (APH) formulation of Pack and Parker for quantum reactive scattering in three dimensions (3D) is used to obtain converged results for the reaction of helium with H+2 (v=1–4) for total angular momentum J=0. The ab initio potential energy surface computed by McLaughlin and Thompson and fitted by Joseph and Sathyamurthy is utilized for the HeH+2 interaction potential. The predicted energy dependence of the accurate 3D state-to-state reaction probabilities show clear evidence for quantum resonances. These resonances are even more numerous than those reported earlier for reduced dimensionality studies of this reaction. The calculated time delays for several of these resonances are found to be over 1 ps. Bending corrected rotating linear model (BCRLM) studies of this same reaction are also reported. These results provide useful insight in sorting out the nature and contribution of the resonances found in the 3D studies. [ABSTRACT FROM AUTHOR]

The photoelectron spectra of the ions BrHI-, ClHI-, and FHI-, along with their deuterated counterparts, are presented. These spectra provide information on the transition state region of the potential energy surfaces describing the exothermic neutral reactions X+HI→HX+I(X=Br, Cl, F). Vibrational structure is observed in the BrHI- and ClHI- spectra that corresponds to hydrogen atom motion in the dissociating neutral complex. Transitions to electronically excited potential energy surfaces that correlate to HX+I(2P3/2,2P1/2) products are also observed. A one-dimensional analysis is used to understand the appearance of each spectrum. The BrHI- spectrum is compared to a two-dimensional simulation performed using time-dependent wave packet propagation on a model Br+HI potential energy surface. [ABSTRACT FROM AUTHOR]

We have calculated state-to-state reaction cross sections for two new energies and state-to-state vibrationally inelastic cross sections for four energies for H+D2 collisions on the most accurate available potential energy surface. The dynamics calculations are based on the quasiclassical trajectory quadratic smooth sampling method. We present a detailed analysis of the theoretical inelastic and reaction cross sections and compare the trends in the reactive scattering results to those calculated from Gerrity and Valentini’s experiments and extrapolations thereof. The agreement between theory and experiment for the reactive scattering at 0.98–1.3 eV relative translational energy is stupendously good. [ABSTRACT FROM AUTHOR]

Measures rate constants for the chemical reaction of chlorine, carbon, hydrogen and bromine. Reaction increase by over two orders of two magnitude when temperature is reduced; Calculations done with the rotating bond approximation.

Examines the angular distributions for individually resolved states from the reaction between fluorine and hydrogen. Achievement of vibrational and rotational resolution by applying laser and bolometer detection techniques to crossed-beam reactive scattering; Observation of products scattered into the forward hemisphere.

We investigated spin-orbit-induced intersystem crossing effects in the title reaction by the time-dependent wave-packet method combined with an extended split operator scheme. We performed non-adiabatic calculations of the fine-structure-resolved cross section and adiabatic calculations of integral cross section. The calculations are based on the potential energy surfaces of 3A′ and the two degenerate 3A′′ states [S. Rogers, D. Wang, A. Kuppermann, and S. Walch, J. Phys. Chem. A 104, 2308 (2000)], together with the spin-orbit coupling matrix [B. Maiti and G. C. Schatz, J. Chem. Phys. 119, 12360 (2003)] and singlet 1A′ potential energy surface [J. Dobbyn and P. J. Knowles, Faraday Discuss. 110, 247 (1998)]. The results of the O(3P) + D2 are similar to those of the O(3P) + H2 reaction. The product spin state-resolved reaction cross section and the total reaction cross section both show that the adiabatic channel is dominant in all cases, and the non-adiabatic channels have cross sections of several orders of magnitude smaller than the adiabatic channels at high collision energy. Although the cross sections caused by the intersystem crossing effects in the O(3P) + D2 reaction are larger than those in the O(3P) + H2 reaction, the differences in non-adiabaticity between these two reaction systems are quite modest. Based on the results of the O(3P) + H2 reaction, we can predict that the influence of spin-orbit on the total reaction cross sections of the O(3P) + D2 reaction is also insignificant. However, these non-adiabatic effects can be reflected in the presence of some forward-scattering in the angular distribution for the OD product. [ABSTRACT FROM AUTHOR]

In this work we present a dynamically biased statistical model to describe the evolution of the title reaction from statistical to a more direct mechanism, using quasi-classical trajectories (QCT). The method is based on the one previously proposed by Park and Light [J. Chem. Phys. 126, 044305 (2007)]. A recent global potential energy surface is used here to calculate the capture probabilities, instead of the long-range ion-induced dipole interactions. The dynamical constraints are introduced by considering a scrambling matrix which depends on energy and determine the probability of the identity/hop/exchange mechanisms. These probabilities are calculated using QCT. It is found that the high zero-point energy of the fragments is transferred to the rest of the degrees of freedom, what shortens the lifetime of H5+ complexes and, as a consequence, the exchange mechanism is produced with lower proportion. The zero-point energy (ZPE) is not properly described in quasi-classical trajectory calculations and an approximation is done in which the initial ZPE of the reactants is reduced in QCT calculations to obtain a new ZPE-biased scrambling matrix. This reduction of the ZPE is explained by the need of correcting the pure classical level number of the H5+ complex, as done in classical simulations of unimolecular processes and to get equivalent quantum and classical rate constants using Rice-Ramsperger-Kassel-Marcus theory. This matrix allows to obtain a ratio of hop/exchange mechanisms, α(T), in rather good agreement with recent experimental results by Crabtree et al. [J. Chem. Phys. 134, 194311 (2011)] at room temperature. At lower temperatures, however, the present simulations predict too high ratios because the biased scrambling matrix is not statistical enough. This demonstrates the importance of applying quantum methods to simulate this reaction at the low temperatures of astrophysical interest. [ABSTRACT FROM AUTHOR]

A previously reported potential energy surface (PES) and a new barrierless PES (both based on ab initio data and describing the CH[sub 3] group as a pseudoatom) were used to study the O([sup 1]D)+CH[sub 4]→OH+CH[sub 3] reaction with the quasiclassical trajectory (QCT) method. The new PES accurately reproduces the experimental rate constant values, in contrast to the previous PES. The QCT study was mainly performed at the relative translational energy (E[sub T]) resulting from the photodissociation of N[sub 2]O at 193 nm (=0.403 eV), although the collision energy obtained from the photodissociation of O[sub 3] at 248 nm (=0.212 eV) was also considered. Good agreement between theory and experiment was obtained for the OH vibrational populations and for the OH rotational populations for the v[sup ′]>=2 vibrational levels, while the rotational distributions for v[sup ′]=0-1 are more excited than in the experiment. The QCT results at E[sub T]=0.403 eV satisfactorily reproduce the experimental kk[sup ′] angular distribution of the state-specific channel OH(v[sup ′]=4, N[sup ′]=8) and the corresponding E[sub T][sup ′] distribution. For OH(v[sup ′]=0, N[sup ′]=5) the reproduction of these properties is poorer, especially for the E[sub T][sup ′] distribution. At 0.403 eV the contribution of the abstraction mechanism to the reaction mode is negligible and two insertion like mechanisms (with fast or slow elimination) are found to be predominant, as suggested experimentally. The discrepancies observed between the QCT and experimental results can be explained on the basis of the defective description of the insertion/slow elimination mechanism provided by the model. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]