Theoretical study on the reaction path and rate constants of the hydrogen atom abstraction reaction of CH2O with CH3/OH (original) (raw)
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Using a recently developed full-dimensional accurate analytical potential energy surface [Gonzalez-Lavado, E.; Corchado, J. C.; Espinosa-Garcia, J. J. Chem. Phys. 2014, 140, 064310], we investigate the thermal rate coefficients of the O( 3 P) + CH 4 /CD 4 reactions with ring polymer molecular dynamics (RPMD) and with variational transitionstate theory with multidimensional tunneling corrections (VTST/MT). The results of the present calculations are compared with available experimental data for a wide temperature range 200−2500 K. In the classical high-temperature limit, the RPMD results match perfectly the experimental data, whereas VTST results are smaller by a factor of 2. We suggest that this discrepancy is due to the harmonic approximation used in the present VTST calculations, which leads to an overestimation of the variational effects. At low temperatures the tunneling plays an important role, which is captured by both methods, although they both overestimate the experimental values. The analysis of the kinetic isotope effects shows a discrepancy between both approaches, with the VTST values smaller by a factor about 2 at very low temperatures. Unfortunately, no experimental results are available to shed any light on this comparison, which keeps it as an open question.
The Journal of Physical Chemistry A, 2017
Rate constants and the product branching ratio for hydrogen abstraction from CH 3 OH by O(3 P) were computed with multi-structural variational transition state theory including microcanonically optimized multi-dimensional tunneling. Benchmark calculations of the forward and reverse classical barrier heights and the reaction energetics have been carried out by using coupled cluster theory and multireference calculations to select the most reliable density functional method for direct dynamics computations of the rate constants. The dynamics calculations included the anharmonicity of the zero-point energies and partition functions, with specific-reaction-parameter scaling factors for reactants and transition states, and multi-structural torsional anharmonicity was included for the torsion around the CO bond in methanol and in the transition states. The resulting rate constants are presented over a wider range than they are available from experiment, but in the temperature range where experiments are available, they agree well with experimental values, which is encouraging for their reliability over the wider temperature range and for future computations of oxygen atom reaction rates. In contrast to a previous computational prediction, the branching ratio predicted by the present work shows that the formation of CH 2 OH + OH is the dominant channel over the whole range of temperature from 250 K to 2000 K.
The Journal of Chemical Physics, 2014
Discrete variational quantum reactive scattering method with optimal distorted waves. II. Application to the reaction H+O 2 → OH+O This work reports a detailed theoretical study of the hydrogen abstraction reactions from ethanol by atomic hydrogen. The calculated thermal rate constants take into account torsional anharmonicity and conformational flexibility, in addition to the variational and tunneling effects. Specifically, the kinetics calculations were performed by using multi-path canonical variational transition state theory with least-action path tunneling corrections, to which we have added the two-dimensional non-separable method to take into account torsional anharmonicity. The multi-path thermal rate constant is expressed as a sum over conformational reaction channels. Each of these channels includes all the transition states that can be reached by internal rotations. The results show that, in the interval of temperatures between 250 and 2500 K, the account for multiple paths leads to higher thermal rate constants with respect to the single path approach, mainly at low and at high temperatures. In addition, torsional anharmonicity enhances the slope of the Arrhenius plot in this range of temperatures. Finally, we show that the incorporation of tunneling into the hydrogen abstraction reactions substantially changes the contribution of each of the transition states to the conformational reaction channel.
The Journal of Chemical Physics, 2006
This article presents a multi-faceted study of the reaction H + C 2 H 6 → H 2 + C 2 H 5 and three of its deuterium-substituted isotopologs. First we present high-level electronic structure calculations by the W1, G3SX, MCG3-MPWB, CBS-APNO, and MC-QCISD/3 methods that lead to a best estimate of the barrier height of 11.8 ± 0.5 kcal/mol. Then we obtain a specific reaction parameter for the MPW density functional in order that it reproduces the best estimate of the barrier height; this yields the MPW54 functional. The MPW54 functional, as well as the MPW60 functional that was previously parameterized for the H + CH 4 reaction, are used with canonical variational theory with small-curvature tunneling (CVT/SCT) to calculate the rate constants for all four ethane reactions from 200 to 2000 K. The final MPW54 calculations are based on curvilinear-coordinate generalized-normal-mode analysis along the reaction-path, and they include scaled frequencies and an anharmonic C-C bond torsion. They agree with experiment within 31% for 467-826 K except for a 38% deviation at 748 K; the results for the isotopologs are predictions since these rate constants have never been measured. The kinetic isotope
The Journal of Physical Chemistry A, 2017
Rate constants and the product branching ratio for hydrogen abstraction from CH 3 OH by O(3 P) were computed with multi-structural variational transition state theory including microcanonically optimized multi-dimensional tunneling. Benchmark calculations of the forward and reverse classical barrier heights and the reaction energetics have been carried out by using coupled cluster theory and multireference calculations to select the most reliable density functional method for direct dynamics computations of the rate constants. The dynamics calculations included the anharmonicity of the zero-point energies and partition functions, with specific-reaction-parameter scaling factors for reactants and transition states, and multi-structural torsional anharmonicity was included for the torsion around the CO bond in methanol and in the transition states. The resulting rate constants are presented over a wider range than they are available from experiment, but in the temperature range where experiments are available, they agree well with experimental values, which is encouraging for their reliability over the wider temperature range and for future computations of oxygen atom reaction rates. In contrast to a previous computational prediction, the branching ratio predicted by the present work shows that the formation of CH 2 OH + OH is the dominant channel over the whole range of temperature from 250 K to 2000 K.
The Journal of Physical Chemistry A, 2004
Theoretical calculations were carried out on the H-, Cl-, and F-atom abstraction reactions from a series of seven substituted halogenated methanes (CH 3 Cl, CH 2 Cl 2 , CHCl 3 , CCl 4 , CHF 3 , CHF 2 Cl, and CHFCl 2 ) by H atom attacks. Geometry optimizations and vibrational frequency calculations were performed using unrestricted Møller-Plesset second-order perturbation theory (UMP2) with the 6-311++G(d,p) basis set. Single-point energy calculations were performed with the highly correlated ab initio coupled cluster method in the space of single, double and triple (pertubatively) electron excitations CCSD(T) using the 6-311++G(3df,3pd) basis set. Canonical transition-state theory with a simple tunneling correction was used to predict the rate constants as a function of temperature (700-2500 K), and three-parameter Arrhenius expressions were obtained by fitting to the computed rate constants for elementary channels and overall reaction.
Journal of Molecular Structure: THEOCHEM, 2004
A computational approach to calculating potential energy surfaces for reactive systems is presented and tested. This hybrid approach is based on integrated methods where calculations for a small model system are performed by using analytical potential energy surfaces, and for the real system by using molecular orbital or molecular mechanics methods. The method is tested on a hydrogen abstraction reaction by using the variational transition-state theory with multidimensional tunneling corrections. The agreement between the calculated and experimental information depends on the quality of the method chosen for the real system. When the real system is treated by accurate quantum mechanics methods, the rate constants are in excellent agreement with the experimental measurements over a wide temperature range. When the real system is treated by molecular mechanics methods, the results are still good, which is very encouraging since molecular mechanics itself is not at all capable of describing this reactive system. Since no experimental information or additional fits are required to apply this method, it can be used to improve the accuracy of molecular orbital methods or to extend the molecular mechanics method to treat any reactive system with the single constraint of the availability of an analytical potential energy surface that describes the model system.
Chemical Physics, 2009
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.
The Journal of Chemical Physics, 1995
We present direct ab initio dynamics studies of vibrational-state selected reaction rates of the OH+H2→H+H2O reaction. Rate constants for both the OH+H2(v=1) and OH(v=1)+H2 reactions were calculated based on a full variational transition state theory plus multidimensional semiclassical tunneling approximations within a statistical diabatic model. The potential energy surface information was calculated at an accurate level of molecular orbital theory. In particular, geometries and frequencies along the minimum energy path were calculated at the quadratic configuration interaction level including all single and double excitations (QCISD) with the 6-311+G(d,p) basis set. Energies along the minimum energy path were further improved by a series of single point projected fourth-order Möller–Plesset perturbation theory (PMP4) calculations using the 6-311++G(2df,2pd) basis set. Our present results of vibrational excited state rate enhancements agree very well with previous experimental data...