The hydrogen abstraction reaction H+CH4. II. Theoretical investigation of the kinetics and dynamics

Departamento de Química Física, Universidad de Extremadura, Badajoz, Spain.
The Journal of Chemical Physics (Impact Factor: 2.95). 06/2009; 130(18):184315. DOI: 10.1063/1.3132594
Source: PubMed


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 + CH(4) 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 + CD(4) 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 CD(3) 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.

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Available from: Jose C Corchado, May 23, 2014
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    • "With the current information we can conclude that the CBE surface adequately describes the dynamics of this reactive system in the high energy regime, while the ZBB3 and ZFWCZ are less accurate. Finally, note that the present results do not modify the whole dynamics description of this system, which is a direct hydrogen abstraction reaction with a rebound mechanism at low collision energy (0.7 eV), evolving to a stripping mechanism at higher energies , by opening the acceptance cone [5]. "
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    ABSTRACT: The excitation functions of the H+CH4 reaction and its isotope analogue H+CD4 have been analyzed by several groups by means of quasi-classical trajectory and quantum mechanical calculations using different potential energy surfaces. Our conclusion is that reactivity does not monotonically increase with collision energy, but it shows a maximum, at relatively high and low energies for the CH4 and CD4 reactants, respectively, and that the analytical surface developed in our lab reproduces recent experiments with improvements in behaviour compared to previous surfaces.
    Full-text · Article · Oct 2015 · Computational and Theoretical Chemistry
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    • "-of-plane bending and valence bending terms and it was constructed by a multi-beginning least-squares optimisation of the parameters to CCSD(T)/cc-pVTZ energies and derivatives. The PES-2009 was subjected to great variety of tests [8] [9] [10], with both kinetics and dynamics (QCT and QM) results compared with the experimental information available for the H + CH 4 reaction and its isotopomers. In general, PES-2009 reproduced the wide variety of experimental properties, which lent confidence to the surface. "
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    ABSTRACT: Using a recent ab initio based potential energy surface, PES-2009, quasi-classical trajectory calculations were performed to analyse the effects of the C–H stretch excitation on the reactivity and dynamics of the H + CHD3 abstraction reaction at a collision energy of 1.53 eV. Firstly, we found that the C–H stretch mode excitation has little influence on the product rotational distributions and on the scattering distribution for both channels. However, it has significant influence on the product energy distribution for the CHD2 + HD channel, indicating that the reaction shows mode selectivity, reproducing the experimental evidence. Finally, excitation of the C–H stretch by one quantum increases the reactivity of the vibrational ground-state for both channels reproducing the experimental evidence, although for the H-abstraction channel we report an enhancement of reactivity somewhat lower than other theoretical results.
    Full-text · Article · Feb 2013 · Computational and Theoretical Chemistry
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    ABSTRACT: The angular scattering of a state-to-state chemical reaction contains fundamental information on its dynamics. Often the angular distributions are highly structured and the physical interpretation of this structure is an important and difficult problem. Here, we report a surprising finding for the benchmark F + H(2) --> FH + H reaction, when the product molecule FH is in a vibrational state with quantum number = 3 and a rotational state with quantum number = 3. We demonstrate that the differential cross section (DCS) is an example of (attractive) rainbow scattering, being characterized by an Airy function and its derivative. The rainbow reveals its presence in the DCS by interference with the repulsive (or nearside) scattering producing characteristic diffraction oscillations. The rainbow is broad, which explains why it has not been recognized in the many earlier theoretical and experimental investigations of this reaction. There is an angular region in the DCS where the rainbow dominates, but with the unusual property that the DCS is less intense than in adjoining angular regions. The reaction investigated is F + H(2)(v(i) = 0, j(i) = 0, m(i) = 0) --> FH(v(f) = 3, j(f) = 3, m(f) = 0) + H, where v(i), j(i), m(i) and v(f), j(f), m(f) are initial and final vibrational, rotational and helicity quantum numbers, respectively. The relative translational energy is 0.119 eV. We use rigorous semiclassical (asymptotic) techniques that provide physical insight as well as a mathematically sound and numerically accurate description of the angular scattering. The semiclassical DCS agrees very closely with the exact quantum DCS. The semiclassical scattering amplitude is used to assess the physical effectiveness of the Fuller nearside-farside decomposition for the partial wave series of the F + H(2) reaction, including the effect of one resummation. We also compare the semiclassical and exact quantum nearside, farside, and full local angular momenta and find good agreement. Although our new rainbow has unusual and unexpected properties, similar rainbows are predicted to occur in the DCSs of many state-to-state chemical reactions, since the semiclassical analysis is generic and not specific to the present F + H(2) example.
    No preview · Article · Nov 2009 · The Journal of Physical Chemistry A
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