Effects of C–H stretch excitation on the H+CH4 reaction

Department of Chemistry, Stanford University, Stanford, California 94305-5080, USA.
The Journal of Chemical Physics (Impact Factor: 2.95). 11/2005; 123(13):134301. DOI: 10.1063/1.2034507
Source: PubMed


We have investigated the effects of C-H stretching excitation on the H+CH4-->CH3+H2 reaction dynamics using the photo-LOC technique. The CH3 product vibrational state and angular distribution are measured for the reaction of fast H atoms with methane excited in either the antisymmetric stretching fundamental (nu3=1) or first overtone (nu3=2) with a center-of-mass collision energy of Ecoll ranging from 1.52 to 2.20 eV. We find that vibrational excitation of the nu3=1 mode enhances the overall reaction cross section by a factor of 3.0+/-1.5 for Ecoll=1.52 eV, and this enhancement factor is approximately constant over the 1.52-2.20-eV collision energy range. A local-mode description of the CH4 stretching vibration, in which the C-H oscillators are uncoupled, is used to describe the observed state distributions. In this model, the interaction of the incident H atom with either a stretched or an unstretched C-H oscillator determines the vibrational state of the CH3 product. We also compare these results to the similar quantities obtained previously for the Cl+CH4-->CH3+HCl reaction at Ecoll=0.16 eV [Z. H. Kim, H. A. Bechtel, and R. N. Zare, J. Chem. Phys. 117, 3232 (2002); H. A. Bechtel, J. P. Camden, D. J. A. Brown, and R. N. Zare, ibid. 120, 5096 (2004)] in an attempt to elucidate the differences in reactivity for the same initially prepared vibration.

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Available from: Richard Zare, Oct 03, 2015
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    • "As a first test of the quality of the surface, we compared the enhancement of the reactivity when the antisymmetric C–H stretch mode is excited in the H + CH 4 reaction for which experimental data are available, which is not the case for the title reaction . Thus, at 1.52 eV, Zare's group [11] reported an enhancement of 3 ± 1.5. Using QCT calculations (i.e., the same dynamical method ) the ZBB3 [12] and PES-2009 surfaces give similar results, 2.3 and 1.9, respectively, consistent with the experimental evidence. "
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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.
    Computational and Theoretical Chemistry 02/2013; 1006:123–126. DOI:10.1016/j.comptc.2012.11.023 · 1.55 Impact Factor
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    ABSTRACT: We compare experimental photoloc measurements and quasi-classical trajectory calculations of the integral cross sections, lab-frame speed distributions, and angular distributions associated with the CD3 products of the H + CD4(nu = 0) --> CD3 + HD reaction at collision energies ranging from 0.5 to 3.0 eV. Of the potential energy surfaces (PES) we explored, the direct dynamics calculations using the B3LYP/6-31G** density functional theory PES provide the best agreement with the experimental measurements. This agreement is likely due to the better overall description that B3LYP provides for geometries well removed from the minimum energy path, even though its barrier height is low by approximately 0.2 eV. In contrast to previous theoretical calculations, the angular distributions on this surface show behavior associated with a stripping mechanism, even at collision energies only approximately 0.1 eV above the reaction barrier. Other potential energy surfaces, which include an analytical potential energy surface from Espinosa-García and a direct dynamics calculation based on the MSINDO semiempirical Hamiltonian, are less accurate and predict more rebound dynamics at these energies than is observed. Reparametrization of the MSINDO surface, though yielding better agreement with the experiment, is not sufficient to capture the observed dynamics. The differences between these surfaces are interpreted using an analysis of the opacity functions, where we find that the wider cone of acceptance on the B3LYP surface plays a crucial role in determining the integral cross sections and angular distributions.
    The Journal of Physical Chemistry A 01/2006; 110(2):677-86. DOI:10.1021/jp053827u · 2.69 Impact Factor
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    ABSTRACT: We report the methyl radical product state distributions for the reactions of H and Cl with CHD3(ν1 = 1,2) at collision energies of 1.53 and 0.18 eV, respectively. Both reactions demonstrate mode selectivity. The resulting state distributions from the H+CHD3(ν1 = 1,2) reactions are well described by a spectator model. The reactions Cl+CHD3(ν1 = 1,2) exhibit similar behavior, but in some aspects the spectator model breaks down. We attribute this breakdown to enhanced intramolecular vibrational redistribution in the Cl+CHD3(ν1 = 1,2) reactions compared to the H+CHD3(ν1 = 1,2) reactions, caused by the interaction of the slower Cl atom with the vibrationally excited CHD3, which is promoted either by its longer collision duration, its stronger coupling, or both.
    The Journal of Chemical Physics 01/2006; 124(3):034311-034311-7. DOI:10.1063/1.2155434 · 2.95 Impact Factor
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