Communication: Imaging the effects of the antisymmetric-stretching excitation in the O(3P) + CH4(v3 = 1) reaction

Abstract

Effects of one-quantum excitation of the antisymmetric-stretching mode of CH4(v3 = 1) on the O(3P) + CH4 reaction were studied in a crossed-beam, ion-imaging experiment. In the post-threshold region, we found that (1) the product state distributions are dominated by the CH3(00) + OH(v′ = 1) pair, (2) the product angular distributions extend toward sideways from the backward dominance of the ground-state reaction, and (3) vibrational excitation exerts a positive effect on reactivity, but translational energy is more efficient in promoting the rate of this central-barrier reaction. All major findings agree reasonably well with recent theoretical results. Some remaining questions are pointed out.

Communication: Direct comparison between theory and experiment for correlated angular and product-state distributions of the ground-state and stretching-excited O(3P) + CH4 reactions

“…[7][8][9][10] In 2014 an experimental study also appeared investigating the effects of the antisymmetric stretching excitation on the angular distributions of the O( 3 P) + CH 4 (v 3 = 0, 1) reactions. 11 The experiment found that the angular distributions are backward scattered for the groundstate reaction and shift toward sideways and forward directions upon stretching excitation. However, a recent theoretical study found very similar backward scattering for both the O + CH 4 (v = 0) and O + CH 4 (v 3 = 1) reactions.…”

“…Furthermore, we report predictions for the symmetric-stretchingexcited O + CH 4 (v 1 = 1) reaction, for which experiment is planned, 11 and provide OH rotational distributions, which can be directly compared to experiment 12 in the case of O + CH 4 (v = 0).…”

Perspective: Vibrational-induced steric effects in bimolecular reactions

“…In addition to the diverse barrier locations, the barrier heights of the three reactions are also significantly different. The theoretically calculated adiabatic barriers 30 (the experimentally measured thresholds) are 1.03 kcal mol −1 (<0.5 kcal mol −1 ), 31,32 3.4 kcal mol −1 (∼2.5 kcal mol −1 ), 33,34 and 10.1 kcal mol −1 (∼8.0 kcal mol −1 ) 35,36 for the ground state reactions of CHD 3 (v = 0) with F, Cl, and O( 3 P), respectively. [The zero-point-energy corrections have been added to the calculated classical barrier heights 30 for the theoretically adiabatic barriers.]…”

“…30,54,55 Again, the ground-state reaction has been extensively investigated and is characterized by a direct abstraction pathway with the rebound mechanism. 35,36,[56][57][58][59][60][61][62][63] This is to be contrasted to the ground-state reaction of F + CHD 3 , for which dual mechanisms partake: a direct abstraction and a resonance-mediated pathway that prevails at lower collision energies. 38,39 Figure 3 illustrates the effects of CH stretch-excitation of CHD 3 (v 1 = 1) on the CD 3 (v = 0) product, which is the dominant product channel.…”

“…More recent report on an analogous O( 3 P) + CH 4 (v 3 = 1) reaction (v 3 is the anti-symmetric stretch mode) also showed a similar effect. 36,62 The idea that a reactant vibration can open up the cone of acceptance to reaction is not unprecedented. Using a model PES for the O( 3 P) + HCl reaction, Schechter and Levine found in a QCT study a new mechanism for vibrationally enhanced reactivity and attributed it to the enlarged reactive cone of acceptance.…”