A QCT study of the role of the symmetric and antisymmetric stretch mode excitations of methane in the O(³P) + CH₄ (ν_i = 0, 1; i = 1, 3) reaction

Abstract

Quasi-classical trajectory calculations based on a full-dimensional analytical potential energy surface have been performed at different collision energies to analyze the role of symmetric (ν1 = 1) and antisymmetric (ν3 = 1) stretch modes of methane in reactivity and dynamics of the O((3)P) + CH4 (νi = 0, 1; i = 1, 3) gas-phase reactions. Both CH stretch modes increase reactivity with respect to the methane vibrational ground-state by factors between 1.5 and 3. Additionally, the ν1 = 1 mode is slightly more reactive than the ν3 = 1 mode by factors between 1.4 and 1.1 depending on the collision energy. Both stretch modes give similar pictures of OH product vibrational and angular distributions. The former finding shows inverted OH (0, 1) vibrational population, discarding mode selectivity, and the latter shows a shift of the scattering angle from backward to sideways with the vibrational excitation and therefore a change in the mechanism. For the dynamic properties analyzed, the theoretical results for the ν3 = 1 mode reproduce the experimental evidence, while those for the ν1 = 1 mode await confirmation.

Revisiting Adiabatic Switching for Initial Conditions in Quasi-Classical Trajectory Calculations: Application to CH4

“…This is illustrated in the 2014 review of recent QCT calculations of the X + CH 4 (and isotopologs) reaction for both the vibrational ground and excited-state reactions, where X is H, F, Cl, Br, and O( 3 P), 4 and also more recent application to mode-specific effects in O( 3 P) + CH 4 . 5 Experiments and QCT calculations of mode-specificity in reactions of CH 4 (and isotopologs) have been the focus of numerous studies in the gas phase, e.g., refs 3−9. In the QCT studies, normal-mode sampling has been performed, using accurate analytical potential energy surfaces.…”

“…In the QCT studies, normal-mode sampling has been performed, using accurate analytical potential energy surfaces. [3][4][5]9 The modespecific dissociation of methane on a metal surface is also a prototype system when investigating the mode selectivity in gas−surface reactions, and it is also the rate-limiting step in steam re-forming. 10−13 This apparent success of normal-mode sampling notwithstanding, it has been known for years that rigorous phase-space sampling of initial states in QCT calculations should be based on semiclassical quantization of ro-vibrational states of polyatomic molecules.…”

“…Thus, normal-mode sampling generally does provide good and robust results. This is illustrated in the 2014 review of recent QCT calculations of the X + CH 4 (and isotopologs) reaction for both the vibrational ground and excited-state reactions, where X is H, F, Cl, Br, and O­( 3 P), and also more recent application to mode-specific effects in O­( 3 P) + CH 4 …”

“…Experiments and QCT calculations of mode-specificity in reactions of CH 4 (and isotopologs) have been the focus of numerous studies in the gas phase, e.g., refs − . In the QCT studies, normal-mode sampling has been performed, using accurate analytical potential energy surfaces. − , The mode-specific dissociation of methane on a metal surface is also a prototype system when investigating the mode selectivity in gas–surface reactions, and it is also the rate-limiting step in steam re-forming. − …”

Thermal Rate Constants for the O(3P) + CH4→ OH + CH3Reaction: The Effects of Quantum Tunneling and Potential Energy Barrier Shape

“…Full dimensional quasiclassical trajectory method is also widely used for the title reaction, it is an important complement to the reduced dimensional quantum dynamical calculation. Many important conclusions − have been drawn. Varandas et al showed that the topology of the entrance channel had strong implications on the dynamics of the title reaction.…”