Mechanism and Kinetics of Methane Combustion, Part I: Thermal Rate Constants for Hydrogen-Abstraction Reaction of CH₄ + O(³P)

Abstract

The mechanism and kinetics of gas-phase hydrogen-abstraction by the O(P) from methane are investigated using ab initio calculations and dynamical methods. Not only are the electronic structure properties including the optimized geometries, relative energies, and vibrational frequencies of all the stationary points obtained from state-averaged complete active space self-consistent field calculations, but also the single-point energies for all points on the intrinsic reaction coordinate are evaluated using the internally contracted multireference configuration interaction approach with modified optimized cc-pCVDZ basis sets. Our calculations give a fairly accurate description of the regions around the A″ transition state in the O(P) attacking a near-collinear H-CH direction with a barrier height of 12.53 kcal/mol, which is lower than those reported before. Subsequently, thermal rate constants for this hydrogen-abstraction are calculated using the canonical unified statistical theory method with the temperature ranging from 298 K to 1000 K. These calculated rate constants are in agreement with experiments. The present work reveals the reaction mechanism of hydrogen-abstraction by the O(P) from methane, and it is helpful for the understanding of methane combustion.

Mechanism and Kinetics of Methane Combustion. Part II: Potential Energy Surface for Hydrogen-Abstraction Reaction of CH4 + O(3P)

“…Since 2017, a series of studies on the stationary points of the ground state and the first electronic excited state for the O­( 3 P) + CH 4 → OH + CH 3 reaction have been carried out by our group using the internally contracted multireference configuration interaction (icMRCI) method. Specifically, more than two saddle points along the reaction path and several key kinetic properties including thermal rate constants are described accurately. , Our work has found that when the oxygen atom O­( 3 P) attacks the hydrogen atom with C 2 v symmetry along the CH bond direction, one 2p orbital forms a σ bond with H atom, while the other two 2p orbitals remain degenerate. When the OHC bond angle deviates from the linear shape, the Jahn–Teller conical intersection point related to the electronic state of 3 E appears between the two lowest surfaces. , According to reported literature, ,,, there are some obvious differences in the configurations of the reaction transition state, the barrier heights, and the corresponding virtual frequencies.…”

“…The SA-CASSCF method, with the above basis set and full valence active space, has achieved good calculation accuracy and results for this reaction system. 25,26 Further correlations such as the icMRCI method with the same basis set and active space has far exceeded the current computer capacity.…”

“…Specifically, more than two saddle points along the reaction path and several key kinetic properties including thermal rate constants are described accurately. 25,26 Our work has found that when the oxygen atom O( 3 P) attacks the hydrogen atom with C 2v symmetry along the CH bond direction, one 2p orbital forms a σ bond with H atom, while the other two 2p orbitals remain degenerate. When the OHC bond angle deviates from the linear shape, the Jahn−Teller conical intersection point related to the electronic state of 3 E appears between the two lowest surfaces.…”

“…The reaction O­( 3 P) + CH 4 → OH + CH 3 may not be limited to the ground state surface. However, due to the large number of electrons and the high dimensions of the surface, no research on the high-accuracy ab initio calculations involving the excited state for this reaction process has been reported, apart from a few studies ,,, on the nonadiabatic effect on the first excited state (2 3 A) surface. The intermediates in the methane combustion process are free radicals and involve the electronic structure of the excited state, which are difficult to be correctly described by the single-reference ab initio method .…”

Kinetic studies of few-layer graphene grown by flame deposition from the perspective of gas composition and temperature

“…Experimentally, gas phase analysis during methane combustion also shows this mechanism transition 31 which was corroborated by numerical analysis. 32 The transition as temperature was increased beyond 600 C was attributed to the mass transport as being the limiting step at such accelerated reaction rates. The fact that BIN20J remains linear could be due to it being a heavier hydrocarbon (i.e.…”