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.

This paper represents an attempt to extend the mechanisms of reactions and kinetics of a methane combustion reaction. Three saddle points (SPs) are identified in the reaction CH 4 + O( 3 P) → OH + CH 3 using the complete active space selfconsistent field and the multireference configuration interaction methods with a proper active space. Our calculations give a fairly accurate description of the regions around the twin first-order SPs ( 3 A and 3 A ) along the direction of O( 3 P) attacking a near-collinear H-CH 3 . One second-order SP 2nd ( 3 E) between the above twin SPs is the result of the C 3v symmetry Jahn-Teller coupling within the branching space. Further kinetic calculations are performed with the canonical unified statistical theory method with the temperature ranging from 298 K to 1000 K. The rate constants are also reported. The present work reveals the reaction mechanism of hydrogen-abstraction by the O( 3 P) from methane, and develops a better understanding for the role of SPs. In addition, a comparison of the reactions of O( 3 P) with methane through different channels allows a molecule-level discussion of the reactivity and mechanism of the title reaction.

The dynamics of the ground-state reaction of CH 4 + O( 3 P) → CH 3 (ν = 0) + OH(ν = 0) have attracted a great deal of attention both theoretically and experimentally. This rapid communication represents extensive quasi-classical trajectory calculations of the vibrational distributions on a unique full-dimensional ab initio potential energy surface for the title reaction, at the collision energy of relevance to previous crossed molecular beam experiments. The surface is constructed using the all electrons coupled-cluster singles and doubles approach plus quasi-perturbative triple excitations with optimized basis sets. A modified Shepard interpolation method is also employed for the construction. Good agreement between our calculations and the available experimental results has been achieved, opening the door for accurate dynamics on this surface.

Kinetic investigations including quasi-classical trajectory and canonical unified statistical theory method calculations are carried out on a potential energy surface for the hydrogen-abstraction reaction from methane by atom O(3P). The surface is constructed using a modified Shepard interpolation method. The ab initio calculations are performed at the CCSD(T) level. Taking account of the contribution of inner core electrons to electronic correlation interaction in ab initio electronic structure calculations, modified optimized aug-cc-pCVQZ basis sets are applied to the all-electrons calculations. On this potential energy surface, the triplet oxygen atom attacks methane in a near-collinear H–CH3 direction to form a saddle point with barrier height of 13.55 kcal/mol, which plays a key role in the kinetics of the title reaction. For the temperature range of 298–2500 K, our calculated thermal rate constants for the O(3P) + CH4 → OH + CH3 reaction show good agreement with relevant experimental data. This work provides detailed mechanism of this gas-phase reaction and a theoretical guidance for methane combustion.

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