Quantum dynamical study of the O(1D) + CH4 → CH3 + OH atmospheric reaction

Abstract

Time independent quantum mechanical (TIQM) scattering calculations have been carried out for the O((1)D) + CH4(X(1)A1) → CH3(X(2)A2″) + OH(X(2)Π) atmospheric reaction, using an ab initio ground potential energy surface where the CH3 group is described as a pseudo-atom. Total and state-to-state reaction probabilities for a total angular momentum J = 0 have been determined for collision energies up to 0.5 eV. The vibrational and rotational state OH product distributions show no specific behavior. The rate coefficient has been calculated by means of the J-shifting approach in the 10-500 K temperature range and slightly depends on T at ordinary temperatures (as expected for a barrierless reaction). Quantum effects do not influence the vibrational populations and rate coefficient in an important way, and a rather good agreement has been found between the TIQM results and the quasiclassical trajectory and experimental ones. This reinforces somewhat the reliability of the pseudo-triatomic approach under the reaction conditions explored.

Theoretical and experimental investigations of rate coefficients of O(1D) + CH4 at low temperature

“…The convergence of small number of beads should be caused by the small zero point energy (ZPE) differnece (3.16 kcal/mol) [11] between the reactants and the complex. Moreover, the small number of beads for both ∆W (n) and κ (n) implies that quantum effects do not strongly affect the rate coefficients, in a similar manner to previous 3D TIQM [10] simulations.…”

“…Next, turning to the TIQM results, since TIQM calculations were performed for the OH + CH 3 channel only, they are first scaled by a branching ratio of 0.78, which was previously determined [10]. After scaling the TIQM rate coefficients, reasonably good agreement between RPMD and TIQM [10] results is found. The relative differences are smaller than 28%, except for the one at 50 K (roughly 37%).…”

“…All other previous studies used a range of indirect detection methods to follow the progress of the O ( 1 D) + CH 4 reaction. Now, let us compare the RPMD rate coefficients with the previous QCT and [8] and TIQM [10] results. As shown in Table I, the previous QCT results [8] at 300 K and 200 K are given to compare with the RPMD ones at 296 K and 210 K. Although there is temperature difference (smaller than 10 K), good agreement between RPMD and QCT [8] can be found.…”

“…Due to its significance in both atmospheric and combustion chemistry, over the past decade the reaction of methane with O ( 1 D) has attracted great attention both experimentally [1][2][3][4][5][6][7] and theoretically [8][9][10][11], helping to unravel the dynamics and kinetics of this typical polyatomic complex-forming barrierless reaction. These studies [1][2][3][4][8][9][10] have determined an approximately temperature-independent rate coefficient of ∼ 1.5 (unless otherwise specified, the rate is given in 10 −10 cm 3 • s −1 ) at the room temperature.…”

“…On the basis of a pseudo-triatomic three-dimensional (3D) ground-state ( 1 A ) potential energy surface (PES) [8], González et al [8] and Ben Bouchrit et al [10] reported rate coefficients of the O ( 1 D) + CH 4 → OH + CH 3 reaction from quasi-classical trajectory (QCT) [8] and timeindependent quantum mechanical (TIQM) [10] reactive scattering calculations, respectively. They [8,10] found (i) that quantum effects do not strongly affect the resulting rate coefficients and (ii) a rather good agreement between computations and experiments was obtained. Despite the existence of agreement at room temperature, it is still necessary to perform full dimensional (12D) quantum dynamics calculations because the CH 3 fragment was treated as a pseudo-triatom in previous calculations [8,10].…”