A global full-dimensional potential energy surface and quasiclassical trajectory study of the O(¹D) + CH₄ multichannel reaction

Abstract

We report a new global, full-dimensional ground-state potential energy surface (PES) of the O((1)D) + CH4 multichannel reaction, based on high-level ab initio calculations and fitting procedures. The PES is a permutationally invariant fit to roughly 340 000 electronic energies calculated by the MRCI + Q/aug-cc-pVTZ level of theory. Extensive quasiclassical trajectory calculations were carried out on the new PES at the collision energy of relevance to the previously universal crossed molecular beam experiments. The product branching ratios, translational energy distributions and angular distributions of OH + CH3, H + CH2OH/CH3O and H2 + HCOH/H2CO product channels were calculated and compared with the available experimental results. Very good agreement between theory and experiment has been achieved. The O((1)D) + CH4 reaction mainly proceeds through the CH3OH intermediate via a trapped abstraction mechanism, starting with the abstraction of the hydrogen atom, rather than the direct insertion pathway with the O((1)D) atom directly inserting into the C-H bond of CH4. The process with a very short lifetime behaves like an abstraction reaction, producing a pronounced forward scattering peak as found in the OH + CH3 channel, while the process with a relatively long lifetime produces reaction products with nearly forward and backward scattering symmetry, similar to an insertion reaction, as found in other reaction channels.

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

“…In this work, the rate coefficients of the O ( 1 D) + CH 4 reaction are computed based on a local PES in the reactant asymptotic part because of its barrierless and exothermic properties [8,9,11].…”

“…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.…”

“…Recently, Shao et al [11] constructed a global full-dimensional ground-state PES of the O ( 1 D) + CH 4 system by permutationally invariant fitting to roughly 3.4 × 10 5 ab initio energy points with the maximum fitting error of dozen of meV. Since the fitting error of this global PIP PES is rather large, the dynamics simulations for the title reaction may only predict the rate coefficients with rather a large uncertainty.…”

“…Since the fitting error of this global PIP PES is rather large, the dynamics simulations for the title reaction may only predict the rate coefficients with rather a large uncertainty. In this work, we re-construct a new, more accurate PES on the basis of the previous ab initio energy points [11]. Based on this new PES, ring-polymer molecular dynamics (RPMD) [12][13][14][15][16] simulations are performed to accurately and efficiently compute the rate coefficients.…”

When Classical Trajectories Get to Quantum Accuracy: The Scattering of H2on Pd(111)

“…[1][2][3][4][5][6][7][8][9][10] For chemical reactions involving a small number of degrees-of-freedom (DOF) (typically less than or equal to 6), quantum dynamics calculations might be now regarded as exact since they generally provide results in excellent agreement with experiments. 11,12 However, reaction dynamics studies focus on systems of ever increasing size [13][14][15][16][17][18][19][20][21][22][23][24][25][26][27][28] for which the quasi-classical trajectory (QCT) method is at this time the only practical tool to apply. A crucial issue is thus to develop theoretically justified corrections to carry out classical dynamics studies of chemical reactions in a quantum spirit.…”