Theoretical study of the dynamics, stereodynamics, and microscopic mechanism of the O(1D)+CH4(X 1A1)→OH(X 2Π)+CH3(X 2A2″) reaction

Abstract

A previously reported potential energy surface ͑PES͒ and a new barrierless PES ͑both based on ab initio data and describing the CH 3 group as a pseudoatom͒ were used to study the O( 1 D)ϩCH 4 →OHϩCH 3 reaction with the quasiclassical trajectory ͑QCT͒ method. The new PES accurately reproduces the experimental rate constant values, in contrast to the previous PES. The QCT study was mainly performed at the relative translational energy (E T ) resulting from the photodissociation of N 2 O at 193 nm (͗E T ͘ϭ0.403 eV͒, although the collision energy obtained from the photodissociation of O 3 at 248 nm (͗E T ͘ϭ0.212 eV͒ was also considered. Good agreement between theory and experiment was obtained for the OH vibrational populations and for the OH rotational populations for the vЈу2 vibrational levels, while the rotational distributions for vЈϭ0 -1 are more excited than in the experiment. The QCT results at E T ϭ0.403 eV satisfactorily reproduce the experimental kkЈ angular distribution of the state-specific channel OH(vЈϭ4, NЈϭ8) and the corresponding E T Ј distribution. For OH(vЈϭ0, NЈϭ5) the reproduction of these properties is poorer, especially for the E T Ј distribution. At 0.403 eV the contribution of the abstraction mechanism to the reaction mode is negligible and two insertion like mechanisms ͑with fast or slow elimination͒ are found to be predominant, as suggested experimentally. The discrepancies observed between the QCT and experimental results can be explained on the basis of the defective description of the insertion/slow elimination mechanism provided by the model.

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

“…36 PES2 reproduces the experimental dynamic results somewhat better than PES1. Good agreement between QCT and experiments was obtained for the OH(v ) vibrational populations and OH(v ≥ 2) rotational populations, while the rotational populations for v = 0-1 are somewhat more excited than experimentally.…”

“…Thus, the analysis of the microscopic reaction mechanism at 0.403 eV (PES1, PES2) 36 and 0.212 eV (PES1) 4 shows that the reaction takes place almost exclusively through the insertion of the O( 1 D) atom into a C-H bond to yield the CH 3 OH collision complex, which dissociates into products following a fast elimination or a slow elimination process, with almost the same probability. The substantially lower collision complex lifetimes obtained in the QCT calculations (0.2 ps (PES1) and 0.3 ps (PES2) at 0.403 eV) in comparison with the experimental one (≈0.8 ps) 40 may result from the inadequacy of the model to accurately account for the insertion-slow elimination process.…”

“…[4][5][6] The CH 3 OH reaction intermediate can be formed rather easily via insertion of O( 1 D) into a C-H bond of methane. 4,6,24,36,37 The particular importance of the CH 3 + OH channel (which has a kinematics nearly identical to reactants) is probably due to the fact that the other channels are less exoergic or present a barrier with respect to CH 3 OH.…”

“…The same type of function (many-body expansion) 39 and most points of the previous ab initio grid were considered to construct a new pseudo-triatomic representation of the ground PES (PES2), 36 which does not present any energy barrier along the MEP (even if the ZPE is included).…”

Rotational spectral studies of O(1D) insertion reactions with methane and ethylene: Methanol and vinyl alcohol in a supersonic expansion

“…This reaction has been studied extensively through both experimental and theoretical [42][43][44][45][46][47][48] methods. An energy diagram showing an overview of the insertion reaction and subsequent dissociation pathways is shown in Figure 1.…”