Potential energy surface, kinetics, and dynamics study of the Cl+CH4→HCl+CH3 reaction

Abstract

A modified and recalibrated potential energy surface for the gas-phase Cl+CH4-->HCl+CH3 reaction is reported and tested. It is completely symmetric with respect to the permutation of the four methane hydrogen atoms and is calibrated with respect to updated experimental and theoretical stationary point properties and experimental forward thermal rate constants. From the kinetics point of view, the forward and reverse thermal rate constants and the activation energies were calculated using the variational transition-state theory with semiclassical transmission coefficients over a wide temperature range of 150-2500 K. The theoretical results reproduce the available experimental data, with a small curvature of the Arrhenius plot which indicates the role of tunneling in this hydrogen abstraction reaction. A dynamics study was also performed on this PES using quasiclassical trajectory (QCT) calculations, including corrections to avoid zero-point energy leakage along the trajectories. First, we found a noticeable internal energy in the coproduct methyl radical, both in the ground-state [CH4 (v=0)] and vibrationally excited [CH4 (v=1)] reactions. This CH3 internal energy was directly precluded in some experiments or oversimplified in previous theoretical studies using pseudotriatomic models. Second, our QCT calculations give HCl rotational distributions slightly hotter than those in experiment, but correctly describing the experimental trend of decreasing the HCl product rotation excitation in going from HCl (v'=0) to HCl (v'=1) for the CH4 (v=1) reaction. Third, the state specific scattering distributions present a reasonable agreement with experiment, although they tend to make the reaction more forward and backward scattered than found experimentally probably because of the hotter rotational distribution and the deficiencies of the QCT methods.

Tracking the energy flow along the reaction path

“…3, red bars) show significantly larger ratios, in support of this interpretation. In conjunction with the strong curvature couplings of both (stretching and umbrella-excited) active modes to the reaction coordinate (19)(20)(21)(22), a cascading energy flow could lead to a higher population in the ground-state pair (0, 0 0 ) s than the (0, 2 i ) s pairs, which is exactly shown in Fig. 3.…”

“…Theory predicted that as the Cl atom approaches the H end of CHD 3 , the chemical interaction induces a static curvature coupling (i.e., coupling of a vibrational mode to the reaction coordinate induced by the curvature of the reaction path) between the C-H stretching (v 1 ) motion and the reaction coordinate, resulting in a significant decrease of its frequency in the transition-state region (19)(20)(21)(22) and allowing energy flow between this mode and the reaction coordinate. Similar behavior was found for the CD 3 umbrella mode (v 3 ), yet other modes show little variation in frequencies.…”

“…By using the reaction path Hamiltonian approach (26), previous ab initio calculations of isotopically analogous reactions mapped out the minimum energy path and the evolution of the vibrational frequencies of various modes along the reaction path (19)(20)(21)(22). By adding the theoretically calculated vibration frequencies (with isotope corrections) to the minimum energy path, we connected the vibrational energy levels according to their symmetries, employing the rule that energy levels for vibration of the same symmetry do not cross (26).…”

“…The notation of 2 i refers to the vibrational mode 2 (the umbrella-bend) with i-quantum excitation.) Pair-correlated state and angular distributions were then exploited, in light of available ab initio theory (19)(20)(21)(22), to unveil the microscopic reaction pathways. We further sharpened the comparison with the result of a ground-state reaction at either the same initial translation energy (E c ) or the higher E c with an equivalent amount of total energy (vibration ϩ translation).…”

Direct dynamics simulations using Hessian-based predictor-corrector integration algorithms

“…7,8 Exact fitting of the ab initio data is feasible for systems with a small number of atoms or a high degree of symmetry. [9][10][11][12][13] For large systems, the number of internal degrees of freedom to which the data are fitted becomes large ͑3N-6 unique degrees of freedom for a nonlinear system of N atoms with no symmetry͒. In such cases, often a model analytic potential is derived for a few degrees of freedom that are important to describe, the reaction and empirical potentials are used for the other degrees of freedom.…”