Classical Trajectory Study of the Dynamics of the Reaction of Cl Atoms with Ethane

Abstract

We present an electronic-structure and dynamics study of the Cl + C2H6 --> HCl + C2H5 reaction. The stationary points of the ground-state potential energy surface have been characterized using various electronic-structure methods and basis sets. Our best calculations, CCSD(T) extrapolated to the complete basis limit, using geometries and harmonic frequencies obtained at the MP2/aug-cc-pVTZ level, are in agreement with the experimental reaction energy. Ab initio information has been used to reparameterize a semiempirical Hamiltonian so that the predictions of the improved Hamiltonian agree with the higher-level calculations in key regions of the potential energy surface. The improved semiempirical Hamiltonian is then used to propagate quasiclassical trajectories. Computed kinetic energy release and scattering angle distributions at a collision energy of approximately 5.5 kcal mol(-1) are in reasonable agreement with experiments, but no evidence was found for the low translational energy HCl products scattered in the backward hemisphere reported in recent experiments.

Quasi-classical trajectory study of the dynamics of the Cl + CH4→ HCl + CH3 reaction

“…Previous QCT calculations of the reaction of Cl atoms with methane 15 and with ethane 35,36 showed evidence for a chattering mechanism in which the light H atom moves repeatedly between the C and Cl atoms before forming products. All reactive trajectories were analysed for signatures of chattering 35 and a total of 154 trajectories showed this behaviour, 59 of which satisfied the above-mentioned criterion for sufficient ZPE in the products.…”

“…Analysis of product scattering angles (the scattering angle,  is defined as that between the velocity vectors of reagent Cl and product HCl in the COM frame), the translational energy of the products, and the internal energy of HCl were carried out as reported in our previous paper. 36 Our analysis of the internal energy of the radical product (CH3), however, incorporates additional calculation of the energy disposal into the normal modes of CH3. For the reactive trajectories, energy disposal in the nascent CH3 was determined by projecting its space-fixed cartesian velocities, ) (t q  , and coordinates, ) (t q , onto the 3N translational, rotational, and vibrational degrees of freedom of CH3 in its SRP-AM1 COM frame equilibrium geometry, ) (t eq q .…”

“…16 The optimization of the parameters of a semi-empirical Hamiltonian, typically by fitting to energies computed by high-level electronic structure calculations at selected molecular geometries, has been employed for the study of a number of chemical reactions. 16,[34][35][36][37] This specific reaction parameter (SRP) approach enables trajectories to be propagated in direct dynamics calculations with lower computational demand at each step of the trajectory than would be required for energy calculations using methods based on ab initio computation (e.g. Hartree-Fock approaches) or density functional theory (DFT).…”

Velocity map imaging the dynamics of the reactions of Cl atoms with neopentane and tetramethylsilane

“…From Eq. (6) the forms of the opacity function for the reactions of Cl atoms with neopentane and TMS can be derived, and the outcome (deduced to be the same for both reactions) is shown in Figure 11 Fully dimensional quasi-classical trajectory (QCT) calculations of the reaction of Cl atoms with ethane, 59 showed that the relationship in Eq. (5) did not apply well, however, and therefore the scattering could not be described simply by a hard sphere model for collisions.…”

“…Cl + C 2 H 6 → C 2 H 5 + HCl (4) have been well studied both experimentally 17,18,22,[25][26][27]45 and computationally, [57][58][59][60][61] and the reactions of Cl atoms with propane 1,62-64 and n-butane 21,25,62 have been similarly investigated. The energetics and kinetics of these reactions are very similar to those for reactions (2) and (3); the reactions are all rapid and direct, have either a low or no barrier to reaction and the exothermicities of the primary hydrogen abstraction reactions are typically about -3 kcal mol -1 .…”