Computational Study of Competition between Direct Abstraction and Addition–Elimination in the Reaction of Cl Atoms with Propene

Abstract

Quasi-classical trajectory calculations on a newly constructed and full-dimensionality potential energy surface (PES) examine the dynamics of the reaction of Cl atoms with propene. The PES is an empirical valence bond (EVB) fit to high-level ab initio energies and incorporates deep potential energy wells for the 1-chloropropyl and 2-chloropropyl radicals, a direct H-atom abstraction route to HCl + allyl radical (CH2CHCH2·) products (∆ 298 = 63.1 kJ mol -1 ), and a pathway connecting these regions. In total, 94000 successful reactive trajectories were used to compute distributions of angular scattering and HCl vibrational and rotational level populations. These measures of the reaction dynamics agree satisfactorily with available experimental data. The dominant reaction pathway is direct abstraction of a hydrogen atom from the methyl group of propene occurring in under 500 fs. Fewer than 10% of trajectories follow an addition-elimination route via the two isomeric chloropropyl radicals. Large amplitude motions of the Cl about the propene molecular framework couple the addition intermediates to the direct abstraction pathway. The EVB method provides a good description of the complicated PES for the Cl + propene reaction despite fitting to a limited number of ab initio points, with the further advantage that dynamics specific to certain mechanisms can be studied in isolation by switching off coupling terms in the EVB matrix connecting different regions of the PES.

Primary vs. secondary H-atom abstraction in the Cl-atom reaction with n-pentane

“…45, 46 We have previously tested and validated this efficient method for PES generation against experimental data for reactions of Cl atoms with methane and propene. 10,24 The QCT calculations successfully reproduce most of the experimental results, and explore the competition between primary and secondary H atom abstraction pathways.…”

“…10,24,58 Firstly, the diagonal terms of the EVB matrix were parameterized; these diagonal terms correspond to the reactants, and the six primary and six secondary radical product states. Fits of the rigid SCS-MP2-F12/ aug-cc-pVDZ scans along the internal coordinates of the pentane molecule and pent-1-yl, pent-2-yl and pent-3-yl radicals returned the bond stretching, angle bending, out-of-plane bending and torsion force constants.…”

“…3,4 Cl can also add across C=C bonds in unsaturated organic compounds such as isoprene, 5,6 and recent studies examined the subsequent pathways to HCl elimination. [7][8][9][10] The Cl-atom reactions with methane, ethane, larger alkanes and other organic compounds serve as model systems for the study of dynamics of gas-phase reactions of polyatomic molecules, [11][12][13] with detailed investigations performed in a number of laboratories. [14][15][16][17][18][19][20][21][22][23] Selected examples have also been the subjects of computer simulation using global potential energy surfaces 10,24 or direct dynamics calculations, [25][26][27] with calculated product quantum state populations and state-resolved differential cross sections that are in reasonable accord with experimental data.…”

Dynamical Effects and Product Distributions in Simulated CN + Methane Reactions

“…We have applied this technique to both gas-and liquid-phase reactions previously, showing excellent agreement with experimental results. 12,14,22,30,32 Most of the procedures we use for fitting the CN + CH4 → HCN + CH3 surface have direct analogies to building our Cl + CH4 EVB surface.…”

“…It is the goal of the current work to explore some, but not all, of these ideas in hydrogen abstraction from hydrocarbons by CN. In so doing, we build on our recent quantum-state-resolved dynamics measurements and simulations of halogen-atom reactions with saturated 12 and unsaturated polyatomic reactants in gas [13][14] and liquid phases. [15][16][17] A lag exists in our understanding of the gas-phase CN hydrogen-abstraction dynamics relative to those in the condensed phase.…”