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

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 (ΔrH298K⊖ = −63.1 kJ mol–1), and a pathway connecting these regions. In total, 94 000
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. Less 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.