Does the Cl + CH₄ → H + CH₃Cl Reaction Proceed via Walden Inversion?

Area of Science:

• Chemical Dynamics
• Quantum Chemistry
• Reaction Mechanisms

Background:

• The Cl + CH4 reaction is a benchmark system for studying chemical reaction dynamics.
• Previous potential energy surfaces (PES) had limitations in describing high-energy reaction pathways.

Purpose of the Study:

• To develop a chemically accurate, full-dimensional ab initio PES for the Cl + CH4 reaction.
• To investigate novel substitution pathways, particularly those involving retention of configuration.
• To improve the description of the high-energy region of the PES.

Main Methods:

• Global ab initio potential energy surface (PES) calculations.
• Quasiclassical trajectory simulations.
• Analysis of reaction channels, cross sections, and angular distributions.

Main Results:

• The new PES accurately describes abstraction, Walden inversion substitution, and novel retention substitution pathways.
• Substitution channels open around 40 kcal/mol via Walden inversion, with retention pathways emerging around 50 kcal/mol.
• At 80 kcal/mol, retention pathways contribute significantly to the substitution cross section, potentially becoming dominant at higher energies.

Conclusions:

• Novel retention substitution mechanisms are identified and characterized for the Cl + CH4 reaction.
• The developed PES provides a more accurate description of the reaction dynamics across a range of collision energies.
• Understanding these pathways is crucial for predicting reactivity in similar systems.