Indirect dynamics in a highly exoergic substitution reaction

Area of Science:

• Chemical Dynamics
• Reaction Mechanisms
• Quantum Chemistry

Background:

• Nucleophilic substitution reactions are fundamental in chemistry.
• Previous studies on halogen anion substitutions (e.g., Cl- + CH3I) showed different dynamics.
• Understanding reaction pathways is crucial for predicting chemical behavior.

Purpose of the Study:

• To investigate the reaction dynamics of F- + CH3I.
• To compare the dynamics of F- substitution with other halogen anions.
• To elucidate the atomic-level mechanisms governing this reaction.

Main Methods:

• Crossed-beam imaging experiments were conducted.
• Direct chemical dynamics simulations were performed.
• Density Functional Theory (DFT) calculations at the B97-1 level were utilized.

Main Results:

• The F- + CH3I reaction exhibits significant indirect scattering via a hydrogen-bonded complex across a wide energy range.
• This contrasts with Cl- + CH3I, particularly at higher collision energies.
• Simulations identified three dominant mechanisms: indirect via complex, direct rebound, and direct stripping.
• The indirect mechanism contributes substantially (~50%) to the overall rate, surprisingly even at high energies.

Conclusions:

• The F- + CH3I reaction pathway is distinct from heavier halogen substitutions.
• A substantial contribution of indirect scattering, mediated by a hydrogen-bonded complex, is a key feature.
• Experiment and simulation results show strong agreement, validating the proposed mechanisms.