UV-Induced Reaction Pathways in Bromoform Probed with Ultrafast Electron Diffraction.

Area of Science:

• Chemical Physics
• Photochemistry
• Reaction Dynamics

Background:

• Predicting reaction rates and product distributions is challenging, especially for short-lived intermediates requiring ultrafast techniques.
• The UV photochemistry of bromoform (CHBr3) is well-studied, but pathways like isomerization versus direct C-Br bond scission remain debated.

Purpose of the Study:

• To directly observe the structural dynamics of bromoform following UV excitation using ultrafast methods.
• To elucidate the competing reaction channels, specifically isomerization and direct dissociation, and their respective contributions.

Main Methods:

• Gas-phase ultrafast megaelectronvolt electron diffraction (MeV-UED) was employed.
• Single 267 nm photon excitation was used to initiate the photoreaction.
• Femtosecond temporal resolution allowed for the tracking of structural changes.

Main Results:

• Direct C-Br bond breaking occurs within 200 femtoseconds (fs).
• Isomerization to iso-CHBr3 (Br-CH-Br-Br) was observed on the same timescale, with a lifetime exceeding 1.1 picoseconds (ps).
• The branching ratio favors isomerization (approx. 60%) over direct dissociation.

Conclusions:

• Isomerization is a significant early-stage pathway in bromoform UV photochemistry.
• The observed isomer formation and lifetime support a roaming reaction mechanism.
• Results align well with ab initio molecular dynamics simulations.