Criegee Intermediates in Autoxidation Reactions: Mechanistic Considerations.

Area of Science:

• Atmospheric Chemistry
• Chemical Kinetics
• Computational Chemistry

Background:

• Criegee intermediates (CI) are key species in atmospheric chemistry.
• Recent studies detected CI products in autoxidation reactions involving β-hydroxyperoxy radicals.
• The formation mechanism of these CI products remained unclear.

Purpose of the Study:

• To investigate the thermodynamic feasibility of direct CI formation from β-hydroxyperoxy radicals.
• To elucidate the reaction mechanism leading to CI products in autoxidation.

Main Methods:

• High-level G4 computational chemistry methods were employed.
• Thermodynamic stability and reaction pathways were analyzed.
• Potential energy surfaces for key intermediates were explored.

Main Results:

• Direct scission of β-hydroxyperoxy radicals to form Criegee intermediates is thermodynamically highly unfavorable.
• An alternative pathway involving hydrogen abstraction is proposed.
• This pathway proceeds via reversible formation of a dimeric tetroxide intermediate.
• A subsequent [1,6] hydrogen shift of the hydroxy hydrogen atom is crucial for CI formation.

Conclusions:

• The direct pathway for CI formation from β-hydroxyperoxy radicals is not viable.
• A multi-step mechanism involving hydrogen abstraction and a dimeric tetroxide intermediate is the likely route.
• This finding clarifies the formation mechanism of Criegee intermediates in radical-driven autoxidation processes.