Dynamics of hydrogen shift reactions between peroxy radicals

Area of Science:

• Atmospheric Chemistry
• Chemical Kinetics
• Quantum Chemistry

Background:

• Peroxy radicals are vital atmospheric intermediates, influencing new particle formation (NPF).
• Radical-radical reactions form tetroxide intermediates, decomposing into alkoxy radicals and O2.
• Understanding the detailed mechanisms of alkoxy radical reactions, including H-shift pathways, is incomplete.

Purpose of the Study:

• To elucidate the microscopic mechanisms of H-shift and dissociation pathways in methoxy radical self-reactions.
• To investigate the roles of singlet and triplet electronic states in these reaction dynamics.
• To provide insights into the formation of accretion products relevant to new particle formation.

Main Methods:

• Time-dependent dynamics simulations using multireference XMS-CASPT2.
• Application of the mixed reference spin-flip time-dependent density functional theory (MRSF-TDDFT) method.
• Energetic calculations employing the metadynamics method.

Main Results:

• Both XMS-CASPT2 and MRSF-TDDFT simulations provided consistent dynamic results.
• The methoxy radical reaction, initiated in the triplet state, predominantly yields products on the singlet surface.
• Efficient intersystem crossing (ISC) facilitates the transition between electronic states.
• Calculated branching ratios for H-shift versus dissociation channels align well with experimental observations.

Conclusions:

• The study successfully established the microscopic mechanisms governing methoxy radical self-reactions.
• MRSF-TDDFT is a promising method for simulating dynamics of larger radical systems.
• The findings clarify the role of electronic state transitions in atmospheric radical chemistry and NPF.