Calculated Hydrogen Shift Rate Constants in Substituted Alkyl Peroxy Radicals

Area of Science:

• Atmospheric Chemistry
• Chemical Kinetics
• Organic Chemistry

Background:

• Peroxy radical hydrogen shift (H-shift) reactions are fundamental to atmospheric chemistry.
• These reactions contribute to the formation of highly oxidized organic molecules and atmospheric particle growth.
• Understanding H-shift reaction kinetics is vital for accurate atmospheric modeling.

Purpose of the Study:

• To calculate rate constants for a systematic set of H-shift reactions using a theoretical approach.
• To investigate the influence of various substituents on H-shift reaction rates.
• To determine the impact of radical stability and transition state ring size on H-shift prevalence.

Main Methods:

• Utilized an experimentally verified theoretical approach based on multi-conformer transition state theory.
• Calculated rate constants for a diverse range of H-shift reactions.
• Analyzed the effect of substituents (OH, OOH, OCH3, C=O, C=C) and radical type (primary, secondary, tertiary) on reaction rates.

Main Results:

• Substituents like OH, OOH, and OCH3 increased rate constants by over 50 times.
• C=O and C=C substituents led to resonance stabilization and rate increases exceeding 400 times.
• Reactions forming tertiary radicals were 30 times faster than those forming secondary radicals, which were 100 times faster than those forming primary radicals.
• H-shift rate constants exceeded 0.01 s⁻¹ for secondary carbons with specific substituents, and reached 1 s⁻¹ for 6-8 atom transition states.

Conclusions:

• H-shift reactions are significantly influenced by molecular structure and substituents.
• The calculated rate constants indicate H-shift reactions are more prevalent in the atmosphere than previously assumed.
• These findings necessitate re-evaluation of H-shift reactions' role in atmospheric chemistry and particle formation.