Competition between H2O and (H2O)2 reactions with CH2OO/CH3CHOO

Area of Science:

• Atmospheric Chemistry
• Chemical Kinetics
• Quantum Chemistry

Background:

• Criegee intermediates (CIs) play a vital role in atmospheric oxidation processes.
• Understanding the reaction kinetics of CIs with water is essential for atmospheric modeling.
• Previous studies have lacked comprehensive data on CI reactions with water monomers and dimers.

Purpose of the Study:

• To determine bimolecular rate coefficients for CH2OO and CH3CHOO reactions with water monomer and dimer.
• To experimentally validate theoretical calculations for CH2OO reaction rates with water vapor.
• To assess the atmospheric significance of water-mediated CI removal pathways.

Main Methods:

• Ab initio calculations using QCISD(T)/CBS//B3LYP/6-311+G(2d,2p) for energy calculations.
• Anharmonic vibrational corrections using second-order perturbation theory for partition functions.
• Direct experimental measurement of CH2OO reaction rates with water vapor at elevated temperatures (348-358 K).

Main Results:

• Theoretical rate coefficients for CH2OO reactions with water agree well with experimental data across a broad temperature range.
• Rate coefficients for anti- and syn-CH3CHOO with water dimer and monomer were determined at room temperature.
• Estimated effective first-order rate coefficients indicate water vapor dominates removal for certain CIs at atmospheric concentrations.

Conclusions:

• Water vapor significantly influences the atmospheric lifetime of Criegee intermediates.
• The reaction pathway involving water monomer is particularly important for CH3CHOO decay.
• Theoretical calculations provide reliable estimates for CI reaction rates with water, aiding atmospheric models.