UV absorption of Criegee intermediates: quantitative cross sections from high-level ab initio theory

Area of Science:

• Atmospheric Chemistry
• Combustion Chemistry
• Theoretical Chemistry

Background:

• Criegee intermediates (CIs) are key in atmospheric and combustion processes.
• Accurate UV absorption cross sections are needed for chemical modeling.
• Existing experimental data for CI UV spectra show discrepancies.

Purpose of the Study:

• To resolve discrepancies in experimental UV absorption cross sections of CIs.
• To provide reliable theoretical characterization of CI UV spectra.
• To calculate absolute intensities for individual conformers of CH3CHOO.

Main Methods:

• Semiclassical reflection principle approach with path integral molecular dynamics.
• Kernel density estimation for UV spectra processing.
• Advanced quantum chemical methods including multireference and single-reference approaches with triple excitations.

Main Results:

• Multireference character and electron correlation significantly impact CI spectra description.
• Single-reference methods become reliable with triple excitations.
• CC2/ADC(2) results conflict but reconcile at CC3/ADC(3) levels.
• Composite EOM-CCSD/T and CC2/3 methods yield accurate cross sections at lower cost.
• First absolute intensities for CH3CHOO conformers are provided.

Conclusions:

• The study provides a reliable theoretical framework for calculating CI UV absorption cross sections.
• The findings resolve experimental contradictions and improve atmospheric/combustion models.
• Accurate spectral data for CIs, including CH3CHOO conformers, are now available.