The effect of substitution on the unimolecular reaction rates of stabilized Criegee intermediates

Unimolecular reactions of stabilized Criegee intermediates (SCIs) with diverse substitution were studied using quantum chemical methods. The multiresonance electronic structure of SCIs was investigated by conducting both multireference and single reference benchmark calculations. A computationally inexpensive linear-scaling DLPNO-CCSD(T) method depicts SCIs sufficiently compared to other CCSD(T) levels of theory benchmarked in this study. Unimolecular reaction rate coefficients were calculated at the DLPNO-CCSD(T)/aug-cc-pVTZ//UM06-2X/aug-cc-pVTZ level of theory using lowest conformer transition state theory. We found several SCIs with slow unimolecular reactions and investigated how the molecular structure of a SCI affects its unimolecular reaction rate. Unimolecular structure-activity relationships were supplemented with reactivity trends for monocyclic saturated SCIs, conjugated cyclic SCIs, open chain aldehyde SCIs, and bicyclic monoterpene-derived SCIs. Structures with slow unimolecular reactions can react bimolecularly: long-lived SCIs are efficient oxidants of SO₂ leading to sulfate aerosols and accretion reactions with oxygenated organics lead to low volatility organic compounds capable of secondary organic aerosol formation. This study highlights the different reactivity of SCIs, and their various pathways to impact Earth's radiation budget.