Multidimensional tunneling and reaction-path dynamics in oxygen-insertion reactions of Criegee intermediates with small alkanes

Abstract

The alcohol-forming reactions of formaldehyde oxide (CH2OO) and its halogenated analogues (CCl2OO and CF2OO) with methane and ethane were investigated using dual-level variational transition state theory with multidimensional tunneling treatments. High-level coupled-cluster single, double, triple/complete basis set energetics were incorporated along minimum-energy paths computed at the MP2/aug-cc-pVDZ level to provide a consistent description of barrier profiles and reaction exothermicities. Representative systems were further analyzed using microcanonical multidimensional tunneling, while small-curvature tunneling treatments were applied to the remaining reactions. The results reveal that the reactions proceed through an asynchronous oxygen-insertion transition region in which hydrogen transfer and O–O bond weakening occur simultaneously, leading to narrow effective barrier regions and pronounced tunneling sensitivity. For CH2OO + CH4, tunneling enhances the canonical rate constant by nearly two orders of magnitude at 200 K, and deuterium substitution yields kinetic isotope effects exceeding one order of magnitude at low temperature. Oxygen-isotope effects approaching two at 200 K further indicate significant heavy-atom tunneling contributions. These reactions provide well-defined theoretical benchmark systems for evaluating tunneling and reaction-path effects in which hydrogen and oxygen tunneling jointly influence rate behavior. Halogen substitution substantially lowers the insertion barrier by up to 16 kcal/mol and produces rate constants approaching those of benchmark hydrogen-abstraction reactions, suggesting that faster halogenated systems may offer more accessible conditions for future experimental validation. These findings highlight the importance of reaction-path curvature and multidimensional tunneling effects in oxygen-insertion reactions and demonstrate that multidimensional tunneling treatments are essential for reliable kinetic predictions.